[11] What do Chromatograms tell us? Deformed Peak due to Adsorption Interactions in the Liner

2011-jaap-pasfoto4Chromatograms are like fingerprints.  If you can “read” chromatograms well, you often can find a plausible cause. In this series, we will show a series of GC-chromatograms that are obtained from users and discuss some potential causes for the phenomena. Then we can move into some solutions for improvement.

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Fig.1 Poor transfer of analyte from liner. Interactions with wool cause delay.

The injection of the sample plays not only an important role on using the full efficiency of the capillary column, it also should transfer the compounds in a defined band. This is especially important when using the splitted injection technique.

In split injection the sample must evaporate, homogenize and be splitted in a fraction of a second. If this process is not done fluently, the injection band will be broader and will compromise efficiency of the capillary.  The moment the solvent evaporates a lot of energy is take away from the liner and as a result, the local temperature reduces drastically (compare what happens if you put a μl acetone on your finger tips and blow air.. you feel the cooling).

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Fig. 2 Best generic liner for Splitted injection. The wool makes sure evaporation is smooth

That’s why they often add some deactivated wool in the liner exactly at the position where the evaporation takes place. The mostly used liner for splitted injection is therefore the “precision” liner, see fig 2.  The wool is exactly positioned where the needle tip injects the sample. This liner is most widely used in splitted injection systems, as the wool also will keep some dirt from entering the column.

 

There are however also samples that interact with the wool. If the matrix and/or analyte is highly polar, the wool may cause injection issues. An example is shown in figure 1.  A highly polar analyte (glycolaldehyde) is analyzed in a highly polar matrix. The peak shape obtained was caused by the delayed sample transfer occurring in the liner that was filled with wool.

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Fig 3. If interactions are expected from matrix or component with wool, use a liner without wool.

 

Using a double gooseneck (open) liner already improved this, see fig 3. The peak shape is now near symmetrical.

Other ways to improve the injection of this type of samples:

 

–          Use the cyclo splitter  It’s a more expensive liner, but performs quite well;

–          Inject using the “hot needle” technique;  Inject the needle, wait for 2-3 sec. and then transfer the sample;

–          Consider to use new generation deactivated liners, like Restek Premium Liners

–          Inject smallest possible amount:  evaporation will always be better, reducing risks of discrimination


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