Microanalysis by monochromatic microprobe x-ray fluorescence—physical basis, properties, and future prospects

A monochromatic microprobe for x-ray fluorescence is obtained by a doubly-curved crystal diffractor which focuses characteristic radiation from a small laboratory-based x-ray source. Monochromatic microprobe x-ray fluorescence (MMXRF) provides unique advantages over conventional XRF, i.e., smaller analytical volume, higher sensitivity for the detection of impurities, and more accurate quantitation. Possible photon energies, voltage for the x-ray source, and type of diffractor geometry are discussed. Calculations of geometric aberration, collection solid angle, and beam intensity are given for a Johann-based diffractor. Properties of a mica diffractor used to focus Cu Kα1 x rays are predicted by ray tracing and experimentally verified by x-ray topographs and images of the focal spot. With the mica diffractor and a 20 μm x-ray source at 30 kV and 0.1 mA, ∼1.1×108 photons/s were obtained in a probe of 57 μm×43 μm and probes less than 10 μm appear to be theoretically possible. Energy dispersive spectra for bulk specimens of Si, GaAs, Mg, and Muscovite obtained with the Cu Kα1 probe exhibited extremely high signal/background ratios. The sources of background and reasons for low values are discussed. The low background resulted in predicted detection limits as low as 1.6 ppm for a measurement time of 500 s. Detection limits in the ppb range should be possible with higher power for the x-ray source, better detectors for energy dispersive spectrometry, improved diffractor fabrication and appropriate selection of the exciting photon energy.