Evidence supporting the occurrence of Coulomb fission during conventional sample introduction in inductively coupled plasma mass spectrometry

Abstract

Axial and radial profiles of ion (Al⁺, V⁺, Cr⁺, Mn⁺, Fe⁺, Ni⁺, Co⁺, Cu⁺, Zn⁺, As⁺, Mo⁺, Sb⁺, La⁺, LaO⁺, Ce⁺ and CeO⁺) distribution in inductively coupled plasma mass spectrometry were examined for solutions containing 1% v/v HNO₃ (i.e. 0.16 M) with and without 0.02 M NaNO₃, KNO₃, CsNO₃, HI or HCl. None of the latter matrices induced a significant change in sample introduction efficiency compared to 1% v/v HNO₃ alone. Nonetheless, although analyte signal suppression was predominant in the presence of the K and Cs matrices, 0.02 M Na induced an enhancement by 1.6 ± 0.3 across the mass range, which was irrespective of the analyte ionization potential (i.e. even analytes that should be completely ionized were enhanced). This may be rationalized using a Coulomb fission mechanism within the spray chamber, which shifts the droplet size distribution towards smaller ones that are then more efficiently transported through the spray chamber and ultimately result in more analyte ions in the plasma. Furthermore, desolvation of these smaller droplets is completed sooner in the ICP, as axial profiles of the oxide fraction of La and Ce were shifted lower in the plasma in the presence of a non-volatile ionic matrix (0.02 M NaNO₃, KNO₃, CsNO₃) but were unaffected by 0.02 M HCl or HI. The broadened analyte radial profile in the presence of 0.02 M Na, which was observed at the optimum sampling depth for a 1% v/v HNO₃ matrix, also points to earlier desolvation. These results suggest that the Coulomb fission mechanism may be significant with a conventional sample introduction system.