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Understanding Our Planet Through Chemistry

X-ray Fluorescence Technique Spectrometry

Major-element chemical analysis is a front-line tool in the study of volcanoes and volcanic hazards. The analysis of a volcanic rock provides a fundamental common ground for comparing the styles and violence of previous eruptions of similar composition. During the first half of the 20th century, these analyses were performed exclusively by classical wet chemical analyses chemically separating each element of interest from the other elements in the sample. This procedure was extremely laborious. A good analytical chemist could analyze only a couple of hundred rocks per year for their complete major element chemistry. U.S. Geological Survey scientists now use technology called X-ray Fluorescence Spectrometry (XRF) to perform the same type of analyses.

XRF Spectrometry starts at the atomic level. Atoms consist of protons and neutrons in a central nucleus with electrons in different orbitals around that nucleus. If an electron from an inner orbital is knocked out, the vacancy created is filled by an electron previously residing in a higher orbit. The excess energy resulting from this transition is dissipated as an X-ray photon with a characteristic wavelength. In X-ray fluorescence analyses, the electron vacancies are created by bombarding the sample with a source of X-rays or gamma rays most frequently from an X-ray tube or a radioactive isotope. By detecting the characteristic X-rays that are fluoresced, the element of interest is shown to be present in the sample. The more abundant the X-rays are, the more of that element is present in the sample.

Bombarding the sample with X-radiation does not require a liquid sample. In fact, because solid samples are more stable than liquids, virtually all samples presented to X-ray spectrometers are solids. Furthermore, there is almost no permanent change that takes place in a solid sample analyzed by XRF, allowing it to be saved and reanalyzed. This is especially important for the repeated analysis of the same calibration standards over periods of years, permitting the use of the same analysis protocol. Homogeneity requirements are frequently solved by dissolving a portion of the pulverized sample in molten flux that is then poured into a mold and cooled to form a solid glass disc with a precise, flat, analytical surface.

Photo of scientist pouring molten sample to prerpare for XRF analysis. To analyze samples by X-ray fluorescence spectrometry, samples are fused at 1120°C with a flux; the chemist then pours the molten mixture into special molds to produce solid glass discs with a precise analytical surface. [43k] [75k]

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