C, N and P from a simple single-vial persulfate digestion

A simple one-vial digestion that yields total organic carbon, nitrogen and phosphorous has been developed. The method is based on the established alkaline persulfate oxidation with 1 to 2 hour incubation at ~120˚C. The carbon, oxidized to carbon dioxide, is analyzed from the reaction vial headspace by GCMS using argon gas as an internal standard. The nitrogen and phosphorous are measured as nitrate and phosphate directly from the reaction solution by either ion chromatography or standard colorimetric methods. All three analytes were calibrated using mixed solutions of glucose, serine and sodium tripolyphosphate. Typically, CO2 calibration spans from 1.3 to 64 micromoles of carbon with a 10-mL digestion solution in a 20-mL vial. We have found linearity out to at least 230 micromoles C as well. Nitrate, measured by IC, is calibrated from 2 to 8 micromoles N. The phosphate was measured by both the colorimetric method of Strickland and Parsons (1972) and by IC. The phosphate standard curve spanned from 0.5 to 4 micromoles of P in the 10-mL digestion solutions. All calibration curves yielded linear regression R² of better than 0.99. The method has been used on dissolved nutrients in water samples, particulate matter on filters and solid material.  Precision of this method allows us to analyze C:N:P from a single daphnia.