Exploring the boundaries of double excitation in TXRF

Abstract

Double excitation is an established concept in Total-Reflection X-Ray Fluorescence Analysis (TXRF) and one of the key advantages of TXRF over common XRF methods. It is realized by setting the incident angle of the excitation beam to an angle below the critical angle of total reflection of the reflector material (e.g. Si wafers or quartz glass), external total reflection can take place. Consequently, the excitation beam can excite the sample prepared on the reflector twice: once by the incoming beam, and once by the reflected beam. Are there boundaries to this concept however? One obvious boundary is the realm of the X-Ray Standing Wave Field (XSW) that leads to oscillating excitation conditions in the nanometer range above the reflector surface, this may be identified as a “lower” boundary. Here however, the authors study an “upper” boundary for specimens in the micrometer range. This “upper” boundary was probed with binary mixtures of Ni and Ga with metal masses between 300 and 10,000 ng. These sample masses translate into dried sample residues with heights of about 5 to 100 μm and widths of up to several hundred μm. Using laboratory based angle-resolved (T)XRF two distinct deviations from the angle dependent double excitation were observed. A fundamental parameter model was created to give comprehensive insights into the why and wherefore of this “upper” boundary. This includes the approximation of the sample 3D shape, excitation geometry, reflectivity of the sample carrier and matrix effects. The model suggests that sample widths of several hundred micrometers induce a dampening of double excitation, i.e. only a relative signal intensity of approx. 1.75 is observed below the critical angle instead of twice the intensity (both relative to large angles above the critical angle). Furthermore, heights above several ten micrometers lead to an angle-dependency of the signal intensity with a maximum just below the critical angle, due to insufficient illumination of the sample by the reflected beam.