We are presenting a method to the FDA and need a few references. Our method uses a SPE extraction method. A deuterated internal standard is used. The analyte and its deuterated internal standard are measured using GC-MS/MS.
The analyte recovery through the extraction process is about 55% but we have more than enough sample to measure (~900,000 area counts)on the GC-MS/MS. The results are accurate and reproducible. The reviewer is stating that recovery should be 70-130%. I'm sure that they are thinking of an assay without an internal standard where if you don't recover all of your analyte, you will have a low result. With an internal standard even if you extraction process had a 10% recovery you would still have an accurate answer because in theory you would have 10% of your analyte and 10% of your internal standard (provided that you still have an adequate signal).
Dose anyone have any references I can cite that states that the recovery of a SPE method using an internal standard can be as low as or lower than 50%?

I've found some that say it can be "lower" but none that give some numbers.

Thanks