Corn leaf nitrate reductase--a nontoxic alternative to cadmium for photometric nitrate determinations in water samples by air-segmented continuous-flow analysis.
Development, characterization, and operational details of an enzymatic, air-segmented continuous-flow analytical method for colorimetric determination of nitrate + nitrite in natural-water samples is described. This method is similar to U.S. Environmental Protection Agency method 353.2 and U.S. Geological Survey method 1-2545-90 except that nitrate is reduced to nitrite by soluble nitrate reductase (NaR, EC 1.6.6.1) purified from corn leaves rather than a packed-bed cadmium reactor. A three-channel, air-segmented continuous-flow analyzer-configured for simultaneous determination of nitrite (0.020-1.000 mg-N/L) and nitrate + nitrite (0.05-5.00 mg-N/L) by the nitrate reductase and cadmium reduction methods-was used to characterize analytical performance of the enzymatic reduction method. At a sampling rate of 90 h(-1), sample interaction was less than 1% for all three methods. Method detection limits were 0.001 mg of NO2- -N/L for nitrite, 0.003 mg of NO3-+ NO2- -N/L for nitrate + nitrite by the cadmium-reduction method, and 0.006 mg of NO3- + NO2- -N/L for nitrate + nitrite bythe enzymatic-reduction method. Reduction of nitrate to nitrite by both methods was greater than 95% complete overthe entire calibration range. The difference between the means of nitrate + nitrite concentrations in 124 natural-water samples determined simultaneously bythe two methods was not significantly different from zero at the p = 0.05 level.
Patton
Charles J
CJ
US Geological Survey, National Water Quality Laboratory, Denver Federal Center, Colorado 80225-0046, USA. cjpatton@usgs.gov
Fischer
Anne E
AE
Campbell
Wilbur H
WH
Campbell
Ellen R
ER
D002104:Cadmium / Q000737:chemistry; D003124:Colorimetry / Q000379:methods; D004784:Environmental Monitoring / Q000379:methods*; D050901:Nitrate Reductase; D009565:Nitrate Reductases / Q000494:pharmacology*; D009566:Nitrates / Q000032:analysis*; D018515:Plant Leaves / Q000201:enzymology; D014873:Water Pollutants / Q000032:analysis*; D003313:Zea mays
D009566:Nitrates; D014873:Water Pollutants; D002104:Cadmium; D009565:Nitrate Reductases; D050901:Nitrate Reductase