Submitted by anthias on
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 into a gas chromatograph. However, as with most analytical equipment, each function needs fine tuning to produce the best results.
Injection style.
The most basic injection style is to load a volume of sample (either liquid or vapour) and inject into the instrument (auto-injector loading below). 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. Discrimination may occur if a wide range of compound volatilities is present in the sample. However this may be overcome by the use of pre- and post-injection delays, or by using a solvent plug (below)
However for liquid samples, the possibility of loading several different aliquots into the syringe at the same time exists as shown below:
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 derivitisation reagent if within-inlet derivitisation is to be performed. By using the autosampler to make these additions, improved precision is obtained together with a saving of operator time.
NB when using multiple aliquots, ensure that the total vapour volume generated from all of 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 out of the inlet liner into the cold gas lines, generating carry-over and ghost peaks.
To learn more about sample preparation for GC & GC-MS, attend Day 3 / Module 8 of our Complete GC & GC-MS course.
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