Injection Technique

Initial vaporization of the sample is identical with that in split injection and so many of the factors described above are relevant. The text below covers points of special relevance to obtaining satisfactory solvent trapping.

The solvent boiling point should be 20°C below the column temperature at the start of chromatography.

Because, irrespective of oven temperature, the solvent will condense quantitatively only if its vapour pressure is sufficiently high, the amount of solvent injected must be adequate (1-2.5 |L is usually sufficient); excessive dilution of the sample vapour cloud with carrier gas should be eliminated by use of an injector liner of appropriate volume. If the solvent boiling point is too high the solvent peak might interfere with those of the sample components, although this might be overcome by choosing a solvent only sparingly soluble in the liquid phase and thus eluted quickly.

To reduce discrimination against less volatile sample components 90-95% of the sample must be transferred to the column. Because of dilution of sample in the injector, such transfer requires a split-less period four times that necessary to transfer undiluted sample; this could lead to excessive solvent tailing. In practice, the pressure drop caused by solvent condensation results in sample being sucked into the column faster than would otherwise occur; this offsets band broadening caused by sample dilution such that it is rarely a problem for sample volumes <4 |L. The optimum splitless period should be determined experimentally, starting at ca. 20-30 s and increasing it for larger volumes and less volatile compounds. Optimum column position, needle length, etc., must also be determined experimentally.

If the injector has a separate septum purge and its flow is maintained sufficiently low, it may be possible to leave it open during injection without significant loss of sample, thus almost eliminating the effects of septum bleed.

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