1.4. X-ray Fluorescence Analysis

X-ray Fluorescence Analysis (XRF) uses x-rays generated by an X-ray tube to excite analytical X-rays from the sample analysis (Figure1.4a). The tube consists of a cathode running a current of 40 to 60 milliamps (mA) producing electrons that are accelerated by a voltage of 5 to 100 keV and fired at a target anode. The target generates broadband continuum X-rays and lots of heat (about 99% of the energy output). Both the cathode and target are put in vacuum to avoid oxidization and to minimize absorption of X-rays by air. The resulting X-rays are directed at the sample, causing it to emit X-rays characteristic of its constituent elements. The X-rays emitted by the sample are collimated and diffracted by an analyzing crystal in the spectrometer and their intensities measured using detectors mounted on a goniometer. The bulk chemistry of the sample is determined by comparing the intensities of the X-rays to those from standards.

Schematic cross-section of x-ray fluorescence unit

Figure 1.4a. Schematic diagram of the components of a typical wavelength-dispersive X-ray fluorescence (XRF) spectrograph.

The XRF requires large homogeneous bulk samples, but can determine trace elements to the parts-per-million (ppm) level. Electron microprobe detection limits are less good, being about 100 parts-per-million (ppm) in routine analysis. However, in grams this is pretty impressive: 100 ppm in a volume of 10 µm3 corresponds to about 10-14 grams of material! In addition, the amount of power required for direct electron excitation (microprobe) is much less than that for X-ray excitation (XRF).

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Last update: 01/18/2006 01:47 PM.