Calcium isotopes, determination

Isotopic methods for estimating calcium absorption have been evaluated by several researchers (49,55-58). From the human data of Harrison et al. (55), the relationship between percent calcium absorption determined by isotope dilution (Y) and excreta counting... 

Miller YM, Ustinov VI, Artemov YM (1966) Mass spectrometric determination of calcium isotope variations. Geochem Inter 3 929-933... 

Figure 9. (a) Box model of Skulan and DePaolo (1999) used to explain the 6 Ca variations found in the hone and soft tissue of vertehrates. The 6 Ca values of bone and soft tissue are generally determined by the ratios of the dietary Ca flux (V ), to the Ca fluxes due to bone formation (Pi,), and bone dissolution or loss (Vi). It is hypothesized that only the step associated with bone grovrth involves isotopic fractionation (lOOOlna = = -1.5). (b) Calculated effects on calcium isotopic fractionation due to variations of the... 

Lee T, Papanastassiou DA, Wasserburg GJ (1977) Mg and Ca isotopic study of individual microscopic crystals from the Allende meteorite by the direct loading technique. Geochim Cosmochim Acta 41 1473-1485 Lee T, Russell WA, Wasserburg GJ (1979) Calcium isotopic anomalies and the lack of aluminum-26 in an unusual Allende inclusion. Appl J Lett 228(L93-L98) 661-662 Marshall BD, DePaolo DJ (1982) Precise age determinations and petrogenetic studies using the K-Ca method. Geochim Cosmochim Acta 46 2537-2545... 

Schmitt AD, Bracke G, Stille P, Kiefel B (2001) The calcium isotope composition of modem seawater determined hy thermal ionisation mass spectrometry. Geostandard Newsletter 25 267-275 Schmitt A-D, Stille P, Venneman T (2003a) Variations of the " Ca/ Ca ratio in seawater during the past 24 million years evidence from 5 Ca and values of Miocene phosphates. Geochim Cosmochim Acta 67 2607-2614... 

Physiologically important calcium flow rates can be determined using calcium isotopic tracers without resorting to classical metabolic balance methodology Calcium stable isotopic tracers are sufficiently benign to permit their use in studies of calcium metabolism in children The measurement techniques and clinical protocol for our studies of skeletal development in children are described ... 

In order to test the accuracy of the SCSI-MS technique, the masses of the calcium isotopes " a+ and " a+ were measured with reference to the mass of the Ca ion one of the isotopes was trapped together with a Ca ion and the COM resonance frequency was determined by the phase-detection method (see Ref. [3]). As a reference, the COM frequency for two " Ca ions was determined also by the phase-detection method. The experimental oscillation frequencies from a series of measurements... 

An example where nonlinear phenomena in connection with laser-rf spectroscopy have been used in atomic beams, is the recent work on calcium isotopes carried out in our laboratory. The goal of these experiments was to determine nuclear electric quadrupole moments from precise hyperfine structure data of the atomic spectrum. This is of some importance in the case of calcium, since Ca as well as Ca are so-called double-magic nuclei, i.e., with closed proton and neutron shells. Radioactive Ca (t = 1.03 X 10 yr) and the stable isotope Ca have been investigated by laser-rf spectroscopy. The measurements allow to study the influence of a single neutron and three neutrons, respectively, on the double-magic °Ca core. 

Most elements in the periodic table have one, two, or even three isotopes that arc free from isobaric overlap. An exception is indium, which has two stable isotopes, " In and " In. The former overlaps with " Cd and the latter with " Sn. Mote often, an isobaric interference occurs with the most abundant and thus the most sensitive isotope. For example, the very large peak for Ar (see Figure ll-15b) overlaps the peak for the most abundant calcium isotope Ca (97%), making it necessary to use the second-most abundant isotope Ca (2.1%). As another example, the most abundant nickel isotope, Ni, suffers from an isobaric overlap by "Fe. This interference can be corrected by measuring the peak for Fc. From the natural-abundance ratio of the Fe isotope to that of Fe isotope, the contribution of Fe to the peak at m/z. 58 can be determined and the correction made. Because isobaric overlaps are exactly predictable from abundance tables, corrections for the problem can be carried out with appropriate software. Some current instruments arc capable of automatically making such corrections. 

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... 

Likewise, helium has very little effect on reducing the Ar interference in the determination of °Ca+. So, when using a collision cell with helium, the quantification of calcium must be carried out using the " " Ca+ isotope, which is about 50 times less sensitive than °Ca . For this reason, in order to achieve the lowest detection limits for calcium, a low-reactivity gas such as pure hydrogen is the better option. By initiating an ion-molecule reaction, it allows the most sensitive calcium isotope... 

By experimentally determining the ratio of abundances of C and isotope peaks for CO2 dissolved in sea water at various temperatures, a graph can be drawn relating the solubility of CO2 compared with that of CO2 (the ratio described above). On extracting the CO2 from sediment containing the shells (calcium carbonate) of dead sea creatures by addition of acid, a ratio (R) of abundances of CO2 to CO2 can be measured. If this value is read from the graph, a temperature T is extrapolated, indicating the temperature of the sea at the time the sediment was laid down. Such experiments have shown that 10,000 years ago the temperature of the Mediterranean was much as it is now. 

One method for measuring the temperature of the sea is to measure this ratio. Of course, if you were to do it now, you would take a thermometer and not a mass spectrometer. But how do you determine the temperature of the sea as it was 10,000 years ago The answer lies with tiny sea creatures called diatoms. These have shells made from calcium carbonate, itself derived from carbon dioxide in sea water. As the diatoms die, they fall to the sea floor and build a sediment of calcium carbonate. If a sample is taken from a layer of sediment 10,000 years old, the carbon dioxide can be released by addition of acid. If this carbon dioxide is put into a suitable mass spectrometer, the ratio of carbon isotopes can be measured accurately. From this value and the graph of solubilities of isotopic forms of carbon dioxide with temperature (Figure 46.5), a temperature can be extrapolated. This is the temperature of the sea during the time the diatoms were alive. To conduct such experiments in a significant manner, it is essential that the isotope abundance ratios be measured very accurately. 

Calcium exists in the human body as Ca(II) protein-bound and free Ca (II) ions (Dilana et al. 1994). For total extracellular Ca in plasma, serum and urine a definitive isotope dilution-mass spectrometry (ID-MS) method exist. Free Ca(II) in plasma/serum can be determined with PISE, but no definitive and reference methods exist. For Ca in faeces, tissue and blood flame atomic absorption (FAAS) is used widely. 

Calcium carbonate is also the main constituent of the shells of sea animals, which make their shells from elements acquired from the surrounding waters. Now, the degree of fractionation of the oxygen isotopes as well as the formation of mineral carbonates and of animal shells in sea waters are determined on the basis of the temperature-dependent fractionation of the isotopes of oxygen the oxygen isotope composition of these materials reflects, therefore, the temperature at the time of their formation. Thus determining the isotope ratio between the stable isotopes of oxygen... 

Determining the temperature at which ancient sediments were formed entails two successive experimental stages (1) extracting the carbon dioxide from the sediments and (2) determining the relative amounts of the oxygen isotopes in the extracted carbon dioxide. Treating a sediment with a standard acid, such as hydrochloric or sulfuric acid, dissolves the calcium and magnesium carbonates in the sediments and... 

To determine how the degree of exchange between an extrinsic isotope and the intrinsic calcium in the food or meal is affected by the method of incorporating the isotope, the calcium source itself or foods fed with it. 

The observed calcium/phosphate ratio of 4.5 at the intercept of the calcium and phosphate retention curves that should minimize the sum of the urine calcium plus urine phosphate losses was difficult to believe in view of both the known Ca/P ratio of bone and the amounts we were adding to these solutions. This disparity between the optimal ratio determined experimentally and what we had assumed this ratio should be on the basis of known body composition is partially reconciled by the experiment of Sutton and Barltrop. They fed preterm infants stable Ca46 and observed that up to 20% of the isotope absorbed was subsequently excreted in the stool. Our infants also were undoubtedly having unmeasured calcium losses from the bile, pancreatic juice and succus entericus secreted into their intestine... 

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... 

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