problem with persistent water in 5971

[jkloepping]

How long should it take to get rid of the water signal in a 5890/5971 setup?

I am using helium with a new water trap and oxygen trap, I have conditioned the trap in the 5890, I have conditioned the column, and I've run the system now for almost four days and water is still at 50% of peak 69. Oxygen and Nitrogen are below 2%, so I don't think I have a leak. The water signal doesn't change with the calibration valve open or closed, and it doesn't change when column flow is reduced from 1ml to .5ml. It certainly seems like the water must be in the MS, but where could it be hiding? Could water vapor diffuse into the aluminum casting over time?

The signal went down pretty rapidly the first day, to a little over the peak for 69, and then dropped to its current value late on the second day. Since then, it's been pretty stable. I notice that when I close the valve for the PFTBA, the signal at 69 goes away almost instantly, but the water remains.

[Bigbear]

It depends. Has this system spent much time open to atmosphere? If so the ceramic insulators can take on water that will take quite a while to pump out.
New water trap? Try taking it out to see if this is the water source.

[James_Ball]

Assuming the source was cleaned recently, did you use Methanol or Water on the tan colored insulators? If so, they will hold on to water for quite some time. Do you have the Ion Gauge to measure high vacuum? If so you can degas it once the vacuum is below 5x10^-5torr and you will cook off any water that may have adsorbed onto the filaments or the glass.

[jkloepping]

Ok, good suggestion on degas. I did that for 15min, and out of curiosity I scanned the ions from 10 - 70 and saw m/z 28 had shot way up, above the water signature in fact, and some other peaks were also appearing. I scanned again as I shut off degas and the spikes subsided. However, this so far has not helped the water, still unchanged at around 40%.

According to the ion tube manual, degas should be done for around 15min. I wonder, shouldn't degas be continued until no more degas ions are showing up in the scan? I only scanned for a moment, as I wasn't sure that it was a good idea to do that during degas. Is 15 minutes some sort of important upper limit, or could I just leave it on for 30-45 minutes?

[James_Ball]

The excessive heat during degass can eventually cause the filaments to sag and not work, but for short times like 10-15 minutes it is ok. I used to monitor the ions present during desorb to see that they fell off as they should.

Was this system left at atmospheric pressure for a long time? If so it could have absorbed a lot of water that will take time to cook out. Do you have the interface heated up to 150-180C, that will put more heat into the system and help drive out the water if you haven't done that yet. The source temperature should read about 170C if everything is heated up and making good connections.

If at atmospheric pressure for a long time you could also have water absorbed into the diffusion pump oil which will take time to cook out.

Have you run Autotune or merely used manual tune to view the background? If no autotune, it could be that the tune is biased to low masses and makes Water look unusually high.

[jkloepping]

Yes, the system was left at atmospheric pressure for quite a long time. So, now I'm leaning towards patience. After a couple of days stuck at 40%, water is now down to 28%. I've had the interface heater on for the whole time at 240C, and I'm running the oven and injector port at 100C just to keep them warm.

I have run a successful autotune, so I think I'm ok there.

[James_Ball]

If it is still falling, I am betting on it being in the ceramic ends that hold the quads in plus in the vespel items and probably the diffusion pump oil too if that is old.

[jkloepping]

I thought I'd provide a final update for those who were kind enough to reply to my questions.

The water level declined very slowly for about a week, from 40% down to 17% at which point I decided to simply try a few runs and see what happened. I injected 1ul of a 10% Wintergreen/Hexane solution, and got pretty good data. While I was fiddling with getting the 7673 working properly I did two more runs (each about 45min), and towards the end of the second run, the plot suddenly showed what looked like a huge increase in noise.

I reran the air and water check, and the water was down to < 2%, Nitrogen and Oxygen < 1%. I retuned and checked again, and noticed that the EMV had dropped to 2200.

Since then the air and water check has been stable, and the sensitivity seems to have increased dramatically. I discovered on the ETP website (the multiplier manufacturer) a document that warns that the detector can absorb moisture during long periods of storage. Ideally, the detector should be baked out under vacuum before being energized, or there is a possibility of permanent damage. I wish I had seen that before I started! In any event, everything seems fine now.

[James_Ball]

I didn't think about that you might have the ETP multiplier in it. Those have ceramic side boards that would pick up moisture and since they are not directly heated would take a long time to desorb. Once you began using it you were getting more heat from the high voltage there and that probably kicked out the last of the moisture.

If you replace it you will find the new one will tune about 1000v or less. Set the tune you will be using(Atune, Stune, BFBtune, ect) down to 800v or 1000v and save the parameters before you ever actually turn on the new multiplier, then run autotune and you should be fine.

With the current one you can probably get away with even less voltage on the EM once it settles in.