Cobalt and Copper Ions Synergistically Enhanced Photochemical Vapor Generation of Molybdenum: Mechanism Study and Analysis of Water Samples

Herein, we report a discovery that photochemical vapor generation (PCVG) of molybdenum (Mo) can be synergistically enhanced dramatically by cobalt and copper ions sourced from acetates in the medium of formic acid, utilizing a flow-through reactor. The nature of this new PCVG was probed for the first time by an electron paramagnetic resonance (EPR) spin trapping technique for its possible reaction mechanism. Carboxyl and hydroxyl free radicals were verified during PCVG processes, and the results indicate that variations in the relative amounts and proportions of free radical species may account for the synergistic effect from concomitant Co2+ and Cu2+, as well as the generation of molybdenum hexacarbonyl from the UV-induced photolysis of formic acid. Under “dry” plasma conditions, the simultaneous spiking of Co2+ and Cu2+ to 20% (v/v) formic acid solutions and a 60 s irradiation time could give rise to a 15-fold enhancement in signal intensities, together with a blank-limited but still impressive limit of detection of 6 ng L–1 (6 ppt) by inductively coupled plasma mass spectrometry (ICP-MS). The accuracy and reliability of this methodology was validated by analysis of molybdenum in a seawater certified reference material, CASS-6, as well as two varieties of drinking water and seawater samples, with satisfactory spike recoveries (91–101%).