In liquid chromatography (LC), the syringe functions primarily as a pipette or liquid-transfer device that loads a sample loop. The syringe generally does not take an active role in injection, which occurs only after sample has been displaced from the syringe. The same is true of most gas chromatography (GC) gas-sampling valves — the time at which the gaseous sample is injected into the column is separate from the moment that it is transferred into the injection system. In GC analysis of liquid samples, however, the syringe becomes an integral part of the inlet during injection: sample, in liquid or gaseous form, starts to enter the column as soon as the syringe enters the inlet.

In GC inlet systems for liquids, the injection technique, choice of syringe, and inlet operating conditions all play a crucial role in the injection process. Two principal sample-transfer mechanisms move sample from the syringe into the inlet while the syringe is in the inlet. First, liquid-sample transfer takes place as the syringe plunger is depressed and liquid is expelled from the syringe tip. In cold injection, where the inlet temperature is not high enough to produce significant solvent vaporization, this pipette-like action is the only major sample-transfer mechanism. A competing process occurs in a hot injector, however. Within a few tenths of a second after the needle enters a hot inlet, sample begins to evaporate inside the needle. Bubble formation and concomitant increased internal pressures force some liquid out along with the vapor, so that part or all of the sample contained in the syringe needle volume is introduced into the inlet as a mixture of liquid and vapor. As the plunger is depressed, additional room-temperature liquid sample is forced from the syringe through the needle, which cools the needle and suppresses but does not entirely stop in-needle evaporation. The needle heats up again once syringe plunger motion ceases, which causes additional sample vaporization from the needle into the inlet. All of these processes take place in a matter of seconds. The total amount of sample that actually is injected into the inlet depends strongly upon these two processes, their timing, the volumes involved, and the inlet conditions. Along with judicious injection condition control, a good understanding of the role of the syringe in these processes will help gas chromatographers obtain better injections.

Sample Distortion During Injection

Unfortunately, for many gas chromatographers, the same kind of thermal fractionation process can occur in the syringe needle during injection. When in-needle vaporization occurs, the lighter components vaporize first and leave the syringe needle quickly. Heavier compounds take longer to evaporate and leave the syringe needle more slowly. The effect of initial needle heating during injection is remediated largely by subsequent bulk liquid transfer through the needle: straggling heavy compounds will be rinsed out. At the end of injection, however, if the needle is withdrawn from the inlet before complete transfer of all compounds can occur, then the sample that enters the inlet system will contain more of the volatile components and less of the heavier components than were present in the sample before injection. This effect is called "mass discrimination" because it tilts the sample composition according to components' vapor pressures, which are related largely to their masses.

Two other thermal side-effects go hand-in-hand with needle fractionation. Many compounds are sensitive to thermal stress, especially in the presence of a hot steel syringe needle, and decompose rapidly when subjected to high inlet temperatures: high molecular weight glycerols are one good example. Polar compounds can become strongly adsorbed on the needle surface. Many of the polar polyaromatic hydrocarbons (PAHs) as well as a host of chlorinated pesticides suffer from this problem. In both cases, the use of nickel syringe needles, or needles that have been deactivated by advanced surface treatments, can help tremendously, as can deactivation of inlet liners and injector inner surfaces exposed to the sample. In these situations, thermally mild injection techniques such as cold on-column injection or programmed temperature vaporization usually produce results superior to what can be obtained by careful deactivation of needles and other hot injection materials, but the specialized inlets needed for these somewhat more complex techniques might not be available.