Problem of GC/MS performance

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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Recently, two analytes among 20 in the same mixture has been found with irreproducible results which the signal becomes larger and larger on repeat injections with the same concentration. This phenomena occurred without any repairing or changing in GC/MS setting and haven't happened before. This only shows in certain analytes but remains stable for the rest. Please advice.
If you can tell us a bit more about the nature of the analyts and the way you analyse them we might be able to give some advice.

are they e.g. pesticides, PAHs, FAME, .....
what is the matrix
what type of injector, column, detector do you use,

.....
Does a blank injection, after injecting your mixture, gives any signal for those particular analytes?
We are using the GC/MS to determine the organochlorine pesticides.

Injection type: pulsed split
Inlet temperature: 250oC
Injection vol: 2 microliter
Column: Agilent HP-5MS
Detector: MS with SIM mode

The signal has become larger when the analysis in progress, the signal is about 50% larger after 30 repeating injections of the same solution in different vials.

Please advice, thank you.
Some organochlorine pesticides are prone to breakdown in the inlet. If the breakdown decreases with conditioning then you will see an increase in response for those analytes. Does changing your liner help or make it worse?
You may be overloading your inlet, and causing carry over. If you over load, the sample will go out the split vent line and condense there, and possibly carry over.
What is your solvent, and liner?
Steve Reimer wrote:
Some organochlorine pesticides are prone to breakdown in the inlet. If the breakdown decreases with conditioning then you will see an increase in response for those analytes. Does changing your liner help or make it worse?


I agree, it sounds like the repeated injections are capping off active sites in the inlet and giving better peak response.

Would the peaks giving the increased response happen to be DDT or Endrin? Those are especially susceptible to breakdown in the inlet.
The past is there to guide us into the future, not to dwell in.
Thank you for your advice, we use hexane as the solvent and we've tried to replace with a new liner (Agilent 5190-2293), however, this doesn't help with the situation.

This problem occurs to most of the compounds of OC and even the internal standard (Triphenylphosphate). However, the percentage of signal increase various from each analyte, (from 20% to 50%), so it is difficult to obtain a solution even with the correction of internal standard. Are there any possible check/amendment of the method?
Did you only replace the inlet liner?

If the injection port becomes contaminated you will also need new gold seal at the bottom, and to scrub the inner surface with swabs and solvent. We even have to remove the port and scrub it out with a brush and polishing compound or something like Bar Keepers Friend to remove very bad contamination.

Also look at the hole where the split vent line attaches, (the copper line coming from the side of the injection port) crud can build up in there that will cause the same problems, it can be cleaned by removing the split vent line and using a small pipe cleaner.
The past is there to guide us into the future, not to dwell in.
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