Total reflection X-ray fluorescence analysis with a glass substrate treated with a He atmospheric pressure plasma jet
Total reflection X-ray fluorescence (TXRF) is a powerful technique for trace elemental analysis of various liquid samples. A small volume of the liquid sample is dropped onto a flat substrate, and the dried residue is measured using TXRF. Usually, the substrate surface is treated with a silicone solution to make it hydrophobic. Such hydrophobic substrates are useful to concentrate the analyte at a small point, leading to a dot-type residue. However, the solution sample will provide a residue with a height of several tens of μm depending on the matrix of the solution, in which self-absorption is a serious problem. In this paper, the authors applied a He atmospheric pressure plasma jet (APPJ) treatment to a glass substrate to obtain a hydrophilic surface. The chemical properties of the glass substrate surface were drastically changed by the application of the He APPJ. The contact angle of the liquid droplet was 5.2° on the APPJ-treated glass, while it was 88.8° on the glass with a silicone layer. A droplet of the liquid solution smoothly spread over the APPJ-treated glass substrate, leading to a thin film-like residue. The recovery and detection limits for TXRF analysis of a standard solution sample were improved especially for low-Z elements by using the APPJ-treated glass substrates. This new sample preparation technique was also applied for TXRF analysis of a red wine sample. We found that this hydrophilic glass substrate gave a film-like residue from a small droplet of the red wine. Reasonable TXRF quantitative results were obtained by the Ga internal standard method. The preliminary experimental results suggested that the film-type residue produced on the APPJ-treated substrate was effective in decreasing the matrix effect especially for low-Z elements.