Comparison of Dumas and Kjeldahl methods with automatic analyzers on agricultural samples under routine rapid analysis conditions

Abstract

With the improvement in dry combustion nitrogen (N) analyzer technology and because of the expense of disposing of hazardous laboratory waste chemicals, the Dumas method is replacing the traditional Kjeldahl method as the method of choice for N analysis. Consequently, comparison of the Dumas method with the Kjeldahl method is important. Typically such comparisons have been done on small numbers of distinctive samples. The objective of this research was to evaluate the performance of instruments which use automated Dumas (Macro-N) and Kjeldahl (KjelFoss) methods for N analysis of agricultural materials in a routine high-throughput laboratory situation. Samples of manure, sewage sludge, plant tissue, plant seeds, and fibers of feedstuffs were analyzed for N concentrations using both instruments. The statistics of the N concentrations were evaluated for all samples as a group and as specific sub-populations. The N results from the all-samples population rendered a simple linear correlation coefficient of 0.979, with the Macro-N instrument measuring larger N concentrations in some samples than the KjelFoss instrument. The correlation improved very little when the sewage sludge and manure samples were excluded. The linear correlation was less when the N concentrations of only the plant tissue samples were evaluated: r=0.965. Again the Macro-N instrument measured greater N concentrations than the KjelFoss instrument for some samples. An investigation of the plant tissue samples revealed that some samples contained substantial nitrate-N. Removing plant samples that contained 0.4% or more nitrate-N improved the linear correlation: r=0.996. Since manure and sewage sludge usually contain less than 0.4% nitrate-N, the higher N concentrations obtained with the Macro-N suggests that there may be a N component in these materials other than nitrate-N that the Kjeldahl method cannot measure. Further study of just the manure and sewage sludge samples showed a relatively low correlation of 0.972, with the Macro-N instrument again measuring considerably greater N concentrations than the KjelFoss instrument for some samples. The correlation coefficient for grain samples was 0.999, whereas, it was only 0.799 for N associated with fiber. Since the Macro-N Dumas method appears to measure the N associated with nitrate, caution is advised when using N concentration as a measure of protein in some plant materials containing high concentrations of nitrate-N.