Quantification of nitrite and nitrate in human urine and plasma as pentafluorobenzyl derivatives by gas chromatography—mass spectrometry using their N-labelled analogs

For the quantification of nitrite and nitrate, the stable metabolites of -arginine-derived nitric oxide (NO) in human urine and plasma, we developed a gas chromatographic—mass spectrometric (GC—MS) method in which ¹⁵N]nitrite and ¹⁵N]nitrate were used as internal standards. Endogenous nitrite and ¹⁵N]nitrite added to acetone-treated plasma and urine samples were converted into their pentafluorobenzyl (PFB) derivatives using PFB bromide as the alkylating agent. For the analysis of endogenous nitrate and ¹⁵N]nitrate they were reduced to nitrite and ¹⁵N]nitrite, respectively, by cadmium in acidified plasma and urine samples prior to PFB alkylation. Reaction products were extracted with toluene and 1-μl aliquots were analyzed by selected-ion monitoring at m/z 46 for endogenous nitrite (nitrate) and m/z 47 for ¹⁵N]nitrite (¹⁵N]nitrate). The intra- and inter-assay relative standard deviations for the determination of nitrite and nitrate in urine and plasma were below 3.8%. The detection limit of the method was 22 fmol of nitrite. Healthy subjects (n = 12) excreted into urine 0.49 ± 0.25 of nitrite and 109.5 ± 61.7 of nitrate (mean ± S.D., μmol/mmol creatinine) with a mean 24-h output of 5.7 μmol for nitrite and 1226 μmol for nitrate. The concentrations of nitrite and nitrate in the plasma of these volunteers were determined to be (mean ± S.D., μmol/l) 3.6 ± 0.8 and 68 ± 17, respectively.