Determination of sulphur in plant material by X‐ray emission spectrometry

Abstract

A simple, rapid technique for the routine determination of total sulphur in pasture samples is described. Eighty samples a day could be measured, with little additional time needed for the sample preparation.Measurements were made directly on loosely packed plant material. The intensity of emission at the sulphur-Ka line for each plant sample was related to that of a fused reference standard used in silicate analysis.Variations in sample density, due mainly to particle size of the material and ambient humidity, influenced the results, but were eliminated readily.A linear expression, Y = 0.4162X-0.0047 (r = 0.9968; P <0.001) relating X, the ratio, and Y, the chemically determined sulphur content of the sample, was derived.Introduction Jenkins et al.1 have shown that the use of X-ray emission spectrometry could be extended satisfactorily to the determination of elements of low atomic number in plant material, where many of them occur in trace amounts. Sulphur was not among the elements tested in that study and its measurement by colorimetric procedures2~3 is time-consuming.Brown & Kanaris-S~tiriou~ have developed a technique for the measurement of sulphur in soils, and Kobuta & Lazar5 have described a method for plant material in which the ground sample is packed loosely into a Mylar-backed sample holder, and the intensity ratio of the sulphur-Ka line to background is measured by X-ray fluorescence. The relation between the ratio and the chemically determined sulphur content was linear (r = 0.994; R = 98.8%). When this technique was adapted to the authors' instrument system for the measurement of sulphur in subterranean clover (Trifolium subterraneum L.) it proved less satisfactory. Individual correlation coefficients were acceptable, but there was a displacement between linear expressions depending upon when they had been made. This displacement effect was eliminated when the simple procedures outlined below were followed. The one standard relationship could now be applied on any occasion when measurements were made. Experimental and Results Sample preparationStored, dried plant material was heated for 15 min in an oven at 105"c priorto grinding. Thesamples were then ground in a high-speed mill to pass a 60-mesh sieve. Once the particle-size range had been established, it was sufficient to estimate the size visually, thereby eliminating sieving of individual samples.As required, samples of the ground material were taken and further dried in a vacuum oven at 70" for 0.5 h before transfer to the spectrometer for measurement.In the range 0-3-4.0 g, sample weight did not influence the intensity of the sulphur-Ka line. A convenient sample weight (1-1 * 5 g) was estimated visually. Weighing was not necessary. MethodAll measurements were made with a Philips X-ray Spectrometer PW1540, using the accessories and settings shown in Table I. The intensity of emission at the sulphur-Ka line for each sample was compared directly with that of a fused reference standard, FS31, used in silicate analysis. A fixed counting time of 40...