Spectroscopy, X-Ray Fluorescence

X-ray fluorescence (XRF) spectroscopy is a widely used method for identifying the chemical composition of almost any material, regardless of its quantity or form. The analysis is useful for two primary reasons: it is noninvasive and nondestructive to the sample material; it yields an easily identifiable and reliable atomic composition of materials, excluding only those primarily constituted of elements lighter than aluminum.

The history of XRF spectroscopy begins with Wilhelm K. Roentgen’s 1895 discovery of x-rays, which earned him the 1901 Nobel Prize. Less than ten years later, Charles G. Barkla discovered a relationship between the x-rays emitted from a sample material and that sample material’s atomic weight. Then in 1913, Henry Gwyn Jeffreys Moseley renumbered the periodic table of elements by measuring the x-ray fluorescence of each element. Prior to his reorganization, the periodic table was arranged by atomic weight rather than atomic number.

Moseley is also credited with inventing the first XRF spectrometer, which used electrons as a rather inefficient energy source. Other scientists continued to build variations of x-ray detectors with little progress in efficiency until 1948 when Herbert Friedman and LaVerne S. Birks built an XRF spectrometer with a highly sensitive x-ray detector that utilized a Geiger Counter. Their innovation increased the number of elements that could be identified.

Then in the 1960s and 1970s, computers could rapidly analyze data, allowing for readings of multiple elements at a time and larger quantities of sample materials to be tested. Widespread usage of XRF spectrometers increased as they become more and more efficient and even portable. In fact, the National Aeronautics and Space Administration (NASA) sent XRF spectrometers on both the Apollo-15 and -16 missions, as well as to investigate the asteroid Eros, and to Mars onboard the Pathfinder in 1997 and the Rover in 2004.

XRF spectroscopy is a relatively accessible process for data collection. To conduct research, one needs a sample, an energy source, an x-ray tube, an x-ray detector, and a computer for data analysis. As noted above, one of the advantages of XRF spectroscopy is that nearly any kind of unknown material may be identified, whether it is in solid, gaseous, or liquid form. The choice of energy source is largely dependent on the form of the sample material and the particular application.

Energy sources vary from x-rays to alpha-particles to beams of electrons. Via an x-ray tube, the energy source bombards the sample material. An x-ray detector or x-ray fluorescence spectrometer then measures the fluorescent light emitted from the sample atoms. Computers then are used to compute the identity of the element and its quantity within the sample. One disadvantage to this process is that most energy sources for XRF spectrometers are radioactive and must be replaced on a regular basis due to the normal decay of radioactive materials.

The basic process of XRF spectroscopy begins when, depending on the particular application, the sample is initially bombarded through an x-ray tube with an energy source such as x-rays, alpha- particles, or beams of high-energy electrons. When electrons in the innermost orbital shells of sample atoms absorb the energy, they are ejected from the atom.

When this happens, electrons from the higher-energy outer shells of the atom move inward to fill vacancies in the lower-energy orbitals to stabilize its atomic structure. The movement of the electrons from higher-energy orbitals to lower- energy orbitals causes energy to be emitted in the form of fluorescent light. This fluorescent light, or x-ray, is the characteristic signature of that particular element.

The x-rays are reliable identifiers of the element because of their relationship with other properties of atomic physics. The energy emitted by the transitioning electron will be equal to the difference between the binding energies of the two orbitals occupied by that electron. Because the difference of two specific orbital shells of a given element is constant, the energy emitted by a transitioning electron is also constant and is therefore characteristic of that element. Thus, XRF spectroscopy is used to qualitatively identify the elemental content of a sample material.

XRF spectroscopy can be used to quantitatively measure the amount of a given element within a sample material. Scientists can assess the count rate or peak intensity of the wavelength of the fluorescent x-ray emitted by the transitioning electron. Specifically, the count rate refers to the number of emitted fluorescent photons per unit of time. The count rate can then be used to establish the quantity of a particular element within the sample. Analysis of count rates is easily accomplished with the help of computers.

Applications from the evaluative processes of x- ray fluorescence spectroscopy touch the everyday lives of people everywhere. Portable versions of XRF spectrometers are widely used for field applications, whether the site is an ancient city long buried by volcanic activity or the surface of Mars. The primary industrial use of XRF spectrometry is for quality control of material composition, from raw forms to finished products.

Examples include NASA’s use of XRF spectroscopy to evaluate the geology of Mars during the 2004 Rover expedition, and museums’ use of the process to identify pigments in rare paintings for purposes of restoration. The U.S. Food and Drug Administration evaluates the content of vitamins and other drugs with XRF spectroscopy while archeologists use XRF spectrometers to identify and date artifacts by their mineral contents.

In just over one hundred years since the discovery of x-rays, XRF spectroscopy has become one of the most reliable and widely used methods for chemical composition analysis. Biologists, chemists, museum curators, health inspectors, forensic scientists, medical doctors, ecologists, mineralogists, archeologists, and many university students all use XRF spectroscopy to identify elemental constituents of a vast range of materials.

The process is vital for quality control of raw materials and finished products within numerous industrial settings. By traveling into space to classify yet-to-be explored worlds as well as back in time to identify ancient artifacts, scientists use XRF spectrometers to expand the domain of knowledge.