Photochemical Vapor Generation of Halides in Organic-Acid-Free Media: Mechanism Study and Analysis of Water Samples.

Abstract

Herein, we report a discovery that photochemical vapor generation (PCVG) of halides (bromide, chloride, and fluoride) can be realized in organic-acid-free media, with figures of merit comparable to those in classical scenarios employing acetic acid. Metal acetates, copper salts, and mixtures of different acetates and copper salts were evaluated for their performance in facilitating PCVG of halides; the formation of copper acetate complexes turned out to play a crucial role. Methyl halides (CH3X, X = Br, Cl, F) were identified by gas chromatography-mass spectrometry (GC-MS) as principal volatile compounds. Several important intermediate species, including cuprous ions (Cu+), methyl (•CH3), and hydroxyl (•OH) free radicals, were confirmed using cuproine, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO), and coumarin as a chromogenic agent, radical tracer, and fluorescence probe based on UV-vis, GC-MS, and fluorescence spectroscopy, respectively. The ligand to metal charge transfer (LMCT) between acetate and copper and the charge transfer to solvent (CTTS) excitation of halides were considered to account for the generation of methyl halides in organic-acid-free media. The presence of 100 and 200 μg mL-1 of CuAc2, as well as sample delivery rates of 10.7 and 3.3 mL min-1, yielded limits of detection of 0.03 and 3 μg L-1 for Br- and Cl-, respectively, by inductively coupled plasma mass spectrometry (ICPMS). The method was applied to the analysis of bottled water and seawater, achieving spike recoveries between 92 and 101%.