components) and very small samples (nanograms); useful for both qualitative and quantitative analysis.
Relative precision 2–5%.
Disadvantages
Samples must be volatile and thermally stable below about 400°C; most commonly used detectors are
non-selective; published retention data are not always reliable for qualitative analysis.
Gas chromatography, so called because the mobile phase is a gas, comprises gas-liquid
chromatography (GLC) and gas-solid chromatography (GSC). For GLC the stationary phase is a high-
boiling liquid and the sorption process is predominantly one of partition. For GSC the stationary phase
is a solid and adsorption plays the major role. Samples, which must be volatile and thermally stable at
the operating temperature, are introduced into the gas flow via an injection port located at the top of the
column. A continuous flow of gas elutes the components from the column in order of increasing
distribution ratio from where they pass through a detector connected to a recording system. A schematic
diagram of a gas chromatograph is shown in Figure 4.17 and details of the components are discussed
below.
Figure 4.17
Schematic diagram of a gas chromatograph.
(From R. M. Smith, Gas and Liquid Chromatography in Analytical
Chemistry, Wiley, 1988).(1)—
Mobile Phase and Flow Control
The mobile phase or carrier gas is supplied from a cylinder via a pressure-reducing head at a pressure
of 10–40 psi giving a flow rate of 2–50 cm^3 min-^1. Fine control of carrier-gas pressure is achieved either
by a needle-valve or by a mass flow controller. The latter enables a constant flow rate to be