[25] What do Chromatograms tell us? My peaks broaden and look worse after every analysis: Impact of the Transfer Line in GC-MS

 

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.

 

This was an interesting case.  The customer makes a sequence of 8 analysis and every next analysis the chromatogram looked different. Not a little different but the last peaks completely broadened and disappeared, see fig 1 for the first 4 runs out of the series of 8.  The early eluting peaks look similar.

Figure 1. First 4 runs of series of 8 Semivolaatiles. Note with every run more compounds broaden and disappear.

It was an analysis of semi-volatiles using GC/MS, using splitless injection on a 30m x 0.25mm Rxi-5Sil MS, film 0.25 micron, a column used a lot for this type of work because it offers inertness and low bleed.

 

As the change happened with every new injection, it could have been contamination of liner caused by highly dirty sample.  In my life I have never seen such big differences just between a few injections.

An other way to get such extreme “smeared” peaks, is when there is a lot of solvent condensation on the column. This can happen if the oven temperature is much below the boiling point of the solvent. Because of excessive condensation, peaks can look very broad. As methylene chloride was used here, this could also not be the case.

The last option must be a cold spot or a “zone” that changes in temperature after each analysis.

 

After checking that, it became clear: it was the temperature of the transfer line which was “OFF”.  After turning it “ON” the results were as in figure 2 and peak width/response was back to normal.

 

Fig. 2 After transfer line temperature was turned on, peaks were again narrow and sharp

Probably at the first analysis the transfer line was still hot, but every next run, it became colder. When components pass through a hot section, nothing will usually happen, but passing a colder section, the retention increases a factor 2 every 15 C lower temperature.

That means that a sharp peak can easily be converted into a “blob” as we see happening in fig 1.

As the temperature of transfer line was decreasing in time, the effect became more visible also at more volatile compounds.

It's very important to eliminate cold spots, so make sure the transfer line temperature is at least similar as the final oven temperature. This is also a reason why detector temperature must be above final column temperature.