A sensor comprising: a first electrode formed of an electrically conductive material and configured to be located in contact which a solution to be analysed; a second electrode configured to be in electrical contact with the solution to be analysed; an electrical controller configured to apply a potential difference between the first and second electrodes to electro-deposit chemical species from the solution onto the first electrode, and an x-ray fluorescence spectrometer configured to perform an x-ray fluorescence spectroscopic analysis technique on the electro-deposited chemical species, the x-ray fluorescence spectrometer comprising an x-ray source configured to direct an x-ray excitation beam to the electro-deposited chemical species on the first electrode and an x-ray detector configured to receive x-rays emitted from the electro-deposited chemical species and generate spectroscopic data about the chemical species electro-deposited on the first electrode, wherein the sensor is configured such that in use the x-ray excitation beam incident on the electro-deposited chemical species on the first electrode is attenuated by no more than 60%.