John V. Hinshaw

In GC, samples can be liquids, solids, or gases. The type of sample and its concentration, taken along with the column and detector, determine which types of injection will give the best results. There often is more than one choice of appropriate injection technique, of which only one might be available in a particular laboratory or instrument. Sometimes none of the suitable inlet systems is available, in which case the analyst might have to choose between compromising the injection or upgrading their chromatographic equipment. Where they have the methodological latitude, they also can solve this dilemma by modifying preparation procedures to make the sample compatible with available inlet systems, or by choosing a different, more compatible column. For example, analysis of a trace-level sample might best be performed with splitless or on-column injection onto a 25-m, 0.25-mm i.d. high-resolution column. Without an on-column or split–splitless inlet on-hand, the chromatographer could choose to sacrifice speed of analysis by using a longer 60-m, 0.53-mm i.d. megabore column with the same phase ratio and direct injection. Such a column could produce about the same resolution as the narrow-bore column in about twice the time, and it is compatible with the larger injection volumes produced by direct injection.

Liquid sample injection techniques for open-tubular (capillary) column GC can be classified by, among several variables, the fraction of the vaporized sample that enters the column. For trace-level analyses, it is desirable for all or at least a substantial portion of the sample to enter the column to maximize method sensitivity. But at higher concentrations the amounts of analytes that traverse the column can become large enough to affect separation quality by causing peak overloading and peak shape distortion, so in these cases only a fraction of the sample is passed into the column by using split injection. To complicate the situation, passing liquid sample volumes greater than about 1 μL directly onto the column can cause a number of solvent-induced side effects that detract from the column's resolving power: controlling these effects can be critical. Often, it is very clear which type of injection technique will be the best choice for a specific sample and analysis, but there are many samples that could be handled by multiple injection types. Deciding which to choose can be difficult, and understanding the side effects that can occur is vital for troubleshooting inlet problems.