Analytical application of total reflection and polarized X-rays

Abstract

Detection limits in photon induced X-ray fluorescence analysis (XRF) using energy dispersive spectrometer systems (ED) can be improved by the application of special excitation conditions. 1. Total reflection of the primary beam of the plane and smooth surface of a reflector reduces the radiation induced background. In total reflection geometry the beam scarcely penetrates into the medium. Taking the reflector as the substrate, only an extremely thin layer of the reflector (20–70 nm) acts as scatterer of the primary photons. The geometry allows efficient excitation of the sample and optimal detection of the fluorescent signal as the detector can be mounted close to the sample. Thin film samples are placed on the reflector-substrate. The sample volume needs only to be a few microliter. Concentrations in the ppb level or absolute amounts of pg can be detected in special cases. Total reflection XRF will be a method of choice if low concentrations in small volumina have to be detected. 2. Linear polarized X-rays are used in an orthogonal beam geometry (x-y-z) to reduce the background from scattered radiation from sample and substrate. Several possibilities to produce polarized X-rays are discussed and proposals made to increase the intensity of the polarized beam after a 90° Bragg reflection from a single crystal. Samples can be of any kind and shape. Detection limits are at present under good excitation conditions at the sub ppm level or in the range of about 100 pg absolute. The application of XRF in the pg range may therefore be extended to practically all kinds of samples if only special care in the sample preparation technique is taken.