Empirical evaluation of the TXRF detector field of view – a coffee-ring case study

Abstract

TXRF is a microanalytical method for trace elemental determination. Common sample preparation comprises drying a sample volume in the microliter range on a hydrophobized reflector. However, such specimens often dry as “coffee-ring”-like deposits. It is reported in the literature that coffee-ring specimens are correlated with poor accuracy. Nonetheless, models suggest that coffee-ring specimens provide the least errors from primary absorption. With picoliter derived specimens, the position dependent analyte signal response can be investigated. Here we study the elemental sensitivities obtained from ring-like deposits prepared by pL-printing with respect to their radius and position relative to the detector field of view (FOV). To understand the influence of a ring-like deposit on the TXRF signal we (a) determined the detector FOV and (b) modelled the declining response with increasing offset and finally (c) designed a multiple ring specimen for experimental validation. The detector FOV was determined from 128 individual micro specimens (Co, Cu, Ga, and Ni) with defined offsets using an in-house-build pL-printer. The 2d Gaussian intensity profile of the FOV exhibits a mean of 26.7 cps⁻¹ ng⁻¹ (Co Kα), offset from the reflector centre by +0.62 mm (x-axis) and +0.28 mm (y-axis). The σ of the intensity profile is located at 2.4 mm (x-axis) and 2.2 mm (y-axis). At a distance of 9.47 mm (x) and 8.71 mm (y) (4σ) from the centre the counts are down to 10%. The local relative sensitivity (e.g., Co Kα_offset normalized to Ga Kα_offset) (RS) corresponded well to the mean RS of all positions (e.g., Co Kα_mean normalized to Ga Kα_mean). The position-dependent errors in signal intensities were evaluated experimentally with concentric ring structures (Co, Cu, Ga, and Mn) with up to 6 mm distance from the centre. The bias originating from a total spatial separation of the IS and the analyte and location of the IS in the centre and analyte on circles of different radii was determined. A negative bias of 33% was observed for the 2 mm radius circle, 80% for the 4 mm radius and 99% for the 6 mm radius circle. The observation for the two inner circles matches the determined FOV profile.