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 K alpha(1) 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 mu m x-ray source at 30 kV and 0.1 mA, similar to 1.1 x 10(8) photons/s were obtained in a probe of 57 mu m x 43 mu m and probes less than 10 mu m appear to be theoretically possible. Energy dispersive spectra for bulk specimens of Si, GaAs,Mg, and Muscovite obtained with the Cu K alpha(1) 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. (C) 1998 American Institute of Physics. [S0021-8979(98)07014-5].