Instant Notes: Analytical Chemistry

Section D – Separation techniques

D4 GAS CHROMATOGRAPHY:

PRINCIPLES AND

INSTRUMENTATION

Principles Gas chromatography(GC) is a separation technique where volatile, thermally
stablesolutes migrate through a column containing a stationary phase at rates
dependent on their distribution ratios (Topic D2). These are inversely propor-
tional to their volatilities, which in turn are determined by their partial vapor

Key Notes

Gas chromatography is a technique for the separation of volatile

components of mixtures by differential migration through a column

containing a liquid or solid stationary phase. Solutes are transported

through the column by a gaseous mobile phase and are detected as they

are eluted.

The mobile phase is an inert gas, generally nitrogen or helium, supplied

from a cylinder via pressure and flow controls, and passing through

purification cartridges before entering the column.

Gaseous, liquid and solid samples are introduced into the flowing mobile

phase at the top of the column through an injection port using a

microsyringe, valve or other device.

Columns are either long, narrow, capillary tubes with the stationary

phase coated onto the inside wall, or shorter, larger diameter tubes

packed with a particulate stationary phase. Stationary phases are high-

boiling liquids, waxes or solid sorbents.

The column is enclosed in a thermostatically-controlled oven that is

maintained at a steady temperature or programmed to increase

progressively during a separation.

Solutes are detected in the mobile phase as they are eluted from the end

of the column. The detector generates an electrical signal that can be

amplified and presented in the form of a chromatogram of solute

concentration as a function of time.

A dedicated microcomputer is an integral part of a modern gas

chromatograph. Software packages facilitate the control and monitoring

of instrumental parameters, and the display and processing of data.

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

and applications (D5)

Principles

Mobile phase

Sample injection

Column and

stationary phase

Temperature control

Solute detection

Instrument control

and data processing