RESUMO
Automated, colorimetric analysis of nutrients in samples with high and variable salinity can be time consuming due to the need to matrix match calibration and reference solution matrices with those of samples-particularly when using flow-based analyzers that are prone to detector artifacts caused by optical inhomogeneities, "schlieren," that form at interfaces between samples and deionized water carrier or wash solutions. Such detector artifacts do not occur in discrete analyzers. Here we report spike recoveries when nitrite plus nitrate, nitrite, ammonia, orthophosphate, and silica were determined in estuarine waters, seawater, and hypersaline surface water samples without calibrator matrix matching using an automated discrete analyzer set up with standard colorimetric methods. Salinities of these samples varied from 0 to 22%. Spike recoveries and precision were excellent for nitrite and nitrite plus nitrate analyses in samples with salinities up to 15%, for ammonia in samples with salinities up to about 1.7%, and for orthophosphate in samples with salinities up to about 3.5%. Orthophosphate spike recoveries were high biased in hypersaline matrices (salinity >3.5%), likely due to an unidentified sample matrix interference. After applying linear correction factors to account for chemistry-related salt effects, spike recoveries were acceptable for ammonia analyses in samples with salinities in the range of 1.7-3.5%, and for silica analyses in samples with salinities in the range of 0-3.5%.
RESUMO
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.
