Application of a quadrupole ion trap mass spectrometer to ICP-MS and the direct detection of x-rays using a charge-injection device

This work describes the construction of an inductively-coupled plasma tandem quadrupole mass spectrometer where a quadrupole ion storage trap acts as a second sector and collision cell to achieve neutralization or collisional dissociation of interfering species before mass analysis. Because most elements exist as singly-charged ions in an inductively-coupled plasma (ICP) plume, the ICP can be used as an ionization source for mass analysis (ICP/MS). By reducing the sample to elemental ions before mass analysis, ICP/MS spectra tend to be simple compared with those obtained by ICP-optical emission spectrometry (ICP-OES) where elements may have hundreds to thousands of emission lines and spectral overlaps can be severe. This is especially troublesome in the analysis of rare earth elements which have the largest numbers of active emission lines when excited in an ICP. In addition, detection limits by ICP/MS are often up to 3 orders of magnitude lower than by ICP-OES. ICP/MS analysis is not immune from isobaric and isotopic interferences or matrix effects. For most analyses, an acid digestion precedes aspiration as an aqueous solution into an argon plasma gas. This can lead to large amounts of Ar+ etc., which may interfere to varying degrees with analytes of interest. Oxides, argides and hydrides of matrix ions or other analytes may also form and interfere. These same processes can also split peak areas between the atomic form of an analyte and the molecular. In isotope ratio studies where precise measurements on more than one isotope per analyte are needed, these effects may be compounded. Isobaric interferences normally require high resolution mass analysis to resolve if they cannot be separated prior to sample introduction. However, the interface between a high vacuum, high resolution sector or ion cyclotron resonance mass spectrometer and an atmospheric pressure plasma is non-trivial and such instruments are expensive. The focus of this work is new approach which uses a collision cell where weakly-bound molecular species can be dissociated and ions with relatively high electron affinities, such as Ar+, can be neutralized through charge exchange reactions