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

Abstract

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%).

Impact of Gas–Liquid Interface on Photochemical Vapor Generation

“…The results suggest that the major products formed from irradiation of As, Mo, and Sb are (CH 3 ) 3 As, and Mo(CO) 6 and (CH 3 ) 3 Sb Figure 5, respectively. 28,32,34,41,42 No new volatile species of As, Sb, and Mo were detected in the system with the gas−liquid interface but the generated volatile species in the system with gas−liquid interface are more than that obtained in bulk solution. It implies that similar photochemical processes may happen, but the gas−liquid interface(s) may facilitate the photochemical reduction of analytes.…”

“…Photochemical vapor generation (PVG), utilizing the generated radicals/electrons as reductants from photo-decomposition of low-molecular weight organic compounds (LMWOCs), is emerging as a simple and green sample introduction method for improving the analytical sensitivity and alleviating the spectral interferences during the past few years. − Hydride-forming elements (Se, Pb, Cd, Hg, As, Te, Sb, and Sn), transition ions (Cu, Ni, Co, and Fe), and nonmetal elements (Cl, Br, I, and S) could be derivatized to volatile species under UV irradiation in the presence of LOWOCs. ,,− Nevertheless, the PVG efficiencies of some elements were low and could not meet the requirement for trace/ultra-trace analysis. The use of semiconductor photocatalysts including TiO 2 /doped-TiO 2 , ZrO 2 , and metal–organic frameworks is beneficial for the efficient PVG of Se, Hg, and As. ,− Alternatively, metal ions such as Cu 2+ , Fe 2+ , Fe 3+ , Ni 2+ , Co 2+ , and Cd 2+ have recently been found acting as “sensitizers”, significantly promoting the photochemical reduction of elements, especially for those influenced inertly by semiconductor-based photocatalysis. ,, The analytical sensitivity for Hg, As, Se, Te, Mo, Bi, Ir, Cl, Tl, Pb, W, and Rh was increased from 1.4- to 30000-fold using metal ion-enhanced PVG as the sample introduction method with atomic spectrometry measurement. − However, the PVG and the mass transfer of elements with the gas–liquid interface are poorly understood. The photochemical behavior of elements with the gas–liquid interface is not clear.…”

Cadmium Assisted Photochemical Vapor Generation of Tungsten for Detection by Inductively Coupled Plasma Mass Spectrometry

“…34 This was immediately followed by a study by Hu et al which described a synergistic effect of added Co 2+ and Cu 2+ ions on PVG of Mo and considered mechanistic aspects of its generation. 35 Although the identification of the volatile Mo species has not yet been confirmed, it is likely Mo(CO) 6 . Together with W(CO) 6 , these species have earlier been detected in gaseous effluents from landfill and sewage sites.…”

“…Very recently, efficient generation of stable volatile molybdenum species from a solution of 30–50% formic acid was reported using a flow-through photoreactor . This was immediately followed by a study by Hu et al which described a synergistic effect of added Co 2+ and Cu 2+ ions on PVG of Mo and considered mechanistic aspects of its generation . Although the identification of the volatile Mo species has not yet been confirmed, it is likely Mo­(CO) 6 .…”

“…Generation strategies usually focus on composition of the photochemical medium and/or on the addition of various photocatalysts (such as TiO 2 or similar nanomaterials) or metal sensitizers to the sample to optimize efficiency. The following metal sensitizers have so far been utilized with significant enhancement effects on PVG of several elements and include Cd 2+ and Fe 3+ on PVG of As; , Cd 2+ on Se 6+ ; Co 2+ and Ni 2+ on Pb; Cu 2+ on Cl; Cu 2+ on Ir and Rh; Fe 2+ on Te; Fe 3+ and a mixture of Co 2+ and Cu 2+ on Mo; , Fe 3+ on Bi; , and very recently Co 2+ on Tl . Apart from the evident change in the UV–vis absorption spectrum of the reaction medium, the mechanism of their action remains rather speculative. ,, …”