Long-Term Observations of Isotope Ratio Accuracy and Reproducibility Using Quadrupole ICP-MS

High precision isotope ratio and trace element determination can be achieved with modern quadrupole ICP-MS provided that short and long-term instrument performance is accurately monitored. Here we present results for the isotope ratios 6Li/7Li, 147Sm/149Sm, 160Dy/161Dy, 207Pb/206Pb, 208Pb/206Pb, 206Pb/204Pb and 235U/238U with which we determined long-term isotope ratio stability of relevance to both trace element and isotope determination. With respect to trace element determination, we first present long-term observations regarding oxide formation rates of Ba and Nd on light REE and heavy REE, as well as Zr on Ag. These showed good correlations and could be used to correct effectively the interference. The efficacy of this correction was demonstrated with analyses of the rock reference material BHVO-2 at both low and high oxide formation rates. Next, we studied the long-term reproducibility of a Dy isotope ratio that was measured to correct for the isobaric interference on Gd. It was found that, regardless of tuning condition, the ratio reproduced very well (0.58% RSD, 1s) and that the estimate of the Gd concentration did not suffer from the large correction (> 10%) caused by the Dy isobar. Long-term reproducibilities of Li, Sm and U isotope ratios, required for accurate mass bias correction when isotopically enriched internal standards of these elements are employed, were measured in the rock reference materials AGV-2 and JA-3 over a time period of up to 3 years. As expected, the Li isotope ratio showed the largest variability (RSD = 7%), but the other two ratios had relative external reproducibilities of only 1.01% (1s, U) and 0.67% (Sm). The mass bias-induced scatter in measurements for Sm and U was so small that the internal standard correction was effective, even for samples with high concentrations of these elements. With regard to Pb-isotope ratio determination, we also present long-term reproducibility for NIST SRM 982, run as an unknown and two accuracy tests for Pb separated from granitoids and from meteorites. It is demonstrated that the obtained ratios, including those involving 204Pb, are accurate relative to MC-ICP-MS determinations and of comparable precision to conventional TIMS analysis. The excellent agreement between all data sets shows the potential of modern quadrupole ICP-MS instrumentation for Pb-isotope determination, particularly for samples with very low Pb content.La determination precise des rapports isotopiques et des teneurs en elements traces peut etre realise avec les ICP-MS quadripoles modernes a condition que les performances a court et a long terme des instruments soient controlees avec precision. Ici, nous presentons des resultats pour les rapports isotopiques 6Li/7Li, 147Sm/149Sm, 160Dy/161Dy, 207Pb/206Pb, 208Pb/206Pb, 206Pb/204Pb et 235U/238U avec lesquels nous avons determine la stabilitea long terme de rapports isotopiques presentant un interet pour les etudes des elements traces et des isotopes. En ce qui concerne les elements traces, nous presentons d'abord les effets a long terme des taux de formation des oxydes de Ba et Nd sur la determination des terres rares, ainsi que de ceux du Zr sur celle de l'Ag. Les resultats ont montre de bonnes correlations et pourraient etre utilise pour corriger efficacement les interferences. L'efficacite de cette correction a ete demontree par des analyses du materiel rocheux de reference BHVO-2 a la fois a faible et fort taux de formation d'oxydes. Ensuite nous avons etudie la reproductibilitea long terme d'un rapport isotopique du Dy qui a ete mesure afinde corriger l'interference isobarique sur Gd. Il a ete constate que, independamment des conditions de reglage, ce rapport est tres bien reproduit (0.58% RSD, 1s) et que l'estimation de la concentration de Gd ne souffre pas de la large correction (> 10%) causee par l'isobare Dy. Comme prevu, le rapport isotopique du Li a montre la plus grande variabilite (RSD = 7%), alors que les deux autres rapports avaient des reproductibilites externes relatives de seulement 1.01% (1s, U) et 0.67% (Sm). La dispersion induite par le biais de masse dans les mesures de Sm et U etait si petite que la correction standard interne a ete efficace, meme pour les echantillons contenant des concentrations elevees de ces elements. En ce qui concerne la determination du rapport isotopique du Pb, nous presentons egalement la reproductibilitea long terme pour le NIST SRM 982, analyse comme un echantillon inconnu et deux essais de precision pour du Pb separea partir de granitoides et de meteorites. Il est demontre que les rapports obtenus, y compris ceux impliquant le 204Pb, sont similaires a ceux obtenus par MC-ICP-MS et d'une precision comparable a ceux obtenus par une analyse TIMS conventionnelle. L'excellent accord entre toutes les series de donnees montre le potentiel des ICP-MS quadripoles modernes pour la determination des isotopes du Pb, en particulier pour les echantillons ayant une teneur tres faible en Pb.