Characterization of ionization and matrix suppression in inductively coupled ‘cold’ plasma mass spectrometry

Abstract

A parametric study of plasma power and central gas flow was carried out to study the transition from normal analytical conditions to cooler plasma conditions using an inductively coupled plasma mass spectrometer having a balanced load coil. ‘Cold plasma' conditions (low power and high central gas flow) permit the determination of K, Ca and Fe at trace levels. The effect of changing the position of the ground reference of the load coil was investigated. Trace element ionization is consistent with thermal ionization at low electron density. Ion–molecule chemistry (charge transfer) with NO⁺ or O₂⁺ may be important at the cooler plasma temperature. Suppression of analyte signals by concomitant matrix elements is partially correlated with the ionization potentials of the matrix element. If the analyte ion signals are normalized to that for NO⁺, the suppression of signals appears to be independent of the matrix element, and a modest dependence on the ionization potential of the analyte element is apparent. For high concentrations of elements of low ionization potential, an additional or enhanced mechanism of ionization is evident. The onset for this enhanced ionization is sharply defined by a characteristic ionization potential near 6.0 eV. The sensitivity for trace elements does not appear to be affected by this enhanced ionization. The appearance of the enhanced ionization is made evident by a change in the ratio of the NO⁺ and O₂⁺ signals. Use of cold plasma conditions for the determination of K, Ca and Fe in high-purity waters and acids is evident. It appears that the method may also be used for samples having moderate salt content if the analytical protocol includes measurement of the background ions NO⁺ and O₂⁺.