Effect of methanol addition on generation of isobaric polyatomic ions in the analysis of arsenic with ICP-MS

In the analysis of arsenic (As) with ICP-MS, it was observed that methanol addition to the sample decreases the amount of 40Ar35Cl+ formed in the previous study. To investigate the effect of methanol addition on generation of isobaric polyatomic ions and its mechanism, a diluted HCl solution and As solutions with varying concentrations of methanol (0, 1, 2, 3, and 4 v/v%) were analyzed using ICP-MS at a variety of mass spectra (m/z = 35, 47, 75, 82, 117, and 152). With addition of methanol to the sample, the signal intensity of 12C35Cl+ increased 11.1-fold while that of 40Ar35Cl+ only increased 1.17-fold. In this regard, carbon atoms that are not ionized in the plasma (12C) originated from methanol seemed to convert 40Ar35Cl+ to 12C35Cl+ through the substitution reaction. Furthermore, carbon atoms ionized in the plasma (12C+) converted 75As into 75As+ via the charge transfer reaction and thus increased signal intensity for As. At the same time, the amount of 40Ar35Cl+ formed in the plasma decreased since the 12C regenerated from 12C+ during the charge transfer process can act as a reactant in the substitution reaction aforementioned.