SIMULTANEOUS DETERMINATION OF ARGON AND NITROGEN

Denitrification can be confirmed by the presence of excess N-2 as indi cated by measurement of argon and nitrogen. Previously reported Ar/N-2 ratios in ground water have been measured by static gas mass spectrom etry, quadrupole mass spectrometry, and gas chromatography. The detect or geometry and flared flight tube of the VG Optima isotope ratio mass spectrometer (IRMS) were determined to be fortuitously engineered for the simultaneous and highly precise measurement of Ar+ (mass 40) and N-2(+) (mass 28). Certified gas mixtures containing known Ar/N-2 ratio s were analyzed to assess instrument accuracy. A preparation system fo r the extraction and purification of Ar and N-2 from water samples was developed and tested. delta(Ar/N-2) values for prepared air-saturated water (ASW) samples (24 degrees C) averaged +1149 + 8 parts per thous and and did not significantly differ from the predicted value relative to atmospheric Ar/N-2 of +1144 parts per thousand (p < 0.001). The de lta(Ar/N-2) from in situ microcosms averaged +1280 +/- 22 parts per th ousand versus atmospheric Ar/N-2 and approximated the expected level f or ASW of +1218 at 12 degrees C. Much lower delta(Ar/N-2) values were measured in 18 ground-water samples collected from a site where denitr ification had previously been documented. Samples collected from a sit e where conditions indicate little or no denitrification have delta(Ar /N-2) values close to those for ASW.