PLANT-SAMPLE N-15 MEASUREMENT RESPONSE TO TUBE PRESSURE WITH OPTICAL-EMISSION SPECTROSCOPY

Optical emission spectroscopy provides a rapid and relatively precise method for determining N-15/N-14 ratios of N-15-enriched plant and/or soil samples. The objectives of this study were to determine the optim um amount of nitrogen (N) in measurement tubes which would generate a usable operating pressure range after baking and to measure N enrichme nt of plant samples with a large range of N concentrations and % atom N-15 enrichment using a commercial emission spectrometer. Ten plant sa mples with large ranges of N concentration of 0.1 to 3.5% and % atom N -15 enrichment of 0.37 to 3.54% were used to determine the optimum amo unt of N in the tube. Another 350 plant samples contributed to a compa rison of success rate of tube lighting during emission spectroscopy an d of determination of the N-15 baseline at normal (7 mu g) and low (3. 5 mu g) N levels in the tube. Results of our analysis of plant samples suggest that for routine N-15 analysis by emission spectrometer, tube s (1 mL internal volume) containing 3 to 4 mu g total N generate good pressures (0.3 to 0.45 kPa) after baking, and thus provide more reliab le estimates of N-15 enrichment than tubes containing 7 mu g N (>0.6 k Pa), especially when N concentration is low (<3 g/kg). Lower N in the measurement tube may also reduce the need to concentrate N in analysis of N-15 enrichment in soil samples.