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For capillary columns able to handle only a small capacity of sample, even the smallest volume that it is possible to inject with a microsyringe (0.1 μl) can saturate the column. Special injectors are used that can operate in two modes, with or without flow splitting (also called split or splitless). This helps to adjust the sample fraction sent into the column as compared to the total quantity introduced in the chromatogram.

In split mode, a flow of carrier gas (e.g. 100 ml/min) arrives in the vaporization chamber where it mixes with the vapours of the injected sample (Figure 2.5). A vent valve separates this flow into two fractions, of which the largest portion is vented from the injector, taking with it the majority of the sample introduced. The split ratio typically varies between 20:1 and 500:1. Only the smallest fraction, containing an amount of sample equal to the split ratio, will penetrate into the column.

Figure 2.5 Injectors. Above left, injection chamber with splitter (vent 2 regulates the split). Below, a typical chromatogram obtained in splitless mode. For solvent peaks that are superimposed upon those of the compounds, a selective detector which does not “see” the solvent is recommended.

Splitless mode is used only for very dilute samples. The contents of the microsyringe are injected very slowly from the microsyringe, during which bleeding valve 2 (Figure 2.5) is maintained in a closed position for 0.5–1 minute in order for the vaporized mixture of compounds and the carrier solvent to be concentrated in the first few decimetres of the column, where the temperature is lower than the boiling point of the solvent. The opening of valve 2 provides an outlet for an excess of sample. The solvent precedes the compounds in the column and on the chromatogram. For either one of these injection techniques, chromatographs with software‐controlled parameters (flow rates and pressures) have a gas saver, which reduces the split ratio after the injection phase.

The split injector may lead to a poor evaluation of mixture composition, following a discrimination between compounds with varying volatilities: the composition of the fraction entering the column is not the same as the composition of the eliminated fraction. Therefore, it is a problem in quantitative analysis, where external standards are not advised.