Avaliação crítica da quantificação elementar (incluindo hidrogênio, carbono e oxigênio) por emissão de raios-X induzidos por prótons, PIXE, e por fluorescência de raios-X, XRF

Abstract

Among various analytical techniques, X-Ray Fluorescence (XRF) and Particle-Induced X-Ray Emission (PIXE) allow a highly sensitive, multi-elementary analysis, traditionally used in areas dealing with thin films, mineral, geological, archaeological and biological samples, etc. PIXE and XRF techniques allow quantifying the elemental composition of a sample measuring their characteristic X-rays. In thick homogeneous samples, the determination of the elemental mass concentrations requires prior knowledge of several parameters involved in the interaction of radiation with matter, such as the proton stopping power in the sample for PIXE and the X-ray mass absorption coefficients for both PIXE and XRF, which in turn depend on the elemental concentrations of the sample. In this work we will review and evaluate different methods for determining the mass concentrations that have been developed over the years, obtaining good accuracy in most cases, especially with geological and mineral samples. In samples with low-Z elements, like H, B, C, N, O and/or F, not detected in PIXE nor XRF, the methods to determine elemental concentrations do not have the same level of accuracy, except for major elements. Knowing the concentrations of these major elements, we can extract information of the undetected elements in the samples. We suggest a method for determining the concentrations of trace elements based on the knowledge of at least two major elements, constructing an equivalent sample-matrix constituted by hydrogen, carbon and oxygen with the same X-ray mass absorption coefficients and proton stopping power of the original sample-matrix made of hydrogen, carbon, nitrogen and oxygen, which are the major elements in biological and organic samples. Hence, the trace elements of the sample can be fully analyzed with an accuracy better than 1%. In PIXE analysis, under certain conditions, numerical calculations on thick and homogeneous samples show that on common materials, it is possible to retrieve the hydrogen amount in the sample with acceptable accuracy. The calculation of the uncertainties indicates that the method of determining hydrogen is quite sensitive to instrumental uncertainties, stressing the need for more precise PIXE measurements. A generalization of the method is also considered for all kinds of low-Z sample-matrix, not only biological or organic, having accuracies lower than 10%. Other applications and limitations are also discussed.