MEASUREMENT OF NITRITE AND NITRATE IN BIOLOGICAL-FLUIDS BY GAS CHROMATOGRAPHY-MASS SPECTROMETRY AND BY THE GRIESS ASSAY - PROBLEMS WITH THEGRIESS ASSAY - SOLUTIONS BY GAS CHROMATOGRAPHY-MASS SPECTROMETRY

Assay methods based on the Griess reaction are frequently used to meas ure nitrite and nitrate in urine, plasma, and other biological fluids. With minor exceptions, careful attention has not been paid in extendi ng the Griess assay from aqueous solutions to biological fluids. In th e present study, parallel measurements of nitrite and nitrate were per formed in urine, plasma, and aqueous solutions with a published batch assay based on the Griess reaction and with gas chromatography-mass sp ectrometry (GC-MS). We report here further interferences by free reduc ed thiols, proteins, and other plasma constituents in the Griess assay but not in GC-MS. The best correlation (r(2) = 0.985) between the Gri ess assay and G:C-MS was observed for aqueous solutions in the absence of thiols. Unlike GC-MS, the Griess assay was not applicable to whole human plasma and urine samples. For the measurement of nitrate in dil uted human urine samples, reduction by cadmium was performed both unde r acidic (pH 2 or 5) and alkaline (pH 8.8) conditions. The mean recove ry rate of nitrate from urine samples was quantitative in the GC-MS bu t amounted to only 30-80% in the Griess assay. Measurement of nitrate in human urine samples (n = 33) resulted in an excellent correlation b etween two GC-MS techniques (r(2) = 0.979) but only in a poor correlat ion (r(2) < 0.64) between the Griess assay and GC-MS. Unlike GC-MS, th e batch Griess assay is associated with many problems in measuring nit rate in biological fluids. (C) 1997 Academic Press