Chromatography Forum

We have an Agilent 6890/5973 hooked up to a purge & trap for running volatiles analysis. For the past couple of years we have been having sporadic problems with low levels of random noise which occur across the entire scan range (30-350 amu). Most of the peaks are of around the same height, say around 1000, so a bit too high to remove by cutting the signal threshold. On any given scan there are anywhere from 25-50 peaks evenly spread out, all at approximately the same height, so it looks a bit like a cross-section of someone's lawn. "Illustration": (|||| || ||| |||| ||||||| || |||| |||||| ||| ||||||). The amu's of the peaks are random as well, so it doesn't look like any physical contamination, i'm fairly certain it's electronic noise. I've made sure that everything is on the same circuit and completely swapped circuits to known "clean" circuits, so it doesn't seem to be outside interference. I'm running an Edwards 1.5L pump connected to a turbo, my vacuum gauge reads 2.3 e-6, so the vacuum looks good, no air/water leaks, cleaned the source multiple times, reversed polarity, changed filaments, changed EM's, (even swapping between the OEM and SGE mounts), wiped down the inside surfaces of the analyzer with MeOH (not the quads of course), blown out the quads with computer duster, dipped the coils, swapped exterior cables connecting the analyzer to the boards, blocked off the mass spec from the column, etc. etc. etc. I'm figuring at this point it's got to be the HED power supply, except that this has been going on at varying levels for a couple of years now (we mostly use this instrument for screening) and was wondering if that would have burned out by now if something was wrong with it? Before I break down and just buy some new boards, anyone have any other suggestions?

We once had this happen with a 5973 attached to a GC purge and trap for 8260. We had Agilent come in and it turned out it was a bad main board (the board attached to the MS side plate). We also saw a similar problem later and it turned out to be excess water getting into the MS from the P&T. In this case it also looked like electronic noise with the same symptoms you are describing. I have only limited experience running VOCs but it always seemed that the answer to MOST troubleshooting issues was excess water. If it's been happening for "years" and you only use it for screening I would seriously investigate the possibility of excess water. Have you tried running direct injections or a no-injection run from the GC without running the P&T cycle? If you disconnect the P&T you can eliminate water as a variable.

If this doesn't work then I would say you are probably right on track with suspecting the HED or a board. If you end up needing a new sideboard then I would suggest getting the new one that allows for fast scanning and dual SIM/Scan acquisition modes. It requires that you have MSD Chemstation Productivity v D.03 or higher though to run the "Fast" scanning options.

I hope you are able to figure it out. Those P&T GC-MS systems can be a real pain to troubleshoot.

Ty

Karl,
Did you ever figure out this problem?
D.R.

Hi, karlh

I've had this problem a couple of times. It was caused by dirt in the "whatyoucallit" pin just after the quadrupole that turns the ion beam 90º to impact in the multiplier. Just blowing it with clean gas solves the problem. I am sorry I cannot give the pin a more technical name, I couldn't find it in the Agilent web page.

Hope it helps.