Gas Chromatography

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Principle of GC

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Sample complex mixture with multiple components (inert gas) evaporated and introduced into a system with a column containing a stationary components slowed down, separation into distinct bands, and detection and quantification is made possible

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James and Martin (1952)

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GC Devised separation of carboxylic acids on a packed column (1-3m)
Invention of the thermal conductivity detector (TCD) later replaced by flame-ionization detector (FID) (Golay)

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1957

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Golay derived packed capillary GC columns dominated until the middle of the 1980’s when fused-silica becomes the dominant material for analysis and causes capillary columns to become dominant on the market

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1990

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application of gas chromatography linked to mass spectrometric detection (GC/MS) petrochemical industry, impurity analysis in pharmaceutical industry, residue analysis of environmental samples, trace evidence analysis in forensic laboratories and arson investigations

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Separation process

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analyte molecules partition between the mobile and the stationary phases as a function of the distribution coefficient K of the particular analyte.

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distribution coefficient

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K = Cs / Cg

Cs - analyte concentration on stationary phase mol/L
Cg - analyte concentration in mobile phase mol/L

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retention time of the analyte molecule Tr

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Tr = Tm + Tr'
Tm - dead-volume time, i.e. unretained peak time (min)
Tr' - time the analyte molecules spends in the stationary phase (min) reduced retention time

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dead-volume time, i.e. unretained peak time (min)

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Tm = L/u Tr = L/u (1+K')
L length of the GC column (m)
u linear velocity of the mobile phase movement through the column (ms-1)
k’ the analyte’s capacity factor

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k’ the analyte’s capacity factor

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the measure of the retardation of the analyte’s molecule movement through the column with respect to the mobile phase (dimensionless)

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Capacity Factor Equation

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K' = (Tr - Tm) / Tm = Tr' / Tm

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Average Carrier Gas velocity

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U = L / Tm

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djusted retention time

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Tr' = Tr - Tm

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Capillary column

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30 - 100m long, fused silica high quality glass coil of 0.2mm width. Stationalphase coated on the walls with hollow space in between the walls

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Packed columns contain

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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 - 4mm.

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Capillary column Wall-coated columns consist of

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a capillary tube whose walls are coated with liquid stationary phase
SCOT columns are generally less efficient than WCOT columns. Both types of capillary column are more efficient than packed columns.

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Capillary column - support-coated columns

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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.

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Capillary vs Packed column

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Packed columns - higher sample capacity than capillary column.Packed columns may have an advantage is in analysis of gas samples.Capillaries provide much better efficiency (narrow peaks) which leads to greatly improved peak separation - short columns in very brief runs with great separation capacity.

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Capacity factor is determined by

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volume of stationery phase, structure of analyte, stationery and mobile phase as well as solubility of analyte

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High distribution constant

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The longer it takes for elution to occur thru column