LC-MS TSQ Vantage sensitivity loss

[bsxenia]

Hello everyone,


I am working with an LC-MS/MS TSQ Vantage HESI II since 8 months and a sensitivity loss appeared suddenly for two developed and accreditated methods, nicotine (positive mode) and bisphenol A (negative mode). Same concentration of standards is giving 10-factor lower surface and a lower S/N ratio.

Source and optics are cleaned, LC is checked, standards are checked and new columns are used. Direct injection with polytyrosine seems ok, no charging and intrument's specifications with reserpine are passed.

The technician cannot excplain this difference with the two other methods. Another detail is that the multiplier voltage has jumped in one day at 1660V form 1300V.


Do you have any idea what would it be?


Thank you in advance!

[leadazide]

Sometimes people will have their TSQ's multiplier gain run in negative mode.. This will give a higher multiplier voltage also for running in pos mode as a TSQ only has one EM setting which is used for Pos and Neg.

The correct way to do a calibration is to calibrate the system in Pos mode (coping the setting to neg if the system is not used in neg and NOT coping them if the system IS used in neg) and then run a Gain. Now the system is set to Neg and the source parameters are changed to get a stable spray in neg mod. Expect a 10 fold decrease in intensity for PolyT in neg. Now run the Neg polyT calibration. Do NOT copy to positive mode and do NOT run Gain in neg mode.

Again some people that only ever run samples in neg can do the gain in neg mode but this is not the normal way..

[bsxenia]

Thank you very much for your answer.

I didn't calibrate gain in neg mode. I calibrate PolyT in pos mode without copying parameters in neg mode. I run gain in positive and then calibration of PolyT in neg mode without the gain in neg mode.

That's why this behaviour is really strange.

[leadazide]

Hmm.. I have seen sometimes the systems get calibrated in neg mode without anyone noticing as it is just a quick slip of the mind and you accidently tune the gain with PolyT-neg selected..

What sort of intensities are you getting on PolyT in Pos/Neg? What is your distribution of 182/508/997 and 180/506/995?

I have seen PolyT solutions go bad so there are ions very close to the calibration ions that interfere.. When the mass calib is done it might select a 179 instead of 182 and screw up the whole mass calib.

Make sure your PolyT is fresh and you have no interfering ions in a +/- 25 amu window og each PolyT ion.. I usually setup a 3 SIM window with the PolyT peaks centered and width 25.. here you should only get on dominating peak and it MUST be the peak of PolyT..

Hope this helps.

[bsxenia]

In pos I get 5e7 and in neg 4e6. Obviously in negative is not so good and lately with a new std of PolyT I have masses like 90, 113, 249, 497 (negative mode) and 129 - 130 (positive mode). So the dominating peaks are not the peaks of PolyT. Nut that happens this week, later than the presence of the sensitivity problem.

As you told in negative mode it was needed to do a mass calibration as it took 179 in order of 180 and I guess also 497 instead of 506 and so it screw up my calibration.

I will order a new solution of PolyT and try again.

In the case that gain calibration was made in negative mode, how should I proceed to go back to pos? just redo positive gain calibration?

Thank you very much for your time and advice!

Kind regards,

bsxenia