Specialized Techniques Used in Gas Chromatography
Temperature Programming
The effect of column temperature on chromatographic retention is pronounced in that there is an inverse
exponential relation with the distribution coefficient, KD (p. 91). This results in a shortening of retention
times as the temperature is increased. Temperature programming, which is a form of gradient elution
(p. 91), exploits this relation. In practice, the oven temperature is progressively raised during a
chromatographic run to improve the resolution of mixtures where the components have a wide range of
boiling points, and to shorten the overall analysis time by speeding up the elution of the higher boiling
compounds. It is particularly useful for the separation of multicomponent mixtures on capillary
columns. In such cases, it is very likely that no isothermal conditions will be entirely satisfactory for
separating and detecting all components for the following reasons:
(1) If the isothermal temperature is too high, early eluting peaks may not be fully resolved as these
solutes will elute quickly.
(2) If the isothermal temperature is too low, later eluting peaks may have unacceptably long retention
times, and detection limits will be poor because of excessive peak-broadening. Peak shape may also be
adversely affected.
(3) Intermediate isothermal temperatures may result in part of the chromatogram having acceptable
resolution and detection limits whilst other parts do not.
The separation of a series of n-alkanes isothermally at 150°C on a packed column is shown in Figure
4.23(a) and illustrates both of the first two problems. The first four alkanes, n-hexane (C 6 ) to n-decane
(C 10 ), are incompletely resolved, C 6 and C 7 co-eluting. Beyond n-dodecane (C 12 ), the peaks are smaller,
broader and show fronting (p. 82). In the temperature programmed chromatogram (Figure 4.23(b)), a
complete separation of all the n-alkanes up to C 21 has been achieved and in about a third of the time
taken to separate only up to C 15 isothermally. Note also that C 6 , C 7 and C 8 are fully resolved and that the
later peaks are sharper and more symmetrical. For mixtures where the individual components are not, as
in the case of the n-alkanes, members of a homologous series, temperature programmes are often more
complex. They may involve initial, intermediate and final isothermal periods separated by temperature
ramps of varying rates, usually between 2 and 30°C per minute. The optimum conditions for a
particular sample are generally established by trial and error.
Two potential disadvantages of temperature programming are the inevitable delay between consecutive
chromatographic runs whilst the oven is cooled down and a stable starting temperature re-established,
and the possible decomposition of thermally-labile compounds at the higher temperatures. Computer-
controlled systems have improved the reproducibility of temperature programming, and automated
forced cooling of the oven