Sensitivity issues at higher concentrations.......urgent

[jbshan]

Dear friends,
I am doing the LCMSMS analysis of two drug molecule (analyte & metabolite) in a single run. I am using API 2000, ESI source,MP of ACN : 0.1% Formic acid , chromolith C18 performance column, run time 2.5 min . The extraction process is by LLE by diethyl ether.
the problem I face is that the linearity is totally out since sensitivity of the analyle is reduced to almost 25% at higher CC conc CC7 CC8.,when compared to CC1 & CC2. whereas the metabolite is coming perfect.

One more information is that my machine was calibrated few days ago and earlier to that i had got the same method with good linearity.

Please send me your valuable suggestions at the earliest possibe....very urgent.

Shan

[bert]

Try to find whether the linearity problem is due to your MS or to problems with the extraction. If you inject your analyte as standards with comparable conc. to your extracted plasma samples, is that linear?

regards Bert