Experimental evidence of enhanced water dissociation and spatially dependent charge-transfer reactions in mix-gas inductively coupled plasma optical emission spectrometry

Abstract

Water dissociation and spatial dependence of charge-transfer reactions among nitrogen species (N⁺/N₂⁺) and argon levels in inductively coupled plasma are experimentally demonstrated in the present study. The emission of species with energy near 5 eV (close to that of OH and H–OH), the ratio ion/atom or ion/ion along the ICP axis, and signals of argon spectral lines covering a broad energy range were evaluated. A microporous membrane fitted to a pneumatic nebulizer/desolvation system (PN/DES-MD) was employed for introducing the sample solution into the ICP, reducing the water loading and magnifying the effect of N₂ in the drier ICP. It was deduced that the addition of N₂ promotes the dissociation of OH or H–OH especially at low distances from the load coil. The ICP robustness was deteriorated due to water removal through the membrane, but the effect was compensated by adding a low flow of N₂ into the central channel of the ICP. Matrix effects were mitigated when the desolvation membrane was used in parallel with the addition of N₂. Emission intensity of argon spectral lines revealed that charge-transfer reactions among N⁺ and medium-energy argon levels are probably not spatially dependent along the ICP axis while those involving N₂⁺ and high-energy argon levels probably are. The charge transfer reaction among N₂⁺ and Ar requires a longer time of interaction, suggesting that it may occur in a two step process. The precision was better when membrane desolvation was used, but the sensitivity was worse, keeping the instrumental limit of detection at the same level as that obtained without using the membrane. The accuracy was not degraded by using the PN/DES-MD as the sample introduction system and adding a low flow of N₂ to the nebulizer gas.