Proton‐InducedX‐Ray Emission in Environmental Analysis

Abstract

Particle‐induced X‐ray emission (PIXE) is an analytical method based upon X‐ray spectrometry. In the vast majority of applications, a proton beam is used to eject inner‐shell electrons from atoms in a solid specimen (target). When the resulting vacancies are filled by outer‐shell electrons, characteristic X‐rays whose energies identify the particular atom are emitted. Proton PIXE is performed by a small particle accelerator (the most widely used is the Van de Graaff machine at a 2–3 MV voltage, more recently also small tandem accelerators), that provides a beam of protons. The beam emerging from the accelerator, stabilized and directed by electrostatic and magnetic steering elements in a high vacuum line, enters the specimen chamber through suitable collimators. The X‐ray detection system, housed in the chamber, is generally a Si(Li) device, that combines the advantage of high efficiency in the X‐ray energy region of interest (usually 2–20 keV) with a good energy resolution. A typical PIXE spectrum consists of characteristic X‐ray peaks superimposed on a background due to bremsstrahlung radiations and nuclear reactions induced by the beam. The area of each peak is related directly to the concentration of the corresponding element in the specimen. Various software codes have been developed to deconvolute spectra and to accurately calculate peak areas. PIXE intrinsic detection limit (DL) is not very much below 1 ppm (mg kg −1 ) in a given matrix. It offers its maximum sensitivity in two atomic number (Z) regions: 20 < Z < 35 and 75 < Z < 85. Measurement errors are in the order of 10%, depending mainly on the target preparation procedure and on the slight variability of the proton flux. The PIXE technique allows fast (few minutes), nondestructive, highly sensitive simultaneous determinations of a wide group of elements (12 ≤ Z ≤ 85), without large variations in sensitivity among different elements. The total element concentration is measured by PIXE, independent of different oxidation states. Adequate sample preparation procedures may be required in environmental PIXE applications since solid targets have to be exposed to the particle beam. Thin targets are preferable, in order to minimize matrix effects. For thin samples and for light‐element matrices, PIXE has superior sensitivity compared to X‐ray fluorescence (XRF). In heavy‐element matrices XRF is preferable. DLs for inductively coupled plasma atomic emission spectroscopy (ICPAES) may be lower than for PIXE (depending on the sample dilution factor), but the topic of interferences is much more complex for ICPAES than for PIXE.