I am new to the Mass spectrometry and I am now using a VARIAN 1200L Triple quadrupole ESI-MS-MS for ABA determination. I have a problem with the MS sensitivity between different runs. Sometimes there is a difference of factor two for the same concentration of substance if I run them on two different days (MS is kept in standby mode if the two runs are done with a gap of one week). I could have easily accepted the sensitivity difference if it was consistent. But it keeps changing. May be today I have reduced sensitivity but for my next run the sensitivity again increases by a factor of two and after that it may again go down. All the MS parameters are kept constant and even for HPLC. Could anyone suggest me the possible reason for this difference in sensitivity? Thanks for your suggestions.