Standard Peak Areas Declining During Run

[MattyD617]

Hello,

Our group has been running GC-MS to analyze artemisinin content in samples. We often run long sequences up to 100 samples at a time and have been doing so for years. Usually, we run several sets of external standards in between sets of samples to make sure the detector is performing properly throughout a run. In the past, the peak areas of the standards showed no decrease over a long run however, during the past few months, the areas of the standard peaks decrease over the course of the run. For example, a 20 ug/mL standard injected at the start of the run shows a peak area of 1.5x10^9 but a 20 ug/mL standard injected towards the end of the run has a peak area about 10 fold lower. This has never happened in the past and we have not changed any of our methodology or sample preparation. We have tried changing the column and cleaning the ion source to no avail.

Any help would be greatly appreciated!!!

Thanks,
Matt

[Peter Apps]

Check the voltage on the electron multiplier - it might be getting near (or if deterioration is that rapid, past) the end of its working life.

Peter

[mckrause]

Have you changed liner types or suppliers? Normally when we see this we associate it with a liner issue.

I did have a MS that would not hold tune. If you start to lose tune you will see symptoms very similar to this. In my case it was a quad driver board that was starting to go bad and got hot during the run.