Multiple Collector Inductively Coupled Plasma Mass Spectrometer Research Articles

Thorium isotopic measurements on silicate rock samples with a multi-collector inductively coupled plasma mass spectrometer

Chemical and mass spectrometric procedures for thorium isotopic analysis using a highly sensitive multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) equipped with a retardation lens were developed. They were applied to thorium isotopic measurements in volcanic rock samples. Rock samples were digested in screw-capped Savillex beakers with HF–HClO4. Thorium was then isolated from the rock matrix by a conventional anion-exchange procedure. Purified thorium was analyzed using an MC-ICP-MS (Micromass, IsoProbe) with a micro-concentric nebulizer (Cetac Aridus). The sensitivity for thorium was about 80 V ppm−1, which enabled us to obtain a strong enough ion current to measure 230Th/232Th ratios with a 10 ppb solution. With these procedures, we were able to reduce the amount of thorium required for precise isotopic analysis to <10 ng. Mass-fractionation correction and Daly/Faraday multiplier gain calibration were applied using natural uranium standard solution. The procedures were applied to standard rock samples whose thorium isotopic ratios had been reported using thermal ionization mass spectrometry (TIMS). Thorium isotopic compositions measured for JB-1 and AGV-1 gave coincident values with the TIMS values. In addition, the 230Th/232Th radioactivity ratios of the three standard rocks including JR-2, with eruption ages of older than 350 kyr, were in equilibrium with those of 238U/232Th, confirming the accuracy of our analysis.

Precise Determination of Cadmium, Indium and Tellurium Using Multiple Collector ICP‐MS

New sample preparation and ion‐exchange separation methods as well as instrumental measurement protocols were established for the determination of trace‐level Cd, In, and Te concentrations in geological materials by isotope‐dilution mass spectrometry. High precision isotope ratio measurements were performed with a multiple collector inductively coupled plasma‐mass spectrometer (MC‐ICP‐MS). The mass biases incurred for In and Te were corrected by adding and monitoring Pd and Sb standard solutions, respectively. Mass fractionation of Cd was corrected by using the mass fractionation factor calculated from the measurement of a standard solution. The measurement precision was better than 1 % for Cd, In and Te. Detection limits were &lt; 1 ng g‐1 for Cd, &lt; 0.02 ng g‐1 for In and Te. Using these new analytical techniques, the concentrations of Cd, In and Te were determined in six international geological reference materials. Concentrations could be reproduced within 3% for Cd, 4% for In and 10% for Te. Sample heterogeneity and volatility problems might have been the reason for the relatively large differences between Te replicates. Our results displayed excellent reproducibility compared with those of other techniques and agree well with data from previously published recommended values.

Determination of Cu and Mo Isotope Fractionations in Natural Materials Using MC-ICP-MS

Our aim is to determine the deviations (8) in the isotopic compositions of Cu and Mo between samples and standards, using the VG Plasma54 multiple-collector inductively-coupled plasma mass spectrometer (MC-ICP-MS) at the University of Rochester. MC-ICP-MS systems are well-suited to this research because they feature high ionization efficiency for all transition metals, simple sample introduction, simultaneous collection of multiple ion beams for precise determination of isotope ratios, and no fractionation of the sample reservoir during analysis (Halliday et al., 1995). As a result, the largest source of uncertainty in isotopic analyses by MC-ICP-MS is the variability of instrumental mass bias with time. We can correct for this effect if we take advantage of another unique characteristic of MC-ICP-MS: The minimal variation in instrumental mass bias among different elements over a limited mass range (Hirata, 1996). As a result, if samples and standards are 'spiked' with an element of similar mass to the element of interest, the magnitude of mass bias, ~, can be determined by comparing the measured isotopic composition of the spike element to the 'true' value. The measured composition of the element of interest is then corrected using cz and the appropriate mass fractionation law. This 'elemental spike' method, analogous to use of an isotopic 'double spike', has proven useful in precise and accurate determination of Pb isotopic compositions (Longerich et al., 1987; Hirata et al., 1996). For fractionation studies, the use of elemental spikes avoids some of the complexities of a double spike. It also facilitates isotopic analysis of elements with fewer than four stable isotopes. Significantly, 8 values do not depend on cc being identical for the element of interest (cci) and the spike element (%). Although differences in ~z produce inaccuracies in isotope ratio determinations, such errors cancel in the 8 notation as long as ~i/CCs is constant. Similarly, inaccuracy in the 'true' value of the spike element will not affect 8, as long as the isotopic composition of the spike is identical in samples and standards.

Isotopic variations of germanium in iron and stony iron meteorites

Germanium isotopic ratios for six iron meteorites of various groups, metallic phase of pallasite stony iron meteorites, three chemical reagents, and terrestrial germanite sample from Tsumeb mine, Namibia have been determined using a multiple collector-inductively coupled plasma mass spectrometer (MC-ICPMS). In this study, Ga-external correction technique, which enables us to detect the isotopic fractionation of the elements during the sample formation, was applied for the isotopic analysis of Ge. Resultant germanium isotopic data for terrestrial and meteoritic samples were found to have significant variations. Variations in germanium isotopic ratios for three chemical reagents and germanite samples were about 0.1–0.3% for 70Ge73Ge and 0.5% for 72Ge73Ge ratios, whereas germanium isotopic variations for meteoritic samples were about 4% for 70Ge73Ge and 3% for 72Ge73Ge ratios. Several points emerge from the germanium isotopic data. The first is that the germanium isotopic data for meteorites and terrestrial samples were found to define a straight line having slope of 13 on 70Ge73Ge vs. 72Ge73Ge diagram (Ge three isotopes plot). This indicates clearly that the meteoritic Ge had been isotopically fractionated during the sample formation. A second feature of the germanium isotopic data is that all the meteoritic Ge has systematically heavy isotopic compositions (low in 70Ge73Ge and 70Ge73Ge ratios) compared with those for terrestrial Ge. If the germanium isotopic ratios for germanite and chemical reagents reflect the germanium isotopic compositions for crustal materials of the Earth, this systematic deviations between meteorites and the terrestrial samples could be explained by the separation of crust from mantle. Alternatively heavy isotopic compositions for meteorites might be attributed to the larger evaporation loss of Ge from the meteoritic materials. Detailed mechanisms for the systematical difference in germanium isotopic ratios between meteorites and terrestrial materials will be given in this paper. The germanium isotopic data presented here demonstrate clearly that the isotopic fractionation data for medium- or heavy-mass range elements can give many clues to the origin and thermal evolution sequence of the metallic meteorites.