signal issue

[gcguy]

I have never seen this problem before so I am looking for possible suggestions. Or have I overlocked something obvious?

Agilent system 5973 MSD. On some runs I am seeing the baseline drift below zero. The source is clean and I have swapped the filaments and the multiplier recently. The problem seems to come and go with no correllation to the samples being run, in fact if I re-run the same samples the signal can be ok.
It does not seem to suddenly switch off but gradually drifts down and then re-appears after about 4-5 min.

Any ideas anybody?

GCguy

[JI2002]

Check the threshold, you may want to reduce it a little bit.

[AICMM]

What does the early spectra look like, what does a later spectra look like.

How low are you scanning. When did you last do inlet maintenance on the split line.

What is the water level, nitrogen level, oxygen level, argon level, in the spectra at low temperature and high temperature with the PFTBA valve off? What is the 69 value with PFTBA on?

Best regards.

[gcguy]

Every thing is pretty much as it should be, the system is clean and the tune is ok. This is an intermitant fault that has me scratching my head.

I have superimposed a zoomed in chromatogram.

GCguy

[AICMM]

Gcguy,

Based on the chromatogram JI2002 could be right and solve all of your problems with an increase in threshold.

I wonder more now than ever about the earlier questions, what does the baseline spectra look like, how low are you scanning, what are your MS conditions....

Best regards.

[gcguy]

I would agree that there could be an issue with the ms parameters, unfortunately this is an intermittant problem and the parameters have not changed from the time when this did not occur. At the low points of the baseline the signal is zero so no spectra, on the points that can be seen it is "normal".

We are scanning from 40 up to 600.

GCguy