Improving Sample Preparation for Total Reflection X-ray Fluorescence by Selective Surface Functionalization

Abstract

For reliable (trace) elemental analysis using total reflection X-ray fluorescence (TXRF) spectrometry, cautious sample preparation is crucial: the sample needs to fulfil the thin film approximation [1]. Commonly, this is achieved by pipetting a small volume of the aqueous analyte solution on a silanized reflector. Although this method works well for dilute analytes, more concentrated solutions or suspensions suffer from the so-called coffee-stain effect [2]. In addition, manual pipetting results in limited spatial control over the deposition. In this work, the physicochemical properties of both the analyte solution and the surface of the reflector were altered to overcome these two challenges. To control the spatial distribution of the aqueous analyte, the analyte was forced to dry in a specific hydrophilic region on a functionalized reflector [3]. To create the functionalized reflector, it was first made hydrophobic by constructing a self-assembled monolayer (SAM) on the surface. Afterwards, UV-ozone cleaning, combined with a self-developed photomask, was used to remove the SAM creating a clean, hydrophilic area in the center of the reflector (negative of the photomask). The area of the hydrophilic region was matched to the field-of- view (FOV) of the detector to maximize the spread of the analyte and avoiding violation of the thin-film approximation for concentrated samples. The formation of a coffee-stain around the periphery of the hydrophilic area on the reflector was prevented by Marangoni flow assisted drying [4]. We anticipate that this combinational approach will provide enhanced control over sample preparation for TXRF. The developed method is easy to implement, low cost, versatile and applicable to various reflector materials and analytes.