An LC/MS/MS question

[Shannon78]

Hi everyone,
I'm using a ThermoFinnigan quadrupole mass spectrometer to quantitate analyte in plasma sample, my trouble is the response of internal standard is instable (ranging from 12000 to 4000) when a same sample is injected, and the response of my analyte is stable. It has been tested that the decomposition of my internal standard will never happen under my equipment conditions. What’s troble with my MS system?
Any reply is welcome!
Thanks.

[moraespack]

Is the signal of your internal std unstable when you´re doing infusion or column injecting ?
If you´re doing infusion and your internal std. is chemically similar to the analyte, they should have the same behaviour in the MS.
If you´re doing column injection, a moiety might be interfering at the same retention time of the internal std. because your sample is plasma. this might happen even with LC/MS/MS. One more thing, the concentration of the internal std is about the same that the analyte?

[Shannon78]

Thanks for reply,
If you´re doing column injection, a moiety might be interfering at the same retention time of the internal std. it means matrix effect?