12.4: Other X-Ray Methods

The application of X-rays to the analysis of materials can take forms other than X-ray fluorescence. In X-ray absorption spectrometry, the ability of a sample to absorb radiation from an X-ray source is measured. Absorption follows Beer's law (see Section 12.1) and, compared to emission, is relatively free of matrix effects. X-ray absorption, however, is a less selective technique than atomic fluorescence because we are not measuring the emission from an analyte's characteristic lines. X-ray absorption finds its greatest utility for the quantitative analysis of samples that contain just one or two major analytes.

In powder X-ray diffraction we focus the radiation from an X-ray tube line source on a powdered sample and measure the intensity of diffracted radiation as a function of the transducer's angle (\(2 \theta\)). A typical powder X-ray diffraction spectrum is in Figure \(\PageIndex{1}\) for the mineral calcite (CaCO₃). Qualitative identification is obtained by matching the \(2 \theta\) peaks to those in published databases. A quantitative analysis for the compound—not the elements that make up the compound—is possible using the intensity of a unique diffraction line in a sample to that for a pure sample. Figure \(\PageIndex{2}\) for a mixture of calcite and magnesite (MgCO₃) shows that a simultaneous quantitative analysis for both compounds is possible using the diffraction line at a \(2 \theta\) of 29.44 for calcite and of 32.65 for magnesite.

Figure \(\PageIndex{1}\): Powder X-ray diffraction spectrum for the mineral calcite (CaCO₃). The original data used to construct this spectrum is here.

Figure \(\PageIndex{2}\): Powder diffraction spectra for calcite (CaCO₃) and magnesite (MgCO₃). A quantitative analysis for calcite is possible using its most intense line at a \(2 \theta\) of 29.44 and a quantitative analysis for magnesite is possible using its most intense line at a \(2 \theta\) of 32.65. The original data for calcite is here and that for magnesite is here.