Low Sensitivity

[Spectro]

Hi all,
The sensitivity of my GC-MS became low gradually. I tried to raise the multiply to make better sensitivity. However, I can't keep raising the multiply from time to time. Do we have the other way to make the sensitivity better.
Thank all, Spectro.

[bigwalt]

when was the last time you cleaned the ion source? if the ion source is dirty, that would be the obvious first place to look.

please post some more details, so we will know better how to help you.

[Don_Hilton]

Hoow are you determining that your system has lost sensitivity? Is the tune voltage up or is it only difficulty in seeing low level chromatographic peaks? Also, how is the air/water leak check on the instrument?

[astroguille]

Hi!

The source of the problem can be in the GC or in the MS.
If the autotune is OK, the problem is in the GC (flows, contamination of the inlet, etc).
If the autotune is not OK, the problem is in the MS (the ion source can be cleaned).

Best regards

[Spectro]

Hi all,
Thank for all replies. I did cleaned the source recently. Actually I can see the very small concentration about 5ppb. What I meant low sensitivity is happened during the day. For a while I saw my internal standard areas becoming smaller and small. After about 3 hours, I had to raise my voltage multiply to keep the areas stable. Then for 3 hours later, I raised it one more time. After I left and the machine kept running overnight, the sensitivity was about half of theirs before.
I run the GC-MS, so the inlet looked quite clean when I opened it.
Hopefully I will receive more replies. Thanks all.
Spectro.

[thohry]

Hi all,
Thank for all replies. I did cleaned the source recently. Actually I can see the very small concentration about 5ppb. What I meant low sensitivity is happened during the day. For a while I saw my internal standard areas becoming smaller and small. After about 3 hours, I had to raise my voltage multiply to keep the areas stable. Then for 3 hours later, I raised it one more time. After I left and the machine kept running overnight, the sensitivity was about half of theirs before.
I run the GC-MS, so the inlet looked quite clean when I opened it.
Hopefully I will receive more replies. Thanks all.
Spectro.

I just say that, sometimes when these things look quite clean and in fact they are not. You have to isolate them.

[Don_Hilton]

The better indication of the mass spectrometer condition is the tune voltage. The PFTBA level in the ion source of an instrument with a dedicated PFTBA inlet is failry constant and does not involve passage through the GC, so it is a good indicator of mass spec performance.

What you describe sounds like it is more likely to be an inlet issue -- but you need to get a measuement that separates issues.

The rate of change you describe would lead me to suspect a GC issue, particularly relating to the inlet - like a cored septum.