Chromatography Forum

Hello everybody, i'm currently running a LC/MS/MS method for cilazapril, cilazaprilat determination in plasma. I have observed an ion enhancement phenomenon at the retention time of my analytes which differs between different lots of plasma. The problem is that the plasma lot in which standards and QC's have been spiked displays a much greater enhancement and for a longer period than the plasma taken from volunteers. This causes a great variability in the internal standard (enalaprilat) area between different lots. I've tried to move my peaks out of the enhancement area but the only analyte that was succesfully moved was cilazapril. The other peaks remained in the dager zone. The extraction method is LLE with ethyl acetate after acidification with formic acid. Does anyone have any idea on how do i minimize ion enhancement in the first plasma lot?

LLE is not a bad method for sample prep, but it also has its limitations. Generally, SPE is better, because it gives you much more flexibility in manipulating the separation of your compound(s) from interferences. I suggest to look at the collection of Oasis SPE methods that you can find online in the Oasis book at the Waters website.

i diluted the plasma to be analyzed (0.25 ul) with 75 ul of the purchased plasma which seemed to help with similar problems.

See my paper on my web page about matrix effects and especially the book referenced in the paper.

http://users.chartertn.net/slittle/default.htm