PROTON INDUCED X-RAY EMISSION (PIXE) ANALYSIS
What is PIXE (Proton Induced X-ray Emission spectroscopy)?
PIXE (Proton Induced X-ray Emission spectroscopy or Particle Induced X-ray Emission spectroscopy) is an analytical technique for identifying the elements in a given sample. Through the application of an ion beam, PIXE measures the radiation emitted by electron state changes and identifies each element based on its unique emissions recorded as a spectral peak. This technology is extremely useful as it is a non-destructive analytical method, and can, in fact, analyze samples either in or outside of a vacuum.
The External Beam PIXE has the ability to analyze the whole sample externally. This technology is widely used in geology, archaeology, and art, while the standard PIXE is focused mainly in the chemistry and pharmaceutical fields. The technology has been used to analyze everything from dinosaur eggs to commercial polymers and plastics. The range of applicable uses for PIXE is what classifies it as a valuable instrument. It can be used to analyze toxins, identify elemental content of an unknown sample, examine materials for coatings and oils, as well as quantify atmospheric levels of harmful elements. The instrumentation also allows for solid samples to be analyzed without any pre-treatment and can detect many elements below 1ppm.
How does PIXE (Proton Induced X-ray Emission spectroscopy) work?
PIXE works by precisely detecting x-rays emitted through electron state change. Quantum theory states that within any stable atom electrons must maintain constant energy levels. PIXE uses an ion accelerator to disrupt this stability. The ion beam sends the necessary energy to excite the inner shell electrons. In turn, to maintain equilibrium outer shell electrons transition to replace the excited K-shell electrons, emitting energy in order to do so. The electromagnetic radiation that is emitted is in the X-ray light spectrum. These transitions and subsequent energy emissions are characteristic of independent elements. A device called the energy dispersive detector measures the x-rays and their intensities as emitted by the state changes and records the resulting spectral data of the elements’ concentrations. The detection limits can be in the half ppm with +/-10% accuracy. The total X-ray spectrum can be measured at once, and the quantitative data is determined as the area of the spectral peak relating to each independent element.
The limitations of PIXE are primarily in regards to what elements it can identify. PIXE is accurate for elements above sodium. Anything below sodium will not show up as the elements’ rays will be absorbed prior to reaching the detector. While External Beam PIXE spectrometers can be used to analyze whole samples, the ideal sample is homogeneous. If the sample is not homogenous it is best practice to test multiple sections and take the average of the quantitative results. This provides “semi-qualitative” data but is within an acceptable error range. For internal PIXE analysis, homogenous samples are also ideal as x-rays are emitted from only the top few microns and thus non-homogenous samples may produce inaccurate readings.
Scientists in various fields are continuously finding new uses for PIXE. From polymer testing and failure analysis to deformulation and pharmaceutical product development, PIXE is a premium non-destructive method for delivering incredibly accurate information on the elemental level.