Water samples are digested with persulfate oxides to convert all N-forms to nitrate. This nitrate is first reduced to nitrite with a cadmium reduction column and then diazotized to form an azo dye which is then measured colorimetrically using an automated continuous-flow analyzer (Alpkem). It is both safer and more effective than traditional Kjeldahl techniques and is based on the method of Koroleff (1983).
Details of the method
A discussion of principles and practical considerations in continuous-flow colorimetry is available in the Colorimetry Overview. The TN procedure is specified is detailed here.
Sample preparation and handling
Samples must be free of turbidity and particulate matter and must not contain any color that absorbes at the same wavelength as the analytical color chemistry tint. Liquid samples which cannot be analyzed immediately after collection must be preserved for shipment. A list acceptable preservation methods and holding times for many analytes are available from the EPA. Detailed sample-handling instructions specific to this laboratory are available in the Sample collection instructions.