TY - JOUR
T1 - Precise analysis of Ca isotope ratios (δ44/40Ca) using an optimized 43Ca-42Ca double-spike MC-TIMS method
AU - Lehn, G. O.
AU - Jacobson, A. D.
AU - Holmden, C.
N1 - Funding Information:
The authors thank one anonymous reviewer for helpful comments and Scott McLuckey for thoughtful editorial handling. This work was supported by an Environmental Protection Agency STAR Fellowship awarded to G.O.L. as well as a Major Research Instrumentation Grant ( EAR-0723151 ) and a David and Lucile Packard Foundation Fellowship awarded to A.D.J. The study also benefited from NSF EAR-0643317 and NSF-0617585 awarded to A.D.J.
PY - 2013
Y1 - 2013
N2 - We used a Monte Carlo error model to optimize a 43Ca-42Ca double-spike technique for measuring Ca isotope ratios (844/40Ca) by Multi-Collector Thermal Ionization Mass Spectrometry (MC-TIMS). Optimization is non-unique and method-specific because errors for counting statistics and Johnson noise depend on ion beam intensities, integration times, and the total number of duty cycles, while the cumulative effects of measurement-induced Faraday collector damage restrict the double-spike 43Ca/42Ca and spike/sample ratios that can be practically employed. Factors that improve counting statistics and limit Johnson noise also accelerate collector damage and associated drift in measured 844/40Ca values. Accordingly, better internal precision occurs at the expense of accuracy and external reproducibility. For a 20 V 40Ca ion-beam implemented in a three-hop, dynamic multi-collection routine, the model predicts that a wide range of 43Ca/42Ca ratios should yield internal precisions of ±0.020-0.025‰ (2σsem) for a runtime of 2.5h/sample, but theoretical constraints for minimizing drift exclude options having relatively high spike/sample ratios. Using a Thermo Fisher MC-TIMS (Triton), we tested 43Ca/42Ca = 1 [42Ca/(42Ca + 43Ca) = 0.50mol/mol] and spike/sample = 0.33 [Cadsp/(Cadsp + Casmp) = 0.25 mol/mol] by repeatedly analyzing OSIL Atlantic seawater (sw), NIST SRM 915a, NIST SRM 915b, and CaF2 over 11 sessions spanning 3 months. The average internal precision for 171 measurements is ±0.024‰, in excellent agreement with the model prediction. We adopted an experimental protocol that eliminates drift for a single measurement session (≤30 runs), thereby increasing accuracy and external reproducibility across consecutive sessions. The average 844/40 Ca values agree well with accepted values: δ44/40Casw-sw = 0.000 ± 0.005‰ (n = 62), δ44/40Ca915a-sw = -1.865 ± 0.006‰ (n = 42), δ44/40Ca915b-sw = - 1.134±0.006‰ (n = 37), and δ44/40CaCaF2sw = -1.392 ± 0.008 ‰ (n = 30). The global, long-term external reproducibility for the method is ±0.041 ‰ (2σsd), which represents a two-to ten-fold improvement over Ca isotope measurements made with existing double-spike MC-TIMS methods.
AB - We used a Monte Carlo error model to optimize a 43Ca-42Ca double-spike technique for measuring Ca isotope ratios (844/40Ca) by Multi-Collector Thermal Ionization Mass Spectrometry (MC-TIMS). Optimization is non-unique and method-specific because errors for counting statistics and Johnson noise depend on ion beam intensities, integration times, and the total number of duty cycles, while the cumulative effects of measurement-induced Faraday collector damage restrict the double-spike 43Ca/42Ca and spike/sample ratios that can be practically employed. Factors that improve counting statistics and limit Johnson noise also accelerate collector damage and associated drift in measured 844/40Ca values. Accordingly, better internal precision occurs at the expense of accuracy and external reproducibility. For a 20 V 40Ca ion-beam implemented in a three-hop, dynamic multi-collection routine, the model predicts that a wide range of 43Ca/42Ca ratios should yield internal precisions of ±0.020-0.025‰ (2σsem) for a runtime of 2.5h/sample, but theoretical constraints for minimizing drift exclude options having relatively high spike/sample ratios. Using a Thermo Fisher MC-TIMS (Triton), we tested 43Ca/42Ca = 1 [42Ca/(42Ca + 43Ca) = 0.50mol/mol] and spike/sample = 0.33 [Cadsp/(Cadsp + Casmp) = 0.25 mol/mol] by repeatedly analyzing OSIL Atlantic seawater (sw), NIST SRM 915a, NIST SRM 915b, and CaF2 over 11 sessions spanning 3 months. The average internal precision for 171 measurements is ±0.024‰, in excellent agreement with the model prediction. We adopted an experimental protocol that eliminates drift for a single measurement session (≤30 runs), thereby increasing accuracy and external reproducibility across consecutive sessions. The average 844/40 Ca values agree well with accepted values: δ44/40Casw-sw = 0.000 ± 0.005‰ (n = 62), δ44/40Ca915a-sw = -1.865 ± 0.006‰ (n = 42), δ44/40Ca915b-sw = - 1.134±0.006‰ (n = 37), and δ44/40CaCaF2sw = -1.392 ± 0.008 ‰ (n = 30). The global, long-term external reproducibility for the method is ±0.041 ‰ (2σsd), which represents a two-to ten-fold improvement over Ca isotope measurements made with existing double-spike MC-TIMS methods.
KW - Calcium
KW - Double-spike
KW - Isotopes
KW - MC-TIMS
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U2 - 10.1016/j.ijms.2013.06.013
DO - 10.1016/j.ijms.2013.06.013
M3 - Article
AN - SCOPUS:84885598454
VL - 351
SP - 69
EP - 75
JO - International Journal of Mass Spectrometry and Ion Processes
JF - International Journal of Mass Spectrometry and Ion Processes
SN - 1387-3806
ER -