Isotope ratios measured by ICP-MS are enriched in heavy isotopes with respect to the true sample due to instrumental mass fractionation. This process is thought to occur mostly during the selec- tion of ions from the plasma through the sample and skimmer cones (?)Andren:2004p1099); varies in magnitude with cone design or guard electrode potential, and drifts with time. In this study, instrumental mass fractionation is corrected by standard-sample bracketing using the 25Mg, 43Ca, and 87Sr enriched coral matrix standard SGSk described in Fernandez et al. (in press). The magni- tude of instrumental fractionation for our tune conditions is typically∼2% per amu for strontium isotopes,∼4% per amu for calcium isotopes, and∼6% per amu for magnesium isotopes, following a mass dependent trend. To correct for drift in instrumental mass fractionation, a time weighted linear model is used to interpolate between bracketing SGSk measurements, where:
ft=s= f t=1 ts−t1 + ft=2 t2−ts / 1 ts−t1 + 1 t2−ts
andf = (True Ratio)/(Measured Ratio).
Drift correction uncertainty was tested in a typical analytical session by treating every other SGS as a “sample”. Bracketing SGS measurements on each side of the “sample”-SGS were used to correct for instrumental mass fractionation. Drift correction error was estimated from residuals between the calculated drift corrected ratio of each SGS and the true value, for a typical error<0.2 permil. Drift error improves when samples and standards are ratio matched, so blank effect is probably limiting the precision of drift correction as well as sample ratios.
Figure 2.4: Blank memory studied by the repeated analysis of the deep-sea coral consistency standard (DSC-CS) with and without ratio matching. TOP: Sr/Ca of DSC-CS during a single analytical session. Samples and standards for the first half of the session all have similar isotope ratios. Half way through the session, samples of DSC-CS with twice the amount of spike (black squares) were alternated with the previous samples, yielding very different isotope ratios. Both accuracy and precision are affected. BOTTOM: Blank 87/88 ratio during the same session. At the beginning of the session the blank ratio matches the ratio of the isotopic standard SGSk (dark horizontal line), which is the most frequently analyzed material. The blank ratio is also similar to the ratio of normally spiked DSC-CS (horizontal dashed line), so sample Sr/Ca are unchanged by blank and memory effects. As sample and blank ratios differ from each other, Me/Ca ratios are affected. If this memory issue is addressed, current∼1 per mil precision may be improved to the∼0.1 per mil precision demonstrated with ratio matched samples.
re-calibrated in this study. Calcium isotopes were standardized using several natural standards: calcium flouride standard from the same batch of material as analyzed by Russell et al. (1978) (Henry Ngo, personal communication), deep-sea coral consistency standard, Aldrich CaCO3, and Durango Apatite. The original matrix of the Henry Ngo Calcium Standard is dilute HCl, and I have observed lower precision results when mixing acid matrices during an analytical session. To match the 5% nitric acid matrix of all other samples, the CaF sample was dried, re-dissolved in concentrated HNO3, dried again, and finally dissolved in 5% HNO3. In three different standardization sessions,
three natural samples were measured between each SGSk analysis. To minimize a systematic memory bias due to the measurement of different ratios, samples were analyzed in random order, with different samples following SGSk throughout the run. The 43/48 and 44/48 value of SGSk was adjusted until the average drift corrected ratio of all natural abundance standards equaled natural ratios 43/48 = 0.7311±.0002 and 44/48 = 11.273±0.002 (Russell et al., 1978). The 43/48 ratio of all the natural abundance materials are the same within 1 permil after drift correction (Figure 2.5), this agrees with the small fractionation observed for terrestrial samples (Russell et al., 1978). The resulting re-calibrated SGSk ratios are 43/48 = 9.376 and 44/48 = 11.69. SGSk 43/48 is within 0.7% of the previous calibration for this standard, 9.44±0.03, reported in Fernandez et al. (in press). Strontium isotopes were standardized against NBS SRM 987 and against marine isotope abundances (Table 2.2) using a dissolved sample of deep-sea coral. New isotope values are 88/87 = 1.1116 ±0.0001 and 86/88 = 0.1282 ± 0.001 for SGSk. Again, these values compare well to the previous calibrations on a single-collector mass spectrometer, differing only 0.1% from the value reported by Fernandez et al. (in press) of 1.113 for 88/87. This re-calibration was necessary to ensure accurate isotope ratio measurements for the experiments described in Chapter 4.
Two related calibrations affect the final Me/Ca ratio of a mixed spike isotope dilution measure- ment: the isotope ratio reference standard (SGSk) and the isotopic abundances of the mixed spike. Spike abundances are calibrated by isotope dilution measurements on a series of Me/Ca solutions gravimetrically prepared form pure solid standards. Like any other isotope dilution measurement, spike abundances are referenced to a particular SGSk. Since spike abundances were not re-calibrated
0 100 200 300 400 500 600 700 800 0.73 0.7305 0.731 0.7315 0.732 0.7325
run time −−> (min)
43/48 CaF2 Coral Aldrich CaCO 3 Apatite
Figure 2.5: The 43/48 of several natural calcium containing materials show limited fractionation. The materials measured in this is experiment differ by less ± 1 permil, (indicated by the dashed lines). The mean 43/48 of the natural samples measured here is assumed to be 0.7311, the natural abundance 43/48 reported in Russell et al. (1978). This value is then used to calibrate the 48/43 of SGSk.
in this study, the SGSk values of Fernandez et al. (in press) are used with the spike calibrations of Fernandez et al. (in press) to calculate Me/Ca ratios of carbonates. Where just calcium or strontium isotope ratios are measured, in Chapter 4 for example, the new SGSk values are used.