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

Abstract

An accelerated procedure for the determination of total Kjeldahl nitrogen in certified reference Chinese loam soil and real samples has been developed. This method makes use of open-vessel single-mode microwave technology. When a direct comparison was made with a fully optimised thermal Kjeldahl digestion method an 8-fold decrease in digestion time was observed, reducing the time required for the digestion process from 120 min to 15 min for certified reference soil, and a 2-fold decrease for a real soil sample of S.E. England soil. Extracted nitrogen was determined colorimetrically in a flow injection 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 development 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 microwave-heated system. The limit of detection by the 3ς method was 14 µg l–1 of nitrogen for the thermally heated system while a limit of detection of 6 µg l–1 was achieved on the microwave-based system. A linear calibration of 2 orders of magnitude was achieved for both systems. The combination of the microwave Kjeldahl and flow injection with microwave-enhanced colour development greatly reduced the time required for the determination of total nitrogen in Chinese loam soil, and recoveries of 100 ± 2% of the certified value of 640 mg kg–1 were obtained. Similar recoveries and reduced times to determination were also achieved for real soil samples.