Positive Material Identification (PMI)
ENERGY DISPERSIVE X-RAY FLUORESCENCE ANALYSIS
Published on 21st July 2020
The basics of X-ray fluorescence analysis and the most important instrument properties
In the past, X-ray fluorescence analysis (XRF) was mainly used in geology. Today it’s firmly established as a key technology for use in both industry and in the laboratory. This method is extraordinarily versatile: it can detect all relevant chemical elements from sodium to uranium.
XRF is often used for material analysis, i.e. to determine the amount of a given substance in the sample, like measuring the gold content in jewelry or detecting hazardous substances in everyday objects in line with the Restriction of Hazardous Substances (RoHS) directive. In addition, the thickness of coatings can be measured with XRF: it’s fast, clean and non-destructive.
THIS IS HOW THE MEASUREMENT WORKS
As an X-RAY device starts a measurement, the X-ray tube emits high-energy radiation, which is also called the ‘primary’ radiation. When these X-rays hit an atom in the sample, they add energy – i. e. they ‘excite’ it – causing the atom to eject an electron close to its nucleus, a process known as ‘ionization’. Since this state is unstable, an electron from a higher shell moves in to fill the gap, thereby emitting ‘fluorescence’ radiation.
The energy level of this secondary radiation is like a fingerprint: it’s characteristic for the respective element. A detector sees the fluorescence and digitizes the signal. After the signal has been processed, the device creates a spectrum: The energy level of the detected photons is plotted on the x-axis and their frequency on the y-axis (count rate). The elements in the sample can be identified from the positions (along the x-axis) of the peaks in the spectrum. The levels (along the y-axis) of these peaks provide information about the elements’ concentration.
THE MOST IMPORTANT DEVICE PROPERTIES FOR THE BEST MEASUREMENT RESULT
Many factors influence how well the device can differentiate between elements. Components such as the X-ray tube, optics, filters and the detector play a major role in this.
The materials in the X-ray tube determine the energy spectrum of the primary X-ray radiation with which the sample is excited. A tungsten anode is commonly used because it produces a particularly intensive and broad spectrum that can be employed for general applications. For specialized applications, e.g. in the semiconductor or printed circuit board (PCB) industries, molybdenum, chromium or rhodium anodes are also used; these anodes are particularly suitable for measuring light elements and material analysis.
On the way from the anode to the sample, the primary X-rays pass through a filter. Fischer generally uses filters made from thin metal foils, e. g. from aluminum or nickel. These filters modify the characteristics of the primary radiation by absorbing part of the spectrum. This way the background noise can be significantly reduced. Thus, a higher sensitivity to weak signals can be achieved. For example, aluminum filters help to detect lead in particularly low concentrations
Apertures and X-ray optics
The aperture (collimator) lies between the X-ray tube and the sample. It controls the size of the primary beam and ensures that only a specific, focused spot on the sample is excited.
When the measurement spot is necessarily small, the radiation that reaches the sample is minimal and the resulting fluorescence signal is correspondingly weak. To achieve high enough counts for reliable evaluation, the measurements need to take longer.
The solution to this problem is polycapillary optics. Polycapillaries are bundles of glass fibers that focus the almost entire primary radiation like a magnifying glass on a small spot. There are only two manufacturers of such optics worldwide – and Fischer is one of them!
The last crucial component is the detector, which is the part that ‘sees’ the fluorescence radiation. There are three types.
The tried-and-tested proportional counter tube (PC) has a large sensitive area and therefore achieves high count rates. It is well suited for the measurement of thicker layers with small measuring spots. However, since it offers a comparatively low energy resolution and limited sensitivity, especially for light elements, it is only partly suitable for more demanding measurement tasks.
The silicon PIN diode is a mid-tier detector. It has a much better resolution than the PC but only a small measurement area. It can be used for both material analysis and coating thickness measurement, but it requires a relatively long measurement time for small measuring spots.
The highest quality X-ray fluorescence devices use a silicon drift detector (SDD). This type of detector has excellent energy resolution, which means that it can detect the radiation even from elements in the sample that are present in very low concentrations. In addition, such devices can determine the thickness of coatings in the nanometer range and easily allow the evaluation of complex multilayer systems.