Development of a Rapid Fast Neutron Activation Analysis Method for the Determination of Nitrogen in Food Products

A fast neutron activation analysis method, developed for rapid (9 min) determination of nitrogen in the presence of phosphorus, is described. The results obtained by applying the method to nitrogen determination in some flour samples are presented and compared with those given by the Kjeldahl method. Also indicated are possible applications of the method and the overall value of the investigation.
The basis of the method is the count ratio technique employed in this investigation for the resolution of two-component annihilation spectra. The irradiated mixture is counted over two separate time intervals, using two scalers and three timers. The ratio of the contributions from each component to the composite annihilation peak is then calculated from a set of two linear equations. A plot of the count ratios against the corresponding mass ratios yields a novel calibration procedure. By adding an additional known amount of one of the elements as an internal standard, it is possible to determine the amounts of both elements in the sample.
The experimental procedures designed to achieve the rapid analysis are fully described. Also fully reported are investigations of the count ratio technique and the recoil proton effect which are likely to affect the performance of the new method. The method gave results accurate to within 1 % of the Kjeldahl value. Above 5% nitrogen concentration, the experimental relative standard deviation was ca. 3%. Though the analysis time achieved is 9 mins, the method can easily complete the analysis in 5 minutes if the neutron flux and the nitrogen concentration are high enough.
The study of the count ratio technique has shown that the count ratio does not change significantly when the neutron flux changes with time during the irradiation. This is true whether the two half-lives are close or not.
In the study of the recoil proton effect, it has been shown both theoretically and experimentally that the recoil proton reaction yield varies as the square of the mass irradiated and that, for a given matrix, there exists a definite hydrogen/carbon ratio that makes the yield a maximum.
The count ratio technique, as developed in this study, could also be used to compare nuclear cross sections.
In short, the results of the research show a clear shortening of the current analysis time. They have also thrown some light on the effectiveness of the count ratio technique and taken our understanding of the recoil proton effect one step further.