The determination of nitrogen-15 in plant material with an emission spectrometer

Abstract

Instrumental and procedural modifications for the use of the Statron, Model NOI–5, emission spectrometer in the determination of nitrogen-15 are reported. An interpolative method is recommended for estimating the background of the nitrogen-14-nitrogen-15 peak. This method gives a linear relationship between the apparent percentage of nitrogen-15 and the actual percentage of nitrogen-15 over the lower, most commonly used range. Assays repeated at intervals over 2 months had standard errors of 0·75 per cent.

The preparation of nitrogen gas samples for the emission spectrometer has been treated as a separate operation from the determination of total plant nitrogen. After a standard Kjeldahl digestion of the plant material, the ammonium ion is precipitated with a Nessler reagent. The supernatant is rejected and the precipitate treated with dilute hydrochloric acid to give an ammonium chloride solution, from which an aliquot is taken for a Dumas combustion in a discharge tube. Two operators can prepare and measure up to fifty samples per day. The method has been used to analyse a range of plant tissues from an apple nutrition experiment, during which replicate digestions and analyses gave values for nitrogen-15 enrichment that differed by an average of only 2·5 per cent.