TechTip 12: Use of liquid autosamplers – GC injection style

Figure 1

Published: 1st November 2012 | © Anthias Consulting Ltd

Modern autosamplers have become much more versatile and sophisticated than their original antecedents, so that they are not merely a device for replacing manual injection, they can now offer many options and improvements to the introduction of liquid and gaseous sample aliquots. However, as with most analytical equipment, each function needs fine tuning to produce the best results. 

Injection style used in GC injections

The most basic injection style is to load a volume of sample (either liquid or vapour) and inject into the instrument. In gas chromatography (GC) (Figure 1), this technique is associated with “fast injections” so that the solvent slug is fired into the inlet to evaporate, rather than evaporating from the needle. Syringe mass discrimination may occur if a wide range of compound volatilities is present in the sample, however, this may be overcome using post-injection delays, or by using a solvent plug.

For liquid samples, the possibility of loading several different aliquots into the syringe at the same time exists (Figure 2):

Figure 2

Using a solvent plug and air-gap, ensures that the entire sample is ejected from the syringe and needle, the solvent aliquot effectively flushing-out the sample from the syringe barrel and needle into the inlet.

By using the multi-sandwich technique, several aliquots may be combined.  As the example shows, this could be adding an aliquot of internal standard at the injection stage (saving manual additions) and even adding a derivatisation reagent if within-inlet derivatisation is to be performed.  By using the autosampler to make these additions, improved precision is obtained together with a saving of operator time.

NB In GC, when using multiple aliquots, ensure that the total vapour volume generated from all the solvent aliquots does not exceed 75% of the inlet liner volume.  Otherwise, contamination of the inlet system may occur by expansion of the vapour cloud beyond the inlet liner and condensing in the cold gas lines, generating carry-over and ghost peaks as well as reducing the amount of sample to transfer to the column.

To learn more about GC autosampler methods attend:

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