Hi all, I have a colleague having trouble passing their EPA 533 PT sample, HFPO-DA has been failing low. I took a look at their data, and in all of their PT extractions, the area for labeled HFPO-DA is elevated (~135-155%) as compared to calibration standards & QC samples, and is skewing the native compound value low.
Switching the IS to any other nearby one drastically increases the native HFPO-DA's calculated value.
No peak seems to be co-eluting with the labeled IS.
They've tried PTs from both Phenova and ERA with the same issue.
Instrument: Sciex 7500, standard EPA 533 mobile phases, 10cm C18 column.
The PTs are made in basic methanol, but the ammonium acetate brings them back to the right pH.
I know HFPO-DA itself is sensitive to source parameters, but her calibration and QC look just fine, so I don't think it's an MS issue.

My only recommendation so far was to spike less PT into the sample and hopefully dilute out whatever is causing the enhancement (the PT levels are usually high enough that 5x dilution is fine for DLs)

Has anyone seen this before? Any thoughts on why it could be occurring?

Thanks all!