Deuterium Labelling Experiments high-density acid (440 mg) was dissolved in D2O (25 mL). TBAP-α (25 mg) was dispersed in this solution by ultrasonication before degassing thoroughly by N2 bubbling for 30 minutes. The mixture was placed in a quartz vessel and sealed in a reactor under nitrogen. The sample was illuminated with a 300 W Newport Xe light source (Model: 6258, Ozone free) for the time specified at a fixed distance under atmospheric pressure. The Xe light source was cooled by water circulating through a metal jacket. Gas samples from the 1.3 mL headspace of the reactor were analyzed at the time periods specified by a customized HPR-70 batch sampling system from Hiden Analytical using a HAL3F/301 triple filter Mass Spectrometer with a Faraday detector for analysis.