progressive loss in sensitivity in solution

[H-KP]

Hi everebody,
We face problem of constat signal decrease both for the drug and the ISTD when we inject standard solution in a LC/MS/MS system (C18, mobile phase : ACN/Formic acid 0.1% in a gradient from 5% to 60% ACN in 5min. detection MS/MS with a Micro Quattro in the positive mode).

For exemple, the first injection in the morning gives an area of 1000, the second 750 (25% loss), the third 562 (again 25% loss) etc...(this is not an issue of analyte degradation).
The day after the signal is recovered at 1000 but decreases again...

does anyone has an idea?

[Uwe Neue]

What is your "standard solution"?

[Kostas Petritis]

The very fact that you get exactly 25% decrease of the signal of both the drug and ISTD makes me belive that there is something to do with the injection of your compounds than with the mass spectrometry. I am not very familiar with the autosampler that you are using but maybe you activated a function where the autosampler injects different quantities in order to create a calibration curve (just a thought).

Before trying anything else I would by-pass the autosmpler with a manual injection valve and try to make some injections of the same sample. If you get constant singal, then the problem comes from the autosampler...

[kuldeep_mica]

Form your mail it is evident that the loss in the signal is very significant. I agree with kostas petritis. As the decrease in signal is systematic there are possibilities of such errors in the LC program. But if its due to MS there are following two possibilities:
1. A loss in the vaccum level: If there is any problem in the vaccum assembly in the instrument . Signal of analytes may decrease with the loss in the vaccum level.
2. Charging or contamination of the Quadrupole rails
To confirm the instrument conditions you can infuse diluted solution of your compound in MS through instrument syringe at a constant rate and moniter the base line. If the baseline decreases significantly then its due to instrument problem.

[Kostas Petritis]

Hi Kuldeep_mica,

If it was any of the problems that you mention then the analyte respond would not go back to the original levels (unless you clean or fix the vaccum problem)...