Temperature control is essential in ensuring reproducible separations by GC.
The column is enclosed in an insulated and thermostatically-controlled oven
with a heater and circulating fan to maintain a uniform temperature from
ambient to about 400∞C. For isothermal(constant temperature) chromatog-
raphy, the selected temperature must be maintained to ±0.1∞C, as solute distrib-
ution ratios are highly temperature sensitive, e.g. a 20∞increase in column
temperature results in about a two-fold decrease in distribution ratio and a
corresponding decrease in retention time, tR. Temperature programming is a
procedure used to optimize the separation of complex mixtures (Topic D5).Solute detection The carrier-gas flows through a detectorthat responds to changes in a bulk
physical property, such as its thermal conductivity, in the presence of a solute
vapor, or to a specific property of the eluting solutes themselves, such as their
ability to be ionized. Detectors may be universal, responding to practically all
solutes, or selective, where they respond to solutes with particular characteris-
tics, such as specific elements or structural features. Ideally, detectors should
have the following characteristics:
● a rapid and reproducible reponse to the presence of solute vapors in the
carrier gas;
● high sensitivity, i.e. able to detect very low levels of solutes;
● stablity in operation;
● a signal directly proportional to solute concentration or mass over a wide
range (wide linear dynamic range).Although many types of GC detector have been investigated, only four are in
widespread use. Details of these are summarized below and in Table 5.Temperature
control
D4 – Gas chromatography: principles and instrumentation 145
Table 3. A selection of stationary phases for packed columns
Stationary phase Chemical type Polarity
Apiezon L Branched-chain alkane grease Nonpolar
OV101 Dimethyl silicone Nonpolar
OV17 50% Phenyldimethyl silicone Medium polarity
Carbowax 20M Polyethylene glycol Very polarTable 4. A selection of stationary phases for capillary columns
Stationary phase Chemical type Polarity Applications
BP1 100% methyl Nonpolar Solvents, VOCs, petroleum
products
BP5 5% phenyl Nonpolar Aromatics, PAHs, drugs,
95% methyl perfumes
OV1701 14% cyanopropyl polysiloxanes Medium polarity Alcohols, phenols, esters,
BP10 86% methyl ketones, pesticides
DB17 50% phenyl Medium polarity Esters, ketones, plasticizers
RT50 50% methyl
CP-Wax Polyethylene glycol Very polar Alcohols, esters, acids,
DB-Wax amines, solvents
VOCs =volatile organic compounds; PAHs =polyaromatic hydrocarbons.