Advanced study of the following and their applications

Gas chromatography: Introduction: theory: Column operation: InstrumentationAnd detection . GCMS.

Gas-liquid chromatography (often just called gas chromatography) is a powerful tool in analysis.

Prof. p. Ravisankar Vignan Pharmacy collegeValdlamudiGuntur Dist.Andhra PradeshIndia. banuman35@gmail.com00919059994000

GAS CHROMATOGRAPHY

Chromatograph(or)

Chromatogram

Column

GAS CHROMATOGRAPHY( A separation Technique)

Injection port

Gas separator (or) Aerograph.

Gas chromatography (GC) is most widely used analytical method for the separation of volatile and semi-volatile organic compounds without decomposition

Small amounts of sample For ex.1ml of air 1µl (microlitre) uL; 1/1000 of a mL)the solutions either Liquids in solids in solution

The compounds are separated primarily based on the relative(differences in their) volatilities

What is gas chromatography?

Genarates a written record of analysis

All forms of chromatography involve a stationary phase and a mobile phase. In all the other forms of chromatography you will meet at this level, the mobile phase is a liquid. In gas-liquid chromatography, the mobile phase is a gas such as helium and the stationary phase is a high boiling point liquid absorbed onto a solid.

Introduction

From the column, the separated solutes pass through a detector where they are sensed generating an electronic signal. The signal is then amplified and normally displayed on a strip chart recorder. The trace plotted on the recorder is called a "Chromatogram". It is a plot of the detector response in millivolts as a function of time. Time is the Abscissa(horizontal or X-axis)And millivolts the Ordinate.(y- coordinate(vertical axis)

Gas Chromatograph Components

Flame Ionization Detector

Column

Oven

Injection Port

front view

One milligram in a kg is 1 ppm (by mass). One liter . so 1 mg/L is 1 ppm

One ppb represents one microgram of something per liter of water (ug/l), or one microgram of something per kilogram of soil (ug/kg).

GC

with a TCD the components can continue on to another detector after passing through the TCD; thus it is considered a non-destructive detector (this can be useful for further analysis.

What are advantages of GC?The GC is one of popular instrument used in the worldSeveral advantages includeHigh resolution. High speed.High sensitivityThe GC is robust, flexible, and user-friendly. Most importantly, it has the best value in the market.. High resolution :Many compounds can be resolved nicely for ex: Gasoline has been resolved in to over 300 different peaks complex sample of Petroleum. (complex mixture can be resolved in constituents) The term resolution refers to well separated two peaks are from each other. If two peaks are so close together that you can’t tell when one peak begins and the other ends the resolution is poor; you can also say that the separation is poor. If you can clearly identify two different peaks- the resolution is good. The longer and more narrow the column, the better the resolution. Increasing the carrier gas flow rate and/or the temperature will send the vapours through the column faster, which will lower the retention time and worsen the resolution. Lowering the temperature and/or flow rate increases retention times and broadens the peaks.

High speed: Most of the compounds Analyze in a matter of minutes and few compounds cananalysis in a matter of few seconds also possible.High sensitivity: we see both rapid analysis as well as high sensitivity.(detectability) evenProviding selective(identification)For ex a 2 1/2 min of separation 3 common pesticides Methyl parathion, Malathion, Ethion at pg levels. Pg 10-12 grams means these are parts per billion. (One trillionth of a gram )This is a very good ex of both the high speed as well as a very sensitive detection.

.

Methyl parathion

Malathion

Ethion

High accuracy: It gives good precision and accuracy. (free from errors)Accuracy just means we do quantitative analysis we get a very good results write answer, good quantitative results. (possible or actual deviation from the exact answer(exactness).GC is also a very easy technique compare to some very well known . most widely used instruments in the world today

GC and liquid chromatography together just been the premier techniques for Trace analysis of organic and inorganic compounds.

All the work which has been done by Air pollution and water pollution and food safety we hv. to analyze for pesticides toxic chemicals founds in Food and food products all of these things are done by routinely daily and rapidly by gc and or liquid chromatography.

In essential role of chromatography is the QC and foods, and drugs control in raw material and finished products ensuring the safety of the people. we are so dependent on the world today on chemicals synthetic chemicals made by chemist. Primarily pesticides very good for agriculture and very harmful for humans. So Chromatography is the best separation technique for quantitative trace analysis of toxic chemicals.

1. GC the samples must be volatile. Allow to heat the samples up to 3000 and 3500C at that point we must generate Vapors that can be easily carried by the carrier gas.2. Dirty samples require cleanup. dirty samples Duran, waste water Extracts of many things. and these samples normally contaminate the system and may even Plug up the column and destroy the column and these case often times we will extracts with the Solvents to take off the imp components.3. Another limitation is we must use another instrument for ex a Mass spectrometer for conformation Typically we use the retention time of standards of un knows to decide what the peak can be. but Legally in the us the retention times are not considered a conformation. A MS is need for conformation if necessary., and finally of course some training/ and some experience is necessary in order to get good results. Samples for GC: They can be gases Almost every gas has been analyzed liquids and some solids solids are Usually dissolved in a low boiling solvents and analyzed. Molecular wt. has been done easily from Molecular weight 2 hydrogen, up to over 800 (2 to~800).

But we must be honest also limitations….

In exceptional cases simple hydrocarbons MW up to 1200 have been separated.Samples can be organic and inorganicBut most of the things in GC are organic compounds.Inorganic would be water or gases.The samples always must be volatile but you cant do rocks , sticks or stones andYou can’t also do proteins, peptides and biological molecules those are better done by liquid chromatography.For Quantitative analysis peak area is proportional to the concentration I would like to show u some data here this is a very simple sample of hydrocarbonsDecane,(C10H22)undecane(C11H24) ,dodecane(C12H26), tridecane(C13H28) which would blended up volumetrically andUsing density is calculated mass in grams this is determined by GC plus 1 SDand results and relative error is less than 1% in this case. This is a simple sampleDone by flame ionization to gather with aid of computer typical very high accuracy One can obtain.Quantitative analysis is the major advantages of GC.

ChromatographyIntroduction:The term chromatography is derived from a Greek words Chromatos= meaning colour.Graphos = written.Initially used for analysis of coloured compounds now due to vast Developments it is also applied to colourless compounts.Chromatography is the separation technique of a mixture of compounds (solutes) into separate components By using a stationary phase and a mobile phase so it is easier to identify (qualitative) and measure the amount (quantitate) of the various sample components.Chromatography makes use of 2 phases. The mobile phase and the stationary phase.Mobile phase refers to the mixture of analyte while stationary phase consists of fixed solid orLiquid medium. The mobile phase run over the stationary phase and the mixture gets distributedb/n these 2 phases resulting in the separation of analyte/solute. Finally separated analyte is Identified qualitatively or quantitatively by Massspectrometry,IR,NMR. The technique for GC is similar to that of column Chromatography except that the liquid mobile phasein the column chromatography is replaced by a moving gas.

History– Russian botanist and physical chemist

Scientist Mikhail Semenovich Tswett is credited for the discovery of chromatography in the early 1900’s (1903).

– Germangraduate student Fritz Prior is credited for developing

solid state gas chromatography (1947).– However the foundation of the GSC

was laid down by Damkohler and Thiele in 1943.

Mikhail Tswett, Erika Cremer, A.J.P. Martin og Richard L. M. Synge

• Modern GC was invented in 1952 by Martin and James.

• The father of modern gas chromatography is Nobel Prize winner(1952) John Porter Martin,Synge who also developed the first liquid-gas chromatograph. (1950).

• The gas chromatography technique was first carried out in Austria in 1944 by the chemist Erika Cremer, who used a solid stationary phase.

• Griffin and George (London, UK) probably manufactured the first commercial GC system in 1954, and several companies, including Perkin Elmer, Fisher/Gulf, Barber Coleman, Podbelniak (all U.S.-based) and Pye Unicam (UK), followed shortly in 1955 and 1956.

• Harold McNair was lucky to be around in the beginning of gas chromatography (GC). Harold heard about it in 1956, made his first injections in 1957, and he is still working with it today.

• Dr. R. Gohlke had introduced the first GC–mass spectrometry (MS) experiment in 1959 using a packed column Harold McNair

H C H

H

H

H H

O3.

Bonding electrons are not shared evenly.The end of the bond with electrons becomes partially negative.The end of the bondwithout electrons becomes partially positive.

Polar

Polar compound

Non-polar compound

Polar compounds are soluble in polar solvents.Non-polar compounds are soluble in non-polar solvents.

Basic rule in organic chemistry is that “like dissolves like rule .” Thus the polar solvent water dissolves the polar solute ethanol but not the hydrocarbon octane. The nonpolar solvent benzene will dissolve octane but not ethanol. Polar stationary phases will retain polar solutes and pass those that are nonpolar. The order of emergence is reversed with nonpolar stationary phases.

Principle: 5 Principle 1.Adsorption in GSC 2.Partition of molecules between gas (mobile phase) and liquid (stationary phase) in GLC (gas) MOBILE PHASE Sample in Sample out STATIONARY PHASE (solid or heavy liquid coated onto a solid or support system)

Packing material

The most popular packing material is silica gel.It is believed that silanol radicals ( -Si-OH ) on the surface of silica gel act as the active site and the sample is separated.

SiSi

SiOH

OHOH OHOH

OH

the surface of silica gel

Gas-liquid chromatography (often just called gas chromatography)

Depending upon the nature of stationary phase used gas chromatography is divided in to 2 types.1.Gas liquid chromatography (GLC) or (VPC) or (GC): Principle of separation is PARTITION. In this technique an inert porous solid is coated by a high boiling viscus liquid orNon volatile liquid(carbowax 200m(1500C)Applications –CHO,>C=0,PEG(2000C) Alcohols,pestisides, poly siloxane(2500C)steroids,glycols,pestisides, Silicon rubber gum(300-3500C maximum temp.)Alkaloids,vitamines,gums,fattyacids.,or polymer .The inert solid support which acts as a stationary phase ,then this chromatography Is termed as GLC (or) (VPC) vapour-phase chromatography . Separation is based on the relative volatilities.Commonly used support for liquid phase is Diatomaceous earth (or) Kieselguhr.supports may be either firebrick materials such as (chromosorb-p ,Anakrom- ABS)

Generally this technique is most widely used.

Gas-solid chromatography the solid adsorbent is used as a stationary phase , it is termed as GSC.In this technique the components of the mixture get distributed between the gas and the solid phase due to differences in their adsorptive behavior. This technique is mainly used for the separation of gases and it is rarely used.The common adsorbants are Zeolite, activated alumina, carbon, Granular silica gel etc. It has few applications because-The active gases get retained on the solid surfaces. -Reproduction of surface area is difficult.-Elution peaks results in tailing

2. Gas solid chromatography (GSC): In GSC the principle of separation is ADSORPTION

In gas chromatography, the basis for separation is the distribution of solutes between two phases. One of these phases is a stationary bed of large surface area (stationary phase) and the other is a gas (mobile phase) which percolates through the stationary phase. If the stationary phase is a solid adsorbent, then it is known as gas- solid chromatography (GSC). Here, the adsorptive properties of the stationary phase areresponsible for separating solutes, primarily gases. Common solid stationary phasesare silica gel, molecular sieves, porous polymers, alumina and charcoal. In case, thestationary phase is liquid, it is called gas- liquid chromatography(GLC). The liquid isspread (coated) as thin film over an inert support. The basis of separation is thepartitioning of the solutes in and out of the liquid film. There is a wide range of theliquid phases with usuable temperatures up to 400ºC. This makes GLC the mostversatile and the selective form of chromatography. It is used for analysis of gases,liquids and solids.

Explain the basic principle of gas chromatography? Or Describe the principle of GC…. According to the Henry’s law of partition when certain amount of gas comes in contactWith a liquid in a stationary phase a specific amount of gas gets dissolved in the liquid.

= Kc Where

X = Amount of gas m = Amount of liquidC = Concentration of liquid gasK = Constant.The principle of GC is similar to the Column chromatography,HPLC as well as TLC with the following differences.1. In GC the separation of mixture of components occurs b/n gas mobile phase and liquid stationary phase where as in other chromatographic techniques it occurs b/n liquid mobile phase and a solid stationary phase.2. The concentration of solute(sample) of components in the carrier gas is entirely a function of vapour pressure of the carrier gas.3. In GC the column is placed in an oven whose temperature can be controlled where as in other chromatographic techniques temperature programming is not essential.

Mover ever principle of GC is also similar to the principle of fractional distillation.Where in the components of the mixture are separated depending up on the differencesIn their boiling points. The GC is used on micro scale and the fractional distillationon the large scale basis.

After the known volume of sample to be analyzed has been injected in to the injection Port which is maintained at temp. higher than the boiling point of the sample.In the column the partitioning of the sample components occurs in between the carrier gasAnd the high boiling liquid in accordance with the equilibrium law.The partitioning properties of sample components differs from one another.The components that have greater tendency to get dissolved in the liquid stationary phaseMove slowly through the column while negligible solubility move rapidly. Hence the Components are carried along the column at different rates and at different retention times.

Basic Chromatography Theory

A GC separation, like extraction, involves a partitioning of solutes between phases. In the case of GC, one phase in stationary and the other is mobile. The more a solute is partitioned in the mobile phase, the more it moves; in other words, the partitioning between the stationary and mobile phases affects the time required for a solute to travel through the instrument.

A solute that interacts very little with the stationary phase (via van der Waals forces such as dispersion, dipole-dipole interactions, etc.) moves relatively quickly through the column. Such a solute is not retained by the stationary phase material. A solute with strong interactions with the stationary phase is retained by that phase; such a solute will take longer to travel through the column. This is essentially the same for all types of chromatography (thin layer, paper, liquid-liquid, etc). Gas chromatography is more precisely described as gas-liquid chromatography.

Distribution of analytes between phases

The distribution of analytes between phases can often be described quite simply. An analyte is in equilibrium between the two phases;

Amobile A stationary

The equilibrium constant, K, is termed the partition coefficient; defined as the molar concentration of analyte in the stationary phase divided by the molar concentration of the analyte in the mobile phase

Diatomaceous earth which is a very porous rock)

The process where a substance divides itself between two immiscible solvents because it is more soluble in one than the other is known as partition. Now, you might reasonably argue that a gas such as helium can't really be described as a "solvent". But the term partition is still used in gas-liquid chromatography.

You can say that a substance partitions itself between the liquid stationary phase and the gas. Any molecule in the substance spends some of its time dissolved in the liquid and some of its time carried along with the gas

when a mixture is introduced into the hot column, a component that does not dissolve in the liquid would be vaporized by the heat and carried straight though the capillary column at the same speed as the helium gas which is called the carrier gas. A compound of the mixture that dissolves in the liquid called the stationary phase and has less interactions with the gas phase would remain near the start of the column and move through it with difficulty. Consequently, different compounds are separated within the column because they move through it at different rates, depending of the partition between the stationary phase and the mobile carrier gas.

Principle of searation in Gas liquid chromatography: The principle of separation in GLC is Partition. Gas is used as mobile phase. Liquid which is coated on to a solid support is used as stationary phase. The mixture of components to be separated is converted to vapor and mixed with Gaseous mobile phase. The component which is more soluble in the stationary phase travels slower and eluted later. The component which is less soluble in the stationary phase travels faster and eluted out first. No two components has the same partition co-efficient for a fixed combination of Stationary phase, mobile phase and other conditions. Hence the components are separated according to their partition co-efficients. Partition co-efficient is the ratio of solubility of a substance distance between two immiscible liquids at a constant temperature.

2 important criteria for compounds To be analysed by GC are

Volatility: unless a compound is volatile,It cannot be mixed with mobile phase.Hence volatility is important.Thermostability: All the compounds willNot be in the form of vapour. The solidAnd liquid samples convert them to a vapourForm higher temparature is required. That’sWhy compounds have to be thermostable.4m

4mm

Theory of gas chromatography:The column efficiency (or) performance is usually measured in terms of No. of TheoreticalPlates as well as HETP.( Hight equivalent to theoretical plates).

Plate theory was initially developed According to plate theory The vapors come in ,they partition in to the stationary phase and red Stationary phase ,Vapours come to equilibrium with moving gas stage and come back again in the series of steps adsorbing- disrobing ,adsorbing disorbing each of the steps is called a theoretical plate . (Parallel layers of discrete and Continuous horizontal plates)Theoritical plate is an imaginary or hypothetical unit of a column and they do not reallyExist.They help to understand the functioning of the column and serve to measure the Column efficiency.A theoretical plate can also be called as a functional unit of the column.The efficiency of the column can be increased by increasing the no. of theoretical plates.The no of theoretical plates can be calculated (determined) by using the formula:

the chromatographic column is contains a large number of separate layers, called theoretical plates

No of theoretical plates also known as column efficiency. HETP

It is important to remember that the plates do not really exist

Measuring column efficiency, either by stating the number of theoretical plates in a column, N (the more plates the better), or by stating the plate height; the Height Equivalent to a Theoretical Plate (the smaller the better).

2

Where

N = Number of theoretical platestR = Retention time.W = Peak width obtained upon drawing tangents from 2/3rd height of the peak up to the base line.

W

The main reason why different compounds can be separated this way is the interaction of the compound with the stationary phase“(like-dissolves-like”-rule). The stronger the interaction is the longer the compound remains attached to the stationary phase, and the more time it takes to go through the column (=longer retention time).

Theory of Operation• Velocity of a compound through the column

depends upon affinity for the stationary phase

Area under curve is ______ of compound adsorbed to stationary phase

Gas phase concentrationCarrier gas

mass

How does peak look ..lets look at the peak. you see here Selectivity and efficiency is imp. In the case of the peakWhat are the desired characteristics of how peak look like… If the response is distinguished from all other responses the method is said to be selective. selectivity obtained by choosing optimal columns and setting chromatographic Conditions such as mobile phase composition, column temparature and detector wavelength.

Who will decide this one efficiency of the detector selectivity of the detector andAlso the partition ratio.

the sharpness of the peak is an indication of how good, or efficient a column is. The plate number depends on column length: the longer the column, the larger the plate number. Therefore, the plate height term has been introduced to measure how efficiently column has been packed, h = L/N .The lower the plate height and the higher the plate number, the more efficient the chromatographic column.

Scheme of a chromatogram

tR Total retention time of the compound (in the whole chromatographic system)t’R Adjusted retention time of the compound (retention time in the stationary phase)tM "Dead time" (retention time in the mobile phase)W0,5 Peak width at half heighth Height of a signal

This yields in following equation: tR = t’R + tM

What influences the separation?1. Polarity of the stationary phasePolar compounds interact strongly with a polar stationary phase, hence have a longer retention time than non-polar columns. Chiral stationary phases based on amino acid derivatives, cyclodextrins, chiral silanes, etc are capable to separate enantiomers, because one form is slightly stronger bonded than the other one, often due to steric effects.2. TemperatureThe higher the temperature, the more of the compound is in the gas phase. It does interact less with the stationary phase, hence the retention time is shorter, but the quality of separation deteriorates.3. Carrier gas flow If the carrier gas flow is high, the molecules do not have a chance to interact with the stationary phase. The result is the same as above.4. Column lengthThe longer the column is the better the separation usually is. The trade-off is that the retention time increases proportionally to the column length. There is also a significant broadening of peaks observed, because of increased back diffusion inside the column.5. Amount of material injectedIf too much of the sample is injected, the peaks show a significant tailing, which causes a poorer separation. Most detectors are relatively sensitive and do not need a lot of material (see below).6. ConclusionHigh temperatures and high flow rates decrease the retention time, but also deteriorate the quality of the separation.

Another term affecting the column efficiency is the HETP.HETP may be considered as the Height or thickness of the imaginary layers of a columnGreater the no. of theoretical plates (N) or smaller the HETP means the more efficient is the column.

N and HETP are related by the following formula:

(Height equivalent to theoretical plates ) HETP Where L is the length of the column.

Factors that affect the N and HETP include the

Injecting techniqueDesign of the column.Gas flow rateCarrier gasLiquid stationary phaseTemperature programming Detector.

Rate theory:Sharp and symmetrical peaks of the chromatogram are indicates of optimal separation In GC. Column efficiency is affected by the amount of band broadening that occurs as the sample passes through the column.This requires limited band broadening.Retention time (band separation)Van Deemter et. Al. Proposed the rate theory to overcome this drawback of plate theory.According to rate theory describes the influence of variables that affect the band separation(retention time ) and band broadening.Van Deemter equation is useful in optimizing the chromatographic performance and can be Expressed as

+ Cu

Where A = Coefficient of Eddy diffusion B = Molecular diffusion(longitudinal diffusion) C = Resistance of mass transfer u = Average gas velocity or flow rate.

(average velocity of the mobile phase)

it is ideal to have the bands of the individual components as narrow as possible. This is to say that it is best to have each component occupying as little space as possible within the column:

From this figure it can be seen that a better separation between narrow bands of components is ideal for easier collection of the individual samples.

H is the height equivalent of a theoretical plate4 (HETP) and u is the velocity (flow rate) of the mobile phase. The lower the resulting value of H is, the greater the efficiency of the procedure. So, ideally, a scientist will want to minimize all three terms in order to minimize H.

The following figure helps in visualizing Eddy diffusion:The A factor is determined by a phenomenon called Eddy Diffusion. This is also called the multi-path term. Solute molecules will take different paths through the stationary phase at random. This will cause broadening of the solute band, because different paths are of different lengths

In the figure, particle B will be eluted before particle C, and both will be eluted before particle A. Since it is improbable for all particles of one compound to find the shortest path, there will be fractions of the component that will behave like particles A, B, and C. This leads to the broadening of the band. There is little a scientist can do to minimize the Eddy Diffusion factor, can be decreased by using smaller particle size(particle size should be optimum) Shape and manner of packing,Column diameter.

very smaller size particles also increase the pressure drop leading to disturbance in linear gas velocity that’s why decrease the column efficiency. Generally Eddy diffusion can be minimized by using small particles of uniform size and smaller diameter columns. columns should be 1/8inch inner diameter and particle size up to 100-200 mesh range are used for good resolution. as

granular bed of the packing particles

The A term is loosely affected by the flow rate of the mobile phase, and sometimes the affect of the flow rate is negligible.

B/u is called the longitudinal diffusion term, and is caused by the components' natural migration from a place of high concentration (the center of the band) to a place of lower concentration (either side of the band) within the column. Diffusion occurs because molecules in a place of high concentration will tend to spread out to areas of lower concentration to achieve equilibrium.

B - Molecular diffusion( Longitudinal diffusion)

. Longitudinal diffusion is a chief cause of band broadening in Gas Chromatography, as the diffusion rates of gaseous species are much higher than those of liquids.The magnitude of the term B/u can be minimized by increasing the flow rate of the mobile phase. Increasing the velocity of the mobile phase does not allow the components in the column to reach equilibrium, and so will hamper(restrict the activity or free movement) of longitudinal diffusion. Hence the molecular diffusion can be decreased by using the optimum linear gas velocity (flow rate) And using high molecular weight carrier gas eg: Nitrogen, argon than hydrogen or helium.

At time zero in the figure above, the particles of a compound are generally localized in a narrow band within the separating column. If the mobile phase flow rate is too small or if the system is left at rest, the particles begin to separate from one another. This causes a spread in the concentration distribution of that compound within the column, thus bringing about band broadening for the band of that particular compound. As the time that the system is left still approaches infinity, the compound reaches complete concentration equilibrium throughout the entire column.

B - Longitudinal diffusionThe concentration of analyte is less at the edges of the band than at the center. Analyte diffuses out from the center to the edges. This causes band broadening. If the velocity of the mobile phase is high then the analyte spends less time on the column, which decreases the effects of longitudinal diffusion.

. Nonequilibrium or mass transfer: The analyte takes a certain amount of time to equilibrate between the stationary and mobile phase. If the velocity of the mobile phase is high, and the analyte has a strong affinity for the stationary phase, then the analyte in the mobile phase will move ahead of the analyte in the stationary phase. The band of analyte is broadened. The higher the velocity of mobile phase, the worse the broadening becomes.The flow rate of the mobile phase should not be increased in excess, however, as the term Cu is maximized when u is increased. Cu is referred to as the mass transfer term. Mass transfer refers to when particles are so strongly adhered to the stationary phase that the mobile phase passes over them without carrying them along. This results is particles of a component being left behind. Since it is likely that more than a single particle of any given compound will undergo this occurrence, band broadening results. This results in a phenomenon called tailing, in which a fraction a component lags behind a more concentrated frontal band. Non-equilibrium effects can be caused by two phenomena: laminar flow and turbulent flow. Laminar flow occurs in tubular capillaries, and so is most prominent in Capillary Electrophoresis. Turbulent flow occurs as a result of particles becoming overwhelmed by the stationary phase and is more common in column chromatography.

In the above figure, particles of the adsorbent solid become occupied by particles of the sample. If too many particles of the adsorbent are occupied, particle A will have nothing hindering it from flowing through the column. So, the particles of a single compound separate from one another. Also, as the mobile phase moves through the column, particles of the sample leave the stationary phase and migrate with the mobile phase. However, if the flow rate of the mobile phase is too high, many of the sample particles are unable to leave the stationary phase and so get left behind. These occurrences result in band broadening, as the individual particles of a single compound become less closely packed. The high flow rate of the mobile phase makes it more difficult for the components within the column to reach equilibrium between the stationary and mobile phase. It is for this reason that the Cu term is also called the non-equilibrium factor. Minimization of this factor can be achieved by decreasing the flow rate of the mobile phase. Decreasing the flow rate of the mobile phase gives sample components more time to leave the stationary phase and move with the mobile phase, thus reaching equilibrium.

By observing the Van Deemter equation, it can be deduced that an ideal mobile phase flow rate must be determined to yield the best (lowest) value of H. Decreasing the flow rate too much will result in an increase of the longitudinal diffusion factor B/u, while exceedingly increasing the flow rate will increase the significance of the mass transfer term Cu. So, H can be minimized to a finite limit depending on the various parameters involved in the chromatography being performed.

van Deemter profile for hydrogen, helium, and nitrogen carrier gases.

The curves were generated by plotting the Height Equivalent to a Theoretical Plate (H.E.T.P., the length of the column divided by the total number of theoretical plates) against the column's average linear velocity. The lowest point on the curve indicates the carrier gas velocity at which the highest column efficiency is reached.

Hydrogen is the fastest carrier gas (upto : 40cm/sec.) and exhibits the flattest van Deemter profile. Helium is the next best choice (uopt: 20cm/sec.). The head pressures at optimum flow rates are similar for hydrogen and helium because hydrogen has half the viscosity and double the linear velocity of helium. Nitrogen's performance is inferior for capillary columns and is usually not recommended because of the slow optimum linear velocity (uopt: 12cm/sec.) and steep van Deemter profile.

Van Deemter plotsA plot of plate height vs. average linear velocity of mobile phase.Such plots are of considerable use in determining the optimum mobile phase flow rate.

The van-Deemter-equation demonstrates the dependence of getting sharp peaks (low HETP) from the following terms:small particle’s diameter (particle size), small column’s diameter, small film thickness of the stationary phase.

When HETP is plotted against u we get Hyperbola with minimum HETP.This minimum is the optimum flow rate (u)At which column efficiency is maximum.

This graph represents the van deemter curveWhich is a hyperbola with minimum HETP.It shows that the affects of A,B,C on the relation ship b/n HETPAnd the gas flow rate.Minimum HETP indicates max. efficiency of The column.Hence the ideal flow rate Corresponding to the minimum value of HETP is used.

Such plots are of considerable use in determining the optimum mobile phase flow rate.

A- Eddy diffusion

C-Resistance to mass transferB- Molecular diffusion

2.Symmetry Factor:Apart form the the N and HETP another term used to measure the column efficiency orColumn performance is symmetry factor(S) of a peak.

H1/20

It is given by the following equation S = -------------- 2A

S = Symmetry factorH = Peak width at 1/20th level of its height.A = Distance between the perpendicular dropped from the maximum peak height upto 1/20th peak height.When the symmetry factory of a peak is equal to 1 it implies that the peak is reasonablySymmetrical.Hence peak height may be used for the calculation of chromatogram.However, when the symmetry factor of peak is less or greater than 1, thenFronting( is due to saturation of stationary phase and can be avoid by using less Quantity of sample) or tailing of the peak( is due to more active adsorption sites and can be eliminated by support pretreatment,more polar mobile phased increasing the amount of liquid phase) may be seen.

Asymmetry factor:A chromatograhic peak should be symmetrical about centre and peak should be like anIsosceles triange.But in practice due to some factors peak is not symmetrical and shows tailing and fronting .Asymmetry factor (0. 95 to 1.05) can be calculated by using the formula b AF = ---------- b and a calculated at 5% or 10% of the peak height. a

Solvent Efficiency

In GC the liquid phase or stationary phase may be used ( selective solvents) choice ofInterest.

1. Resolution(R): The true separation of 2 consecutive peaks on a chromatogram is measured by resolution. It is the measure of both the column and solvent efficiencies It is expressed as the distance of separation of 2 consecutive peak maxima in the chromatogram.

D = Distance b/n the 2 consecutive peak maximaW1 +W2 = peak widths of peak 1 and peak 2.When R = 1. 98% resolution is achieve. R= ≥ 99.7% resolution is achieved.

Number of platesNumber of theoretical plates determine the length of the column. It can be calculated byThe following formula 2

Where N is no. of theoretical plates. α=Solvent efficiency R =Resolution K’2 =partition ration of second peak.

K’2 =Adjusted retention time(T’R)----------------------------------------- Retention time in Air

𝑁=4𝑅 αα−1

Interaction forces:Forces that help to carry out the separation process in GCa)Debye forces: Debye or induced dipole forces are the minor forces that result from the interaction of 2 molecules i.e. Induced dipole in in one molecule and dipole in another molecule.b) Orientation forces: These forces are the result of interaction between 2 permanent dipoles.c)Specific interaction forces: These forces are the resultant of the variations that are made to occur in the immediate dipoles of any two interacting species.d)Non Polar forces: These forces are the resultant of the variations that are made to occur in the immediate dipoles of any two interacting species.4.Partition Ratio (K)™Partition ratio is the ratio of total concentration or amount of solute in the column or stationary phase(t’R) to the total concentration or amount of solute in the gas or mobile phase Amount of solute in column t’R

K = ---------------------------------------- K = ----------- Amount of solute in gas tM

Partition ratio is the resultant effect of combination forces i.e., Debyforces ,orientation forces, specific interaction forces and non polar

Forces.

It depends upon the temp.of column ,nature of components of solute and liquid phase as well as

On the quantity of liquid phase in the column.

Retention Time (tR)

The retention time is the total time that a compound spends in both the mobile phase and stationary phase. Retention time is generally reported in minutes.

Dead Time (tm)

The dead time is the time a non-retained compound spends in the mobile phase which is also the amount of time the non-retained compound spends in the column. Dead time is generally reported in minutes.

Adjusted Retention Time (tR')

The adjusted retention time is the time a compound spends in the stationary phase. The adjusted retention time is the difference between the dead time and the retention time for a compound.

Capacity Factor (or Partition Ratio) (k')

The capacity factor is the ratio of the mass of the compound in the stationary phase relative to the mass of the compound in the mobile phase. The capacity factor is a unit less measure of the column's retention of a compound.

Phase Ratio (ß)

The phase ratio relates the column diameter and film thickness of the stationary phase. The phase ratio is unitless and constant for a particular column and represent the volume ratioß.

Distribution Constant (KD)

The distribution constant is a ratio of the concentration of a compound in the stationary phase relative to the concentration of the compound in the mobile phase. The distribution constant is constant for a certain compound, stationary phase, and column temperature.

Selectivity (or Separation Factor) (alpha)

The selectivity is a ratio of the capacity factors of two peaks. The selectivity is always equal to or greater than one. If the selectivity equals one the two compounds cannot be separated. The higher the selectivity, the more separation between two compounds or peaks.

Linear Velocity (u)

The linear velocity is the speed at which the carrier gas or mobile phase travels through the column. The linear velocity is generally expressed in centimeters per second.

Efficiency

The efficiency is related to the number of compounds that can separated by the column. The efficiency is expressed as the number of theoretical plates (N, unitless) or as the height equivalent to a theoretical plate (HETP, generally in millimeters). The efficiency increases as the height equivalent to a theoretical plate decreases, thus more compounds can be separated by the column. The efficiency increases as the number of theoretical plates increases, thus the column's ability to separate two closely eluting peaks increases.

The oven is used for maintaining the precise temparature control around theColumn. Hence the oven should be free from the influence of changing Ambient temparature and should have adequate air flow system.To guard against sample deposition After long use,narrow glass or metal inserts are provided in the injection port. The sample deposition on inserts can be taken out periodically for cleaning purpose

A typical GC system used is shown below (a gas chromatograph)

Carrier gas: He (common), N2, H2

Pinlet 10-50 psigFlow = 25-150 mL/min packed columnFlow = 1-25 mL/min open tubular column

Column: 2-50 m coiled stainless steel/glass/TeflonOven: 0-400 °C ~ average boiling point of sample

Accurate to <1 °CDetectors: FID, TCD, ECD, (MS)

How separation works on the columnOne of three things might happen to a particular molecule in the mixture injected into the column:It may condense on the stationary phase.It may dissolve in the liquid on the surface of the stationary phase.It may remain in the gas phase.None of these things is necessarily permanent.A compound with a boiling point higher than the temperature of the column will obviously tend to condense at the start of the column. However, some of it will evaporate again in the same way that water evaporates on a warm day - even though the temperature is well below 100°C. The chances are that it will then condense again a little further along the column.Similarly, some molecules may dissolve in the liquid stationary phase Some compounds will be more soluble in the liquid than others. The more soluble ones will spend more of their time absorbed into the stationary phase; the less soluble ones will spend more of their time in the gas.The process where a substance divides itself between two immiscible solvents because it is more soluble in one than the other is known as partition. Now, you might reasonably argue that a gas such as helium can't really be described as a "solvent". But the term partition is still used in gas-liquid chromatography.You can say that a substance partitions itself between the liquid stationary phase and the gas. Any molecule in the substance spends some of its time dissolved in the liquid and some of its time carried along with the gas.

A compound with a boiling point higher than the temperature of the column will obviously tend to condense at the start of the column. However, some of it will evaporate again in the same way that water evaporates on a warm day - even though the temperature is well below 100°C. The chances are that it will then condense again a little further along the column.Similarly, some molecules may dissolve in the liquid stationary phase Some compounds will be more soluble in the liquid than others. The more soluble ones will spend more of their time absorbed into the stationary phase; the less soluble ones will spend more of their time in the gas.The process where a substance divides itself between two immiscible solvents because it is more soluble in one than the other is known as partition. Now, you might reasonably argue that a gas such as helium can't really be described as a "solvent". But the term partition is still used in gas-liquid chromatography.You can say that a substance partitions itself between the liquid stationary phase and the gas. Any molecule in the substance spends some of its time dissolved in the liquid and some of its time carried along with the gas.

Retention timeThe time taken for a particular compound to travel through the column to the detector is known as its retention time. This time is measured from the time at which the sample is injected to the point at which the display shows a maximum peak height for that compound.Different compounds have different retention times. For a particular compound, the retention time will vary depending on:the boiling point of the compound. A compound which boils at a temperature higher than the column temperature is going to spend nearly all of its time condensed as a liquid at the beginning of the column. So high boiling point means a long retention time.the solubility in the liquid phase. The more soluble a compound is in the liquid phase, the less time it will spend being carried along by the gas. High solubility in the liquid phase means a high retention time.

the temperature of the column. A higher temperature will tend to excite molecules into the gas phase - either because they evaporate more readily, or because they are so energetic that the attractions of the liquid no longer hold them. A high column temperature shortens retention times for everything in the column.For a given sample and column, there isn't much you can do about the boiling points of the compounds or their solubility in the liquid phase - but you do have control over the temperature.The lower the temperature of the column, the better the separation you will get - but it could take a very long time to get the compounds through which are condensing at the beginning of the column!On the other hand, using a high temperature, everything will pass through the column much more quickly - but less well separated out. If everything passed through in a very short time, there isn't going to be much space between their peaks on the chromatogram.The answer is to start with the column relatively cool, and then gradually and very regularly increase the temperature.At the beginning, compounds which spend most of their time in the gas phase will pass quickly through the column and be detected. Increasing the temperature a bit will encourage the slightly "stickier" compounds through. Increasing the temperature still more will force the very "sticky" molecules off the stationary phase and through the column.

It should be inert and available at low costHigh purityEasily availableLess risk of explosion or fire hazardsPressure: -Inlet 10 to 50 psi -packed column 25 to 150 mL/min. - capillary column 1 to 25 mL/min

CARRIER GAS:The choice of carrier gas determines the efficiency of chromatographic separation.The main purpose of the carrier gas is to transport sample components through the column.Most commonly used carrier gases are Hydrogen, Helium, Nitrogen and Argon.1. It should be chemically inert and should not interact with sample and stationary phase.2. It should be suitable for the detector to be utilized and the type of sample analyzed.3. Easily available.4. It should be readily available, cheap, and of high purity.5. It should not cause the risk of fire or explosion hazard.6. It should give best column performance consistent with the required speed of analysis.Hydrogen: It has a better thermal conductivity, low density.It is useful in TCD,FID. The disadvantage is that it reacts with unsaturated compounds andIt is inflammable.Helium: It is also has excellent thermal conductivity, but it is expensive. It is very goodCarrier gas when used with TCD.Nitrogen: It is inexpensive but has reduced sensitivity.Argon: For electron capture detector argon is used as carrier gas. However, argon is not Readily available in India.

• Impurities in the carrier gas such as air water vapor and trace gaseous hydrocarbons can cause sample reaction, column character and affect the detector performance.

• The carrier gas system should contains a molecular sieve and filter, drier and absorbing tubes to remove water(moisture) and other gases impurities.

• These gases are available in pressurized tanks. pressure regulators and flow meters are required to control the flow rate of the gas.

• The gases are supplied from the high pressure gas cylinder , being stored at pressure up to 300 psi (pounds per sq. inch).

• carrier gas should be better then 99.99% moles % is desirable and 99.999% is often used.

Soap bubble flow meter

Aqueoussolution ofsoap ordetergent 68

• A soap bubble meter is an accurate device for reproducing the rate of the carrier gas. formed indicates the flow rate.

• Glass tube with a inlet tube at the bottom.• Rubber bulb-----store soap solution• When the bulb is gently pressed of soap

solution is converted into a bubble by the pressure of a carrier gas &travel.

• The time required for the soap film to move between• Two graduations on the burette is then measured and • Converted to flow rate.

Soap bubble meter and flow metersAs carrier gases are stored under high pressure flow regulators are used to deliver gas With uniform pressure or flow rate.Flow meters are used to measure the flow rate of carrier gas.They are soap bubble meter and Rotameter.

Burette

Gas from column

Gas exit

69

inlet tube

Rotameter:

Sample injection port Calibrated Micro syringes are used to inject liquid

sample Sample must be introduced as a vapor in the smallest possible volume and minimum of time with out decomposition. Liquid samples, 1-10 microliters in the volume

are usually injected by a micro syringe through a self sealing silicone rubber septum.

The most accurate and precise method for gas samples used a calibrated sample loop (0.5-10 ml) and a multiport rotary valve. Smaller the sample better the peak shape.

Sample injection portFor optimum column efficiency, the sample should not be too large, and should be introduced onto the column as a "plug" of vapour - slow injection of large samples causes band broadening and loss of resolution. The most common injection method is where a micro syringe is used to inject sample through a rubber septum into a flash vaporiser port at the head of the column. The temperature of the sample port is usually about 50°C higher than the boiling point of the least volatile component of the sample. For packed columns, sample size ranges from tenths of a microliter up to 20 microliters. Capillary columns, on the other hand, need much less sample, typically around 10-3 mL. For capillary GC, split/splitless injection is used. Have a look at this diagram of a split/splitless injector; The injector can be used in one of two modes; split or splitless. The injector contains a heated chamber containing a glass liner into which the sample is injected through the septum. The carrier gas enters the chamber and can leave by three routes (when the injector is in split mode). The sample vapourises to form a mixture of carrier gas, vapourised solvent and vapourised solutes. A proportion of this mixture passes onto the column, but most exits through the split outlet. The septum purge outlet prevents septum bleed components from entering the column.

Basic Inlet Techniques

Split: The inlet is continuously purged with vent gas as some flow ratio to the column flow (at this lab, we use 60:1). This means the flow through the column is of the total flow. This results in most of the injected solution being vented rather than deposited on the column, which in turn gives a tight, spatially limited band of analyte on the column.

Split injection typically gives the best chromatography (highest theoretical plates). Most of the sample is lost, so split injection is not used when absolute sensitivity is required. However, this does give 1-2 orders of magnitude of dynamic range in instrument response without changing any actual detector parameters (simply by splitting or not).

Split less: The split vent is closed during the actual injection. Some time after the injection (for example, one minute), the split vent is opened to purge excess solvent. This technique allows a greater amount of the injected sample to be deposited on the column.

Split less injection gives a greater response for a given solution than split since most of the sample is actually deposited (rather than vented). Split less injection gives the most precise quantitative results, but the chromatographic resolution may be less than with split injection. The instrument parameters needed to produce that maximum resolution are compound specific. This means that each analysis must be optimized to achieve maximum resolution.

Pulsed Splitless: Pulsed splitless is similar to splitless injection, but during the vent closed portion of the timing cycle, the column flow is pulsed to a relatively high rate. This a relatively new technique that combines the advantages of split (better chromatographic resolution) and splitless (better quantitative results and greater response).

a. Gas samples: The sample gas can also be injected at the top of the column by means of a hypodermic syringe. The Hamilton Teflon coated gas syringe is particularly suitable.Generally gases are introduced by typical hex port gas sampling valve which is also Installed on the gas chromatograph.b. Liquid sample: Liquid samples are most conveniently introduced by means of micro syringe whichare different sizes.Liquids can be injected through loop or septum devices. Generally high quality siliconeRubber septum through which sample solution is injected. The rubber is made up of good quality silicone rubber which can with stand high temp.c. Solid samples:Solid samples are dissolved in a suitable volatile solvent and inject like a liquid sampleand they are injected through a septum.

For packed columns, sample size ranges from tenths of a microliter up to 20 microliters. Capillary columns, on the other hand, need much less sample, typically around 10-3 mL. For capillary GC, split/splitless injection is used.

Ovens:The oven is maintaining the precise temperature around the column.Hence the column oven should be free from influence of changing ambientTemperature(temp. of the surroundings ) and free and should have well designed and adequate air flow system.

The column temperatureThe temperature of the column can be varied from about 50°C to 250°C. It is cooler than the injector oven, so that some components of the mixture may condense at the beginning of the column.In some cases, as you will see below, the column starts off at a low temperature and then is made steadily hotter under computer control as the analysis proceeds.

Column temperature and temperature program Column selection A gas chromatography oven, open to show a capillary column

The column(s) in a GC are contained in an oven, the temperature of which is precisely controlled electronically. (When discussing the "temperature of the column," an analyst is technically referring to the temperature of the column oven. The distinction, however, is not important and will not subsequently be made in this article.)

The rate at which a sample passes through the column is directly proportional to the temperature of the column. The higher the column temperature, the faster the sample moves through the column. However, the faster a sample moves through the column, the less it interacts with the stationary phase, and the less the analytes are separated.

In general, the column temperature is selected to compromise between the length of the analysis and the level of separation.

A method which holds the column at the same temperature for the entire analysis is called "isothermal." Most methods, however, increase the column temperature during the analysis, the initial temperature, rate of temperature increase (the temperature "ramp") and final temperature is called the "temperature program."

A temperature program allows analytes that elute early in the analysis to separate adequately, while shortening the time it takes for late-eluting analytes to pass through the column.

Column temperatureFor precise work, column temperature must be controlled to within tenths of a degree. The optimum column temperature is dependant upon the boiling point of the sample. As a rule of thumb, a temperature slightly above the average boiling point of the sample results in an elution time of 2 - 30 minutes. Minimal temperatures give good resolution, but increase elution times. If a sample has a wide boiling range, then temperature programming can be useful. The column temperature is increased (either continuously or in steps) as separation proceeds.

The rate at which a sample passes through the column is directly proportional to the temperature of the column. The higher the column temperature, the faster the sample moves through the column. However, the faster a sample moves through the column, the less it interacts with the stationary phase, and the less the analytes are separated.

In general, the column temperature is selected to compromise between the length of the analysis and the level of separation.

A method which holds the column at the same temperature for the entire analysis is called "isothermal." Most methods, however, increase the column temperature during the analysis, the initial temperature, rate of temperature increase (the temperature "ramp") and final temperature is called the "temperature program

How a Gas Chromatography Machine Works:

• How does this column and system separate things.(How does GC work?• How does the column work? What happens inside the column? How do the

compounds move through the column? Why do some compounds stay in the column longer than others? How does the sample get into the column? These are some of the most basic questions asked about gas chromatography.

• This shows us a schematic of a GC packed column.• There are 2 columns:• Packed columns and capillary.• In this packed column we have are 2 phases.• mobile phase which is moving stationary phase which is not moving.• Mobile phase also called a carrier gas typically it is He some times H2 and

some times Nitrogen.• It is now possible to separate hundreds of components of a mixture in a

single chromatographic experiment.

How does this column and system separate things.(How does GC work?)This shows us a schematic of a GC packed column. There are 2 types of columns.Packed columns large sample capacity

preparative workGood packed column will have 1000 to3000 plates/m. capillary columns.(opentubular column)

Packed column-3m in length.Capillary column- 50-150 METER LENGTH liquid stationary 1 Micron thickness.

higher efficiency smaller sample size analytical applications .Good capilary column range from 1000 to 4000 plates/m. More no of plates better separation.

diameter

What are guard columns?Guard columns are short lengths of deactivated, uncoated fused silica or metal tubingplaced between the injection port and the analytical column. They protect andprolong the lifetime of an analytical column .Why use a guard column?Capillary gas chromatography (GC) guard columns protect analytical columns inseveral ways. Guard columns trap non-volatile residues, preventing them fromcollecting at the head of the analytical column. These non-volatile residues may bevery high molecular weight organic compounds, inorganic salts, or particulates.If these contaminants enter the analytical column, they can cause adsorption ofactive compounds, loss of resolution, and poor peak symmetry

A guard column canprotect your analytical column andensure reproducible analyses.

A guard column or retention gap are the same thing, but they serve different purposes. Both are 1-10 meters of deactivated fused silica tubing attached to the front of the column . Deactivated fused silica tubing does not contain any stationary phase; however, the surface is deactivated to minimize solute interactions. A suitable union is used to attach the tubing to the column. In most cases, the diameter of the retention gap or guard column should be the same as the column.

Guard columns are used when samples contain non-volatile residues that may contaminate a column. The non-volatile residues deposit in the guard column and not in the column. This greatly reduces the interaction between the residues and the sample since the guard column does not retain the solutes (because guard column contains no stationary phase). Also, the residues do not coat the stationary phase which often results in poor peak shapes. Periodic cutting or trimming of the guard column is usually required upon a build-up of residues. Guard columns are often 5-10 meters in length to allow substantial trimming before the entire guard column has to replaced. The onset of peak shape problems is the usual indicator that the guard column needs trimming or changing.

Unions There are a variety of unions that can be used to connect fused silica tubing. Stainless steel, stainless steel-glass combinations, glass press-fit and quick connectors are some of the more common types.

All mobile phases, samples and additives should be filtered through a .45µm syringe filter.It is recommended that guard columns are packed with the same stationary phase as the analytical column to be protected. (Eliminate the possibility of any loss of performance or selectivity.)Pump seals and rotor seals should be replaced on a routine basis.A guard column provides saturation of your mobile phase with silica by “bleeding” silica into the mobile phase instead of from the analytical column. This can be achieved without the loss of resolution or performance by using a Guard Column. When the guard column is destroyed, replace it with another cartridge in minutes.

According to most experts, a guard column can increase the usable life of your columns by a factor of four. You will save money and valuable time. Another source of problems are compounds that irreversibly bond to the stationary phase and are often injected into analytical columns. These compounds cause permanent damage to columns that are not protected by a guard column. Shifting of retention time and loss of resolution often results.

85

Coated with 30 micro metersThick adsorbant such as diatomaceous earth.Which consists of singled -celled sea -plant skeletons.Then this adsorbant is treated with liquid stationary phase.

SCOT columns are capable of holding a greater volume of stationary phase than a WCOT column due to its greater sample capacity, WCOT columns still have greater column efficiencies. modern WCOT columns are made of glass, but T316 stainless steel, aluminum, copper

(capillary tube whose walls are coated with liquid stationary phase)

(the inner wall of the capillary is lined with a thin layer of support material such as diatomaceous earth, onto which the stationary

phase has been adsorbed).

More efficientThan scot columns

Columns:There are two general types of column, packed and capillary (also known as open tubular). Packed columns contain a finely divided, inert, solid support material (commonly based on diatomaceous earth) coated with liquid stationary phase. Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 – 4 mm.Capillary columns have an internal diameter of a few tenths of a millimeter. They can be one of two types; wall-coated open tubular (WCOT) or support-coated open tubular (SCOT). Wall-coated columns consist of a capillary tube whose walls are coated with liquid stationary phase. In support-coated columns, the inner wall of the capillary is lined with a thin layer of support material such as diatomaceous earth, onto which the stationary phase has been adsorbed. SCOT columns are generally less efficient than WCOT columns. Both types of capillary column are more efficient than packed columns

In 1979, a new type of WCOT column was devised - the Fused Silica Open Tubular (FSOT) column

These have much thinner walls than the glass capillary columns, and are given strength by the polyimide coating. These columns are flexible and can be wound into coils. They have the advantages of physical strength, flexibility and low reactivity.

One of the most popular types of capillary columns is a special WCOT column called the fused-silica wall-coated (FSWC) open tubular column. The walls of the fused-silica columns are drawn from purified silica containing minimal metal oxides. These columns are much thinner than glass columns, with diameters as small as 0.1 mm and lengths as long as 100 m. To protect the column, a polyimide coating is applied to the outside of the tubing and bent into coils to fit inside the thermo-statted oven of the gas chromatography unit. The FSWC columns are commercially available and currently replacing older columns due to increased chemical inertness, greater column efficiency and smaller sampling size requirements. It is possible to achieve up to 400,000 theoretical plates with a 100 m WCOT column, yet the world record for the largest number of theoretical plates is over 2 million plates for 1.3 km section of column.

Computer Generated Image of a FSWC column (specialized to withstand extreme heat)

For example, the FSWC column is designed specially for blood alcohol analysis. It produces fast run times with baseline resolution of key components in under 3 minutes. Moreover, it displays enhanced resolutions of ethanol and acetone peaks, which helps with determining the BAC levels. This particular column is known as Zebron-BAC and it made with polyimide coating on the outside and the inner layer is made of fused silica and the inner diameter ranges from .18 mm to .25 mm. There are also many other Zebron brand columns designed for other purposes. Another example of a Zebron GC column is known as the Zebron-inferno. Its outer layer is coated with a special type of polyimide that is designed to withstand high temperatures. It contains an extra layer inside. It can withstand up to 430 °C to be exact and it is designed to provide true boiling point separation of hydrocarbons distillation methods. Moreover, it is also used for acidic and basic samples.

The common liquid phases for Gas chromatography:

Liquid stationary phase Maxium temerature Applications

Squalane(C30H62).High molecular wt. hydrocarbon used for non polar hydrocarbons)

140-150 Hydrocarbons.

Corbowax 200Carbowax 20M(PEG)

150oC 200-250

Aldehydes, ketones.Alcohols,aromatics,pesticides,ketones

Poly siloxane 250oC Steroids,pesticides,Glycols.

Polyethylene glycol(PEG)(more effective for polar comounds)

200-250oC Alcohols,pesticides etc.

Silicon rubber gum(SE-30) 300-350oC. Alkaloids,alcohols, gases fatty acids,gums,bile and urinary compounds,vitamins,Sugars,pesticides,

Stationary Phases:

Stationary phase in GC is the main factor determining the selectivity and retention of solutes.

There are three types of stationary phases used in GC: Solid adsorbents Liquids coated on solid supports Bonded-phase supports

1.) Gas-solid chromatography (GSC)

- same material is used as both the stationary phase and support material- common adsorbents include:

‚ alumina ‚ molecular sieve (crystalline aluminosilicates [zeolites] and clay)

‚ silica ‚ active carbon

Magnified Pores in activated carbon

Gas-solid chromatography (GSC):

advantages: - long column lifetimes- ability to retain and separate some compounds not easily resolved by other GC

methods ‚ geometrical isomers ‚ permanent gases

disadvantage: - very strong retention of low volatility or polar solutes- catalytic changes that can occur on GSC supports- GSC supports have a range of chemical and physical environments

‚ different strength retention sites‚ non-symmetrical peaks‚ variable retention times

2.) Gas-liquid chromatography (GLC)

- stationary phase is some liquid coated on a solid support- over 400 liquid stationary phases available for GLC

‚ many stationary phases are very similar in terms of their retention properties- material range from polymers (polysiloxanes, polyesters, polyethylene glycols) to

fluorocarbons, molten salts and liquid crystals

Based on polarity, of the 400 phases available only 6-12 are needed for most separations. The routinely recommended phases are listed below:

NameChemical nature of polysiloxane

Max. temp.

McReynolds’ constants x’ y’ z’ m’ s’

SE-30 Dimethyl 350 14 53 44 64 41

Dexsil300 Carborane-dimethyl 450 43 64 111 151 101

OV-17 50% Phenyl methyl 375 119 158 162 243 202

OV-210 50% Trifluoropropyl 270 146 238 358 468 310

OV-225 25% Cyanopropyl-25% phenyl

250 238 369 338 492 386

Silar-SCP 50% Cyanopropyl-50% phenyl

275 319 495 446 637 531

SP-2340 75% Cyanopropyl 275 520 757 659 942 804

OV-275 Dicyanoallyl 250 629 872 763 1106 849

McReynolds’ constants based on retention of 5 standard “probe” analytes– Benzene, n-butanol, 2-pentanone, nitropropanone, pyridine

Higher the number the higher the absorption.

Preparing a stationary phase for GLC: - slurry of the desired liquid phase and solvent is made with a solid support

‚ solid support is usually diatomaceous earth (fossilized shells of ancient aquatic algae (diatoms), silica-based material)- solvent is evaporated off, coating the liquid stationary phase on the support- the resulting material is then packed into the column

disadvantage: - liquid may slowly bleed off with time

‚ especially if high temperatures are used‚ contribute to background‚ change characteristics of the column with time

3.) Bonded-Phase Gas chromatography

- covalently attach stationary phase to the solid support material- avoids column bleeding in GLC

- bonded phases are prepared by reacting the desired phase with the surface of a silica- based support

reactions form an Si-O-Si bond between the stationary phase and supportorreactions form an Si-C-C-Si bond between the stationary phase and support

- many bonded phases exist, but most separations can be formed with the following commonly recommended bonded-phases:

‚ Dimethylpolysiloxane‚ Methyl(phenyl)polysiloxane‚ Polyethylene glycol (Carbowax 20M)‚ Trifluoropropylpolysiloxane‚ Cyanopropylpolysiloxane

advantages: - more stable than coated liquid phases- can be placed on support with thinner and more uniform thickness than

liquid phases

Si

CH3

CH3

O

n

Si

CH3

CH3

O

n

Si

C6H5

C6H5

O

m

C CHO O

H

H

H

H

H

n

HO-CH2-CH2-(O-CH2-CH2)n-OH

polyethylene glycol

(Polar)

(intermediate polarity)

(non-polar)Polydiphyl siloxane

Polyalkyline glycol

Poly bis cyano propyl siloxane (very polar non-bonded phase upto 250oC.

Phases

Solid Phase:The main function of the solid phase is to provide mechanical support to the liquid phase.The commonly used solid phases include: Diatomaceous earth or Kieselguhr commonly abailable as dicalite,calite,sterchamol etc.Firebrick coated with metalic silver or gold commercially available as Chromosorb P ,, chromosorb W, Kieselguhr ,Anakrom ABS.Others include glass beads,unglazed tiles, porous polymers, sand etc.

The criteria for an ideal support include:1. It should have large surface area2. It should be chemically inert i.e., it should not react with the liquid phase as well as with the body of the column.3. It should be thermostable.4. It should be a poor adsorbent.5. It should get uniformly wet with the liquid phase.6. It should be strong enough to prevent the fractionating of the column.

Liquid phase:Many liquid phases are available of gas chromatography columns. Choice depends upon the trial/error basis.The requirements for a good liquid phases are1. It should be non-volatile 2. It should have low volatility i.e., should be stable at the operating temarature.3. It should be chemically inert.4. It should possess low vapour pressure at column temperature.5. I t should have high decomposition temperatures.

Liquid phases are classified as:Very polar liquids: Glycerols,glycols, Polyphenols.Polar liquids: Alcohols, ketones, esters.Intermediate polar liquids: Aldehydes, ketons, esters.Low polar liquids: Aromatic hydrocarbons, chloroform, dicloromethane.

For getting very good results…………..The liquid phase should be chemically and structurally similar to the slolute(sample) i.e., The polar liquid phase for polar soluteThe non-polar liquid phase for non-polar solute.

3,6,12 foot length

packed with a solid support like Diatomaceous earthSupports may be either fire brick derived materials likeChromosorb –p ; Anakrom ABS etc.

packed columns can only achieve about 50% of the efficiency of a comparable WCOT column. Due to the difficulty of packing the tubing uniformly, these types of columns have a larger diameter than open tubular columns and have a limited range of length. The diatomaceous earth packing is deactivated over time due to the semi-permanent adsorption of impurities within the column. In contrast, FSWC open tubular columns are manufactured to be virtually free of these adsorption problems.

1.6 to 9.5 mm inDiameter.

Capilary columns haveTubing coiled in to an Open spiral ,A basket-coil orFlat pancake shape.

Column temperatureFor precise work, column temperature must be controlled to within tenths of a degree. The optimum column temperature is dependant upon the boiling point of the sample. As a rule of thumb, a temperature slightly above the average boiling point of the sample results in an elution time of 2 - 30 minutes. Minimal temperatures give good resolution, but increase elution times. If a sample has a wide boiling range, then temperature programming can be useful. The column temperature is increased (either continuously or in steps) as separation proceeds.The column efficiency can be improved by variation in temperature, low liquid phase, loading, narrow particle size distribution and tight packing. The length of the column can be, as deemed necessary, from a fraction of an inch (capillary column) to several hundreds of feet.

For a given species, the ratio of the times spent in the moving and stationary regions is equal to the ratio of its concentrations in these regions, known as the partition coefficient.

Packed columns are made of a glass or a metal tubing which is densely packed with a solid support like diatomaceous earth. Due to the difficulty of packing the tubing uniformly, these types of columns have a larger diameter than open tubular columns and have a limited range of length. As a result, packed columns can only achieve about 50% of the efficiency of a comparable WCOT column. Furthermore, the diatomaceous earth packing is deactivated over time due to the semi-permanent adsorption of impurities within the column. In contrast, FSWC open tubular columns are manufactured to be virtually free of these adsorption problems.

Temperature Control

• Isothermal • Gradient

0

40

80

120

160

200

240

0 10 20 30 40 50 60

Time (min)

Tem

p (d

eg C

)

Instrumentation - Oven

The best detector must have a high sensitivity to traces, good stability, and satisfactory response to a wide variety of substances. Detectors are classified as integral or differential, and destructive or non-destructive.

The integral detector measures the total amount of the component .The differential detector measures some property related to the concentration of the resolved components In case of the destructive detector, the sample is destroyed in the process of detection, such as the case of the flame ionization detector (FID). The thermal conductivity differential detector (TCD) is the most widely utilized non-destructive detector. Fast scanning mass

Detectors:The detector senses the presence of the individual components as they Leave(elute) the column. The detector out put after amplification is traced on a recorder.As peaks at intervals on the chromatograph. The duration of the intervals is usually a single second or even less than that.Hence the detector is considered to be the brain of the gas chromatograph.

The most desirable criteria for a gas chromatographic detectors are:1.It should be highly sensitive towards wide range of compounds.2. It should produce uniform and linear responses towards wide range of vaporized solute particles.3. It should be stable during operation conditions.4. It should have concentration reproducibility.5.It should be easy to operate.Generally gas chromatography detectors are about 4-5 orders of magnitude more sensitive than the liquid chromatography detectors.

generally different detectors gives different types of selectivity.

Non-selective detector- It responds to a wide range of compounds except the carrier gas.Selective detector - it responds to a group of compounds with similar physical or chemical properties.Specific detector -specific detector responds to a single chemical compound. .

In general detectors may be divided in to 2 types.(detectors in chromatography operated in 2 ways. Respond either to the concentration of solute or the mass flow rate.1. Concentration dependent detectors: The signal from a concentration dependant detector is related to the concentration of solute reaching detector, and usually these detectors do not destroy the sample. Dilution of response with make-up gas lowers the response.Ex: Thermal conductivity detector (TCD) Electron capture detector (ECD) Argon ionization detector (AID) Helium ionization detector (HID)

2. Mass flow dependant detectors usually destroy the sample, and the signal is related to the rate of solute particles enter the detector. The response of a mass flow dependant detector is un affected by make-up gas. In differential detectors that responds to the mass flow rate, the peak area is directly proportional to the total mass and there is no dependency on the flow rate of the mobile phase.. These detectors are suitable for quantitative analysis. EX:- Flame ionization detector.(FID) Flame Photometric detector (FPD) Nitrogen phosphorous detector(NPD)Depending upon the reason for operation chromatography may be Preparative chromatography = separation of components of a given mixture for future use. This chromatography is also known as purification process.Analytical chromatography = It can work even with minute concentrations of the sample mixture and measures the components of given mixture. Therefore it is used for quantitative estimation of analytes.

Detectors may be non-destructive, whereby sensing does not alter the nature of the solutes, as in the case of light absorption, so they may be collected for further use. Destructive detectors, on the other hand, destroy the solutes. Detectors include not only the component that senses the solutes but also those that perform the associated transduction, electronic amplification, and final readout.

To obtain optimal separations, sharp, symmetrical chromatographic peaks must be obtained. This means that band broadening must be limited. It is also beneficial to measure the efficiency of the column.

Column efficiencyThe efficiency of a column is reported as the number of theoretical plates (plate number), N, a concept Martin borrowed from his experience with fractional distillation:

where tr is the retention time measured from the instant of injection and w is the peak width obtained by drawing tangents to the sides of the Gaussian curve at the inflection points and extrapolating the tangents to intercept the baseline.

The plate number depends on the length of the column. The extreme value of 106 plates was obtained with an open tubular gas chromatographic column 1.6 kilometres (1 mile) long. A more appropriate parameter for measuring efficiency is the height equivalent to a theoretical plate (or plate height), HETP (or h), which is L/N, L being the length of the column. Efficient columns have small h values (see below Theoretical considerations: Plate height).

HETP = L/N ( Efficiencyresolution: Column efficiency).

There are many detectors which can be used in gas chromatography. Different detectors will give different types of selectivity. A non-selective detector responds to all compounds except the carrier gas, a selective detector responds to a range of compounds with a common physical or chemical property and aspecific detector responds to a single chemical compound. Detectors can also be grouped into concentration dependant detectors and mass flow dependant detectors. The signal from a concentration dependant detector is related to the concentration of solute in the detector, and does not usually destroy the sample Dilution of with make-up gas will lower the detectors response. Mass flow dependant detectors usually destroy the sample, and the signal is related to the rate at which solute molecules enter the detector. The response of a mass flow dependant detector is unaffected by make-up gas. Have a look at this tabular summary of common GC detectors:

Type of Detector Applicable Samples Detection Limit

Mass Spectrometer (MS)

Tunable for any sample .25 to 100 pg

Flame Ionization (FID)

Hydrocarbons 1 pg/s

Thermal Conductivity (TCD)

Universal 500 pg/ml

Electron-Capture (ECD)

Halogenated hydrocarbons

5 fg/s

Atomic Emission (AED)

Element-selective 1 pg

Chemiluminescence (CS)

Oxidizing reagent Dark current of PMT

Photoionization (PID) Vapor and gaseous Compounds

.002 to .02 µg/L

Typical gas chromatography detectors and their detection limits

Detector Type Support gases Selectivity Detectability Dynamic range

Flame ionization (FID) Mass flow Hydrogen and air Most organic cpds. 100 pg 107

Thermal conductivity (TCD) Concentration Reference Universal 1 ng 107

Electron capture (ECD) Concentration Make-up

Halides, nitrates, nitriles, peroxides, anhydrides, organometallics

50 fg 105

Nitrogen-phosphorus Mass flow Hydrogen and air Nitrogen, phosphorus 10 pg 106

Flame photometric (FPD) Mass flow Hydrogen and air possibly oxygen

Sulphur, phosphorus, tin, boron, arsenic, germanium, selenium, chromium

100 pg 103

Photo-ionization (PID) Concentration Make-up

Aliphatics, aromatics, ketones, esters, aldehydes, amines, heterocyclics, organosulphurs, some organometallics

2 pg 107

Hall electrolytic conductivity Mass flow Hydrogen, oxygen Halide, nitrogen,

nitrosamine, sulphur

Thermal conductivity detector (TCD),KATHAROMETER (OR) Hot wire detector:Thermal conductivity detector is the one of the oldest detector still using because of Simplicity of system and it is widely used. Data’s are available over the years. It is simple inexpensive, non-selective, accurate, non destructive of the sample.The TCD is based on the changes in thermal conductivity of the gas stream..Thermal conductivity of the most of the compounds is lesser than the commonly used carrier gases (He, H) because the thermal conductivity of He is about 6 to 10 times greater than most organic compounds. When He is used as a carrier gas the presence of small amounts of organic materials causesA relatively large decrease in thermal conductivity of the column effects. As a result of the decrease in conductivity. The detector undergoes a marked rise in temperature.Since detector response depends upon the difference in thermal conduction of the carrier gas and sample, a large difference is essential. An increase in temp. of the detector causes a change In the resistance of wire or thermistor and this resistance gives a measure of the thermal conductivity of the gas.

TCD consists of a temperature controlled metal block into which 2 cylindrical chambersOr cavities are present. Both chambers consists of filaments(thermistors,resistance wires) made up of platinum,tungsten or alloys.The filaments constitute of the reference(R) and the sensing (S) elements.Both these filaments are connected to the arms of the Wheatstone bridge arrangement.At the given temp. when carrier gas alone is flow through the both cylindrical chambers the Resistance of the both the filaments are thermal equilibrium (they are constant).As long as the composition of the gas does not change the rate of heat loss from the filamentsWill not alter.Once the effluent passes through (s) the temp. of the filament (s) changes .Because of thermal conductivities of sample and the carrier gas are different.The differences result in heat loss form the filament eventually changes in the resistance Of (s).This resistance changes are send by the Wheatstone bridge arrangement.The bridge then produces a measurable voltage change that is amplified and signaled to theRecorder which is recorded on the chromatogram.

Widge balance

ameter

Excitation voltage

Unbalanced Voltage.

Same carrierGas is passingThrough the Detectors.

Initially what To do is widgeBalance with theBy varying these Potentiomenr I will balance theBridge.

When ever the elution peak comes out side gases comes detector outPut change there is a thermal conductivity is changed I will get a Unbalanced voltage recorded on the recorder.We can have 4 detectors in one block itsef.

Amlifier and recorder

Battery(direct current source)

.) Thermal Conductivity Detector (TCD) - katherometer or hot-wire detector- first universal detector developed for GC Process

- measures a bulk property of the mobile phase leaving the column.- measures ability to conduct heat away from a hot-wire (i.e., thermal conductivity)- thermal conductivity changes with presence of other components in the mobile phase

Disadvantages:

An oven is essential for the working of detector to attain column temp.The detectors are relatively insensitive i.e., sensitivity is only about 10-6 to 10-8.

Advantages:The detectors are simple in construction having no moving parts and are inexensive.They give accurate results.They are durable and possess long life.They are non- selective hence known as universal detectors.Use of wheatstone bridge increases the sensitivity of the detector.It does not result in the destruction of the sample.Linear response range is about 3 orders of magnitude.The filament resistances supplies a measure of the thermal conductivity of the gas. HenceBy measuring the filament resistance, the changes in the effluent stream can be monitored.

As the name suggests, analysis involves the detection of ions. The source of these ions is a (tiny)small hydrogen-air flame. Sometimes hydrogen-oxygen flames are used due to an ability to increase detection sensitivity, however for most analysis, the use of compressed breathable air is sufficient. Column effluents are led into the flame wherein ionisation of compounds may takes place.In order to detect these ions, two electrodes are used to provide a potential difference. The positive electrode doubles as the nozzle head where the flame is produced. The other, negative electrode is positioned above the flame. The ions thus are attracted to the collector plate and upon hitting the plate, induce a current.When only carrier gas passes through the flame, its molecules are ionised and the resulting ionisation current after amplification is fed to the sutable recorder. This current is measured with a high-impedance picoammeter and fed into an integrator.A FID is sensitive to almost all the organic compounds But insensitive to noble gases,oxygen,N,CO,C02,water,H2S,S02,CS2 and Nitrogen oxides.

The eluent exits the GC column (A) and enters the FID detector’s oven (B). The oven is needed to make sure that as soon as the eluent exits the column, it does not come out of the gaseous phase and deposit on the interface between the column and FID. This deposition would result in loss of effluent and errors in detection. As the eluent travels up the FID, it is first mixed with the hydrogen fuel (C) and then with the oxidant (D). The eluent/fuel/oxidant mixture continues to travel up to the nozzle head where a positive bias voltage exists (E). This positive bias helps to repel the reduced carbon ions created by the flame (F) pyrolyzing the eluent. The ions are repelled up toward the collector plates (G) which are connected to a very sensitive ammeter, which detects the ions hitting the plates, then feeds that signal (H) to an amplifier, integrator, and display system.

FIDs are best for detecting hydrocarbons and other easily flammable components. n FID essentially can only detect components which can be burned. Other components may be ionized by simply passing through the FID's flame, but they tend not to create enough signal to rise above the noise of the detector.

The effluent from the column is mixed with hydrogen and air, and ignited. Organic compounds burning in the flame produce ions and electrons which can conduct electricity through the flame. A large electrical potential is applied at the burner tip, and a collector electrode is located above the flame. (mixture burns at the tipIons and the free electrons are in the formed ) The current resulting from the pyrolysis of any organic compounds is measured. FIDs are mass sensitive rather than concentration sensitive; this gives the advantage that changes in mobile phase flow rate do not affect the detector's response. The FID is a useful general detector for the analysis of organic compounds; it has high sensitivity, a large linear response range, and low noise. It is also robust and easy to use, but unfortunately, it destroys the. Positive ions and electrons are produced in the flame when organic substances are present. The ions are collected at electrodes and produce a small, measurable current. The flame-ionization detector is highly sensitive to hydrocarbons, but it will not detect carrier gases, such as nitrogen, or highly oxidized materials, such as carbon dioxide, carbon monoxide, sulfur dioxide, and water

Parrlar or cylendrical

½ cm 1cm above the tip

HydrogenTip.

This was the restince Across the gap and Causes a current to flow.

mounted

This ensures that Make current flow onlyIonized materials entersThe external resister is sensed voltage droppedAmplified and displayed onThe detector.

Platinum jet-ve electrode.

advantages: - universal detector for organics

-doesn’t respond to common inorganic compounds.- mobile phase impurities not detected.- carrier gases not detected.- limit of detection: FID is 1000x better than TCD.- linear and dynamic range better than TCD

disadvantage: - destructive detector.

Flame ionization detector

Hydrocarbon groups are enter the flame and a complex process takes place which+ve ly charged carbon species and electrons are formed. Now the current is greatlyIncreased .This FID responds only to the substances only for ionizedTo the substances that produced charged the ions that is burned in H2 flame. organic compounds the response is proportional to the no. of oxidisable carbon atoms. This is basic principle of falme ionization detector.

The electron-capture detector, a stream of electrons from a radioactive source is produced in a potential field. Materials in the gas stream containing atoms of certain types capture electrons from the stream and measurably reduce the current. The most important of the capturing atoms are the halogens—fluorine, chlorine, bromine, and iodine. This type of detector, therefore, is particularly useful with chlorinated pesticides. Certain elements will emit light of distinctive wavelength when excited in a flame.

The Flame Thermocouple Detector The "flame thermocouple detector" was the next detector to be reported which was developed by Scott and was, in fact, the forerunner to the flame ionization detector FID. A diagram of the flame thermocouple is shown.bellow

3.) Nitrogen-Phosphorus Detector (NPD)

- used for detecting nitrogen- or phosphorus containing compounds- also known as alkali flame ionization detector or thermionic detector

Process- same basic principal as FID- measures production of ions when a solute

is burned in a flame- ions are collected at an electrode to

create a current- contains a small amount of alkali metal vapor in the flame- enhances the formation of ions from nitrogen- and phosphorus- containing compounds

Alkali Bead

Flame photometric detector:The flame photometric detector measures the intensity of light with a photometric o circuit. Solute species containing halogens, sulphur, or phosphorus can be burned to produce ionic species containing these elements and the ions sensed by electrochemical means. It is a selective detector. It uses the luminous emissions of S or P from the effluent in a low temp. hydrogen flame. These emissions are Serves as a analytical information i.e.,s pecific For compounds containing these atoms. The intensity for band is recorded photometrically.The detector consists of combustion chamber that houses flame. H2 as fuel, air or oxygen to Support combustion. A thermal filter for the separation of UV and visible rays emitted byThe flame.An interchangeable optical filter for the selection of S,P an insulated Photomultiplier tube(PMT) and chimney as exhaust for combustion products.Applications: For the detection of heavy metals like Iron, chromium, selenium, strontium, tin etc. inOrganometallic compounds.For the analysis of pesticides, coal, hydrogenated products as well as air and water Pollutions.

Electron capture detector:Invented by Dr.J.E.Lovelock in the late 1950s.It is one of the most sensitive detectors used in GC.It is specifically used for trace amounts of halaogen containingCompounds like pesticides,polychlorinated biphenyls.ECD contains a radioactive β-emitter for the generation of electrons.This detector functions in 2 modes DC and pulsed mode.

DC mode: constant DC potential is applied b/n 2 electrodes.

The separation of the sample air is accomplished using gas chromatography. Tiny porous beads, sometimes coated with a liquid, interact with the mixture of molecules impeding there movement either because of their size or their solubility. In our case, this separation column is divided into two parts. When the chemicals of interest move onto the second column, the first column's flow is reversed to clean heavier compounds off before the next injection. This is called "backflushing".

Detection of the halocompounds is by an electron capture detector. A radioactive foil of nickel-63 is inside the pin-in-cup detector. The beta decay (an electron) ionizes the carrier gas forming an electron cloud. Periodically a large positive pulse is applied to the center electrode. This causes the free electrons to move to the electrode where they are measured as a tiny current. When a molecule containing halogen atoms, which has the property of enhanced affinity for electrons, enters the detector, it readily attracts and holds a free electron. The background current is thus reduced. In due time the electron capture molecules are flushed out of the detector and the current returns to its previous level.

The measure of the dip in the current curve is a measure of the amount of chemical present. By periodically injecting gas mixtures containing known quantities of the chemical of interest, calibration of the detector is accomplished.

Recording of the inverted current produces a curve called a "chromatogram".

Advantages: It is highly sensitive in its response.Sensitivity is 10-13 g. It is 1 million times more sensitive than TCD and about 10,000times more sensitive Than FID.It is non-desturctive. i.e., lt does not alter the sample significantly.It is highly sensitive for the detection of compounds like Halogens(Cl,Br,I), Quinones,Peroxides,nitrites,nitro and phosphorous compounds, organometallic compounds.

Disadvantages:It is least sensitive for compounds whose molecules have negligible affinity for electrons.It is insensitive to functional groups like alcohols, amines and hydrocarbons.Linear response is limited to 2 orders of magnitude.The carrier gas used should be in absolutely pure form like pure nitrogen or mixture of Pure methane-argon.The temperature limitations of the detector is 2200C.Applications:Analysis of halogenated compounds.Analysis of environmental pollutants.Detection and quantitative determination of chlorinated insecticides.

Electron-capture DetectorsElectron-capture detectors (ECD) are highly selective detectors commonly used for detecting environmental samples as the device selectively detects organic compounds with moieties such as halogens, peroxides, quinones and nitro groups and gives little to no response for all other compounds. Therefore, this method is best suited in applications where traces quantities of chemicals such as pesticides are to be detected and other chromatographic methods are unfeasible. The simplest form of ECD involves gaseous electrons from a radioactive ? emitter in an electric field. As the analyte leaves the GC column, it is passed over this ? emitter, which typically consists of nickle-63 or tritium. The electrons from the ? emitter ionize the nitrogen carrier gas and cause it to release a burst of electrons. In the absence of organic compounds, a constant standing current is maintained between two electrodes. With the addition of organic compounds with electronegative functional groups, the current decreases significantly as the functional groups capture the electrons. The advantages of ECDs are the high selectivity and sensitivity towards certain organic species with electronegative functional groups. However, the detector has a limited signal range and is potentially dangerous owing to its radioactivity. In addition, the signal-to-noise ratio is limited by radioactive decay and the presence of O2 within the detector.

Electron Capture Detector (ECD)- radioactive decay-based detector- selective for compounds containing electronegative atoms, such as halogens

Principle of Operation- based on the capture of electrons by electronegative atoms in a molecule- electrons are produced by ionization of the

carrier gas with a radioactive source 3H or 63Ni

- in absence of solute, steady stream of these electrons is produced- electrons go to collector electrode where they produce a current- compounds with electronegative atoms capture electrons, reducing current

Advantages: - useful for environmental testing

detection of chlorinated pesticides or herbicidesdetection of polynuclear aromatic carcinogensdetection of organometallic compounds

- selective for halogen- (I, Br, Cl, F), nitro-, and sulfur-containing compounds- detects polynuclear aromatic compounds, anhydrides and conjugated

carbonyl compounds.Disadvantages: Could be affected by the flow rate.

e-

e-

e-

e- e-

He

He

HeHe

He+

He+

He+He+

He+

e-

e-

He

He

HeHe

He+

He+

He+He+

He+

Atomic Emission DetectorsAtomic emission detectors (AED), one of the newest addition to the gas chromatographer's arsenal, are element-selective detectors that utilize plasma, which is a partially ionized gas, to atomize all of the elements of a sample and excite their characteristic atomic emission spectra. AED is an extremely powerful alternative that has a wider applicability due to its based on the detection of atomic emissions.There are three ways of generating plasma: microwave-induced plasma (MIP), inductively coupled plasma (ICP) or direct current plasma (DCP). MIP is the most commonly employed form and is used with a positionable diode array to simultaneously monitor the atomic emission spectra of several elements. InstrumentationThe components of the Atomic emission detectors include 1) an interface for the incoming capillary GC column to induce plasma chamber,2) a microwave chamber, 3) a cooling system, 4) a diffration grating that associated optics, and 5) a position adjustable photodiode array interfaced to a computer

Schematic of atomic emission detector

1. Mass Spectrometer (GC/MS)

Many GC instruments are coupled with a mass spectrometer, which is a very good combination. The GC separates the compounds from each other, while the mass spectrometer helps to identify them based on their fragmentation pattern.

2. Flame Ionization Detector (FID)

The detector is very sensitive towards organic molecules (10-12 g/s, linear range: 106 –107), but relative insensitive to a few small molecules e.g. N2, NOx, H2S, CO, CO2, H2O. If proper amounts of hydrogen/air are mixed, the combustion does not afford any ions. If other components are introduced that contain carbon atoms cations are produced in the effluent stream. The more carbon atoms are in the molecule, the more fragments are formed and the more sensitive the detector is for this compound (-- > response factor). However, due to the fact that the sample is burnt (pyrolysis), this technique is not suitable for preparative GC. In addition, several gases are usually required to operate a FID: hydrogen, oxygen (compressed air), and carrier gas

3. Thermal Conductivity Detector (TCD)

This detector is less sensitive than the FID (10-5-10-6g/s, linear range: 103-104), but is well suited for preparative applications, because the sample is not destroyed. It is based on the comparison of two gas streams, one containing only the carrier gas, the other one the carrier gas and the compound. Naturally, a carrier gas with a high thermal conductivity e.g. helium or hydrogen is used in order to maximize the temperature difference (and therefore the difference in resistance) between two thin tungsten wires. The large surface-to-mass ratio permits a fast equilibration to a steady state. The temperature difference between the reference cell and the sample cell filaments is monitored by a Wheatstone bridge circuit.

4. Electron Capture Detector (ECD)

The detector consists of a cavity that contains two electrodes and a radiation source that emits b-radiation (e.g. 63Ni, 3H). The collision between electrons and the carrier gas (methane plus an inert gas) produces a plasma containing electrons and positive ions. If a compound is present that contains electronegative atoms, those electrons are “captured” and negative ions are formed, and the rate of electron collection decreases. The detector is extremely selective for compounds with atoms of high electron affinity (10-14 g/s), but has a relatively small linear range (~102-103). This detector is frequently used in the analysis of chlorinated compounds e.g. pesticides, polychlorinated biphenyls, which show are very high sensitivity.

Submultiples MultiplesValue Symbol Name Value Symbol Name

10−1 g dg decigram 101 g dag decagram

10−2 g cg centigram 102 g hg hectogram

10−3 g mg milligram 103 g kg kilogram

10−6 g µg microgram (mcg) 106 g Mg megagra

m (tonne)

10−9 g ng nanogram 109 g Gg gigagram10−12 g pg picogram 1012 g Tg teragram

10−15 g fg femtogram 1015 g Pg petagram

10−18 g ag attogram 1018 g Eg exagram

10−21 g zg zeptogram 1021 g Zg zettagram

10−24 g yg yoctogram 1024 g Yg yottagram