Total Reflectance X-Ray Fluorescence

Total reflectance X-ray fluorescence spectroscopy (TXRF) is a variation on the standard X-ray fluorescence technique. In both methods, the sample is irradiated with X-rays. The high energy of the X-ray photons is sufficient to knock out electrons from the inner electron shells of the atoms in the sample. As outer electrons make quantum transitions into the vacated states, energy is released in the form of photons, which are generally also in the X-ray band of the electromagnetic spectrum. Thus, the X-ray source gives rise to the release of secondary X-rays from the sample. This release is the fluorescent signal that carries information about the composition of the sample.

The geometry of the system is what differentiates TXRF from XRF. XRF systems use large angles of incidence, but TXRF systems have the X-ray source aligned so that the X-rays are incident on the surface of the sample at a very small angle, generally less than one degree. This grazing angle geometry confers two main benefits. A portion of the incident X-ray beam simply reflects off of the surface and passes through the system in nearly a straight line, while the fluorescent signal is distributed in all directions. Consequently, a detector placed at right angles to the path of the X-rays excludes virtually all of the background X-ray interference from the source. Thus, this geometry produces a relatively high signal-to-noise ratio. The other potential advantage of the TXRF technique stems from the fact that the grazing angle of incidence is much smaller than the Bragg angle, named after the father and son pair who received the Nobel Prize in Physics for this work in 1915. A consequence of using the grazing angle is that the X-rays do not penetrate more than a few nanometers into the sample, making TXRF an excellent technique for analyzing the composition of surfaces.

However, the technique also has drawbacks. The precise geometry is very sensitive to environmental effects, making the technique difficult. Like XRF, TXRF is also relatively insensitive to the lighter elements. Most significantly, the sample must be very flat in order to present a reflective surface to the X-ray beam, although liquid samples can be analyzed when deposited onto an appropriate substrate. For this reason, TXRF is most commonly found in semiconductor laboratories, as silicon wafers are ideal samples because they do not need much further preparation and polishing. Other applications include the analysis of trace elements in environmental samples, as well as geological, biological and other samples. TXRF systems are far less common than standard XRF systems, although the major X-ray players such as Rigaku and Bruker AXS have dedicated systems. Other players in the TXRF market include Ital Structures, Technos and Cameca, the latter two companies being more focused on process tools for semiconductor fabs. The total market for laboratory TXRF instrumentation was about $23 million in 2006.

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