Comparison of LECO FP-228 "nitrogen determinator" with AOAC copper catalyst Kjeldahl method for crude protein.

The LECO FP-228 "Nitrogen Determinator" was compared with the AOAC copper catalyst Kjeldahl method, 7.033-7.037, for the determination of crude protein in feed materials. The completely microprocessor-controlled instrument determines nitrogen by measuring the nitrogen gas following combustion of the sample; it was easy to operate and broadly applicable. A wide variety of feed materials of various nitrogen levels were analyzed in one mixed sequence. Results were precise, accurate, and rapid. Analysis time for one sample was approximately 3 min. Fourteen samples containing 2.5-15.5% N were selected for study and consisted of meals, grains, forages, and standard organic materials. The overall mean for the 14 samples by the LECO combustion method was 8.61% N compared with an overall mean of 8.58% N for the AOAC Kjeldahl method. Within-sample standard deviations for the LECO combustion method ranged from 0.013 to 0.052% N with a pooled standard deviation (SD) of 0.033% N for the 14 samples. Standard deviations for the AOAC Kjeldahl method ranged from 0.006 to 0.035% N with a pooled SD of 0.022% N. Combined average recovery of nitrogen from tryptophan, lysine-HCl, and EDTA determined by the LECO combustion method was 99.94% compared to 99.88% determined by the AOAC Kjeldahl method.

Pollution-reduced Kjeldahl method for crude protein.

Pollution from the Kjeldahl method for crude protein has been reduced by substituting a low level of copper (0.04 g CuSO4) for the mercury (0.7 g HgO) specified in the AOAC official method, 2.049. Adjustments were made in the salt-acid ratio so the new system could handle hard-to-digest samples in a reasonable time. The new method was rugged for lysine. HCl. It is designed to be used for crude protein in feeds or similar Kjeldahl work. Precision and accuracy were equal to or better than that for the official method in a study of 17 samples analyzed in duplicate on 3 different days. The following samples were used in the study: lysine. HCl, tryptophan, NBS standards, urea, meals, mixed feeds, grains, and forage. The average per cent nitrogen found was 9.52 by the official method and 9.53 by the copper method. The average standard deviation was 0.038 by the official method and 0.033 by the copper method, giving the corresponding relative standard deviations of 0.40 and 0.35%.

Quantitative analysis of cystine, methionine, lysine, and nine other amino acids by a single oxidation--4 hour hydrolysis method.

The sulfur-containing amino acids cystine and methionine play important roles in animal, especially avian, nutrition. Because these sulfur-containing amino acids are destroyed to varying extents by 6N HCl hydrolysis, oxidation and hydrolysis of cystine to cysteic acid and methionine to methionine sulfone have been widely used for determination of cystine and methionine. Lysine is considered the next limiting amino acid after the sulfur amino acids in poultry nutrition; therefore, determination of the amino acid content of rations focuses first on these 3 amino acids. The objective of this investigation was to establish whether lysine and other amino acids could be accurately determined in proteinaceous materials which had undergone performic acid oxidation. To perform this evaluation, lysine was determined in a variety of protein-containing materials both with and without performic acid oxidation. Performic acid oxidation followed by 6N HCl hydrolysis at 145 degrees C for 4 h allows accurate measurement of 3 amino acids especially important to poultry nutrition, cystine, methionine, and lysine, in a single preoxidized hydrolysate; this method can be extended to another 9 protein amino acids.