Application of improved Dumas method to the determination of total nitrogen in soy sauce

Abstract

The determination of the total nitrogen in soy sauce by the Dumas method was studied. Eleven samples of soy sauce were used and a half gram of each sample was combusted at 870°C in an oxygen atmosphere. The resulting nitrogen oxides (NOx) were reduced to nitrogen by a copper column at 600°C, and nitrogen was detected with a thermal conductivity detector. Analyses of each sample were repeated 3 times on 2 different days. The samples were also analyzed by the Kjeldahl method under the optimum digestive condition in order to know the differences between the methods. The standard lysine solution was analyzed by the Dumas method, and accurate results were obtained. Two sets of data by the Dumas and the Kjeldahl methods were tested with a two-sided Welch t -test. There were significant differences in six samples between the methods (p < 0.05). However, the differences between the methods were within the repeatability limit, calculated by the Horwitz equation. Also, the correlation between the methods was 0.9999. The Dumas method had excellent intermediate precision, and was equivalent to, or more accurate than, the Kjeldahl method under the optimum digestive condition. © 2007 The Japan Society for Analytical Chemistry.