Gas chromatography (GC) inlets transform a sample from its physical state outside the chromatograph into a state suitable for separation inside the column. This transformation consists of two principal steps: transfer from the outside into the inlet and transfer from the inlet into the column. On the way to the column, the sample undergoes volume, concentration, temperature and pressure changes that convert it into a condition that is more or less compatible with the separation that follows. Chromatographers measure an injection's success by the degree to which it preserves the relative sample composition while not interfering with separation, and by how well the injection process repeats from run to run.

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 is often 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 are available, in which instance the analyst might have to choose between compromising the injection or upgrading their chromatographic equipment. Where they have the methodological latitude, they can also 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 be best 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 instances 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.