GCMS - High matrix recoveries for late eluting pesticide compounds

Hi,

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.

Thanks

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