GC/MS Loss of signal over time

How dirty are your samples? We analyzed smokeless tobacco flavors in glycerin (for vaping) and the source would become completely contaminated (no tune possible) after one injection.You may need to clean your source. Check your tune results to be sure.

How dirty are your samples? We analyzed smokeless tobacco flavors in glycerin (for vaping) and the source would become completely contaminated (no tune possible) after one injection.You may need to clean your source. Check your tune results to be sure.

Thanks for the help gchaplain, james and steven,

1) Have you searched for an air leak through either a sniffer or through an air water check on the MS?

Performed self-evaluation on the MS. It appears to be water free and no problems where detected

it could be ypour split vent trap filling, hindering pressure control in the inlet.

What does this mean exactly? I checked the inlet liner and while it had a couple of "grains' of dirt in it, the liner-cotton still appeared white and clean. Slightly coloured where the needle entered the cotton. how dirty does an inlet need to be before replacing? Or is this a simple manner of trial and error testing?

3) When was your system last tuned? You maybe experience loss of signal becase of loss of detector sensitivity, especially if the areas are low to begin with (<10'000 counts)

This fixed the problem. last tune was on 27mar18

Performed a run, tuned the MS, performed automatic tune self-evaluation (all OK and passed)

Area run pre-tune: 80.000

Area run post-tune: 150.000

I still dont understand why this fixed the problem. if the signal was lower due to detector sensitivity after +-2 weeks, would that mean I would have to re-preform the tune every week(?!) Wouldnt that just mean i am polluting my MS with every measurement?

On a small side note, is an area of 150.000 'relatively' normal for a 100 ppm sample in SIM? Or should i provide more information in order to answer this?

How dirty are your samples? We analyzed smokeless tobacco flavors in glycerin (for vaping) and the source would become completely contaminated (no tune possible) after one injection.You may need to clean your source. Check your tune results to be sure.

We're on the same team here! Im analyzing raw flavours on a glycering base in a 10 times dilution in MeOH, used for vaping. I have analyzed +-50 samples so far with alot of blanks in between (old habit).

Is there a way to 'bake out' the MS for cleaning? Or are you talking about removing the vacuum, opening the MS, cleaning it with solvents and turning it back on? (Which I have never done before) What do I need to check on the tune results to be sure?

THanks for the assistance everyone. Although the direct issue has been fixed, I would be really happy to discover the root cause of the loss of signal, and further assistance is much appreciated!

Koos

"I still dont understand why this fixed the problem. if the signal was lower due to detector sensitivity after +-2 weeks, would that mean I would have to re-preform the tune every week(?!) Wouldnt that just mean i am polluting my MS with every measurement?

On a small side note, is an area of 150.000 'relatively' normal for a 100 ppm sample in SIM? Or should i provide more information in order to answer this?"

Your answer is that over time detector drifts. In an ion source there are many parts that drift or misalign over time due to temperature and vibration fluctuations, so the instrument needs to be tuned, like tuning a guitar to the right note. The auto tune goes through an algorithm to optimise the ion peak shapes, intensity and mass location. Your detector could have drifted in one or more of these points. We tune our instruments weekly, followed by a calibration weekly. Every day we perform a tune evaluation to ensure the detector is doing what it is supposed to be doing. If it isn't we tune it and calibrate it.

What abundance on the tune are you seeing for the 69 ion peak and what are you getting on the EMV parameter( should be on the report around 400'000 abundance, and between 1000-2000 volts)

thanks for the explanation gchaplain.

The 69 ion peak abundance during tune is 334.000 with an EM Volts of 965.8

Not bad. I would tune it two more times just to be sure.

Also just a quick thought, I assume you are using an internal standard? If you are then at least fluctuations in your area response shouldn't matter t oo much for your results.

Fortunately yes, so I am covered from that point of view

The GC over here is a combo MS-FID machine which can be build to either MS of FID mode. It was in FID mode the last couple of days. Just rebuild the GC to MS mode, but I didnt perfom a full-on cleaning. My measurement series is nearly done, and i prefer to wait and not change anything major until it is complete.

Performed a couple more tunes after the reset:

All tunes are within 1 hour of each other

Tune 1, tune 2, tune 3

• Peak shape smooth & symmetric x3
• Etune ion abundance m/z 69 508k, 512k, 513k
• Etune ion abundance m/z 219 456k, 515k, 517k
• Relative ion abundance m/z 502 7.5%, 7.4%, 7.4%
• EM voltage 988, 1067, 1081
• #background peak 167, 220, 203
• Isotopic ratio m/z 502 10.4%, 11.0%, 9.7%