maintained when the temperature is increased during a separation. With a simple needle valve, the flow
rate will decrease with a rise in temperature due to an increase in the viscosity of the carrier gas. Water
vapour, hydrocarbons and other impurities in the gas affect column performance and detector response,
but they can be removed by passing it through a trap containing a suitable adsorbent. Carrier gases
commonly used are nitrogen, helium and hydrogen, the choice depending on type of column (packed or
capillary), cost and the detector to be used. Helium and hydrogen are the preferred gases for capillary
columns because chromatographic efficiency diminishes more slowly with increasing flow rate above
the optimum (Figure 4.15, equation (4.46)) with these gases than with nitrogen thus facilitating faster
separations.
(2)—
Sample Injection Systems
To ensure the best possible efficiency and resolution, the sample should be introduced into the carrier-
gas stream in as narrow a band as possible. Liquids, diluted if necessary with a volatile solvent, and
solids in solution, are injected through a self-sealing silicone-rubber septum using a 1– 10 μl capacity
microsyringe. Gas samples require a larger volume gas-tight syringe or gas-sampling valve as they are
much less dense than liquids.
For packed columns, 0.1– 10 μl of a liquid sample or solution may be injected into a heated zone or
flash vaporizer positioned just ahead of the column and constantly swept through with carrier gas
(Figure 4.18(a)). The zone is heated some 20– 50 °C above the column temperature to ensure rapid
volatilization of the sample. Alternatively, to minimize the risk of decomposing thermally sensitive
compounds and to improve precision, samples can be deposited directly onto the top of the packed bed
of the column (on-column injection).
Several techniques are available for introducing samples into capillary columns which generally have a
much lower sample capacity than packed columns.
- Split injection involves an inlet stream splitter incorporating a needle valve that enables most of the
injected sample to be vented to the atmosphere whilst allowing only a small fraction (2% or less) to
pass into the column (Figure 4.18(b)). Split ratios between 50 : 1 and 500 : 1 are common. A
disadvantage of split injection is that samples with components that vary widely in their boiling points
tend to be split in differing proportions; relatively more of the lower boiling components entering the
column than the high boiling ones. However, this discrimination effect can be assessed by
chromatographing standard mixtures. Split injection is not suitable when the highest sensitivity is
required as most of the sample is vented to the atmosphere. - Splitless injection avoids the problems and several variations of this technique are used. In one system,
up to several microlitres of injected