[21] What do Chromatograms tell us? Loss of Separation, Peaks are Tailing

2011-jaap-pasfoto4-smallChromatograms 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 Peaks develop a strong tail which impact the separation.

A chromatogram is obtained like in figure 1.   Peaks are tailing a lot and separation is compromised. Also response is much lower as expected.  This is a very common problem as it happens with many applications the target analytes are “just” base line separated.

There are a number of reasons that can cause peaks to look like this.  We see here development of tailing on all components. Because of the tailing, the separation of the target components becomes a challenge. We discussed the development of tailing before in several blogs, see:

http://blog.restek.com/?p=8674;      http://blog.restek.com/?p=8866

http://blog.restek.com/?p=9530       http://blog.restek.com/?p=9657

Here the immediate problem is, that peaks cannot be quantified because the tailing causes a problem with the quality of the separation. Systematical check of system is required. Consider the following:

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

 Contamination of injection port: This can happen and contaminates (septum particles) can delay the injection causing such peaks. 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”;

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  http://blog.restek.com/?p=2254

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-type columns are based on a new stabilization process that can take a lot of sample stress because of the solid stabilization of the polymer backbone. Lastly, you may consider to 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: http://www.restek.com/catalog/view/8175

Consider filtration systems positioned close to the GC to ensure pure carrier gas. See: http://www.restek.com/catalog/view/3445

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