5975C Problem

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

8 posts Page 1 of 1
Hello everyone. I've been lurking here for a few months, but finally decided to join the conversation. I'm new to mass spectrometry and have learned a lot here.

I'm looking for some help with troubleshooting my 5975C. Over the course of a day while attempting to calibrate, I see my analyte peak areas continually decrease, making it impossible to get a calibration. I had thought it must be a leak, but my air and water checks come out ok and a varian helium mass spec leak detector couldn't sniff out any leaks.

I now believe the problem is within the mass spec. The other day I did 4 low mass auto tunes about half hour apart and the PFTBA abundances decreased with each one, with lower masses appearing to be affected more. I also have my GC configured such that I can flow calibrant continually to the mass spec so I can manually tune to maximize my target ion (34) abundance. When I do that, I'll profile ions 34, 69, and 131 and can observe them all decreasing over 10 or 20 minutes. Yesterday while doing this, however, ion 34 abundance decreased while the others increased.

The EM is fairly new, and I've cleaned the source and replaced filaments. Does anyone have any ideas what the problem could be?

Thanks
Boliver Shagnast wrote:
Hello everyone. I've been lurking here for a few months, but finally decided to join the conversation. I'm new to mass spectrometry and have learned a lot here.

I'm looking for some help with troubleshooting my 5975C. Over the course of a day while attempting to calibrate, I see my analyte peak areas continually decrease, making it impossible to get a calibration. I had thought it must be a leak, but my air and water checks come out ok and a varian helium mass spec leak detector couldn't sniff out any leaks.

I now believe the problem is within the mass spec. The other day I did 4 low mass auto tunes about half hour apart and the PFTBA abundances decreased with each one, with lower masses appearing to be affected more. I also have my GC configured such that I can flow calibrant continually to the mass spec so I can manually tune to maximize my target ion (34) abundance. When I do that, I'll profile ions 34, 69, and 131 and can observe them all decreasing over 10 or 20 minutes. Yesterday while doing this, however, ion 34 abundance decreased while the others increased.

The EM is fairly new, and I've cleaned the source and replaced filaments. Does anyone have any ideas what the problem could be?

Thanks


After cleaning a source I normally wait 24 hours before I begin to calibrate. Is this happening the next day after a cleaning or just after the initial 2 hour wait that automatically pops up on the screen?

What are your source and quad temperatures?

Also when I begin to tune, I set it in manual tune and let it scan with the PFTBA gas valve open for 15-30 minutes at least to make sure the 69 mass has stabilized, especially if the instrument was open to atmosphere recently. If it has been running samples every day it will stabilize much faster.

It may not hurt to let it scan a few hours since it is a new Electron Multiplier, just to make sure everything is broken in and stable, putting power on the multiplier will warm it slightly and help it degas any traces of atmospheric gasses that might remain. When I do major cleaning on my instruments I try to do it on Friday, and let them pull vacuum and bake out over the weekend so my first tune and calibration will last longer, but almost always have to return and recalibrate about a week later, after that they remain stable for a long time.
The past is there to guide us into the future, not to dwell in.
Hi James. Thanks for your response.

It has actually been a few weeks since I cleaned the source and I did the cleaning on a Thursday, let it pump down over night, baked out the MSD Friday and let it pump down over the weekend before attempting to tune. This problem of decreasing response isn't an isolated event, but has been ongoing for several months.

The EM is about 3 months old and was autotuning at about 1100V when new, but now after only light use is already over 1500V, which also seems a little fishy to me.

Source is at 230C and Quad at 150C. Before cleaning the source, I was keeping it much cooler (~190) thinking that it may cause less fragmentation, which would help with my sensitivity, but I think keeping it hotter and hopefully cleaner may prove beneficial.

I would expect to see the PFTBA fragments start at high abundance when first opening the valve and then take a little while to equilibrate, but should my calibrant also do that? It's a gas coming out of a calibration standard cylinder at a constant pressure and flow rate.

I may try giving the source another cleaning since the last time was my first experience with it and maybe I didn't get it adequately clean. Have you got any other thoughts?

Thank you.
Is the calibrant coming in through a restrictor or other narrow bore column to keep the flow in the 1-2 ml/min range?

Is there any change in peak widths for the masses as the sensitivity drops, when looking at them in profile scan? If the peak widths are not stable then it can be something in the side board where the quad RF is controlled. If the peaks narrow over time, then the response will drop. I had a 5971 with a bad board that would have peak widths change over night causing a similar problem.
The past is there to guide us into the future, not to dwell in.
I've got a manual rotary valve that when in one position allows effluent from one column to pass onto the head of a second column, which then goes through a restrictor to the mass spec. In the other position, it flows my calibrant onto the second column, through the restrictor to the mass spec. In both cases, the flow is about 2.1 sccm.

Today I tried waiting half hour after beginning to profile with the PFTBA valve open and my calibrant flowing before starting to log the abundances and peak widths. While the peak widths were consistent for each mass (all around .59), the abundances just kept dropping. I let it run for 1 hour 40 minutes after starting to log, and in that time the abundances dropped by 27%, 19%, and 24% for masses 34, 69, and 131, respectively.

I haven't had a chance to re-clean the source yet, but I'll hopefully get to that this week. Could it still be something related to the quads that might show this rapid loss? Have you ever heard of an EM degrading this quickly?

I appreciate the replies. Thanks.
Boliver Shagnast wrote:
I've got a manual rotary valve that when in one position allows effluent from one column to pass onto the head of a second column, which then goes through a restrictor to the mass spec. In the other position, it flows my calibrant onto the second column, through the restrictor to the mass spec. In both cases, the flow is about 2.1 sccm.

Today I tried waiting half hour after beginning to profile with the PFTBA valve open and my calibrant flowing before starting to log the abundances and peak widths. While the peak widths were consistent for each mass (all around .59), the abundances just kept dropping. I let it run for 1 hour 40 minutes after starting to log, and in that time the abundances dropped by 27%, 19%, and 24% for masses 34, 69, and 131, respectively.

I haven't had a chance to re-clean the source yet, but I'll hopefully get to that this week. Could it still be something related to the quads that might show this rapid loss? Have you ever heard of an EM degrading this quickly?

I appreciate the replies. Thanks.


What is the calibrant gas?

Does it fall the same if you are flowing helium but without the calibrant gas?

Just wondering if the calibrant gas is doing something to the source causing it to lose efficiency or become dirty/active.
The past is there to guide us into the future, not to dwell in.
The calibrant gas is 1ppm phosphine in helium, which doesn't appear to be causing the problem. Even when using just PFTBA, the abundances continually drop. I pulled the source out and looked over everything and didn't see any obvious damage to insulators and all the metal parts were clean and mirror-like. I cleaned them again anyway, reinstalled the source, and baked the MSD last night. The problem persists.
I had an Agilent ICP-MS before that had a problem with the board that controls the voltage to the EMV, it was the opposite problem where sensitivity would increase overnight, but reset the next morning when restarting the instrument. In that one the measured voltage would steadily increase throughout the run, which increased the sensitivity, then it would reset to the correct voltage when the instrument was restarted. Only way you can detect it is to monitor the high voltage supplied to the EMV which is over 1000V, so need the correct type of meter for that. If the voltage is drifting down below what the instrument thinks it is, it could cause the problem you see. I can't say that is the problem but is something I have seen before, though only once.
The past is there to guide us into the future, not to dwell in.
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