Instrumentation for Gas Chromatography - Forensic Applications of Gas Chromatography - page 10

Figure 2.6 Inlet liner designs for injection port.
A range of different sample injection devices is based around the injec-
tion port, and these will now be discussed.
2.3.1 Split/Splitless Injector
The split/splitless injector (Figure 2.7) comprises a heated chamber contain-
ing a glass liner (Figure 2.6) into which the sample is injected through a sep-
tum by a syringe (manually or by an autosampler). The chamber is heated
independently of the chromatographic oven; typically, this will mean that the
injection chamber may be heated to, for example, 270°C, while the column
oven may be at 90°C. The injected sample vapourises rapidly to form a mix-
ture of carrier gas, solvent vapour and vapourised solutes. A portion of this
vapour mixture passes onto the column but the greater volume leaves through
the split valve exit. These amounts are predetermined by the operator using
the split valve. The ratio of the split flow to the column flow rate is called the
split ratio; ratios of 50:1 and 100:1 are common. For example, in a 50:1 split
ratio, one part of the injected sample enters the column while the other 50
parts are vented, via a trap, to waste. A disadvantage of this type of injector is
the possibility of discrimination (i.e., production of a chromatogram that is
not truly representative of the actual composition of the mixture).
2.3.2 On-Column Injector
The on-column injector is designed to allow the entire sample to be intro-
duced directly into the capillary column. Typically, this requires a special