We have repeatedly observed dramatic and often sudden losses in the sensitivity of our qTOF-MS during long series of measurements in positive mode. Starting from this situation, measurements in negative mode run with normal sensitivity. After operating in negative mode for a while, polarity can be changed into positive mode again and the sensitivity is normal again. It seems as if negative mode operation would cure the problem.
I know from two colleagues who have experienced the same.
Is there an explanation for this phenomenon?
Is the phenomenon limited to specific MS-types or components?
Can this problem also occur with swapped polarities, e .g. sensitivity lost in negative mode is recovered by operation in positive mode?
Thanks in advance for your help.
The general and simplistic answer is likely because of the electrochemistry that may occur in the ESI probe. You don't specify what samples you normally work with, but, for example, loss of sensitivity (and possibly even clogging of the capillary) is well known to occur due to electrochemical deposition of easily reducible metals (Fe, Cu, Ag, Hg) while electrospraying their salts in the negative mode. Reversing the polarity will re-oxidize the metals and put them back into solution. I've had to deal with solutions with high Fe(3+) content, and it's an occasional nightmare when you see the elemental iron actually building up on the tip of the capillary. Of course, you're citing a situation in the positive mode, but I'm pretty sure the problem is related. For more on the subject, see for example Henderson and McIndoe, Mass Spectrometry of Inorganic, Coordination and Organometallic Compounds, Wiley, 2005, pp. 117-119 and refs. therein.
By chance, I was reading something along the lines today as well, which might explain some of it, if not all!
"Think Negative: Finding the Best Electrospray Ionization/MS Mode for Your Analyte": http://pubs.acs.org/doi/abs/10.1021/acs.analchem.7b00096
I used to run a lot of ESI-MS but I am no expert on the instrument. You might have noticed accumulation of garbage solid near the end of the electro field, which needs to be opened and cleaned up from time to time. My thought was, if too much residues accumulated from previous runs would affect the noise level/ sensitivity, inverting the electrofield might help driving some of them away and thus better result.
This is pure amateur thought. Will wait for opinions from MS experts..
Thanks so far for your answers.
Additional information as requested by Manuel David and Julius:
On the last occasion, sensitivity suddenly fell to almost zero after about 50 hours of continuous measurements in positive mode. About 150 injections took place during this time, which mainly were hydroalcoholic herbal extracts plus a few blank injections and QA samples.
After about the same amount of measurements in negative mode (same samples), we tried the positive mode again and the sensitivity was the same as observed in the positive mode analyses before the problem occurred.
It's beginning to smack of plain old instrument getting dirty somewhere. If I were in the same situation, I'd run the samples in several batches and do thorough routine cleaning of the MS in between. Or, run a batch first in the positive, then in the negative mode and observe the behavior of the machine. Incidentally, I found this delightful piece that everybody should have handy:
https://genome.duke.edu/sites/default/files/PractialLCMSTroubleshooting_TAMS_handout.pdf