Instant Notes: Analytical Chemistry

Section D – Separation techniques

D5 GAS CHROMATOGRAPHY:

PROCEDURES AND

APPLICATIONS

The effect of column temperature on chromatographic retention is pronounced

in that there is an inverse exponential relation with the distribution ratio, D,

which results in a shortening of retention times as the temperature is increased.

Temperature programming, which is a form of gradient elution, involves

raising the oven temperature progressively during a separation 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. Isothermal (constant temperature) conditions may

be unsatisfactory for the following reasons:

● if the isothermal temperature is too high, early eluting peaks may not be fully

resolved;

● if the isothermal temperature is too low, later eluting peaks may have unac-

ceptably long retention times, poor detection limits and small, broad and/or

skewed peaks;

● intermediate isothermal temperatures may result in part of the chro-

matogram having acceptable resolution and detection limits whilst other

parts do not.

Temperature
programming

Key Notes

This is a form of gradient elution whereby the temperature of the column

is progressively increased during a separation to optimize

chromatographic performance.

The analysis of nonvolatile materials, volatile components of complex

mixtures, trace levels of solutes and multicomponent samples with very

large numbers of solutes require special sampling procedures.

Unknown solutes can be identified by comparisons of retention times,

spiking samples with known substances, or using retention indices. These

may be ambiguous, and more reliable information can be provided by

interfacing GC with a spectrometric technique.

Quantitative information is obtained from peak area measurements and

calibration graphs using internal or external standards, or by standard

addition or internal normalization.

Related topics Principles of chromatography (D2) Gas chromatography: principles

and instrumentation (D4)

Temperature

programming

Qualitative analysis

Quantitative analysis

Special procedures

used in GC