Zn Isotope Ratios Research Articles

Variation in the isotopic composition of zinc in the natural environment and the use of zinc isotopes in biogeosciences: a review

Zinc (Zn) is a trace element that is, as a building block in various enzymes, of vital importance for all living organisms. Zn concentrations are widely determined in dietary, biological and environmental studies. Recent papers report on the first efforts to use stable Zn isotopes in environmental studies, and initial results point to significant Zn isotope fractionation during various biological and chemical processes, and thus highlight their potential as valuable biogeochemical tracers. In this article, we discuss the state-of-the-art analytical methods for isotopic analysis of Zn and the procedures used to obtain accurate Zn isotope ratio results. We then review recent applications of Zn isotope measurements in environmental and life sciences, emphasizing the mechanisms and causes responsible for observed natural variation in the isotopic composition of Zn. We first discuss the Zn isotope variability in extraterrestrial and geological samples. We then focus on biological processes inducing Zn isotope fractionation in plants, animals and humans, and we assess the potential of Zn isotope ratio determination for elucidating sources of atmospheric particles and contamination. Finally, we discuss possible impediments and limitations of the application of Zn isotopes in (geo-) environmental studies and provide an outlook regarding future directions of Zn isotope research.

Mass Discrimination During MC-ICPMS Isotopic Ratio Measurements: Investigation by Means of Synthetic Isotopic Mixtures (IRMM-007 Series) and Application to the Calibration of Natural-Like Zinc Materials (Including IRMM-3702 and IRMM-651)

A long known way of anchoring isotope ratio values to the SI system is by means of gravimetrically prepared isotopic mixtures. Thermal ionization mass spectrometry (TIMS) is the traditionally associated measurement technique, but multi-collector double focusing inductively coupled plasma (MC-ICP)-MS now appears to be an attractive alternative. This absolute calibration strategy necessitates that mass discrimination effects remain invariant in time and across the range of isotope ratios measured. It is not the case with MC-ICPMS and the present work illustrates, in the case of Zn isotopic measurements carried out using locally produced synthetic Zn isotope mixtures (IRMM-007 series), how this calibration strategy must be adjusted. First, variation in mass discrimination effects across the measurement sequence is propagated as an uncertainty component. Second, linear proportionality during each individual measurement between normalized mass discrimination and the average mass of the isotope ratios is used to evaluate mass discrimination for the ratios involving low abundance isotopes. Third, linear proportionality between mass discrimination and the logarithm of the isotope ratio values for n(67Zn)/n(64Zn) and n(68Zn)/n(64Zn) in the mixtures is used iteratively to evaluate mass discrimination for the same ratios in the isotopically enriched materials. Fourth, ratios in natural-like materials (including IRMM-3702 and IRMM-651) are calibrated by external bracketing using the isotopic mixtures. The relative expanded uncertainty (k = 2) estimated for n(68Zn)/n(64Zn) and n(67Zn)/n(64Zn) ratio values in the synthetic isotopic mixtures and the natural-like zinc samples was in the range of 0.034 to 0.048%. The uncertainty on the weighing (0.01%, k = 1) was the largest contributor to these budgets. The agreement between these results and those obtained with a single detector TIMS and with another MC-ICPMS further validated this work. The absolute isotope ratio values found for IRMM-3702-material also proposed as "delta 0" for delta-scale isotopic measurements-are n(66Zn)/n(64Zn) = 0.56397 (30), n(67Zn)/n(64Zn) = 0.082166 (35), n(68Zn)/n(64Zn) = 0.37519 (16), and n(70Zn)/n(64Zn) = 0.012418 (23). The derived Zn atomic weight value Ar(Zn) = 65.37777 (22) differs significantly from the current IUPAC value by Chang et al. [1]. Remeasurement, with isotopic mixtures from the IRMM-007 series, of the Zn isotope ratios in the same Chang et al. [1] material have revealed large systematic differences (1.35 (27)% per atomic mass unit) that suggest unrecognized measurement biases in their results.

Zn and Cu isotopic variability in the Alexandrinka volcanic-hosted massive sulphide (VHMS) ore deposit, Urals, Russia

Copper and Zn isotope ratios of well-characterized samples from three ore facies in the Devonian Alexandrinka volcanic-hosted massive sulphide (VHMS) deposit, southern Urals, were measured using multi collector ICP-MS (MC-ICP-MS) and show variations linked to depositional environment and mineralogy. The samples analysed derived from: a) hydrothermal–metasomatic vein stockwork, b) a hydrothermal vent chimney, and c) reworked clastic sulphides. As the deposit has not been significantly deformed or metamorphosed after its formation, it represents a pristine example of ancient seafloor mineralization. Variations in δ 65Cu (where δ 65Cu = [( 65Cu / 63Cu) sample / ( 65Cu / 63Cu) standard − 1] * 1000) and δ 66Zn (where δ 66Zn = [( 66Zn / 64Zn) sample / ( 66Zn / 64Zn) standard − 1] * 1000) of 0.63 and 0.66‰, respectively, are significantly greater than analytical uncertainty for both isotope ratios (± 0.07‰, 2 σ). Very limited isotopic fractionation is observed in primary Cu minerals from the stockwork and chimney, whereas the Zn isotopic composition of the stockwork varies significantly with the mineralogy. Chalcopyrite-bearing samples from the stockwork have lighter δ 66Zn by ∼ 0.4‰ relative to sphalerite dominated samples, which may be due to equilibrium partitioning of isotopically light Zn into chalcopyrite during its precipitation. δ 66Zn also showed significant variation in the chimney, with an enrichment in heavy isotopes toward the chimney rim of ∼ 0.26‰, which may be caused by changing temperature (hence fractionation factor), or Raleigh distillation. Post-depositional seafloor oxidative dissolution and re-precipitation in the clastic sediments, possibly coupled with leaching, led to systematic negative shifts in Cu and Zn isotope compositions relative to the primary sulphides. Copper shows the most pronounced fractionation, consistent with the reduction of Cu(II) to Cu(I) during supergene mineralization. However, the restricted range in δ 65Cu is unlike modern sulphides at mid oceanic ridges where a large range of Cu isotope, of up to 3‰ has been observed [ Rouxel, O., Fouquet, Y., Ludden, J.N., 2004. Copper isotope systematics of the Lucky Strike, Rainbow, and Logatchev sea-floor hydrothermal fields on the Mid-Atlantic Ridge. Econ. Geol. 99, 585–600; Zhu, X.K., O'Nions, R.K., Guo, Y., Belshaw, N.S., Rickard, D., 2000. Determination of natural Cu-isotope variation by plasma source mass spectrometry: implications for use as geochemical tracers. Chem. Geol. 163, 139–149].

Precise Zn Isotopic Ratio Measurements of Human Red Blood Cell and Hair Samples by Multiple Collector-ICP-Mass Spectrometry

Precise 66Zn/64Zn and 68Zn/64Zn isotopic ratios of biochemical samples have been measured using multiple collector-ICP-mass spectrometry (MC-ICPMS). In order to eliminate the mass spectrometric interferences on Zn isotopes (e.g., 64Ni+ and 136Ba2+), we chemically purified the analyte using an ion chromatographic technique. The resulting precisions of the 66Zn/64Zn and 68Zn/64Zn ratio measurements were 0.05/1000 and 0.10/1000 (2SD), respectively, which were enough to detect the isotopic variation of Zn in nature. Red blood cell (RBC) samples were collected from five volunteers (four males and one female), including a series of 12 RBC samples from one person through monthly-based sampling over a year. These were analyzed to test possible seasonal changes and variations in 66Zn/64Zn and 68Zn/64Zn ratios among the individuals. The 66Zn/64Zn and 68Zn/64Zn ratios for a series of 12 RBC samples collected over a year were 0.43/1000 and 0.83/1000 higher than the values of highly purified Zn metal (JMC Zn), and no seasonal change could be found. The 66Zn/64Zn and 68Zn/64Zn ratios for RBC samples collected from five volunteers did not vary significantly. In order to investigate Zn isotopic heterogeneity in a human body, Zn isotopic ratios of a hair sample collected from one of the volunteers was also analyzed. The 66Zn/64Zn and 68Zn/64Zn ratios for the hair sample were 0.59/1000 and 1.14/1000 lower than the mean value of RBC samples. This result demonstrates that detectable isotopic fractionation occurs in the human body. The data obtained here suggest that the isotopic ratios of trace metals could provide new information about transportation of metal elements in vivo.

Performance of diffusive gradients in thin films for measurement of the isotopic composition of soluble Zn

The application of diffusive gradients in thin films (DGT) samplers for measurements of the isotopic composition of soluble Zn by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) was tested under laboratory conditions. The effect of diffusion in hydrogels as a possible source of fractionation was investigated by measuring Zn isotope ratios for different fractions of Zn accumulated by DGT and comparing with the bulk isotope composition of the solution. An important finding of this study is that, provided quantitative elution is obtained, no fractionation of Zn isotopes due to the diffusion process is detectable within the reported precision of MC-ICPMS measurements. Consequently, DGT samplers are suitable for studies of the Zn isotopic composition in natural waters.

Comparison of three different instrumental approaches to the determination of iron and zinc isotope ratios in clinical samples

Stable isotopes can be used to investigate the absorption of nutritionally important elements in humans. Although thermal ionisation mass spectrometry (TIMS) is the benchmark technique for accuracy and precision in isotope ratio measurement, the lengthy sample preparation procedures and analysis times required can present significant disadvantages. Inductively coupled plasma mass spectrometry (ICP-MS) is now a well-established technique for isotopic determinations and can be used following only a simple sample preparation (microwave-assisted digestion followed by a dilution step). The advantage of this is that it can provide comparable data to that from TIMS, but in a significantly shorter analysis time. This study assesses the accuracy and precision of two ICP-MS instruments for the determination of Fe and Zn isotope ratios in faecal samples by comparison with TIMS data. Using standard statistical techniques, no significant differences were found between data obtained from TIMS, double-focussing ICP-MS or a single-focussing ICP-MS equipped with a collision cell.

Analysis of undigested honey samples by isotope dilution inductively coupled plasma mass spectrometry with direct injection nebulization (ID-ICP-MS)

Undigested honey samples were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) using direct injection nebulization (DIN). Samples were diluted 25-fold using dilute HNO 3 to which Triton X-100 was added. Isotope dilution (ID) was used to quantify Ba, Cu, Pb and Zn in these natural matrix samples. DIN parameters were optimized to obtain a stable transient signal enough to determine multiple isotope ratios. The Ba, Cu and Zn isotope ratios were measured in a single sample injection of 50 μl diluted honey and the Pb isotopes in a separate injection. The analysis was characterized by precisions given by R.S.D. values from 0.20 to 1.24% for Ba, 0.49–1.23% for Cu, 0.61–2.28% for Pb and 0.50–1.51% for Zn. The results were in agreement, at the 95% confidence level, with those obtained by analyzing digested samples using external calibration. Using this direct method, 30 real honey samples per hour could be analyzed.

Correction of mass bias drift in inductively coupled plasma mass spectrometry measurements of zinc isotope ratios using gallium as an isotope ratio internal standard

Gallium was used as an isotope ratio internal standard to correct the mass-bias drift of an inductively coupled plasma mass spectrometer employed for zinc isotope ratio determinations. The Zn isotope ratios were measured to establish dietary Zn absorption in nutritional studies using 67Zn, 68Zn and 70Zn as tracers and 66Zn as the reference isotope. Natural abundance Zn standards were analysed before and after each group of five samples. All samples and standards were adjusted to a common Zn concentration and spiked with Ga to obtain a 69Ga intensity roughly equal to that of 66Zn. The measured 71Ga:69Ga ratios served to monitor and correct for changes of the instrumental mass bias. Adding the two Ga isotopes to the data acquisition programme increased the cycle time of the mass spectrometer by only 25% and had no discernible effect on the precision of individual Zn isotope ratio determinations. Three different methods for correcting the measured Zn isotope ratios for mass-bias drift were compared: (i) simple division of the Zn isotope ratios by the Ga isotope ratios; (ii) correction based on a power law; and (iii) correction by a regression method which uses the relationships between the temporal changes of the 71Ga:69Ga ratio and the three Zn isotope ratios for each analytical batch. The correction based on regression consistently gave the best results because it avoided undercorrection or overcorrection of mass-bias drift; even when there was little or no correlation between the drift of the Zn and Ga isotope ratios, the range of variation of the corrected data was not increased. On the average, the regression correction method reduced the drift of the Zn isotope ratios by a factor of 2.5. The study included the derivation of equations which can be used to predict the achievable improvement in precision for the simple division and regression correction methods directly from the measured data, i.e., without actually applying the corrections.

Determination of cadmium and zinc isotope ratios in sheep's blood and organ tissue by electrothermal vaporization inductively coupled plasma mass spectrometry. Invited lecture

A method is described for the determination of Cd and Zn isotope ratios in sheep's blood and organ tissue. Samples were digested with nitric acid using a microwave oven. Cadmium and Zn were separated from matrix components using adsorption chromatography prior to isotope ratio measurement by electrothermal vaporization inductively coupled plasma mass spectrometry. A concentration factor of 35 was achieved. Limits of detection for the determination of Cd and Zn in blood were 0.34 and 0.40 pg g–1, respectively. Cadmium isotope ratios (111Cd: 106Cd; 111Cd: 110Cd) were determined with a precision of 2–3% for both peak height and area count measurements. Zinc isotope ratios (68Zn: 67Zn; 68Zn: 66Zn) were determined with a precision of 2% for peak height measurements and 1% for peak area count measurements.