Column efficiency parameters

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COLUMN EFFICIENCY PARAMETERS

PRESENTED BYSUBODH S SATHEESH

MPHARM PHARMACEUTICS

ECPS

PHARMACEUTICAL ANALYSIS-GAS CHROMATOGRAPHY


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It is expressed by the number of theoretical plates It is determined by the formula

The number of theoretical plates is a measure of the “goodness” of the column

If the retention time is high and peak width is narrow then it shows excellent chromatograms

Column efficiency


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where tr is the retention time measured from the instant of injection

w is the peak width

W is determined by SD= σ ie w=4σ


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Resolution is the ability to seperate two signals In chromatography its the ability to seperate two peaks.ie

seperation of constituents

where tr1 and tr2 and w1 and w2 are the times and widths, respectively, of the two immediately adjacent peaks.

If the peaks are sufficiently close w is nearly the same for both peaks and resolution may be expressed as

Resolution


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Greater the distance more resolution and vice versa.


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The rates of migration of substances in chromatographic procedures depend on the relative affinity of the substances for the stationary and the mobile phases

Its the difference in time between the point of injection and the time of emergence of separation of component from the column.

It is actually the time required for 50% of the component to get eluted.

It is measure in minutes or seconds

Retention time


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It is the volume of carrier gas required to elute components from the column to the time the peak maximum is obtained.

Retention volume depends upon flowrate and retention time

VR= tR-FC

Retention volume


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It is the ratio of partition coefficient of two components to be separated.

S= Kb/Ka = K’a/k’b= (tb-to)/(ta-to)

If peaks are far apart ie there is more difference in partition coefficient between compounds hence more seperation factor and viceversa

Separation factor

Less seperation factor

More seperation factor


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HETP is numerically equal to the column length divided by the number of theoretical plates in the column

It varies from to one column to another as well as one solute to other

The more efficient the column the better the resolution and the smaller the HETP.

HETP=Length of column / no of theoretical plates

HETP


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Here we can see the effect of flowrate on HETP. An ideal flowrate happens in the minimal HETP.


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A chromatographic peak should be symmetrical about its centre to follow gaussian distribution

Asymetric factor is the measure of peak tailing or fronting. It is defined as the distance from the centre line of the peak to

the back slope divided by the distance from the centre line of the peak to the front slope.

Asymetric factor


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The chromatographic peak in (a) is an example of tailing, which occurs when some sites on the stationary phase retain the solute more strongly than other sites. The peak in (b) is an example of fronting, which most often is the result of overloading the column with sample. For both (a) and (b) the green chromatogram is the asymmetric peak and the red dashed chromatogram shows the ideal, Gaussian peak shape.


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Its function is to separate sample components to discrete peaks

It should have reasonable chemical and thermal stability

There are a lot liquid stationary phases available for gas chromatography. But there is no solvent that meet all the requirements of a perfect stationary phase

LIQUID STATIONARY PHASES


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Liquid phase should not permeate too deeply into the fine pores of the support structure as slow diffusion in and out of pores affects column efficiency

It should be chemically inert It should be a good solvent for sample component Liquid phase should have low volatility and high stability at

elevated temperatures otherwise they can contribute to interference in analysis

Characters of good Liquid StationeryPhase


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Stationary Phase Trade Name Max Temp Common Applications

Dimethyl Polysiloxane

OV – 1, SE – 30 350oC Hydrocarbons, Polynuclear

aromatics, PCB’s

Poly(phenyl methyl) siloxane

OV – 17 250oC Steroids, Pesticides, Glycols

Poly (Trifluoro propyl dimethyl)

siloxane

OV – 210 200oC Chlorinated Aromatics, Nitro Aromatics, Alkyl

substituted Benzenes

Polyethylene Glycol Carbowax 20 M 250oC Free acids, Alcohols, Essential

Oils, Glycols

5% Diphenyl – 95% Dimethyl

polysiloxane

DB – 5 325oC Flavors, environmental samples and

aromatic hydrocarbons

Typical liquid stationary phases


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Nonpolar solutes like pentane butane etc can be effectively separated by nonpolar solvents like squalene. And polar solutes can be easily separated by polar solvents eg; PEG.

Boiling point is also a factor of consideration. among solutes with similar polarity if there is sufficient difference in BP effective separation can take place.

Eg. Squalene min/max temp= 293/423 SE4 423/573 A solvent that could generate different partition ratios

among solvents can only be useful in GLC

Selection of solvents


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H KAUR Instrumental methods of chemical analysis ninth edition 2013 ;1091-1092

Skoog holler crouch Instrumental analysis 2012 841-846

B K sharma Instrumental methods of chemical analysis twenty fourth edition 2005 c188-191

En.wikipedia.org gas chromatography

Reference


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Many thanks MANY THANKS

Column efficiency parameters

Science

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