Instant Notes: Analytical Chemistry

very small fraction of injected sample reaching the column. A slow purge

stream of gas prevents substances bleeding from the septum accumulating in

the injection port.

● Splitless injectionis used with capillary columns when samples contain low

levels of some components, and maximum sensitivity is required. The

control valveis kept shut as a 0.5–5ml volume of sample in a volatile solvent

is injected. By cooling the top of the column to just above the boiling point of

the solvent, the sample components are trapped while the solvent travels on

down the column. The control valve is then opened to purge any remaining

sample vapors from the injection port, and the column temperature is raised,

releasing the solutes into the gas stream.

● On-column injectionis used with both capillary and packed columns to mini-

mize the possibility of the decomposition of thermally labile solutes as well as

providing increased sensitivity. It is also ideal for quantitative analysis.

● Flash-vaporizationis primarily used with packed columns and is a means of

rapidly volatilizing samples in a zone heated 20–50∞C above the column temper-

ature to provide as narrow a band of vapor as possible (Fig. 3). However, there

is a risk of thermally degrading labile solutes, although a glass liner inserted into

the metal block in which the samples are injected minimizes the risk.

● Automated sample injectionis advantageous for improving precision and

for processing large numbers of samples loaded into autosampler trays. Most

injection ports can be adapted for this purpose, which is often under

computer control.

● Special injection techniquesare employed for headspace analysis, pyrolysis

gas chromatographyand thermal desorptionto concentrate samples (Topic

D5).

The column is where the separation process occurs and it is, therefore, the

central component of a gas chromatograph. There are two types of GC column,

and a comparative summary is given in Table 2.

Column and
stationary phase

D4 – Gas chromatography: principles and instrumentation 141

Syringe

Septum

Septum

retainer

Heated

block

Carrier gas Oven liner

Seal

Column

Fig. 3. Schematic diagram of a flash vaporization

injection port. Reproduced from Instrumental

Methods of Analysis, 2nd edn, by H.H. Willard, L.L.

Merritt, J.A. Dean, F.A. Settle © 1988. Reprinted

with permission of Brooks/Cole, an imprint of the

Wadsworth Group, a division of Thomson Learning.