The use of microwaves in the acceleration of digestion and colour development in the determination of total Kjeldahl nitrogen in soil

An accelerated procedure for the determination of total Kjeldahl nitrogen i n certified reference Chinese loam soil and real samples has been developed . This method makes use of open-vessel single-mode microwave technology. Wh en a direct comparison was made with a fully optimised thermal Kjeldahl dig estion method an 8-fold decrease in digestion time was observed, reducing t he time required for the digestion process from 120 min to 15 min for certi fied reference soil, and a 2-fold decrease for a real soil sample of S.E. E ngland soil. Extracted nitrogen was determined colorimetrically in a flow i njection system making use of the Berthelot reaction. A comparison was made between the use of a heated water bath and a microwave oven for the develo pment of the indophenol dye. Development times were reduced from 140 s down to 15 s, resulting in an increase in sample throughput from 36 samples per hour on the thermally heated system to 60 samples per hour on the microwav e-heated system. The limit of detection by the 3 sigma method was 14 mu g l (-1) of nitrogen for the thermally heated system while a limit of detection of 6 mu g l(-1) was achieved on the microwave-based system. A linear calib ration of 2 orders of magnitude was achieved for both systems. The combinat ion of the microwave Kjeldahl and flow injection with microwave-enhanced co lour development greatly reduced the time required for the determination of total nitrogen in Chinese loam soil, and recoveries of 100 +/- 2% of the c ertified value of 640 mg kg(-1) were obtained. Similar recoveries and reduc ed times to determination were also achieved for real soil samples.