The quality of a calibration procedure was studied in terms of the accuracy of the calibration factors. In particular, the influence of the uncertainties of the incident and emerging angles and of the mass absorption coefficients on the calibration factors was described theoretically for a calibration procedure based on standard samples of any thickness. The calculations were carried out for an x-ray tube and a radioisotope excitation set-up. Practical consequences of the inaccuracies of the calibration factors were also demonstrated for a few test samples of intermediate thickness analysed by the emissiowtransmission method.
Microbeam X-ray spectrometry, energy-dispersive X-ray fluorescence analysis, and neutron activation analysis were evaluated for the detection of selenium contained in the selenoprotein glutathione peroxidase. The glutathione peroxidase had been previously separated using polyacrylamide gel electrophoresis. The use of Bragg-reflected polarized X-ray beams was employed in the X-ray fluorescence measurements to minimize the problem of scatter owing to the gel matrix. Current detection limits of selenium in a gel matrix are 2.1 ng in the bench-top microbeam X-ray system and 30-60 ng using XRF with polarized beams. Neutron activation analysis was used for quality-control measurements, with a detection limit here of < 0.08 ng. The work has in principle established the feasibility of such an approach.
Radioisotopeexcited XRF systems, using annular sources, are widely used in view of their simplicity, wide availability, relatively low price for the complete system and good overall performance with respect to accuracy and detection limits. However, some problems arise when the use of fundamental-parameter techniques for quantitative analysis is attempted. These problems are due to the fact that the systems operate with large solid angles for incoming and emerging radiation, and both the incident and take-off angles are not trivial. In this paper, an improved way to calculate 'effective' values for the incident and take-off angles, using Monte Carlo (MC) integation techniques, is shown. In addition, a study of the applicability of the 'effective' angles for analysing different samples, or standards, was carried out. The MC method allows also calculation of the excitatiodetection efficiency for different parts of the sample and estimation of the overall efficiency of a source-excited XRF set-up. The former information is useful in the design of optimized XRF set-ups and prediction of the response of inhomogeneous samples. A study of the sensitivity of the results due to sample characteristics and a comparison of the results with experimentally determined values for incident and take-off angles is also presented. A flexible and userfriendly computer program was developed in order to perform efficiently the lengthy calculations involved.~~ ~
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