A mysterious MS problem: test your trouble-shooting skill

[yangz00g]

I recently experienced a mysterious MS problem, want to share with you and see how you diagnose it:

The method is an LC-MS MRM method screening over 60 pesticides in a 8 min gredient. LC from Agilent,and QQQ from AB Sciex.

Wired things happend after last two PMs. Right after the PM in last fall, I lost sensitivety of all my compounds from 50 to 80%. No culprit found, and after one or two month, it went back by itself to normal. No tuning or calibration was done during this period.

Just last week, right after this PM, we lost signal (total loss) for about 45 compounds, however, the detectable ones have no RT shift or sensitivity loss. For both group of compounds, their RT and Q3 m/z are all over the range (1 to 7 min, and 60 to 400m/z).

Eevery PM parameter passed the requirment; Re-calibrated the mass; LC was proven to have no problem by infusion and other checks; ESI probe was good but replaced with new one; method was not corrupted,proved by re-writing, instrument was "baked" over weekend, no improvement; manual optimziation for some lost compounds found no signal; no problem with standards......

Problem is still unsolved by now

What do you catch? Thanks and have fun

[Ron]

I tend to believe that one of my favorite rants may be appropriate here, when it comes to mass spectrometers there are at least two conditions the instrument can be in after preventive maintenance, things LOOK clean or things ARE clean. I have seen too many cases over the years when a mass spec is cleaned and the performance is no better, or in some cases like yours worse than before. The really bad thing is that sometimes when you point this out to the person who did the work the response is "The system meets the performance specifications", and the fact the sensitivity on real samples is worse than before.

If you are paying for this I would call the person doing the PM and ask for it to be redone. The instrument performance shouldn't be after PM than before.

[yangz00g]

Ron, thanks for the suggestion.

As you said, I did have a conference call with the technical support and call the service in to re-tune the system. It turned out to be an electronic control parameter was messed up duirng the past PM. Usually this could cause the loss of all the signals in my and their experience, why and how this time ended up with such a uneven or unfair "picking" remain unclear. But anyway, problem is solved.