Agilent 5975C sensitivity drop off

[Geoff2468]

I am new to this discussion group but also new to Agilent MSD's. A lab we collaborate with has an Agilent 5975C which does not hold sensitivity for very long. Sometimes after one injection, sometimes after several injections through the night. Agilent have replaced the whole analyser assembly, the electronics, the multiplier three times and it still does the same thing. We have read that the Pfeiffer roughing pumps Agilent uses are not very good. The client also uses the column back-flush technique and I observed this last week. The instrument vacuum drops to 10e-4 torr when the back-flush is being deployed at the end of a sample run. Having worked on high-end magnetic sector instruments since 1985, this seems bizarre. That much drop in vacuum points to a lack of pumping ability, or capacity, to me. I wonder if this is contributing to the fall off in sensitivity. Fall off is also noted even without using the back-flush facility. Would a poor Pfeiffer pump cause this problem?

[James_Ball]

It almost sounds like they have the plumbing wrong on the backflush setup, so that it is sending the flow to the MS not back to the inlet. But if the loss in sensitivity persists even when not using the backflush that should not be what is causing the problem unless the change in vacuum is happening even when the backflush is not operating.

What column are they using and what are they injecting? If for some reason they are exceeding the max column temp it could be bleeding off stationary phase that is killing the sensitivity. Had a bad column once that did that even without exceeding the temperature limit, but found out what it was because the company issued a recall on the lot number for that column.

If vacuum varies over time then that could be the cause of loss of sensitivity, if it is constant then something else is the problem.

Split or Splitless injection? Inlet problems can also cause these symptoms. If you go into manual tune and allow it to scan for several hours with the tune gas does the sensitivity stabilize after a few minutes or does it drop continuously for hours? If scanning in manual tune gives consistent counts over a few hours time, then begin looking at the injector as the potential problem.

[syringe]

Geoff
In case you still have the problem, I will comment on some issue I have seen.

Depending how is configured the backflush accessory, it can happens that a leak at the siltite ferrule brings air directly to the ion source, diluting your sample.