[Automated determination of total nitrogen in milk by the Dumas method]

A nitrogen analyser based on the Dumas method (LECO FP-428) and equipped with a liquid injector has been tested. The intensive steam production during injection may result in poor nitrogen recoveries for volatile nitrogen compounds such as ethylene diamine. However, recoveries could be improved by slow injection and by filling the combustion tube with Cer(IV)-oxide. For the analysis of milk the instrument was calibrated with NaN3 solutions because their response was found to be less sensitive to burning conditions. Dumas-N and Kjeldahl-N of milk showed good correlation (r = 0.998). However, with the Dumas method about 6.7% (+ 0.035% N) higher nitrogen values were obtained. With a standard deviation of repeatability S(r) = 0.002% N (CVr = 0.33) and a between day reproducibility SD = 0.006% N (CVD = 1.1%) the precision of the Dumas method was similar to that reported for the Kjeldahl method. In order to avoid systematic errors the use of standard milk samples for calibration is recommended. It is concluded that modern instruments using the Dumas method allow precise measurements of nitrogen in milk with a high sample throughout.