sample are collected in a cold trap at the top of the column initially held at over 100°C below the
boiling point of the most volatile sample component. The trap is then heated to boil off the sample
components sequentially. This method is suitable only if all the sample components have relatively high
boiling points. In a variation devised by Grob, the sample is mixed with a high-boiling solvent, e.g.
octane (b.pt. 126°C), which, on injection, condenses in a thick layer on the cooled wall at the top of the
column. This layer retains and concentrates the sample components before the temperature is raised,
thereby, in effect, increasing the sample capacity of the column.
- On-column injection allows very small liquid samples to be placed directly into the cooled top of the
column which is then heated to volatilize the components. A syringe fitted with a very fine quartz
needle is used in conjunction with a specially designed septumless valve through which cooled or
heated air can be passed (Figure 4.18(c)). Air cooled to about 20°C below the boiling point of the
sample solvent is passed through the valve while the sample is being injected, then warmer air is
circulated to volatilize the sample in situ on the column. This technique avoids discrimination and
reduces the risk of decomposition of thermally sensitive compounds. - Automatic injectors, which eliminate variations due to the analyst, improving reproducibility, and
which can be controlled by computer, are of value where large numbers of samples are to be analysed
or unattended operation is required. Samples and standards loaded into racks or turntables can be run in
a predetermined sequence and under different operating conditions. Such devices can also be used for
single samples to improve injection precision.
Solid samples can be introduced as a solution or in a sealed glass ampoule which is crushed in the gas
stream by means of a gas-tight plunger. Only solids which have appreciable vapour pressures at the
operating temperature of the column can be successfully chromatographed.
(3)—
The Column
The column is the heart of the gas chromatograph in which the separation process occurs. It consists of
a coil of stainless steel, glass or fused silica (quartz) tubing which may be 1 m to 100 m long and have
an internal diameter of between 0.1 mm and about 3 mm.
To ensure operation under reproducible conditions, the column is enclosed in a thermostatically
controlled oven whose temperature can be held constant to within ±0.1°C. Operating temperatures
range from ambient to over 400°C and may remain constant during a separation – isothermal operation
- or automatically increased at a predetermined rate to speed the elution process – temperature
programming (p. 106). The latter is a form of gradient elution. Rapid temperature equili-