Fluorescence and absorption spectroscopy. Fluorescence spectra were captured using a Microspectrometer (ST-VIS, Ocean Insight) housed in a custom-built 3D-printed ring with heat skink and a high powered 265 nm LED (Boston Electronics). The UV-C LED was driven at 700mA using a Mightex LED driver. Fluorescence emission was focused directly into the entrance slit of the spectrometer by a collimating lens placed in the fibre-optic input connector. Liquid samples were analyzed in a 1mL crucible made from 3D printed black plastic (non-fluorescent and chemical resistant). Spectral acquisition on Whatman filters containing deposited e-liquid was performed by placing the spectrometer and LED assembly directly on top. Filters were analyzed with on the side facing away from the e-cigarette mouthpiece, as to validate the in-line detection approach employed by the device. Spectra were not averaged, or background subtracted.