X-Ray Fluorescence (XRF) Spectrometry

X-ray fluorescence (XRF) spectrometry determines the elemental composition of minerals and rocks by using a beam of X-rays - rather than electrons - to excite the sample into emitting its own characteristic X-ray spectrum. The result is an average bulk analysis of the whole sample, making XRF the standard method when whole-rock composition is needed rather than the composition of individual mineral grains.

How XRF Works

An XRF spectrometer is functionally similar to the electron microprobe, but differs in two fundamental ways. The excitation source is the continuous X-ray spectrum from an X-ray tube rather than a focused electron beam. This switch minimizes background noise because the sample produces far less continuous spectrum of its own when excited by X-rays than when bombarded by electrons. The second key difference is that the sample must be prepared as a homogeneous material - either finely ground or fused - before analysis. ^[1]

The preparation requirement arises from a physical limitation: an X-ray beam cannot be focused to a fine spot the way an electron beam can. The entire surface of the sample is illuminated and fluoresces simultaneously. Grinding or fusing the sample ensures that every point on the surface is representative of the whole material, so that the resulting analysis reflects the true bulk composition rather than a sampling artifact. ^[1]

The characteristic X-ray spectra emitted by each element in the sample are detected using the same wavelength-dispersive (WDS) or energy-dispersive (EDS) techniques used in an electron microprobe. The instrument is calibrated using standards of known composition to convert X-ray intensities to element concentrations. ^[1]

Detection Performance

Because the X-ray excitation source generates far less background continuous spectrum than an electron beam does, the signal-to-noise ratio is significantly better than in an electron microprobe. XRF can detect as little as 1 ppm (0.0001 wt%) of some elements, a sensitivity substantially better than EPMA. However, XRF is generally limited to sodium and heavier elements - lighter elements produce low-energy X-ray emissions that are too easily absorbed before reaching the detector. ^[1]

Comparison with EPMA

The key distinction between XRF and electron probe microanalysis is spatial resolution versus detection sensitivity. EPMA uses a focused electron beam to target individual spots as small as a few micrometers, yielding spatially resolved compositional data; this makes it ideal for zoned crystals, inclusions, and fine-grained mineral intergrowths. XRF cannot achieve this precision - it always reports an average over the entire prepared sample. XRF is the method of choice when the goal is the bulk composition of a mineral or rock, not the composition of a particular point within it.

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