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Tech Tip - GC Splitless Injections
Getting better peak shape for GC splitless injections
My observation of chromatograms from labs throughout Australia carrying out GC splitless injection is that the peak shape, especially for the early eluting peaks (volatile compounds), can be excessively broad (as shown in this chromatogram). Broad peaks will lead to a loss of resolution and poor quantitation. However, there is a simple fix. Change the type of inlet liner you are using.
Essentially, there are three aims of a good GC inlet liner:
- Vaporise the sample quickly
- Transfer the sample vapor to the column quickly
- Ensure the sample doesn’t degrade in the process
 By choosing a small inner diameter inlet liner, chromatography for the volatile components can improve dramatically. The reason for this is the reduction in diameter results in a smaller liner volume and leaving all other conditions the same (i.e. the inlet pressure and therefore the column flow rate), the velocity of the carrier gas through the liner is increased. Therefore, the vapour cloud, which results from the sample injection, is transferred faster from the liner to the column. This has the effect of sharpening peaks and leads to better sensitivity and quantitation. The transfer rate from liner to column of the late-eluting (semi-volatile) components is less important because of cold trapping when using a temperature programmed method and so a wider diameter liner is often OK. This is the reason wide liners are often described as both 'split / splitless' but narrow liners only as 'splitless'. The drawback for a narrow diameter liner can be you are more limited to the liquid volume of sample that can be injected into the GC and vapour flash back has to be considered.
The typical diameter for split injection for an Agilent Technologies liner is 4mm and this should be reduced to 2mm when doing splitless injection for volatile compounds.
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