GCMS - High matrix recoveries for late eluting pesticide compounds


I am using Agilent 7890B GC and 7000 MS with HP-5MS UI column.

We are using a method with 165 compounds in but for some of the late eluting compounds (bifenthrin, methoxychlor, fenpropathrin, phosalone, permethrin cis & trans, coumaphos, deltamethrin) the reagent spike/recovery works fine but the matrix spike/recovery is high (120-180%). This is over 8 different matrices (all of which are high) so I don't believe the problem is the matrix which is blank. It's only these late eluters  that are the problem, all the earlier spike/recoveries work fine. It also can't be anything to do with the standard as the same standard is used to spike the reagent spike.

Any help would be much appreciated.


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