Split injection port liners

Either split, splitless, or on-column injections can be used in GCMS. Split injections are usually avoided in cases where trace-level components are being analyzed. Splitless or on-column injections are preferred for trace component analysis. Often splitless injections (septum purgeoff) are made with split injection port liners that are packed with a small amount of adsorbent to trap nonvolatile, polar materials that could contaminate the column and/or the mass spectrometer. For the same reasons, a retention gap is recommended for on-column injections, as the initial column section can be replaced with little effect on the chromatogram. When using a retention gap, however, care must be taken to ensure leaktight connections since a small air leak can have greater consequences for the mass spectrometer than for other detectors. 

Different injection port liners are designed for split, splitless, and on-column injection and for use with solid-phase microextraction. 

Figure 2.55 Injection port liner types for split and splitless injections.

Injection ports for OT columns are usually much more complex because OT columns cannot accept very large samples. By using different liners inside the body of the port and different conditions, a variety of modes of sample introduction can be accommodated with one design. A typical one is shown in Figure 8.3. It is set up for split injection the sample is deposited in the region designated S, where it is volatilized. 

The optimum operating conditions for GC analysis must be determined prior to the GC-MS analysis. In general, this is accomplished by injecting the sample solution several times while varying such parameters as carrier gas, splitting ratio of carrier gas at the injection port fitted with a split liner, carrier gas flow rate, and oven temperature profile. In addition, the type of capillary column used and the concentration and volume of the sample to be injected also have to be optimized. 

On-column injection is another admission mode, which prevents component discrimination due to volatility or molecular weight. The sample is applied directly into the column head. This might be seen as an ideal injection, but with a large number of samples there is the risk of severe column contamination. This is caused by non-volatile sample components which deposit inside the column and eventually clog it. Split/splitless injection ports include a liner just before the column, to act as a filter for these interfering substances. In the case of on-column injection, a pre-column, or retention gap, which is a short capillary with no timer coating, is used for this purpose. 

Split injection is the oldest, simplest, and easiest injection technique to use. The procedure involves injecting 1 /xL of the sample by a standard syringe into a heated injection port that contains a deactivated glass liner. The sample is rapidly vaporized, and only a fraction, usually 1-2%, of the vapor enters the column (see Fig. 6.8). The rest of the vaporized sample and a large flow of carrier gas passes out through a split or purge valve. 

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