The use of scattered radiation in x-ray fluorescence analysis of soil and plant materials

Abstract

The isolated atom model of X-ray scatter was used to develop a new analytical strategy for internal standardisation in X-ray fluorescence spectrometry. The method which was applicable to both geological and biological materials, defined quantitatively, by how much and within what limitations, various calibration techniques could improve analyses, for particular elements in specific materials.

The analytical strategy was based on a modified peak-to-background ratio equation, where the background intensity was raised by an exponent T. The variable T, combined as special cases, all existing analytical strategies which used scattered X-rays and the simple linear calibration where T = O. Variation in T was evaluated using extensive computer processing of matrix data for typical soil and plant materials. It was found that values of T varied with analyte, scatter wavelength, type of scatter, and the matrix component contributing to mass absorption error.

The procedure developed to select the optimum calibration strategy, for any combination of mass absorption errors in soil and plant analysis, showed: (i) how much each interfering component contributed to both initial and final analyte error, (ii) which value of T, for various scatter wavelengths and types of scatter, gave optimum reduction in analyte error, and, (iii) if any improvement over the various strategies commonly used, could be achieved.

Consequent limitations of some analytical methods which have been proposed in the literature were discussed.

Extremely variable matrices were specially prepared to test the method for zinc and zirconium analysis. The improvement in quality of matrix compensation was as predicted from theory. Values of T were tabulated for analysis of various groups of elements in soil and plant materials and situations were described where successful calibration using scattered radiation was most unlikely.

The methods developed were among those used to analyse soils and metalliferous sewage sludge in an experimental programme designed to establish for local conditions (i) maximum permissible sewage sludge application rates and, (ii) the effect of sludge application on pasture production and quality. Although the digested sludge contained useful amounts of plant and animal nutrients, metal content was too high to permit its indiscriminate use as a soil additive. The metal contents were not high enough however to prohibit the judicious utilisation of the sludge within prescribed limits.

Highly significant results from both glasshouse and field trials showed that chromium contamination of sludge could decrease pasture production. It appeared that chromium may have blocked plant uptake of nutrient cations. With the sludge chromium concentration at about 0.5%, and the total sludge application at the recommended level of 250 t/ha, no deleterious effects were likely, provided a good supply of nutrient cations was maintained in the soil.

Zinc was found to be the limiting contaminant in herbage, and copper concentrations were near the upper safety limits. Potassium deficiency was probably a greater pastoral constraint than was metal toxicity. No other elements were found in herbage, in concentrations likely to be injurious to animal health, or restrictive to plant yield.