‘Zone model’ as an explanation for signal behaviour and non-spectral interferences in inductively coupled plasma mass spectrometry

The zone model is a simplified representation of the plasma, resulting from the findings of an optimization study for a VG PlasmaQuad PQ1 inductively coupled plasma (ICP) mass spectrometer (VG Elemental, Winsford, Cheshire, UK). According to this model, for every nuclide there is a zone in the central channel of the ICP, where a maximum density of singly charged ions occurs. The position of such a zone of maximum M+ density is a function of the mass number of the nuclide and the zone can undergo a spatial displacement under the influence of an alteration of an instrumental parameter or the introduction of a different matrix. This representation not only enables an explanation of a large number of observations from the optimization study, but also allows an understanding of why both matrix induced signal suppression and enhancement were observed, why for a given matrix the extent to which the signal intensities were altered differed from day to day and finally why the extent to which a signal is influenced by the matrix was seen to be a function of the mass number of the corresponding nuclide. Although the zone model might not completely reflect the genuine physical reality in all its facets, it provides a phenomenological model for the variation of ion signals with mass number, operating parameters and matrix composition.