A number of variations were evaluated in the techniques and procedures of the classical 6N hydrochloric acid, 110°C, 24 h hydrolysis of protein. Variations included the use of glass tubes with Teflon-lined screw caps as the hydrolysis vessel, high-temperature short-time hydrolysis, performic acid oxidation of cystine and methionine, multiple hydrolysis times at 145°C, and interlaboratory preparation of hydrolysates. A diverse sample set used in the study included a range of protein-containing matrices, and automated ionexchange chromatography was used for the amino acid analysis. Results show that for hydrolysis in glass tubes with Teflon-lined screw caps at 110°C for 24 h, recoveries of amino acids were in good agreement with recoveries by classical hydrolysis in sealed glass ampoules at reduced pressure. Recoveries from a higher temperature hydrolysis, i.e., 145°C for 4 h and using sealed ampoules, were also in agreement with 110°C, 24 h, sealed ampoule results; the former procedure yielded increased isoleucine and valine and decreased serine and threonine values. Glass tubes with Teflon-lined screw caps for hydrolysis were found to be a practical and convenient alternative to sealed glass ampoules; the improved precision with the former was probably due to the simplicity of the method. The average recovery of cystine from a wide range of matrices without the use of performic acid was 55.5% compared with results obtained with performic acid oxidation. Similarly, methionine is preferably analyzed as methionine sulfone. Interlaboratory evaluation of 145°C, 4 h hydrolysis, in which one laboratory used sealed ampoules and the other laboratory used Teflon-lined screw-cap tubes, demonstrated excellent agreement of amino acid values.

A rapid automated method for total protein nitrogen has been developed, using the Technicon AutoAnalyzer with block digestor. Ammonia is determined by the automated ammoniasalicylate reaction at a rate of 40 samples/hr. A number of variations in digestion parameters have been evaluated. Hydrogen peroxide was used as a digestion accelerator. A salt-acid ratio of 1:1 and a block temperature of 425°C were chosen. The catalysts evaluated included HgO, CuS04, Se02, and Ti02/CuS04. Maximum nitrogen recovery in the shortest time (30 mill) was achieved with HgO as the catalyst. The results from multiple analyses of 10 experimental samples of different refractoriness with the block digestor method, using HgO or CuS04 as a catalyst, compared well with the results by the official AOAC Kjeldahl method. The accuracy, precision, economy, and saving of space offered by the Missouri-Technicon block digestor method make this an attractive alternative to classical Kjeldahl analysis for large numbers of total protein determinations in samples of different refractoriness. The AutoAnalyzer cartridge manifold is simple and reproducible in its performance, and a large dilution of the sample is made on dialysis which eliminates the matrix effects from the sample digests. At least 250 samples can be analyzed in 9 hr with 1 instrumentation setup.

This laboratory has evaluated 2 automated nitrogen methods, the Kjel-Foss and the Missouri-Technicon with block digestor. A modified Kjeldahl method using a low level (40 mg) of CuS04 as a catalyst was also studied. These 3 methods were compared with the official AOAC Kjeldahl method. The experimental set numbered 22 samples, consisting of amino acids, ammonium salts, meals, grains, forages, and American Association of Feed Control Officials check feed samples. This set was analyzed in duplicate 3 independent times by each method. The accuracy and precision values for the 4 methods were excellent, within a very close range, and were equal or superior to the AOAC method. All 4 methods gave average relative standard deviations of <1 %. The 4 methods were compared for rapidity and timeliness of analysis, applicability, cost per sample, and labor and physical requirements to provide those who are responsible for protein nitrogen analysis with a basis for choosing a nitrogen method that would be most suitable for their needs.

An automated turbidimetric method has been developed for the rapid and accurate determination of sulfate. The method is practical and useful for accurately measuring total sulfur in plant tissues, and extractable sulfate in soils. The principle of intermittent reagent addition is used which eliminates drift and sensitivity changes caused by coating of BaSO 4 on tubing and cell walls. Also, the appropriate chemistry is used to minimize interactions of the wash with the sample at a sampling rate of 30/H. The sensitivity of the method is excellent with a working range of 0 to 15 ppm sulfur for soils. For plant digests the sample solutions are diluted to 0-35 ppm S. The precision as determined by repeated analysis of a soil sample extract was 0.58% RSD with a mean of 9.26 pg/g extractable SO = 4 -S. On another soil sample using a different extractant and extraction procedure the RSD was 0.64%, mean of 9.26 μg/g. Multiple automated sulfur analyses on a plant tissue digest resulted in an RSD of 0.41% for a sample containing 0.21% S. The automated turbidimetric method for sulfate has excellent precision and sensitivity in plant tissue and soil analyses where gravimetric BaSO 4 assays are not practical. INTRODUCTIONIn the last few years there has been an increasing demand for sulfur analyses in agronomic investigations involving plant nutrition and soil fertility. Also, the analysis for sulfur has become increasingly important in the evaluation of the effects of industrial and power generation effluents on the environment. Our concern was directed toward the important need for a reliable, accurate, and sensitive method for the analysis of total sulfur in plant tissues and extractable sulfate in soils. Their exists a great need for a good method to rapidly analyze soils and plant materials because of the large numbers of samples involved and the low levels of sulfur present.A number of new instrumental techniques have been proposed for sulfur determinations. The reported techniques include the

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