X-ray fluorescence spectroscopy

Introduction

XRF spectroscopy is a very common, simple, and relatively quick spectroscopic technique used in a wide range of applications since its invention in the 1920s. It’s based on a very similar principle to EDX and microprobe WDX spectroscopy, in which the interaction of an electron beam with the sample produces a series of X-rays that are used to identify the composition of the sample. In XRF spectroscopy, X-ray radiation is used instead of an electron beam as the primary excitation source, which simplifies the required equipment and lowers the cost of sample preparation.

How it works

XRF spectroscopy consists in studying the interaction between the sample and X-ray radiation. Consequently, every XRF spectrometer is equipped with an X-ray generator, typically with a Rhodium filament and in the range of 20-60 kV, although W, Mo, Cr and others can also be used, depending on the application. Gamma ray sources can be used too without the need for an elaborate power supply, allowing an easier use in small portable instruments.

Much like in the other kinds of spectrometers, the beam travels through a series of lenses which narrow it and focus it on the surface of the sample. X-ray radiation doesn’t require vacuum like the electron beam used in microprobe spectroscopy, and the beam usually can’t be displaced along the surface of the sample like in a SEM. This simplifies and lowers the cost of the equipment.

When excited with the primary X-ray beam, each element emits a series of characteristic X-rays with different wavelengths from the primary beam, which are used to determine the composition of the sample. As explained in the SEM-EDX page, when the primary beam reaches an atom, it can become ionized (i.e. lose an electron). If the electron is lost from one of the inner shells, another electron from a further outer shell usually occupies its place. When this happens, energy is released due to the lower binding energy of the inner electron orbital compared with an outer one, most commonly in the form of an X-ray. As the energy of the emitted X-rays matches the energy gaps between the orbitals of the atoms of a specific element, these X-rays can be used to detect the abundances of elements that are present in the sample.

The detection of the emitted X-rays can be made both via wavelength dispersive (WDX) or energy dispersive (EDX) detectors. In WDX systems, the different wavelengths are selected one by one with the help of an analytical crystal and passed on to the detector, which registers the intensity of the radiation at every wavelength to construct a spectrum. The utilization of this technique grants a better spectral resolution and a higher quantitative power than EDX, which is why it is a bit more common in X-ray spectroscopy. For a deeper view on WDX spectroscopy read the dedicated page on our website.

EDX detection, on the other hand, offers a faster response by yielding the emitted radiation of all wavelengths at the same time to a semiconductor crystal, in which an electron-hole pair is generated for each X-ray, being the energy of these pairs easily measurable and corresponding to the energy of the incoming X-rays. A spectrum is formed by measuring the amount of electron-hole pairs with each energy.

(EDX and WDX spectra pictures)

Sample preparation is a crucial step to correctly perform an XRF analysis, especially when it comes to quantification of the abundances of elements present. In nearly all cases, the sample must be powdered and pressed into a thin disc, and it is common practice to mix the powdered sample with a chemical flux and use a furnace or gas burner to melt the powdered sample. There are other more elaborate sample preparation techniques for materials such as plastics and even liquids.  The main reasons for this need of preparation are the importance of achieving a homogeneous composition, the need to keep the geometry of the X-ray tube-sample-detector assembly constant, and that the secondary X-rays from lighter elements often only emit from the top few micrometres of the sample. In order to reduce the influence of surface irregularities, the sample is often rotated at 5-20 rpm.

Pros and cons

The main strengths of XRF spectroscopy lay in its simplicity and quickness, and the moderate cost of the equipment (20,000-125,000$) and its maintenance. Another advantage when compared to techniques that use an electron beam is the wide range of materials that can be analysed with it. With the appropriate sample preparation, almost any solid material can be studied, and most of the liquids too. However, XRF is most suitable for analyzing major elements and trace elements in rock and sediment.

These factors allow its routine usage in a vast number of fields such as geology and gemmology, the chemical and pharmaceutical industries, art and antique or the food industry. The simplicity of the XRF systems has lead to the development of handheld XRF analyzers. These are the cheapest and more widespread within the field of portable chemical analyzers, where they rival with Raman and FTIR devices.

Olympus portable XRF analyzer
Olympus portable XRF analyzer

As for its weaknesses, XRF cannot generally make analyses at the small spot sizes typical of electron microprobe devices (2-5 microns), so it is typically used for analyses of larger fractions of geological materials. Additionally, the limit of detection, which is the minimal amount of an element required to be detected, is higher than in other techniques. The requirement of powdering the sample in many materials is of course more destructive for the sample than performing a Raman or FTIR analysis. Moreover, the matrix used with the powder can sometimes significantly affect the obtained result, so these matrix effects must be identified and dismissed from the final results.

To overcome some of these problems, especially the one concerning the limits of detection, Total-Reflection XRF (TXRF) has been developed recently. This technique introduces some modifications to classical XRF, with the most important one being that the beam of the X-ray source is directed at the target at very low angles of incidence (≤0.1°), greatly increasing the reflectivity of the sample. By using this setup with a thin film of sample on a flat support, it was possible to detect secondary X-rays emitted from the sample eliminating matrix effects, and reducing the limit of detection by several orders of magnitude. A smaller amount of material is needed in this technique as well, as long as it can be deposited on a thin layer onto a carrier.

References and links