Effect of methanol and sodium dodecylsulfate on radial profiles of ion abundance in inductively coupled plasma mass spectrometry

The radial profiles of some background ions and analytes spanning the mass range and with a wide range of first ionization potentials were investigated using inductively coupled plasma time-of-flight mass spectrometry. In particular, three different matrices were considered to assess the effect of organic modifiers: 1) 1% HNO3, 2) 1% HNO3 with 2%v/v methanol, 3) 1% HNO3 with 0.2% m/v sodium dodecylsulfate (SDS). Although these concentrations of methanol and SDS induced the same sample transport increase (37%), as measured with a silica gel trap at the exit of the spray chamber, neither of them resulted in a 37% increase in signal across the mass range. In fact, the change in analyte signal as a function of m/z followed opposite trends in these two matrices. With 0.2% m/v SDS, suppression was observed at high m/z with an increasing enhancement as m/z decreased, which was ascribed to electrostatic effects in solution. In contrast, little change or enhancement was seen at high m/z while suppression was evident at lower m/z (with the notable exception of As) with 2% v/v methanol, as a result of a widening of the radial profile, which was inversely dependent on m/z. Although, the total carbon concentration was quite different in these two matrices, i.e. 0.8 M with 2% methanol and 0.08 M with 0.2% SDS, it cannot account for the completely different radial profiles that they produced. Indeed, the same bell-shaped distributions of analyte ions were observed with 0.2% SDS as in 1% HNO3 alone. However, a bimodal distribution, with maxima on either side of the central axis, resulted in presence of 2% MeOH. This distribution was found to be similar to that of several background ions (C+, CO+ and ArC+), which further substantiates the suggestion that ionization then predominantly occurs through charge transfer with carbon-containing ions. This bimodal distribution also suggests that a volatile organic solvent such as methanol quickly spreads into the surrounding area of the central channel upon sample introduction in the plasma.