Methanol-Enhanced Liquid Electrode Discharge Microplasma-Induced Vapor Generation of Hg, Cd, and Zn: The Possible Mechanism and Its Application

Despite increased interest in microplasma-induced vapor generation (μPIVG) over the past several years, applications in real sample analyses remain limited due to their relatively low vapor generation efficiency and ambiguous mechanism. In this work, a novel method using methanol for significantly enhancing the liquid electrode discharge μPIVG efficiency was developed for the simultaneous and sensitive determination of Hg, Cd, and Zn by atomic fluorescence spectrometry (AFS). It is worth noting that the possible enhancement mechanism was investigated via the characterizations of volatile products by AFS, microplasma optical emission spectrometry, online gas chromatography, and gas chromatography-mass spectrometry, which involved the reductive species such as electrons, hydrogen radicals (·H), methyl radicals (·CH₃), and other intermediates in the argon plasma adding methanol. Under the optimized conditions, the limits of detection of 0.007, 0.05, and 0.5 μg L^-1 were obtained for Hg, Cd, and Zn, respectively, with relative standard deviations of 3.1, 3.7, and 5.2% for these elements, respectively. Vapor generation efficiencies of 90, 83, and 55% were achieved for Hg, Cd, and Zn, respectively, and improved 2.7-, 4.8-, and 7.9-fold, respectively, compared to those obtained in the absence of methanol. The accuracy and practicability of the proposed method were validated by the determination of Hg, Cd, and Zn in a certified reference material (CRM, Lobster hepatopancreas, TORT-3) and crayfish samples collected from three different provinces of China.