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[6] What do Chromatograms tell us? Peak Shape: Tailing on all eluting peaks

16 May 2013

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Chromatograms 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 After only 15 injections all peaks are showing a tail

 

A peak shape is observed as shown in figure 1.  It just took 15 injections to get the bottom chromatogram. Peaks are tailing and are lower in response.  Important detail is that all peaks are tailing.  There can be several causes for this.

 

Contamination of injection port:  not usual after 15 injections. Check liner and septum;

 Very strong sample matrix: this will contaminate liners rapidly. Use wool and inject smallest possible amount that will give LOD. Consider more sample clean up or use an injection port that can be used also as “back flush”;

Injection port temperature too low:  poor evaporation. check the actual temperature. Use a thermometer. Do not always trust your GC-readout.

Dead volume the system; Column may be broken in bottom of injection port or in the bottom of the detection port liner. Detection ports are often highly active, see Activity of the FID detection port: a big problem if underestimated.

Activation of the system 1:  this can be the detection port, as stated above, but many times is the first section of the capillary column used. Stationary phase/surface gets activated, causing adsorption activity.  This can be initiated by the sample matrix.  Cut a section off the inlet of the column after XX analysis, or use “pre-columns” as “guard column”. Since it happens after 15 injections, you may consider to use a column with more tolerance for matrix. Rxi-XLB is a new generation phase that can take a lot of sample stress because of the solid stabilization of the polymer backbone. Lastly, you may use a 2x thicker film which will produce better peak shape as elution temperatures increase.

Activation of the system 2:  if water or oxygen enters the system, often at higher temperatures the inlet section for GC columns gets activated. This is depending on temperature.  Air can enter systems when changing carrier gas cylinders, when there is a leaking connection or even  via the septum.  Leak detection is important. Special leak detection systems are available, that also allow you to check the septum, see: Restek Catalog No. 25800.

Consider filtration systems positioned close to the GC to ensure pure carrier gas. See Contamination of Injection System Split-Vent-Lines: A Maintenance item not to Underestimate