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Gas Chromatography - General TheoryType of Sample: Vapor or liquidSample Pressure: 0 to 100 psig (0 to 7 bars)Ambient Temperature: -4 to 122 F (-20 to 50C); however, a sheltered environment is recommendedAnalysis Zone: 140 to 356F (60 to 180C) provided desired temperature is at least 18F (10C) above ambien0.1 F ( .05C) at steady ambient, 0.1 F ( .05 C) FOR 4 TO 122F (-20 TO 50 C) ambient changeContacting Material: Stainless steel, TeflonAuxiliary Utilities: Instrument air, dry oil-free, at 50 psig (3.5 bars) minimum and 2 to 3 scfm (1 to 1.5 sl/s). Cagas, zero air and hydrogen for flame ionization detector (FID). Possibly steam tracing for sample linesCycle Time: 2 to 20 minutes depending on application and packed or capillary column operationSpecial Features: Accepts other inputs (e.g., flow rate and density) to include in calculation of heat rate in unitkal/hr or BUT/hr; Multicomponent readoutLocation: Class 1, Groups B, C, and D, Division 1 hazardous areasAnalyzer Cost: Basic analyzer $25,000 to $35,000 (depending on number)Installed Cost: $60,000 to $125,000 (depending on number and application)Suppliers: ABB Process Analytics; Applied Automation/Hartman & Brown; Carlo Erba; Daniels; Delsi; FluidData/Amscor; The Foxboro Co.; Servomex and Beckman/Rosemount; Siemens; YEW Yokagowa Electric

The word "chromatography" has been coined to describe a physical method of separation that is based on thedifferences in solubility (or adsorption) of substances between a mobile and a stationary phase. It has become o

of the most often used procedures in analytical chemistry for separation and analysis. The reasons for its populacan be traced to its ease of use for the separation of complex mixtures, its high sensitivity, and the small sample

required for the analysis.

There are a number of different versions of chromatography depending on the physical characteristic of the mob

and stationary phases and on the mode of sample introduction. We shall be concerned in this section with theelution method of chromatography using a gaseous mobile phase and a liquid (gas liquid chromatography-GLC o

GC) or solid (gas solid chromatography-GSC) stationary phase.

In the elution method of GLC, the sample is vaporized as a narrow plug, injected into a carrier gas (the mobile

phase), and forced to flow through a small diameter tube containing the stationary liquid phase. This liquid phasmay take the form of a thin film suspended on the walls of the tubing (wall-coated open tubular [WCOT] column

simply a capillary column) or it may be distributed over a solid support that is then packed into the tubing (packcolumn).

In either case, separation takes place primarily because of differences in the solubility of the sample componentsthe stationary phase. Thus, the column retains some of the components longer than others, and consequently, t

pass through the column at varying speeds. Provided that a stationary phase has been selected so as to maximizthese solubility differences, complete separation occurs, with the least soluble components coming through first,

followed by the next soluble one, and so on. Obviously the degree of separation (resolution) is a function of the

above-mentioned solubility differences and also the efficiency of the column, that is, its ability to produce very

narrow peaks.Similar results are obtained with GSC. However, the separations are the result of differences in sample compone
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adsorption on active solid surfaces such as charcoal, silica gel, or alumina.

Chromatograph OperationThe basic instrumentation necessary to accomplish the above-described chromatography is extremely simple. It

consists of a supply of carrier gas (commonly helium, hydrogen, or nitrogen) with appropriate pressure regulatio

sample valve (described further on) to inject a fixed volume of sample into the carrier gas stream, and a columncontaining the stationary phase. Since the degree of solubility (and hence the retention time) is temperature-

dependent, a precisely regulated temperature environment must be provided for the column. Finally, a detector

responds to some property difference between the carrier gas and the sample components must be available, so

that it can be placed at the end of the column and indicate when the components are emerging. This detectorsignal, when recorded as a function of time, produces the familiar chromatogram.

The time required for each of the components to emerge is called the retention time, and the magnitude of thedetector signal (peak height or peak area) is related to the amount of component present in the injected sample

these individual retention times and detector responses are made to remain constant (by adequate control of ca

gas flow rate, amount of stationary phase, column temperature, and detector variables), the process can be

automated so that the instrument can provide this information quite precisely and cyclically on a repetitive basisThe process gas chromatograph (PGC) is quite different from its laboratory counterpart. These differences appea

the result of a number of factors, not the least of which are:The need for continuous, reliable operation of the unit

The need for analysis within the time required for the process control system to achieve the proper control actioThe nature of the environment in which the PGC is placed

The diverse user attitudes regarding maintenance, and user capability regarding the proper operation of the unitThe need to interface with modern computer-controlled process control system

Thus, a premium is placed on simple, reliable design to ensure a low mean failure rate but straightforward, rapid

repair when required; important, too, is the strategic placement of appropriate sensors in the system to aid in

diagnosing problems and provide alarms with regard to the operation of the unit.To summarize, the PGC has the following distinguishing attributes: Located in the plant as near as possible to th

sample point

Dedicated to monitor one or more components in one or more process streams

Designed for continuous, unattended operationDesigned for operation in hazardous environments

Designed for exposure to weather, humidity, dust, and corrosive atmospheresContains integral hardware and software to allow communication with the process control system as well as rem

maintenance stations.

Contains alarms and diagnostic aids to continuously monitor the health of the instrument and aid in diagnosing

problems

Components of a Process Gas ChromatographWhat follows is a general discussion of these various parts and their specific function.AnalyzerThe analyzer section contains all of the basic elements of the chromatograph, namely, columns, sample valves,

column switching valves, and detectors, all enclosed in a precisely thermostatted oven. In some designs theassociated pneumatic components are also placed in the oven, and in others they are not. Which option is electe

depends on the quality of the components and/or robustness of the software to tolerate retention time changes

may occur when pressure and flow controllers experience a varying temperature environment.OvenFor economy and design simplicity, a single temperature zone is most frequently used. The basic designs that ha

evolved are the air bath type and the explosion-proof bell enclosure. In the air bath design each of the appropriaelements of the analyzer (such as the oven heater and the detector) is itself designed to be explosion-proof for


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mounting in basically a non-explosion-proofed air bath enclosure, while the alternative design places all the elem

into an explosion-proof enclosure. Both designs satisfy Class 1, Group D, Division 1 classification.Of the two designs, the most popular is the circulating air bath, in which air (at 3 to4 scfm, or l.5 to 2 sl/s), is pa

over an electrical heating element.

The temperature range is usually 122 to 338 F (50 to 170) (up to 437 F, or 225 C on special jobs) with a stab

of 0.2 F (0.1 C) or better. The operation of a typical air-purged oven and heater is illustrated in Figure 11d.Although only isothermal oven operation has been offered in the past, recent market demands for rapid simulate

distillation, Reid vapor pressure, and other volatility measurements has encouraged some vendors to offer a

temperature-programmed version. At this writing none of these units has proven to be as reliable as the isotherm

versions, nor have any been certified as yet for use in Division 1 areas.Temperature programming is a procedure for decreasing the analysis time for a sample consisting of compounds

widely divergent boiling points and polarity. The temperature of the column is usually maintained at some initialvalue for a fixed period of time while the low boilers elute from the column, and the temperature is then raised i

linear fashion (and at a suitable rate) during the remainder of the analysis period so as to ensure the separation

the remaining sample components within a much shorter period of time than if the analysis has been run

isothermally. The oven must then be cooled rapidly to exactly the same starting temperature and programmed aat exactly the same rate in order to obtain reproducible retention times for the various constituents of the sampl

clearly, not a trivial design task.ValvesValves are used for column switching and for the injection of the sample. The most popular of these are the rotasliding plate, and diaphragm mechanical valves and the so-called valveless, or Deans switch . The use of the old

linear, or spool-type, valves has been all but abandoned because of their inability to produce the sharp input samprofiles required for modern, efficient chromatography.

The sample valve is designed to deliver a sample of constant volume. For very small samples (less than 1 microl

the volume is defined internally by a hole or slot machined into the movable member (sliding plate or rotor); for

larger volumes, it is defined by an external loop made from a length of tubing of known dimensions attached to valve.

In the past, the smaller volumes were used for liquids, but with the increasing popularity of capillary columns

(requiring much smaller samples than packed columns these valves are being used for the injection of gases. In

case, liquid samples pose particular problems, from the point of view of sample valve reliability, sample handlingand safety. Thus operators should try as much as possible to avoid the necessity for liquid sample injection by

vaporizing the sample before presentation to the sample valve.

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Gas Chromatography - Detector TheoryThermal Conductivity Detector (TCD)
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Theory of OperationThe Thermal Conductivity Detector uses four spiral wound filament wires supported inside cavities in a metal bloThe filaments are manufactured from a material whose electrical resistance varies significantly with variations in

temperature.

A constant DC current of up to 100 mA is applied to the filaments in an electronic bridge circuit. With pure carrie

and reference gas flowing across the filaments, the heat loss is constant leading to a constant filament temperatThis consistent filament temperature produces a constant filament resistance. The currents in the electronic brid

can be balanced to produce a zero signal level as a reference.

When component peaks enter the TCD with the carrier, the heat dissipated from the filaments on the measure s

changes. This leads to an imbalance in the electronic bridge. The resulting electrical signal is then used to measuthe quantity of the component.8-Port TCDAn 8-port TCD is typically used to measure hydrogen and nitrogen on the same analyzer. This detector functions

connecting a second injection valve and column set to the pair of detector filaments used as the reference in a

standard TCD. One side (train) uses a nitrogen carrier to measure hydrogen. The other side uses hydrogen or

helium carrier to measure nitrogen, carbon dioxide, oxygen, etc. Each side serves as a reference for the other siThis configuration provides the advantage of measuring hydrogen, nitrogen, carbon dioxide, oxygen, etc., with o

analysis method that prints all the results on the same report. The injection time for each train (side) is set to enthat no peaks elute from both trains at the same time.Detector sensitivityThree factors affect TCD sensitivity. The first factor is bridge current. Increasing the bridge current causes an

increase in the voltage across the bridge circuit. This results in an increased signal amplitude and detectorsensitivity. However, if the bridge current is increased too much, the base line signal can become unstable and t

filament life may be shortened.

The second factor affecting TCD sensitivity is the relationship between the thermal conductivity of the carrier ga

and that of the sample. Greater sensitivity results from greater differences between these. Helium or hydrogen hhigh thermal conductivity compared to nitrogen and air. Using a helium or hydrogen carrier permits much higher

bridge current than possible with nitrogen. This results in increased sensitivity.

The third factor affecting TCD sensitivity is the filament temperature. Increasing the bridge current will increase

filament temperature and detector sensitivity. However, increased filament temperature can lead to shortenedfilament life.

Flame Ionization Detector (FID)DescriptionThe FID is a horizontal flame detector incorporating a collector fashioned from stainless steel. The polarizing volt

is applied directly to the flame tip assembly which is electrically isolated from the FID body. Typical polarizingvoltage is +15 volts DC. Higher polarizing voltages may be used although tests show that this has minimal effec

detector sensitivity.Flame On and Flame Off conditions are sensed by a thermocouple junction just outside the flame area. The Exce

monitors the temperature near the flame to sense the presence or absence of the flame. Automatic flame ignitiocan be enabled or disabled for each detector. The ignitor can also be fired manually. Typically, burner air flow is

at 200 to 245 cc/min and burner hydrogen is set at 40 to 45 cc/min.Theory of OperationWhen sample material enters the FID, the compounds are burned by the flame. The ions produced result in a

proportional current flow inside the detector. This current is amplified and converted to a voltage signal. The

amplitude of the signal is proportional to the concentration of the component ionized. For more detailed theory oFID operation, please consult appropriate literature on detectors used with chromatography.

The FID requires a hydrogen flow rate of 30 to 50 cc's per minute and an air flow of 200 to 300 cc's per minute best overall performance. Optimum sensitivity may be achieved by varying the flow rates somewhat; however, t


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model FID is relatively unaffected by this technique.

Column connection to the FID is made by teeing the column effluent with the hydrogen at the FID. Experienceindicates that a standard tubing teecan be used for this connection. Some applications may require a zero volum

teefor reduced peak tailing.

Flame Photometric Detector (FPD)DescriptionThe Flame Photometric Detector (FPD) is a flame FPD used to detect volatile sulfur compounds. It is primarily us

to measure trace levels (< 500 ppm) of sulfur compounds (e.g., H2S, COS , SO2, CS2, and Mercaptans) in vario

hydrocarbon backgrounds. This FPD provides about 10,000 to 1 discrimination of hydrocarbon to sulfur compounThere are two methods for output linearization. Square root extraction uses an electronic circuit to linearize the

output. Sulfur addition uses the addition of a small amount of methyl mercaptan to the FPD air to provide a lineaoutput and increased sensitivity.Theory of OperationThe FPD provides a hydrogen rich flame where sulfur compounds are reduced to the elemental S2 species which

produce a blue chemiluminescent emission in the visible light spectrum. The blue light emission is passed througnarrow band (395 nanometer) optical filter to the photomultiplier tube. The signal from the photomultiplier tube

further amplified by an electrometer and interface board.To minimize condensate buildup in the detector, the FPD block is typically maintained above 100 C. The

photomultiplier tube housing is maintained at a cooler temperature to extend its service life.The intensity of the blue light emission is approximately proportional to the square of the sulfur concentration in

flame. In some applications, a small amount of methyl mercaptan (2 to 5 ppm) is continuously added to the FPDto bias the output level upward to the linear portion of the output curve. Generally, a permeation device is used

provide a consistent addition of methyl mercaptan to the FPD air. A small sample size is also used to ensure

operation in the linear portion of the curve. The FPD is available with a square root extraction option that elimina

the need for sulfur addition.Hydrocarbons entering the detector have a quenching affect causing false or real flame-outs. If a component of

interest and a hydrocarbon peak elute at the same time, erroneous readings can result. Large hydrocarbon peak

can cause a gradual buildup of carbon deposits on the detector wall and the quartz window. As these deposits

reduce the amount of light entering the photomultiplier tube, detector sensitivity decreases. Valve and columnarrangements should be used to minimize the amount of hydrocarbon materials entering the FPD detector.

Sulfur Chemiluminescence Detector (SCD)The Sulfur Chemiluminescence Detector (SCD) is used for detection of sulfur. In this detection process, sulfur

monoxide is reacted with ozone to form an electronically excited species of sulfur dioxide which emits light when

relaxes to the ground energy state. The light emitted by this reaction is detected and amplified by a photomultipThe response from this reaction is linear over at least three to five orders of magnitude and demonstrates a

selectivity to sulfur over carbon of 106 108 to one.A chromatograph injection valve injects the sample into a flow of air carrier gas which passes to a dual chamber

flameless combustion chamber. Additional air is introduced just prior to entry into the first combustion chamber.Hydrogen is introduced into the second combustion chamber.

In the combustion chambers, the hydrocarbon matrix of the sample is oxidized primarily to carbon dioxide andwater, while the sulfur components are oxidized to sulfur dioxide (SO2), water, carbon dioxide (CO2), and some

sulfur monoxide (SO). Sample from the second combustion chamber is drawn to the reaction cell by a vacuum

source. Ozone from an ozone generator is also routed to the reaction cell.

For the chemiluminescence reaction to take place in sufficient quantity for the required sensitivity, the reaction cmust be maintained at a predetermined vacuum. As reaction cell pressure is increased, relaxation of the excited

sulfur dioxide to its ground state increasingly takes place through mechanisms of intermolecular collision rather tthe chemiluminescence reaction. In addition, the SO formed in the combustion chamber is a short-lived free rad


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and must be transferred to the reaction cell quickly before competing reactions can occur. The low pressure of t

reaction cell causes both a fast transfer time and a quenching of possible competing reactions. The primaryreactions taking place in the system can be represented as follows:

RH + RS + O2 SO + SO2 + CO2 + H2O

SO + O3 SO2 SO2 + hv

At the low pressures of the reaction cell, carbon dioxide and water do not cause appreciable interference orintermolecular quenching of the signal, and water vapor condensation does not occur.

Although many sulfur compounds can chemiluminesce directly by reaction with ozone, the combustion steps in t

process are necessary for several reasons. The direct chemiluminescence reactions exhibit a wide variation in mo

response, probably due to different mechanisms of the attack of the ozone oxidant. In each case, the SO radicalprobably the reactive intermediate that ultimately causes the chemiluminescence.

However, differing reaction kinetics and mechanisms of oxidation determine the various sensitivities observed. Tchemiluminescence reaction is not affected by hydrocarbon or carbon dioxide quenching. Combustion of the sam

in the flame serves to break down the large hydrocarbon molecules in order to maximize the sensitivity and

selectivity of the detector. Combustion also serves to provide an equimolar response by breaking down the comp

sulfur containing molecules into a common sulfur species prior to reaction.Since the SCD pre-combusts the sulfur containing compounds in the sample to SO and SO2 prior to the reaction

ozone, the chemiluminescence response is equimolar to all different types of sulfur compounds found in the samThe burner used for combustion is a two-stage, or dual-chamber unit. Use of the dual-chamber combustion cham

has two advantages. Since combustion is more complete, hydrocarbon quenching and non-equimolar response aminimized. Use of combustion chamber with the SCD results in a detector that is equimolar and virtually free fro

quenching.

Thermionic Ionization Detector (TID)The Thermionic Ionization Detector is a highly selective detector. For example, when the appropriate thermionic

source is used, the TID is very sensitive to chlorinated compounds and relatively insensitive to other compoundsThis characteristic of the TID can be used to improve measurement of certain types of components, especially w

these components are obscured by other components when using an FID.

The type of components detected by a TID are determined by the type of mineral source chosen for the detecto

The mineral source is heated to generate negative ions which produce a signal current that is measured with anelectrometer. Measurement components react with the generated ions producing a change in signal flow resultin

production of a negative peak on the chromatogram. (The Excel V can reverse the negative peaks.) Othercomponents have little effect on the ions and produce very little change in signal current.

The TID uses a modified Flame Ionization Detector body. The FID ignitor is removed and a modified Swagelock

fitting is mounted on the top of the FID cap. The TID source is mounted in this fitting. The thermionic source is

heated by passing a precisely controlled current through a wire inside the mineral source. The current heats themineral source to its operating temperature causing it to generate a flow of ions between the source and the

collector in the detector. The heat of the source also burns the hydrogen and air mixture in the detector.In typical applications, the wire providing current to the TID source is routed a short distance inside the analyze

oven. This wire typically has a relatively low operating temperature limit of 80 C. The oven temperature must nbe set higher than the rated operating temperature of the TID source wire portion that is located inside the oven

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