A novel confocal XRF-Raman spectrometer and FPM model for analysis of solid objects and liquid substances

Abstract

A new table-top combined spectrometer was designed and constructed consisting of X-ray fluorescence and Raman spectrometers for spot-analysis of elementary and chemical composition of solid and liquid substances for industrial analytical applications. For XRF analysis a silicon drift detector and an air-cooled low-power transmission type X-ray tube were built in the combined XRF-Raman spectrometer. Both spectrometer units were connected hermetically to a sealed radiation chamber that can be optionally evacuated or it can be filled with helium under atmospheric pressure in order to improve the analytical sensitivity in the region of low-atomic numbers. The XRF and the Raman devices operate in a confocal geometrical arrangement, where the three axes of the Raman laser beam, the exciter X-ray beam, and the X-ray detection channel focus on the same spot of the examined object. The confocal spot of the combined spectrometer can be positioned along the vertical axis with an optional selected step-size of over 0.2 mm. For the precise geometrical adjustment of the focal spot on the sample surface an optical positioning system was designed and built into the irradiation chamber consisting of a mini camera and two mini laser modules. A new FPM model, its algorithm of numerical solution and software were developed for quantitative XRF analysis. The FPM model is based on application of a series of virtual monochromatic X-ray sources used for excitation of the sample elements substituting the experimentally determined spectra of the X-ray tube. The numerical FPM calculations were performed in a matrix-oriented MATLAB programming environment by using self-made software. The model uses both K- and L-lines for the improvement of the accuracy of the calculations and to extend the range of the atomic number of the analysed chemical elements. This FPM model is suitable for the calculation of the concentrations of chemical elements in 4–5 orders of magnitude for liquid and solid materials. In order to maximise the analytical information simultaneous analyses of the Cu content were performed by Raman and XRF spectrometry in an aqueous solution of CuSO₄ and citric acid. This experiment proved the synergy of the combined use of these two analytical methods, since both molecular and atomic composition could be analysed quantitatively in the same liquid sample.