Sample preparation for total nitrogen and 15N-ratio analysis by the automated Dumas combustion method

Dry combustion based on the Dumas (1831) method has become increasingly popular for total nitrogen (N) and isotope-ratio analyses. However, the lack of sample homogeneity with soil and plant samples is often the limiting factor for the accuracy and precision of this method. Moreover, water samples are not directly analyzed by the Dumas method on a routine basis. The objective of this study was to develop an adequate sample preparation protocol for plant and water samples to be analyzed for their total N and N-15-ratios by the Dumas method. Twenty-four corn (Zea mays L.) samples were randomly selected, dried at 60 degrees C, and finely ground. Plant samples were analyzed for total N by the traditional Kjeldahl and Dumas methods. Seven preparations of subsurface drainage and standard nitrate (NO3)-N solution samples were similarly analyzed for their total N content. The effects of reducing agent (copper wire or shot) used on total N and N-15 isotope ratios were also investigated in this study. The average percent total N of plant material as determined by the Kjeldahl and Dumas methods were 0.762 and 0.772, respectively, and were not statistically significantly different. The Dumas method failed to recover 25% of the NO3-N of a drainage effluent, but exhibited recoveries as high as 97% when standard NO3-N solutions were added. The small copper shot yielded 60 mu g higher total N and 0.0074 lower atom percent N-15 values for the drainage plus standard NO3-N samples when compared to when copper wire was used. The Dumas method failed to yield adequate total N values for the drain water samples. Further research is needed regarding total N analyses of water samples by the Dumas method.