Instant Notes: Analytical Chemistry

employed. A nonpolar stationary phase (e.g. a silicone), which separates

compounds chiefly according to their boiling points, is particularly useful.

Since the GC oven is often programmed to fairly high temperatures, some-

times to 400°C, it is essential that the stationary phase is stable and does not

‘bleed’ off into the detector or spectrometer. Columns where the stationary

phase has been chemically bonded to the column wall minimizes this problem.

Infrared spectrometry is usually carried out at atmospheric pressure, unlike

mass spectrometry. This allows direct transfer of the separated components

between the gas chromatograph and the IR spectrometer through a simple

heated tube as an interface. This is shown schematically in Figure 1.

GC detectors, such as the flame ionization detector (FID) are extremely sensi-

tive. By contrast, the infrared spectrometer is far less sensitive. The gas stream is

therefore usually split at the column exit so that about 90% goes on to the spec-

trometer, and only 10% to the FID. The major portion is transferred through the

interface, usually a short heated tube, often an inert piece of GC capillary, into

the FTIR spectrometer. To obtain the maximum response it is necessary to do

one of two things: either (i) the sample must be concentrated by condensation

onto a cooled surface, or by absorption onto an active solid, or (ii) if the sample

is to remain gaseous, it must be passed through a small volume cell, or light

pipe, which gives a long path length to maximize the absorption of IR radiation.

Both these alternatives have their advantages. A condensed sample gives the

spectrum normally obtained by conventional IR sampling (mulls, KBr disks,

melts). The vapor phase spectrum will be rather different, since less intermolec-

ular interactions, especially hydrogen bonding, can occur. However, the vapor

sample is readily and rapidly removed from the cell, and may even be collected

for further investigation.

F4 – Gas chromatography-infrared spectrometry 299

The gas chromatograph

The optical system

Injector

GC column

Splitter

assembly

Flexible

transfer

line

FID

Gold-coated

light-pipe

IR beam

IR beam

Carbon trap

Carbon trap

Bypass valve

Fig. 1. Gas chromatograph-infrared spectrometer system.