Measurement of nutrients in saline and hypersaline waters by discrete analyzer colorimetry without matrix matched calibration standards

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, nitrite plus nitrate, 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 percent (%). 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 to 3.5%, and for silica analyses in samples with salinities in the range of 0 to 3.5%.