Enhancing the sensitivity of X-ray fluorescence detection in HPLC through chromatographic miniaturisation and hardware optimisation.

The use of X-ray Fluorescence Spectroscopy (XRF) in a dynamic, flow-through environment with liquid chromatography (LC) was recently explored and emerged as a promising new strategy for universal quantification in chromatography. However, the transient nature of chromatographic peaks imposes limitations on detector sensitivity, and the detection limits of the initial setup were rather modest. Therefore, in this work, multiple approaches were studied to improve the sensitivity of benchtop LC-XRF, including setup design, flow cell materials and geometry, and separation column variables. In addition, the effect of chromatographic miniaturisation was investigated. The approach was evaluated using a range of brominated compounds, including 2,4,6-tribromophenol for comparison with previous research and tetrabromobisphenol A, selected as a representative for brominated flame retardants, a group of chemicals that could greatly benefit from universal quantification in LC. Firstly, setup optimisation was able to improve sensitivities by a factor of almost seven. Furthermore, it was shown that the optimised setup exhibits a concentration-dependent nature, indicating the potential of further chromatographic miniaturisation through capillary- or even nano-chromatography. Finally, the optimised setup and method were used for the analysis of a mixture of brominated molecules to establish the improved detection limits of benchtop HPLC-XRF. This resulted in LODs between 26 and 54 μM (14-18 ppm) when employing the ISO standard 11843-2, which is a reduction of over 70 % compared to previous work.