Calcium Isotope Ratios Research Articles

Optimisation of dynamic reaction cell (DRC)-ICP-MS for the determination of 42Ca/43Ca and 44Ca/43Ca isotope ratios in human urine

A new method for the measurement of calcium isotope ratios (42Ca/43Ca and 44Ca/43Ca) in urine using DRC-ICP-MS after oxalate precipitation is presented. The isotope ratios were measured with a precision of approximately 0.15% RSD, limited only by counting statistics in samples diluted to a total calcium concentration of 3–4 mg L−1. The observed mass bias was substantial, approximately 5–10% per u. Careful mass bias correction using matrix and concentration matched solutions was essential and necessary in order to obtain accurate results. A methane gas flow of 0.6 ml min−1 was used as a reaction gas in the dynamic reaction cell to remove polyatomic interferences (e.g., 40Ar+ and 40ArH2+). Unfortunately, other interferences are formed in the dynamic reaction cell from secondary reactions with methane adducts. An interfering ion observed at m/z = 43 with a precursor ion of m/z = 17 was removed by applying an Rpq setting of 0.4, while an interfering ion on m/z = 48 with a precursor ion of m/z = 37 could not be removed. Despite the fact that the 40Ar interference was successfully removed by the methane reaction gas, it was not possible to measure the 44Ca/40Ca and 42Ca/40Ca ratios with a precision only limited by counting statistics, most likely due to uncertainties from the detection system when measuring high count rates. The developed method was successfully applied for the calcium isotope ratio analysis of human urine samples enriched in 42Ca and 44Ca. It was found that an atom enrichment of 1.3% was needed in the urine samples in order to obtain reliable results.

The source of calcium in wet atmospheric deposits: Ca-Sr isotope evidence

Tracing the origin of calcium in wet atmospheric deposits is important for environmental studies since it is a dominant acid neutralising cation and an essential nutrient. Here we present the first calcium isotope ratio measurements for rain and snow. Samples from northeastern and southern locations (France, Switzerland, Luxembourg, California and Japan) suggest a carbonate origin. Only at a local scale in a forested watershed (Vosges mountains, France) the Ca isotopic compositions of rainwater indicate that the rainwater becomes admixed with throughfall solution, which is a mixing product of Ca from dissolved atmospheric dust particles and leave excretions. If this proves to be widely true, the contribution of Ca from the canopy could be very important for the neutralisation of acid rain and of the corresponding soil system in forested watersheds. In such cases Ca and Sr cycles are decoupled in the lower atmosphere. In contrast to Sr isotopes, Ca isotopes can be applied to tracing processes occurring in the biosphere.

Temperature dependence of $delta;7Li, $delta;44Ca and Li/Ca during growth of calcium carbonate

Lithium and calcium isotope ratios, together with the Li/Ca element ratio, have been measured in calcites precipitated inorganically at temperatures ranging from 5 to 30 °C, and on seasonal growth bands from a coral. These measurements indicate the potential uses of these three environmental proxies. Carbonate Li isotopes are lighter than the solution from which they grow, probably due to equilibrium fractionation. This fractionation is not significantly influenced by temperature and, because Li is not involved in any known biological process, this suggests that carbonates will provide a recorder for the past Li-isotope composition of natural waters and hence for past weathering conditions. As has been observed previously for inorganic aragonite, the Ca-isotopes in inorganic calcite are lighter than the growth solution, and are only weakly dependent on temperature. This confirms that the temperature dependence observed in some species of foraminifera [Geochem. Geophys. Geosyst. 1 (2001)] reflects additional isotope fractionation due to biological processes during mineralization. Ca isotopes may therefore provide a powerful tool with which to investigate the processes of biomineralization. Finally, Li/Ca ratios in inorganic and biogenic carbonates increase as temperatures decrease with a sensitivity of ≈4% per °C. This temperature dependence of Li/Ca may prove useful to assess past temperatures in cold environments such as the deep ocean.

Temperature dependence of δ7Li, δ44Ca and Li/Ca during growth of calcium carbonate

Lithium and calcium isotope ratios, together with the Li/Ca element ratio, have been measured in calcites precipitated inorganically at temperatures ranging from 5 to 30 °C, and on seasonal growth bands from a coral. These measurements indicate the potential uses of these three environmental proxies. Carbonate Li isotopes are lighter than the solution from which they grow, probably due to equilibrium fractionation. This fractionation is not significantly influenced by temperature and, because Li is not involved in any known biological process, this suggests that carbonates will provide a recorder for the past Li-isotope composition of natural waters and hence for past weathering conditions. As has been observed previously for inorganic aragonite, the Ca-isotopes in inorganic calcite are lighter than the growth solution, and are only weakly dependent on temperature. This confirms that the temperature dependence observed in some species of foraminifera [Geochem. Geophys. Geosyst. 1 (2001)] reflects additional isotope fractionation due to biological processes during mineralization. Ca isotopes may therefore provide a powerful tool with which to investigate the processes of biomineralization. Finally, Li/Ca ratios in inorganic and biogenic carbonates increase as temperatures decrease with a sensitivity of ≈4% per °C. This temperature dependence of Li/Ca may prove useful to assess past temperatures in cold environments such as the deep ocean.

Inductively coupled plasma mass spectrometric analysis of calcium isotopes in human serum: a low-sample-volume acid-equilibration method.

Analytical methods for measuring the calcium isotope distribution in enriched human serum samples that use low blood volumes, simple preparation methods, and rapid analysis are important in clinical studies of calcium kinetics. Previously, sample preparation by oxalate precipitation typically required 500 micro L of serum. This method was time-consuming, and the blood volume required was limiting in circumstances when only a small amount of serum could be obtained. Serum was collected from humans who were administered (42)Ca, and 20 micro L of serum was mixed with 2 mL of 0.22-0.67 mol/L HNO(3) at room temperature for between 1 min and 16 h. The (42)Ca/(43)Ca ratio in the supernatant was measured by a magnetic sector inductively coupled plasma mass spectrometer (ICP-MS). Calcium isotope ratios from these equilibration solutions were compared with data from oxalate-precipitated serum samples to determine the optimum equilibrium time and the effect of acid concentration on equilibrium. Various amounts of aggregated particles developed in different acid-serum mixtures. These affected the time required for isotope equilibration in the mixture. The shortest equilibrium time needed for the calcium isotopes varied from 1 to 6 h for samples acidified with 0.22-0.45 mol/L HNO(3). Data obtained from these solutions were consistent with data from oxalate-precipitated calcium. The precision of (42)Ca/(43)Ca ratio measurements was better than 0.5%. We have developed a simple, rapid sample preparation technique for ICP-MS analysis in which 20 micro L of serum can be used for accurate measurement of the calcium isotope distribution in a sample with good precision and a rapid analysis time.

The calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance

Calcium isotope ratios relative to seawater were studied in natural waters in order to constrain the Ca isotopic flux to the oceans and to discuss the oceanic Ca budget. This study includes the analysis of (1) Ca isotopes in continental waters at the regional scale of the Upper Rhine valley and (2) the determination of the δ44Ca values of some major rivers and hydrothermal vents. The data indicate that the global δ44Ca variability of the studied waters, with a maximal range of 1‰, is limited and must be linked to mass fractionation processes. At the scale of a small watershed (Aubure, Vosges, France), the δ44Ca river flux changes due to the variation of the proportion of waters resulting from chemical alteration of rocks and biologically fractionated soil solutions. Ca isotopes could therefore be an important tool to quantify the impact of the plants and the biosphere on the river water chemistry. At the regional scale of the Rhine valley as well as at the global scale of world rivers, the δ44Ca variability of river samples ranges from 0.5 to 1‰ without any relationship with the lithology of the watershed or the climate. Similarly, the δ44Ca value of hydrothermal vents is also uniform and close to the mean value of river waters. Consequently, the δ44Ca flux to the ocean should remain rather constant through time with a mean of −1.1±0.2‰. This value confirms that the Ca budget of the present-day seawater is in steady-state. This was not necessarily the case in the past. Non-steady-state periods should have been due to intensity variations of incoming and outcoming Ca fluxes rather than to the variations in their isotopic signatures.

Model for kinetic effects on calcium isotope fractionation (δ 44Ca) in inorganic aragonite and cultured planktonic foraminifera

The calcium isotope ratios (δ 44Ca = [( 44Ca/ 40Ca) sample/( 44Ca/ 40Ca) standard −1] · 1000) of Orbulina universa and of inorganically precipitated aragonite are positively correlated to temperature. The slopes of 0.019 and 0.015‰ °C −1, respectively, are a factor of 13 and 16 times smaller than the previously determined fractionation from a second foraminifera, Globigerinoides sacculifer, having a slope of about 0.24‰ °C −1. The observation that δ 44Ca is positively correlated to temperature is opposite in sign to the oxygen isotopic fractionation (δ 18O) in calcium carbonate (CaCO 3). These observations are explained by a model which considers that Ca 2+-ions forming ionic bonds are affected by kinetic fractionation only, whereas covalently bound atoms like oxygen are affected by kinetic and equilibrium fractionation. From thermodynamic consideration of kinetic isotope fractionation, it can be shown that the slope of the enrichment factor α(T) is mass-dependent. However, for O. universa and the inorganic precipitates, the calculated mass of about 520 ± 60 and 640 ± 70 amu (atomic mass units) is not compatible with the expected ion mass for 40Ca and 44Ca. To reconcile this discrepancy, we propose that Ca diffusion and δ 44Ca isotope fractionation at liquid/solid transitions involves Ca 2+-aquocomplexes (Ca[H 2O] n 2+ · mH 2O) rather than pure Ca 2+-ion diffusion. From our measurements we calculate that such a hypothesized Ca 2+-aquocomplex correlates to a hydration number of up to 25 water molecules (490 amu). For O. universa we propose that their biologically mediated Ca isotope fractionation resembles fractionation during inorganic precipitation of CaCO 3 in seawater. To explain the different Ca isotope fractionation in O. universa and in G. sacculifer, we suggest that the latter species actively dehydrates the Ca 2+-aquocomplex before calcification takes place. The very different temperature response of Ca isotopes in the two species suggests that the use of δ 44Ca as a temperature proxy will require careful study of species effects.

Precise and accurate determination of calcium isotope ratios in urine using HR-ICP-SFMS

Precise and accurate determination of calcium isotope ratios in urine is imperative in limiting the expense of enriched isotopes used in human metabolic tracer studies. The calcium isotope spectrum obtained from an ICP-MS (using conventional spray chamber), operated in low resolution mode (LR = 300), is subject to numerous polyatomic, isobaric and doubly charged interferences. The most severe polyatomic interfences can be resolved at resolutions greater than 3500, whereas, isobaric and doubly charged interferences are easily corrected. Precise (<0.1% 1σ) isotope ratios, however, are not routinely obtained in medium resolution (MR = 4300). Here we use a HR-ICP-MS in MR to identify and determine methods to reduce polyatomic interferences. Using oxalate precipitation of urinary calcium and desolvation sample introduction, interferences can be reduced sufficiently to allow determination of the 42, 43, 44, 46 and 48 isotopes of calcium with the precision of LR. Isotope ratio precision and accuracy is < ±0.1% (1σ) for analysis of three isotopes (42, 43, and 44), as required for clinical studies. Propagating this level of uncertainty indicates that >4% enrichments provide < ±10% (2σ) uncertainty in the calculated fractional calcium absorption values. Minimizing unnecessary enrichments can increase the number of subjects in studies without increasing the substantial cost of enriched isotopes. This new analytical method is applied to our on-going clinical trials where 0.017 mg kg−1 of 42Ca (90.8%; intravenous, IV) and 0.012 mg kg−143Ca (52.1%;oral) administered to postmenopausal women show average 24 h (pooled) urinary enrichments of 5.0% and 5.1% from the intravenous and oral doses, respectively.

Measurement of calcium isotopes ( [formula omitted]) using a multicollector TIMS technique

We propose a new “multicollector technique” for the thermal ionization mass spectrometer (TIMS) measurement of calcium (Ca) isotope ratios improving average internal statistical uncertainty of the 44 Ca/ 40 Ca measurements by a factor of 2–4 and average sample throughput relative to the commonly used “peak jumping method” by a factor of 3. Isobaric interferences with potassium ( 40 K + ) and titanium ( 48 Ti + ) or positively charged molecules like 24 Mg 19 F + , 25 Mg 19 F + , 24 Mg 16 O + and 27 Al 16 O + can either be corrected or are negligible. Similar, peak shape defects introduced by the large dispersion of the whole Ca isotope mass range from 40–48 atomic mass units (amu) do not influence Ca-isotope ratios. We use a 43 Ca/ 48 Ca double spike with an iterative double spike correction algorithm for precise isotope measurements.

Uncertainty calculation on the determination of calcium absorption in nutritional experiments with enriched stable isotopes and detection by double focusing sector field ICP-MS with a shielded torch

A new method for the measurement of calcium isotope ratios and total calcium, using a double focusing sector field inductively coupled plasma mass spectrometry (ICP-MS) instrument equipped with a shielded torch, is presented. The method was applied for the measurement of the 44Ca/43Ca, 42Ca/43Ca and 44Ca/42Ca ratios and total calcium by isotope dilution in urine samples from two different nutritional experiments set up to determine the calcium absorption from various foods. The isotope ratios were measured with a precision of 0.25%, 0.23% and 0.05% RSD for 44Ca/43Ca, 42Ca/43Ca and 44Ca/42Ca, respectively. The precision of the 44Ca/42Ca ratio is only limited by counting statistics, whereas the precision is limited by uncertainty from the needed background and interference corrections when the minor 43Ca isotope is used for analysis. Uncertainty calculations on the determination of calcium absorption in the nutritional experiments are presented. These calculations show that, for a double stable isotope procedure (administration of enriched stable 44Ca and 42Ca), the overall uncertainty is controlled by the precision of the ICP-MS measurement of the isotope ratios (44Ca/43Ca, 42Ca/43Ca), whereas, for a mixed radioisotope–stable isotope procedure (administration of enriched stable 44Ca and the 47Ca radioisotope), the overall uncertainty is mainly controlled by the uncertainty in the measurement of total calcium by isotope dilution ICP-MS. From these results, it was concluded that, when optimizing the performance of nutritional experiments, the emphasis should be placed on different parameters depending on the exact analysis involved. The results also show that uncertainty calculations are very useful in pinpointing the critical parameters of an experiment.

Measurements of calcium isotopes and isotope ratios: a new method based on helium plasma source “off-cone” sampling time-of-flight mass spectrometry

Operation of a helium-based low-power microwave plasma under “off-cone” sampling mode can lead to significant background suppression without sacrificing the analyte signal intensity. The nearly clean background spectrum obtained with “off-cone” sampling provides a unique approach for isotope ratio measurements, especially for low-mass elements that are often problematic. In this work, calcium isotopes and isotope ratios are quantitatively measured using “off-cone” sampling with microwave plasma time-of-flight mass spectrometry. The viability of measurements for all calcium isotopes, from the most abundant isotope (40Ca) to the least abundant isotope (46Ca), has been established for this technique. Several calcium isotope ratios are measured and calculated through both elemental calcium ion peaks and molecular calcium ion peaks. The relative standard deviations (RSD) for isotope ratios measured are in a range of 1.2 to 2.5% for commonly used major calcium isotopes. The absolute measurement errors are around 0.8% or less in most cases for major isotopes.

Calcium isotope fractionation between soft and mineralized tissues as a monitor of calcium use in vertebrates.

Calcium from bone and shell is isotopically lighter than calcium of soft tissue from the same organism and isotopically lighter than source (dietary) calcium. When measured as the (44)Ca/(40)Ca isotopic ratio, the total range of variation observed is 5.5 per thousand, and as much as 4 per thousand variation is found in a single organism. The observed intraorganismal calcium isotopic variations and the isotopic differences between tissues and diet indicate that isotopic fractionation occurs mainly as a result of mineralization. Soft tissue calcium becomes heavier or lighter than source calcium during periods when there is net gain or loss of mineral mass, respectively. These results suggest that variations of natural calcium isotope ratios in tissues may be useful for assessing the calcium and mineral balance of organisms without introducing isotopic tracers.

Isotope ratios of calcium determined in calcium-46 enriched samples from infants by automated multiple-collector thermal ionization mass spectrometry

A high-precision method was developed for automated multiple-collector determination of 46Ca enrichment using magnetic sector, thermal ionization mass spectrometry (TIMS). Calcium was separated from biological samples by precipitation as calcium oxalate. The calcium oxalate was washed, then dissolved in nitric acid, and isotope ratios were determined in 10 µg samples. The 44Ca, 46Ca and 48Ca isotopes were collected simultaneously. The 46Ca:48Ca ratios were measured and corrected for fractionation by iterative normalization, using the 44Ca:48Ca ratio, in order to achieve the required precision. Blocks of ten ratios were measured with an internal or within-run precision of 0.14% relative standard deviation (RSD) in urine samples and 0.10% RSD in faecal samples. The external or between-run precision for nine replicates was 0.07% RSD for urine and 0.09% RSD for faecal samples. Enrichment of samples collected following the feeding of 46Ca to pre-term infants ranged from 8 to 179 Δ% excess in faecal samples and from 19 to 91 Δ% excess in urine samples. If another isotope in addition to 46Ca is to be enriched, the identical analytical method can be applied by collecting 42Ca, 43Ca, 44Ca and 46Ca simultaneously, using the two unenriched isotopes to correct for fractionation.

Recent studies of human calcium metabolism using stable isotopic tracers

Stable isotopes of calcium are used safely as tracers for calcium in human populations ranging in age from infants to postmenopausal women. Thermal ionization mass spectrometry is used to measure calcium isotope ratios with relative accuracies of about 1% for natural abundance ratios at precisions of about 1% relative to the mean. Perturbations of natural abundance ratios are determined for the calcium in blood, urine, and feces with a limit of detection of about 2 delta % excess. The mathematical rationale for clinical studies of fractional absorption of dietary calcium and the kinetics of calcium's internal distribution are presented.

Precise age determinations and petrogenetic studies using the KCa method

High precision mass spectrometric determination of calcium isotope ratios allows the 40K → 40Ca radioactive decay to be used for dating a much broader range of geologic materials than is suggested by previous work. 40 Ca 42 Ca is used to monitor enrichments in 40Ca and can be measured to ±0.01% (2σ) using an exponential mass discrimination correction (Russell et al., 1978) and large ion currents. The earth's mantle has such a low K Ca (∼0.01) that it has retained “primordial” 40 Ca 42 Ca = 151.016 ± 0.011 (normalized to 42 Ca 44 Ca = 0.31221 ), as determined by measurements on two meteorites, pyroxene from an ultramafic nodule, metabasalt, and carbonatite. 40 Ca 42 Ca ratios can be conveniently expressed relative to this value as ϵ Ca in units of 10 −4. To test the method for age dating, a mineral isochron has been obtained on a sample of Pikes Peak granite, which has been shown to have concordant KAr, RbSr, and UPb ages. Plagioclase, K-feldspar, biotite, and whole rock yield an age of 1041 ± 32 m.y. (2σ) in agreement with previous age determinations ( λ K = 0.5543 b.y. −1, λ β− λ K = 0.8952 , 40K = 0.01167%). The initial 40 Ca 42 Ca of 151.024 ± 0.016 ( ϵ Ca = +0.5 ± 1.0), indicates that assimilation of high K/Ca crust was insufficient to affect calcium isotopes. Measurements on two-mica granite from eastern Nevada indicate that the magma sources had K/Ca ≈ 1, similar to intermediate-composition crustal rocks. These results show that the KCa system can be used as a precise geochromometer for common felsic igneous and metamorphic rocks, and may prove applicable to sedimentary rocks containing authigenic K minerals. The relatively short half-life of 40K, the non-volatile daughter, and the fact that potassium and calcium are stoichiometric constituents of many minerals, make the KCa system complementary to other dating methods, and potentially applicable to a variety of geologic problems.

Nomenclature des formes multiples des enzymes: Recommandations (1971)

Carbonate minerals constitute a major component of the sedimentary geological record and an archive of a fraction of the carbon and calcium cycled through the Earth's surface reservoirs for over three billion years. For calcium, carbonate minerals constitute the ultimate sink for almost all calcium liberated during continental and submarine weathering of silicate minerals. This study presents >500 stable isotope ratios of calcium in Precambrian carbonate sediments, both limestones and dolomites, in an attempt to characterize the isotope mass balance of the sedimentary carbonate reservoir through time. The mean of the dataset is indistinguishable from estimates of the calcium isotope ratio of bulk silicate Earth, consistent with the Urey cycle being the dominant mechanism exchanging calcium among surface reservoirs. The variability in bulk sediment calcium isotope ratios within each geological unit does not reflect changes in the global calcium cycle, but rather highlights the importance of local mineralogical and/or diagenetic effects in the carbonate record. This dataset demonstrates the potential for calcium isotope ratios to help assess these local effects, such as the former presence of aragonite, even in rocks with a history of neomorphism and recrystallization. Additionally, 29 calcium isotope measurements are presented from ODP (Ocean Drilling Program) Site 801 that contribute to the characterization of altered oceanic crust as an additional sink for calcium, and whose distinct isotopic signature places a limit on the importance of this subduction flux over Earth history.

Search for natural variations in calcium isotope abundances

The calcium isotope ratios of sèveral selected geological samples of different origin have been measured. No variations exceeding the standard deviation of the reference sample have been found. The differences in Ca-isotope ratios of the samples investigated are smaller than ± 1.5‰ pet mass unit.

Calcium isotope ratios in the homestead and pasamonte meteorites and a devonian limestone

Isotope abundance results obtained for “common” calcium (Devonian limestone) were, in atom per cent: 40 96.88 ± 0.05 42 0.655 ± 0.006 43 0.138 ± 0.002 44 2.12 ± 0.04 [46 0.0046 ± 0.0010] 48 0.200 ± 0.006 Calcium extracted from the Homestead, Iowa chondrite was isotopically identical to “common” calcium to within ±1 per cent, except for a better measurement of 46Ca/ 40Ca ratio of (33 ± 1) × 10 −6 . In contrast, 40Ca may be depleted by as much as 3 per cent in the Pasamonte achondrite, which reportedly has a Ca/K ratio of 150.