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
Syringe-needle effects influence not only the injected sample volume; they also can modify the relative amounts of individual
sample components that enter the inlet. To understand this secondary effect, consider that not all sample components have
the same vapor pressures at a given temperature. Lower molecular weight compounds have higher vapor pressures, and conversely,
heavier molecules have lower vapor pressures. This differentiation forms the basis for simple thermal fractionation of a mixture
of compounds — in a distillation column, for example.
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.