For PIXE, typically protons are produced in small energy accelerators. Quadruple magnets focus the protons, and the sample to be analyzed is hit by this proton beam. The protons eject electrons from the K or L orbital of the atoms in the sam­ple. From here, the processes are the same as in XRF: the characteristic X-ray photons emitted by the sample are detected by semiconductor detectors (e. g., SiLi).

Using traditional SiLi PIXE detectors, the concentration of the elements that have an atomic number greater than 13 can be measured. The sensitivity of the K and L lines is the highest in the range of atomic number 20 < Z < 35 and 75 < Z < 85, respectively. The most important trace elements of the biological and geological systems are mostly in just these ranges. This gives the significance of PIXE studies. There are special SiLi detectors with an ultrathin window, which are applied for the measurement of elements from carbon to iron. In Figures 10.25 and 10.26, the spectrum of an aerosol is shown using two detectors: a traditional SiLi PIXE detector and a SiLi detector with an ultrathin window.

The method has 10_6—10_7g/g relative, and 10_9—10_12g absolute detection limits with 5—10% error. The sensitivity can be increased using a very thin proton beam (with a diameter measured in micrometers). The method is called micro — PIXE, and its absolute detection limit is 1015—1016 g. This very high sensitivity is the main advantage of PIXE compared to XRF.

The main field of PIXE applications is the study of atmospheric aerosols. Filtering through different pore-sized membranes, fractions of air with different particle sizes are collected. The quantity of each fraction is small; thus, the analysis requires very sensitive techniques. Using PIXE, the elementary composition of the different fractions is measured directly, positioning the membrane filter to the win­dow in front of the proton beam (Figure 10.27). The sample remains unchanged after the PIXE analysis, so it can be subjected to another analytical technique (gravimetry, microscopy, particle distribution analysis, or other spectroscopic

techniques), which is essential since main light elements (H, C, N, O) composing aerosol particles (soil, humus, etc.) cannot be analyzed by PIXE. Therefore, PIXE and chemical analysis of the light elements (e. g., PGAA and electron microprobe) can be combined.