| Abstract | The fundamentals of photochemical (PCVG), sonochemical (SCVG), and thermochemical (TCVG) vapor generation techniques are reviewed, and based on the impact of their practical implementation, potential mechanisms of action accounting for the identity of analytical products and effects of interferences are discussed. All three VG methodologies have a common basis of action which relies on the formation of radical intermediates to foster reduction reactions.
For PCVG a general model of homogeneous redox reactions facilitated by e⁻(aq), H˙, R˙, and CO₂˙⁻ produced in the irradiated media occurs. Reduction of aquo-metal and oxyanion species to their elemental state is followed by reaction with excess concentrations of H˙, H₃C˙, and CO arising from ultraviolet-C photolysis of typically formic and/or acetic acid to yield (predictable) volatile organometallic products. For some species, notably SeVI, slow kinetics of reduction to Se⁰ are overcome by the intervention of heterogeneous catalysis based on TiO₂ or other “designer” semiconductor photocatalysts that facilitate rapid reactions on their surfaces to which the reaction partners have adsorbed.
Despite currently being limited to only Hg and As, similar radical-based reactions appear operative when using SCVG and TCVG techniques. Acoustic cavitation results in the creation, expansion, and implosive collapse of microbubbles in liquid media, creating high pressure and temperature regions conducive to the thermolysis of the liquid medium and the generation of radicals similar to those involved with PCVG. Hence, TCVG likely follows the same reaction pathway. More extensive testing is needed before firm conclusions can be drawn with regards to either the mechanism(s) or extent of practical applicability of these two VG approaches. Future directions are discussed concerning needed research in these areas. |
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