X-Ray fluorescence Analysis (XRF)

X-Ray fluorescence (XRF) spectrometry has been a powerful technique for elemental analysis for almost 100 years based on Moseley’s well known law which relates “characteristic” fluorescence radiation to the atomic number of the emitting atom. Today composition analysis by measuring fluorescence spectra has become a routine technique utilized in a vast number of research areas ranging from material science to biomedical science. X-Ray fluorescence analysis is nondestructive, high precise and multi-elemental method to analyze most elements of the periodic table (Z>5 (B), most effective for Z>11 (Na)) with simple or even no sample preparation. Moreover, as the fluorescence intensity is proportional to the concentration of an element present in the sample, not only qualitative but also quantitative analysis is possible. As wavelength and energy are equivalent the fluorescence radiation can be evaluated in wavelength or energy dispersive mode. However, classical XRF analysis is not applicable for ultra-trace elemental analysis and susceptible to systematic errors due to sample matrix effects.

 

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Fig.1: Photoeffect excitation and emission of characteristic radiation
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Fig.2: Setup and spectrum of an EDXRF system
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In energy dispersive XRF (EDXRF) the photon energy is directly measured by a suitable detector which selects the photons due to energy and counts them directly and stores the result in a multichannel memory. The result is a spectrum with intensity on the y-axis and energy on the x-axis. The range of detectable elements is limited. It ranges from Be (Z=4) for the light elements and goes up to U (Z=92) on the high atomic number Z side. The concentrations that can be determined with standard spectrometers of both types WD or ED are lying between µg/g to weight percent, thus having a wide dynamic range. In terms of mass the ng range is reached with spectrometers having the typical excitation geometry of incident primary radiation (45° to the flat sample surface).

From its principle XRF is a multielement analytical technique and in particular with ED-XRF inherently the simultaneous determination of all the detectable elements present in the sample is possible.