The use of Pt guard electrode in inductively coupled plasma sector field mass spectrometry: advantages and limitations

Abstract

Performance characteristics of inductively coupled plasma sector field mass spectrometry (ICP-SFMS) were studied with a Pt guard electrode (GE) inserted between the torch and load coil. The importance of the optimisation procedure and the matrix effects caused by a seawater matrix were assessed for 20 elements. Oxide and doubly charged ion formation was also investigated. Use of the GE allows a significant increase in ion transmission, by a factor of three to 20, thus resulting in improved instrumental detection limits. The improvement in sensitivity is mass dependent, with the highest gain observed for lower mass elements. Since, for the majority of analytical applications, actual detection limits depend upon blank levels rather on instrumental sensitivity, the most important factor for the determination of elements at ultra-trace levels is the degree of contamination of reagents and containers used. At the same time, significantly greater oxide formation is observed when operating the GE grounded rather than in the floating mode. For example, the BaO⁺/Ba⁺ ratio is ten to twelve times higher in the grounded mode. This calls for compromised instrumental parameters and the potential for severe spectral interferences from oxide species, which are often unresolved, even in high-resolution mode. Furthermore, non-spectral interferences from the seawater matrix appear to be more pronounced with the grounded GE, yielding a recovery of Ni of 55% compared with 93% in the floating GE mode. Hence all possible advantages and limitations of the use of the GE should be carefully considered prior to the analysis of real samples.