Rapid method for total nitrogen analysis using microwave digestion

A microwave digestion procedure recently proposed for total N analysis of soil using HF-HCl, H₃BO₃, and H₂O₂ was found to give much lower values than conventional Kjeldahl digestion in analysis of soil and plant samples and simple organic N compounds. An alternative microwave procedure was developed that involves Kjeldahl digestion with concentrated H₂SO₄ and a K₂SO₄-catalyst mixture. With this procedure, digestion of five samples can be accomplished in 30 min. Complete recovery (≤98%) of N was obtained in analysis of several organic N compounds. Total N analyses of plant materials by the microwave Kjeldahl method agreed closely with those by conventional Kjeldahl digestion using an Al block; the microwave method gave somewhat lower results than the conventional method for surface soils with a high content of total N. Precision of the analysis was comparable for both methods. Recovery of N added to soil as NO₃ averaged 97% when the microwave procedure was modified to include a pretreatment with salicylic acid and sodium thiosulfate. Although much more rapid, the microwave Kjeldahl method is not necessarily more convenient than conventional digestion with an Al block because of the need for frequent loading and unloading of samples.