Problems in obtaining precise and accurate Sr isotope analysis from geological materials using laser ablation MC-ICPMS

Abstract

This paper reviews the problems encountered in eleven studies of Sr isotope analysis using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) in the period 1995-2006. This technique has been shown to have great potential, but the accuracy and precision are limited by: (1) large instrumental mass discrimination, (2) laser-induced isotopic and elemental fractionations and (3) molecular interferences. The most important isobaric interferences are Kr and Rb, whereas Ca dimer/argides and doubly charged rare earth elements (REE) are limited to sample materials which contain substantial amounts of these elements. With modern laser (193 nm) and MC-ICPMS equipment, minerals with >500 ppm Sr content can be analysed with a precision of better than 100 ppm and a spatial resolution (spot size) of approximately 100 μm.

Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013)

“…The correction is calculated from the measured intensity of a different isotopic peak of the contaminant and the known natural isotopic abundance ratio. From [260].…”

Comments on Richards et al., Journal of Archaeological Science 35, 2008 “Strontium isotope evidence of Neanderthal mobility at the site of Lakonis, Greece using laser-ablation PIMMS”

“…Within the geochemical community considerable effort has been invested in identifying the various molecular interferences that are present in the Sr mass range during laser ablation of a wide variety of materials, including tooth enamel (summarised in Vroon et al, (2008) and references within). As an illustration of the complexity of the Sr mass spectrum during laser ablation of enamel some of the potential interferences that can be present are listed in Table 1 (see also Table 2 Vroon et al, 2008 andHorstwood et al, 2008) but the common feature to all these approaches is the recognition that the majority of the interferences are matrix sensitive and cannot be assumed to be constant during and/or between analyses.…”

“…As an illustration of the complexity of the Sr mass spectrum during laser ablation of enamel some of the potential interferences that can be present are listed in Table 1 (see also Table 2 Vroon et al, 2008 andHorstwood et al, 2008) but the common feature to all these approaches is the recognition that the majority of the interferences are matrix sensitive and cannot be assumed to be constant during and/or between analyses. Where possible, corrections must be applied on-line in real time to each cycle/integration of an individual measurement.…”

“…Richards et al (2008) appear to have made no attempt to identify any of the isobaric interferences present during analysis of the Neanderthal tooth although they do recognise the need to correct for isobaric interferences. Given the cup configuration of the mass spectrometer used by Richards et al (2008 ; Table 1) it would actually not be possible to correct for interferences with the same rigour as developed within the geochemical community since key interference monitor masses are not included (Vroon et al, 2008, Horstwood et al, 2008 If this simple approach for interference corrections were to be applied to the two standard teeth themselves the cattle tooth would be over-corrected by 0.0005 (0.07%) while the sperm whale tooth would be under-corrected by 0.0005 (0.07%). Clearly this is an inherently unsatisfactory approach to interference corrections since it fails to recognise that interferences are matrix sensitive and often time dependant.…”

Petrological and volcanological insights into acid lavas from the Paraná-Etendeka Magmatic Province on the surroundings of Guarapuava city, Paraná, Southern Brazil: a contribution of detailed textural characterization combined with in situ Sr isotopes in plagioclase phenocrysts

“…Rb has only one nonradiogenic isotope ( 85 Rb) and mass bias fractionation of Sr might be applied to correct the Rb fractionation. In order to minimize such problems, this technique must be applied for materials bearing very low Rb/Sr ratios (usually <0.05) (Ramos et al, 2004;Vroon et al, 2008 …”