[2] What do Chromatograms tell us? Peak Shape: Reactivity initiated by sample matrix

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.

Figure 1: Analysis of underivatized amphetamines solvent standard after 71 injections of liver, blood and urine, 65 HFAA Derivatized amphetamines with no cleanup. Note the reaction platform after the injections (green trace)

A peak shape is observed as shown in figure 1.  Here an interesting peak shape develops after the analysis of underivatized amphetamines solvent standard after 71 injections of liver, blood and urine, 65 HFAA Derivatized amphetamines with no cleanup.

Originally the peak shape for amphetamine and methamphetamine was good (red trace) , but it developed in time into the shape shown by the green trace.  Same as in previous blog on peak shape, a “reaction platform” seems to be formed.

This was a pretty dirty sample and the impact on the chromatography is significant.

First suspect is the liner, which for sure can be a problem-maker, but it usually will not give us a platform. Typical reactivity in a liner provides a sharp decomposition peak, which is also focused by the splitless injection.

Here the matrix is changing (contaminating) the first section of the analytical column, which makes it more reactive. This caused the amphetamines to decompose while they are moving through the active inlet-side of the column

To reduce this phenomena, one could:

Cut periodically a coil off the inlet section of the column:  column will get shorter in time which will affect retention times and eventually also separation efficiency;

Couple the column with a guard column:  one can use deactivated fused silica for that, but in case of reactivity one can also use a coated guard for that. Coated columns usually have more “capacity” to deal with contamination/activation, so they last longer.

Inject less sample into the column:  if detection limits can be reached by injecting less, the impact on lifetime of liner and column will be a direct benefit;

Use sample clean up: will take extra work and will be a trade off.