After calibration and cleaning we can only see positive ions but no negative. While calibration, the negative ion mode worked fine without any problem, but after shutting down the system for cleaning, we can only detect positive ions. Maybe wrong reassembling?
Hi,
If by "no negative" you mean not even a single signal, I would guess there is a problem with the electrical connection somewhere.
What is the instrument and what exactly did you do with it?
Hi Fabrizio,
yes, by "no signal" i mean 1 peak every 10 seconds or so. so basically nothing.
we've got a LCQ deca xp+, what i did was: tuning and calibrating it, then immediately afterwards i did a measurement in neg. ion mode which worked fine, but in pos. ion mode i saw some peaks from the calibration solution, so i decided to clean the API source as recommended by the manufacturer. so i, disassembled the API stack, cleaned the relevant parts and reassembled it again. after that i couldnt get a negative ion signal anymore. i thought that maybe i reassemled something wrong, so i did the whole thing over again, but nothing changed.
i found one troubleshoot in the internet that mentioned a rusty adapter nut as the problem, maybe thats it?
http://web.uvic.ca/~mcindoe/qtof.html#press
It might be something similar.
My point is: you see signal in positive mode, therefore I would say that the ionization is happening.
It looks to me that the problem comes afterwards.
You might want to send an email to Bill Mahn From Thermo. He has a lot of experience with that instrument (among the others).
When was the last time you calibrated the instrument for the negative ion mode? If it is too much out of calibration then there can be a problem with the settings of your ion optics, i.e. your ion beam might not be making to the multipliers/detectors. Secondly, ion optics could be dirty (plastered with contaminants).
Definitely give a call to your service engineers!!
Is it working well in positive ion mode? I.e. signal strength per known concentration? When was the last time you ran it in neg. ion mode? We had an LCQ Deca back in 2004, and it was a troublesome unit. If I remember, it has an inline sampling orifice vs. orthogonal which required a lot of cleaning. I've had a similar issue on a different instrument when it had a failed signal discriminator board at the detector (~$1,000), but much different hardware.
If the positive mode works fine, the ion source an ion optics is OK. Since switching to negative mode means the potential applied on all ion optics should be switched, I think the problem is here. As long as one of the negative potential was not generated or applied, or with some error from power supply, usually is a module controlled by PC, your will lose all of the signal. So check output of the power supply module at negative mode.
thank you guys for your input so far, i checked a few things you suggested:
regarding calibration in negative ion mode:
- the last calibration i did was in positive ion mode (about a week ago) and on the same day i did a measurement in negative mode that was ok.
- the problem seems to have occurred after disassembling the API stack for cleaning, unless there's just an odd coincidence with something else
so, i guess, what i will try now is calibration in negative mode and if that doesn't help, cleaning the ion optics.
problem solved
thanks again for your input guys!
i didn't think of tuning the lenses again, because automatic tuning failed due to the weak (almost not existing) signal. so i tuned the lenses manually - et violá - negative ions are back! i'm currently running some test, but it looks very good.
I used the manual tuning option, it is explained pretty well in the user's manual!
Hello, I have same issue in the LCQDeca MS- no signal in the negative ion mode. I tried what is given here. However, I am confused about the "tuning the lenses manually". Is it the "manual tune" of the Tune option?
Also, would anyone suggest any more option?
yes, what i meant here was the manual tune. but before doing that, i would try instead using an older tune file or the default settings. sometimes its even enough to turn up the capillary voltage to like 2 or higher to get a signal again.
Hello Mr Rupart,
I tried all types of tuning and calibration to resolve my problem and have gotten no signal/peak in the positive ion mode even. Would you please give me some suggestion to get the signals back? Let me know if you have anything to know. Thanks a lot for your suggestion and concern.
Ok, so I'm not a technical expert, but from my experience, there is a great number of possible reasons why you don't get a signal. Some of them can be tackled by yourself, for some you need a service technician.
I would suggest you start with running all the test/check options. Look if all read-backs (voltages, vacuum etc.) are ok. If this is the case check the electrode of the needle and the cable leading to it (that everything is properly connected). If you disassembled the MS recently (e.g. for cleaning), check again if everything is properly re-assebled. Check your test solutions and needles.
I have problem with my LC MS MS analysis. The negative mode were appeared accordingly, but no positive mode. Why the compounds did not show positive mode. My extract was water/polar sample. Can anyone give me suggestion whether this phenomena is normal?
Thanks
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