Non-spectral and spectral interferences in inductively coupled plasma high-resolution mass spectrometry : Part I. Optical characteristics of micro-plasmas observed just behind the sampler and the skimmer in inductively coupled plasma high resolution mass spectrometry

Abstract

In order to explain the mechanism of matrix effects in inductively coupled plasma high resolution mass spectrometry (ICP-HRMS), physical properties of micro-plasmas observed just behind the sampler and the skimmer of the MS interface were measured with an optical fiber system. The optical fiber system showed: some remarkable differences between emission characteristics observed behind the sampler and behind the skimmer; band emission of polyatomic species; and spatial distribution of Ar emission lines. In the micro-plasma behind the sampler, band emission due to water (OH and H2O+) was strong and the excitation temperature for Ar (Ar Tex) was 3230 K, while, in the micro-plasma behind the skimmer, extensive N2 band emission was observed and the Tex value was 8120 K. An accelerating voltage added to the MS interface was the most important parameter affecting physical properties and ionization efficiencies of the analytes. In the micro-plasma behind the sampler, all of the temperatures obtained experimentally, Ar Tex, rotational temperature for OH and ionization temperature for Mg, were independent of the accelerating voltage. Ar Tex and the vibrational temperature for N2 decreased with an increase in the accelerating voltage in the micro-plasma behind the skimmer. Also the accelerating voltage affected the formation of polyatomic ion species, because weakly bound polyatomic ion species such as ArX+ were formed in the micro-plasma after the sampler through collision-induced reactions of neutral Ar with X+. Theoretical dissociation equilibrium temperatures for ArX+ showed a tendency to decrease with increasing accelerating voltage, showing the same trend as the observed plasma temperatures in the micro-plasma, not after the sampler but after the skimmer.