Chromatography Forum

I had been running a 5971 with good results for a couple of months, when over the course of a few days the high mass sensitivity dropped. Oddly, the mid mass sensitivity seemed to increase. I am running hydrogen carrier gas, and my vacuum is better now than it has ever been at 2.0x10-5.

So far, I've cleaned the source (it appeared perfectly clean), changed the filaments (they looked fine), changed the every board except the power supply (re-checking results in between). I checked the power supply voltages, and they are fine. Rechecked my column flow (but I get the same results even with the transfer line removed and capped).

I'm pulling my hair out. I finally changed the detector for a new one, but it hasn't made a difference. The odd thing, is that I cannot even raise the EM voltage above 1400 with the new detector, because when I do that, it throws the ratios way off and the auto tune then reduces the EM voltage to bring everything in line.

Perhaps there is some calibration I'm not aware of? Any suggestions would be greatly appreciated.

Hello

Could you please post Autotune Report?

Thanks

Regards

Tomasz Kubowicz

If you're talking about mid/high mass sensitivity, are you referring to mid/high mass fragments of the calibration gas, or mid/high boiling analytes in your standards/samples?

Here is the first scan, from happier days:

http://www.edenbotanicals.com/josh_files/scan0002.jpg

Here is today's tune:

http://www.edenbotanicals.com/josh_files/scan0001.jpg

The first image is when I was using the old electron multiplier, and the second has the new one.

1.6 million counts for 69 is very high as is the EMV in the first report. In the first one you are less than 200v from the maximum which means time for a new EM, but it also looks like the abundance was very high so it could probably be run a few hundred volts lower.

Going from 3.32% 502 down to 2.26% 502 is not bad. Both of those ratios are great for a 5971. The 326,976 abundance for 69 in the second one looks more like a normal autotune number to me.

Running with that first tune, you probably had high background signal which meant your signal to noise was not so good. Give the new tune a try and see if you have smaller chromatographic peaks, but better s/n ratio. I don't think I ever had to run with a 69 tune abundance over 800K and could usually get by with it in the 500k range with no problems.

I assume that the maximum repeller voltage is set for 15V. In the first tune your ion focus is also at max, 120V, whereas in the second it is a much more reasonable 65V. It may be worth your while to find another tune file from an old tune that worked and gave good results. Load it up and see how it compares.

It turned out that the EM ground pin was arcing to the EM frame. I'll have to replace the frame and check, but I hadn't noticed that on this base plate, the pin was straight up instead of bent like the rest that use this style multiplier frame.