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GCMS problem
Posted: Wed Aug 25, 2010 9:21 pm
by chromogirl
I am getting a peak for the same drug at different retention times about 30 sec apart! Both MS are 99% match. The sample is a spiked QC so no metabolite issues. Have been using the same extraction method, same aquisition method, same column, everything for several years. We are running a gradient temperature method which screens for 300 or so compounds. Has anyone ever seen this ?
Posted: Thu Aug 26, 2010 4:00 am
by Don_Hilton
Can the drug have isomeric forms? Or homologs?
Posted: Thu Aug 26, 2010 2:17 pm
by JTM
It's consistent, though?
Posted: Thu Aug 26, 2010 2:43 pm
by gcguy
Are the breakdown patterns exactly the same?
gcms issues
Posted: Thu Aug 26, 2010 8:12 pm
by chromogirl
The QC contains several known drugs. The late eluting drug trazodone shows 2 peaks at least 1 minute apart. The MS pattern for both "trazodone" peaks are very close with the peak at the wrong retention time having a few smaller ions different but major the same.
The result is not consistent, very inconsistent. One day the extract looks fine and the next it exhibits the "double" peaks and also much lower areas for oxycodone and nortriptyline two other drugs present in the QC.
We have changed all reagents used, liners, column, etc.
Also, injected extract on a different GC, same result with low areas and double peaks. So, it seems sample-related.
Just really strange....not isomers or homologs.
Posted: Thu Aug 26, 2010 10:17 pm
by Schmitty
Is the QC amount added known? Can you quantify the peak and calculate a recovery? Can you sum the peaks and calculate the recovery?
Posted: Fri Aug 27, 2010 12:19 am
by Don_Hilton
The rew smaller ions beign different may be telling you something. You may be looking at a different compund. So if you inject the same extract (from the same GC vial on different days) is the chromatogram consistent?
I would want to see the spectra of the duplicate peaks (and you can post them). Was there a change in the lot of QC standard? New box or new vial about the time the problem started? Without knowing any thing about your sample preparation, I don't knwo what could go wrong there - but a change in a source of suppies can sometimes result in problems.
A bit more detail about what your are doing and seeing - and perhaps someone on the site will spot a a good place to look for the problem.