Anyone with GC experience.....?

Question

I recently performed a lab experiment to determine the accelerant used in an arson case.

Five standard accelerants were measured by diluting 2 mL of each fuel to 50 mL in a volumetric flask with dichloromethane.  (The item burnt with the unknown acclerant was extracted with dicholormethane as well.)

The experiment, for the most part, went fine.  My unknown is clearly identifiable.  However, one of the standards is messed up.

For every sample, the DCM peak eluted at ~2.71 min.  But for the gasoline standard, it eluted at ~0.86 min.  In addition to this, the concentrations in the gas standard were much smaller.  The largest peak only had a peak height of about 47 . . . whereas the other standards were well in the hundreds - with some easily breaking 1000.

Can anyone think of a what might have been the problem?  (I didn't run the standards myself, so I'm having trouble thinking of possible causes.  All I can think of is possibly a bad injection - but I can't see why that would change the retention time so drastically.)

Jimbo · Accepted Answer

I don't know if the samples were manually injected or if there is an automatic injection process for the GC but assuming that all factors remained constant such as flow rate, temperature, and stationary phase for all samples, I believe that either the chromatogram was run too far away after the sample had been injected as the separation would have already been occurring before the chromatogram had started leading to a quicker retention time or the sample was not injected immediately after being placed into the GC or if this was the last sample to be injected the rubber septum may have been damaged.

My recommendation would be to run the standard again and if the same thing happens again replace the septum and if the results are still dodgy make another standard.

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Anyone with GC experience.....?

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