Isotope ratio measurements calcium

Thermal ionization has been used to determine isotopic abundance of virtually all the elements We have recently extnded our own capability in this direction by adapting the silica gel/phosphoric acid filament coating technique (5) to our system Five 1 of a fine silica gel suspension is placed on a filament Five l of the analyte ion solution is coated, dried then coated with 2 pi of a 0 7N phosphoric acid solution and heated until dry again The analysis is performed in a similar manner as before, except that the signal is more transient and somewhat less intense than the calcium analysis With this approach, however, we have made natural abundance isotope ratio measurements on zinc, copper, and magnesium Table II shows our measurements compared to the accepted values, shown in parenthesis, for these elements The isotope used as reference... 

Results of isotope ratio measurements from an earlier study that used a similar protocol to the one just described are shown in Figure 3. These curves show plasma Isotope levels for both the i.v.oral tracer, labelled as PTA and CTA respectively. It is generally accepted that isotopic enrichment, atom percent excess, of urine reflects that of plasma, and after the initial period of rapid mixing, urinary atom percent excess is used in lieu of plasma measurements. The curves drawn through the data are those generated using the proposed model for calcium kinetics. 

The results of the described experiments are summarized in Table 8 where the totally extracted amounts of calcium and cryptand are also listed. Additionally, a blank result is given which was obtained under the same conditions, however without a cryptand compound. The e-values were calculated under the assumption that during the passage of the chloroform droplets through the aqueous solution a distribution equilibrium has developed. Only under this condition the measured isotope ratio of calcium in the distillating flask is equal to that of one extraction equilibrium stage. Because of the fact that the establishment of the equilibrium is not completely... 

Kendall, C. E. Grim, 1990. Combustion tube method for measurement of nitrogen isotope ratios using calcium oxide for total removal of carbon dioxide and water. Anal. Chem. 62 526-529. 

Wieser, M.E., Buhl, D., Boumaim, C., and Schwieters, J. (2004) High precision calcium isotope ratio measurements using a magnetic collector inductively coupled plasma mass spectrometer. J. Anal. At. Spectrom.,... 

All of the above taken together may explain why calcium is among the elements most intensively studied using stable isotope techniques. A comprehensive review of the state-of-the-art of calcium isotope ratio measurements was presented recently by Boulyga [99]. Single-collector ICP-MS or TIMS permits the determination of calcium isotope ratios at a precision of the order of 0.1-0.5%, depending... 

In a similar vein, mean seawater temperatures can be estimated from the ratio of 0 to 0 in limestone. The latter rock is composed of calcium carbonate, laid down from shells of countless small sea creatures as they die and fall to the bottom of the ocean. The ratio of the oxygen isotopes locked up as carbon dioxide varies with the temperature of sea water. Any organisms building shells will fix the ratio in the calcium carbonate of their shells. As the limestone deposits form, the layers represent a chronological description of the mean sea temperature. To assess mean sea temperatures from thousands or millions of years ago, it is necessary only to measure accurately the ratio and use a precalibrated graph that relates temperatures to isotope ratios in sea water. 

Table 1. Natural abundances, measured isotope ratios, and enrichment factors used for Calcium isotopes.

Using the three measured ratios, Ca/ Ca, Ca/ " Ca and Ca/ " Ca, three unknowns can be solved for the tracer/sample ratio, the mass discrimination, and the sample Ca/ Ca ratio (see also Johnson and Beard 1999 Heuser et al. 2002). Solution of the equations is done iteratively. It is assumed that the isotopic composition of the Ca- Ca tracer is known perfectly, based on a separate measurement of the pure spike solution. Initially it is also assumed that the sample calcium has a normal Ca isotopic composition (equivalent to the isotope ratios listed in Table 1). The Ca/ Ca ratio of the tracer is determined based on the results of the mass spectrometry on the tracer-sample mixture, by calculating the effect of removing the sample Ca. This yields a Ca/ Ca ratio for the tracer, which is in general different from that previously determined for the tracer. This difference is attributed to mass discrimination in the spectrometer ion source and is used to calculate a first approximation to the parameter p which describes the instrumental mass discrimination (see below). The first-approximation p is used to correct the measured isotope ratios for mass discrimination, and then a first-approximation tracer/sample ratio and a first-approximation sample CeJ Ca... 

Calcium-aluminum-rich inclusions The CAIs reveal a lot of isotopic action in calcium. But 4°Ca itself is taken as a reference for other isotopes because it is the most abundant. What is measured is the ratio of the other Ca isotope abundances to that of 4°Ca therefore attribution of an anomalous isotopic ratio to an anomaly in 4°Ca abundance is seldom made. Because of systematic linear variations with isotopic mass of the measured isotopic abundances, moreover, it is common to remove that effect by defining the ratio 44Ca/4°Ca as normal, a doubtful procedure, but common. Anomalies in other Ca isotopes thus appear as deviations from the linear slope set by the 44Ca/4°Ca ratio. All this is to say that a self-consistent analysis of the entire Ca suite is required. 

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