Measurement of urinary F2-isoprostanes by gas chromatography-mass spectrometry is confounded by interfering substances

Analysis of F-2-isoprostanes in urine using gas chromatography-mass spectrometry is confounded by the presence of endogenous compounds interfering with the internal standard, 15-F-2t-IsoP-d(4) (m/z 573). Previous efforts to resolve the 15-F-2t-IsoP-d(4) from co-eluting peaks with different solid phase extractions were unsuccessful. This study has now used a highly-deuterated, d(9)-analogue of the derivatization agent N,O-Bis(trimethyl-d(9)-silyl) trifluoroacetamide (BSTFA-d(9)) yielding trimethylsilyl ethers, but this was not successful in resolving the 15-F-2t-IsoP-d(4) from co-eluting peaks. It was hypothesized that interfering peaks at m/z 573 could be the tetrahydro analogue of 15-F-2t-IsoP. However, using an authentic standard showed the interfering peaks are not due to this metabolite. In subsequent experiments good resolution was shown of the 15-F-2t-IsoP peak using 8-F-2t-IsoP-d(4) (m/z 573) as the internal standard. These data show that care must be taken when using GC-MS for quantitation of F-2-IsoPs to prevent interfering substances affecting the results.