Splitless injection port liners

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

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. 

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. 

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

Specially designed injection port inlet sleeves have been available on the market for direct and hot on-column injection.In the direct injection mode, 2 -mm-lD inlet sleeves are commonly used, which permit a sufficient space for sample evaporation however, the on-column mode is usually performed by inserting a 26-gauge needle inside a 0.53-mm-lD column. Direct injection is more favorable because it is less problematic than the hot on-column mode. Because the liner can trap nonvolatile residues before entering the column, this technique is suitable for dirty samples. Compared to the splitless mode, the direct injection is advantageous, involving less adsorption of the solutes and better sensitivity. However, with this technique, the adsorption of the sample may occur on the inlet sleeve during the evaporation process.In this case, the hot on-coIumn mode offers more benefits. 

Solvent Effect OTQ. An effect noted in splitless injections for concentrating higher boilers at the head of the column so that the peak band will reflect the efficiency of the column and not the volume of the injecUon port liner. For this effect to occur, the oven temperature must be close to the boiling point of the major solvent component in the system so that it condenses at the head of the column and acts as a barrier for the solute. 

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