Technical Help Needed – IRMS Instrumental Quandry

This is a technical post. I’m writing for other scientists who use the same (or similar) instrument as I do to ask a technical question. I’m having a problem with the mass spectrometer and need the advice of others.

If the mechanics of isotope ratio mass spectrometry aren’t your thing, you might wanna stop reading now. If you’re interested in the types of challenges that arise with these high-falutin’ instruments, you might keep going.

The problem:

I’m getting intermittent power fluctuations through the source. These are little 1-second-long sinusoidal waves of around 10-20mV. These are often overprinted on broader waves or interferences (maybe 100 seconds?) of around 50mV. These are wrecking our analyses.

Initial observation of intermittent waveforms. Here, through the gasbench. Click to enlarge.

Initial observation of intermittent waveforms. Here, through the gasbench. Click to enlarge.

There was an alarm on our UPS/power conditioner, so I had it serviced. I also cleaned the source while the instrument was shut down. There was nothing consequentially wrong with the UPS. The alarm was a pre-programed service alarm.

I restarted the instrument and immediately observed the same pattern.

Observing it in the Gasbench. Click to enlarge.

Observing it in the Gasbench. (Really! I changed to Conflo just before grabbing this screenshot.) Click to enlarge.

I thought maybe it was a Gasbench thing, so for grins I switched over to the Conflo and observed the same thing. Notice, however, how the interference comes and goes.

Noise in the Conflo. Click to enlarge.

Noise in the Conflo. Click to enlarge.

After that, it seemed to go away. I tuned the instrument and got good results. I set it to run on-offs over night. Most of them look great. However, on occasion things would go wrong for a half hour or so, resulting in on-off sets like this:

Those peaks should be perfectly flat. You can see the ~10mV waves on top of ~50mV waves. Click to enlarge.

Those peaks should be perfectly flat. You can see the ~10mV waves on top of ~50mV waves. Click to enlarge.

I’ve cleaned the source and serviced the UPS, as well as done a hard reboot of the system since I had to completely shut it down for the UPS service. What am I missing?

If you have ideas, please let me know in the comments below or by e-mail to pennilyn (dot) higgins (at) rochester (dot) edu.

Thanks!

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