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 T_ex) was 3230 K, while, in the micro-plasma behind the skimmer, extensive N₂ band emission was observed and the T_ex 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 T_ex, rotational temperature for OH and ionization temperature for Mg, were independent of the accelerating voltage. Ar T_ex 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.