Lead Isotope Homogeneity of NIST SRM 610 and 612 Glass Reference Materials: Constraints from Laser Ablation Multicollector ICP-MS (LA-MC-ICP-MS) Analysis

This contribution presents data for laser ablation multicollector ICP‐MS (LA‐MC‐ICP‐MS) analyses of NIST SRM 610 and 612 glasses with the express purpose of examining the Pb isotope homogeneity of these glasses at the ～ 100 μm spatial scale, relevant to in situ analysis. Investigation of homogeneity at these scales is important as these glasses are widely used as calibrators for in situ measurements of Pb isotope composition. Results showed that at the levels of analytical uncertainty obtained, there was no discernable heterogeneity in Pb isotope composition of NIST SRM 610 and also most probably for NIST SRM 612. Traverses across the ～ 1.5 mm glass wafers supplied by NIST, consisting of between 75 and 133 individual measurements, showed no compositional outliers at the two standard deviation level beyond those expected from population statistics. Overall, the measured Pb isotope ratios from individual traverses across NIST SRM 610 and 612 wafers closely approximate single normally‐distributed populations, with standard deviations similar to the average internal uncertainty for individual measurement blocks. Further, Pb isotope ratios do not correlate with Tl/Pb ratios measured during the analysis, suggesting that regions of volatile element depletion (marked by low Tl/Pb) in these glasses are not associated with changes in Pb isotope composition. For NIST SRM 610 there also appeared to be no variation in Pb isotope composition related to incomplete mixing of glass base and trace element spike during manufacture. For NIST SRM 612 there was some dispersion of measured ratios, including some in a direction parallel to the expected mixing line for base‐spike mixing. However, there was no significant correlation parallel to the mixing line. At this time this cannot be unequivocally demonstrated to result from glass heterogeneity, but it is suggested that NIST SRM 610 be preferred for standardising in situ Pb isotope measurements. Data from this study also showed significantly better accuracy and somewhat better precision for ratios corrected for mass bias by external normalisation to Pb isotope ratios measured in bracketing calibrators compared to mass bias corrected via internal normalisation to measured ²⁰⁵Tl/²⁰³Tl, although the Tl isotopic composition of both glasses appears to be homogeneous.     

Compositional Heterogeneity in NIST SRM 610-617 Glasses

Extensive compositional heterogeneity is shown to affect at least twenty four of the doped trace elements in the NIST SRM 610-617 glasses.
Compositional profiling and mapping using laser ablation ICP-MS reveals that all NIST SRM 610-617 wafers examined here contain domains that are significantly depleted in Ag, As, Au, B, Bi, Cd, Cr, Cs, Mo, Pb, Re, (Rh), Sb, Se, Te, Tl and W, and antithetically enriched in Cu (and Pt), with large enrichments in Cd, Fe and Mn also being encountered in some cases. These domains are visible in doubly polished wafers by unaided visual inspection and by transmitted light and schlieren microscopy. They occur in close proximity to the wafer perimeters and also as stretched and complexly folded forms within wafer interiors. The chemical and optical properties of these heterogeneous domains are consistent with those of compositional cords, a phenomenon of glass manufacture where glass bulk composition and physical properties are modified by loss of volatile components from the molten glass surface. The NIST SRM 610-617 glasses may be considered reliable reference materials for microanalysis of only between one half and two thirds of the trace elements with which they were doped, including Be, Mg, Sr, Ba, Sc, Y, REE, V, Zr, Hf, Nb, Ta, Th, U, Ga, In, Sn, Co, Ni and Zn. These elements show no evidence of significant heterogeneity, indicating that the original glass constituents and possible residues remaining in the furnace from preceding glass batch fusions were well homogenised during manufacture.     

New ID-TIMS, ICP-MS and SIMS Data on the Trace Element Composition and Homogeneity of NIST Certified Reference Material SRM 610-611

We present new concentration data for twenty four lithophile trace elements in NIST certified reference material glasses SRM 610-SRM 611 in support of their use in microanalytical techniques. The data were obtained by solution ICP-MS and isotope dilution TIMS analysis of two different sample wafers. An overall assessment of these new results, also taking into account ion probe studies that have been published in the literature, shows that these wafers can be considered to be homogeneous. Therefore, individually analysed wafers are believed to be representative of the entire batch of the SRM 610-611 glasses. Possible exceptions are the alkali metals (and a few volatile or non-lithophile trace elements). The analysed concentrations range between 370 μg g⁻¹ (Cs) and 500 μg g⁻¹ (Sr) and agree well with published values. On the basis of our new data and data recently published in the literature we propose "preferred average" values for the elements studied. These values are, within a few percent, identical to those proposed by other workers.     

Isotope Dilution MC-ICP-MS Rare Earth Element Analysis of Geochemical Reference Materials NIST SRM 610, NIST SRM 612, NIST SRM 614, BHVO-2G, BHVO-2, BCR-2G, JB-2, WS-E, W-2, AGV-1 and AGV-2

We present data for the concentrations of eleven rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er, Yb, Lu) in eleven international geochemical reference materials obtained by isotope dilution multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). We have analysed both rock powders and synthetic silicate glasses, and the latter provide precise data to support the use of these as reference materials for in situ trace element determination techniques. Our data also provide precise measurements of the abundance of mono-isotopic Pr in both glasses and powders, which allows more accurate constraints on the anomalous redox-related behaviour of Ce during geochemical processes. All materials were analysed in replicate providing data that typically reproduce to better than one percent. Sm/Nd ratios in all these materials also reproduce to better than 0.2% and are accurate to < 0.2% and can thus be used as calibrants for Sm-Nd geochronology. Our analyses agree well with existing data on these reference materials. In particular, for NIST SRM 610, USGS BHVO-2, AGV-1 and AGV-2, our measured REE abundances are typically within < 2% (and mostly 1%) of REE concentrations previously determined by isotope dilution analysis and thermal ionisation mass spectrometry, consistent with the higher degree of precision and accuracy obtained from isotope dilution techniques. Close agreement of results between basaltic glass reference materials USGS BHVO-2G and BCR-2G and the BHVO-2 and BCR-2 powders from which they were created suggests that little fractionation, concentration or dilution of REE contents occurred during glass manufacture.     

Isotope Dilution Determinations of Lu,Hf, Zr,Ta and W,and Hf Isotope Compositions of NIST SRM 610 and 612 Glass Wafers

Isotope dilution determinations of Lu, Hf, Zr, Ta and W are reported for nine test portions (five for W) of NIST SRM 610 and 612 glass wafers. Additionally, all test portions were analysed for their Hf isotope compositions. In general, high field strength elemental (HFSE) distributions in NIST SRM 610 and 612 were reproducible to ～± 1%, except for Zr (± 5%) in NIST SRM 612, and absolute reported concentrations agreed with previously published values, but with higher precision. The slightly worse reproducibility of Zr in NIST SRM 612 compared to other HFSE is interpreted to result from analytical scatter, rather than sample inhomogeneity. The analyses demonstrated elemental homogeneity for both glass wafers for samples of 1–2 mg with respect to the precision of the method, i.e., ± 1% or better. Average Hf isotope compositions for both glass wafers agreed within uncertainty and the weighted average of all determinations yielded a mean ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.282111 ± 0.000009 (95% confidence level). However, although mean values for NIST SRM 610 and 612 agreed within analytical limits, NIST SRM 610 test portions showed a tendency of systematically elevated isotope composition of ～ 0.5 _?Hf units when compared to NIST SRM 612, which may indicate a slightly more radiogenic Hf isotope composition of NIST SRM 610. The results of this study suggest that NIST SRM 610 and 612 are valuable calibrators for HFSE in situ analyses within the given uncertainties.     

Strontium, Neodymium and Lead Isotope Analyses of NIST Glass Certified Reference Materials: SRM 610, 612, 614

The NIST glass certified reference materials, SRM 610-617, have been widely adopted by the geological community as calibration samples for a variety of in situ trace element analytical techniques. There is now an urgent requirement for similar reference materials for in situ isotopic analytical techniques. We have analysed SRM 610, 612 and 614 for their Pb, Sr and Nd isotopic compositions using thermal ionisation mass spectrometry. Large differences in isotopic composition were observed between each CRM, suggesting a significant trace element content in the initial starting material (base glass). As a result, isotopic compositions for one CRM cannot be extrapolated to another, and each must be calibrated for use independently. We present the first compilation of working values for these glasses.     

Determination of Forty Two Major and Trace Elements in USGS and NIST SRM Glasses by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry

Forty two major (Na, Mg, Ti and Mn) and trace elements covering the mass range from Li to U in three USGS basalt glass reference materials BCR‐2G, BHVO‐2G and BIR‐1G were determined using laser ablation‐inductively coupled plasma‐mass spectrometry. Calibration was performed using NIST SRM 610 in conjunction with internal standardisation using Ca. Determinations were also made on NIST SRM 612 and 614 as well as NIST SRM 610 as unknown samples, and included forty five major (Al and Na) and trace elements. Relative standard deviation (RSD) of determinations was below 10% for most elements in all the glasses under investigation. Consistent exceptions were Sn and Sb in BCR‐2G, BHVO‐2G and BIR‐1G. For BCR‐2G, BHVO‐2G and BIR‐1G, clear negative correlations on a logarithmic scale exist between RSD and concentration for elements lower than 1500 μg g^‐1 with logarithmic correlation coefficients between ‐0.75 and ‐0.86. There is also a clear trend of increasing RSD with decreasing concentration from NIST SRM 610 through SRM 612 to SRM 614. These suggest that the difference in the scatter of apparent element concentrations is not due to chemical heterogeneity but reflects analytical uncertainty. It is concluded that all these glasses are, overall, homogeneous on a scale of 60 μm. Our first results on BHVO‐2G and BIR‐1G showed that they generally agreed with BHVO‐2/BHVO‐1 and BIR‐1 within 10% relative. Exceptions were Nb, Ta and Pb in BHVO‐2G, which were 14‐45% lower than reference values for BHVO‐2 and BHVO‐1. Be, Ni, Zn, Y, Zr, Nb, Sn, Sb, Gd, Tb, Er, Pb and U in BIR‐1G were also exceptions. However, of these elements, Be, Nb, Sn, Sb, Gd, Tb, Pb and U gave results that were consistent within an uncertainty of 2s between our data and BIR‐1 reference values. Results on NIST SRM 612 agreed well with published data, except for Mg and Sn. This was also true for elements with m/z 85 (Rb) in the case of NIST SRM 614. The good agreement between measured and reference values for Na and Mg in BCR‐2G, BHVO‐2G and BIR‐1G, and for Al and Na in NIST SRM 610, 612 and 614 up to concentrations of at least several weight percent (which were possible to analyse due to the dynamic range of 10⁸) indicates the suitability of this technique for major, minor and trace element determinations.     

Results for Rarely Determined Elements in MPI-DING, USGS and NIST SRM Glasses Using Laser Ablation ICP-MS

New analytical results are reported for rarely determined elements Be, B, Ge, As, Mo, Rh, Pd, Ag, Cd, In, Sn, Sb, W, Re, Ir, Pt, Au, Tl and Bi in MPI‐DING and USGS (BCR‐2G, BHVO‐2G, BIR‐1G) silicate glasses and the NIST SRM 610‐614 synthetic soda‐lime glasses using 193 nm ArF excimer laser ablation and quadrupole ICP‐MS. The method used involved external calibration against GOR132‐G for Ir and NIST SRM 610 for other elements, internal standardisation using Ca, and ablation with a crater diameter of 160 μm and a pulsed laser repetition rate of 10 Hz. Small amounts of nitrogen (5 ml min^?1) were added to the central channel gas of the plasma to improve the limits of detection for most of these elements by a factor of 1.2–2.5 and to reduce the oxide interference level to 0.02% (ThO⁺/Th⁺). Under these conditions, the LODs for most of these rarely determined elements were within the range 0.1 to 10 ng g^?1. The operating conditions that were required to minimise ICP‐induced fractionation (U⁺/Th⁺≈ 1) in the mode without nitrogen were accompanied by a 50–60% reduction in sensitivity for elements such as Ca, Au and Pt. In contrast, ICP‐induced fractionation could be minimised (U⁺/Th⁺≈ 1) with no loss of analyte sensitivity in the nitrogen mode. Interferences of CuAr⁺, ZnAr⁺, Cd⁺, Pb²⁺ and Sn⁺ on Pd⁺, Rh⁺, Cd⁺ and In⁺ were corrected. Oxide interferences were not considered due to their lower production rate. Analytical precision, as given by one relative standard deviation (% RSD) was less than 15% for most of the elements present at concentrations greater than 0.1 μg g^?1. A significant negative correlation was found between logarithmic concentration and logarithmic RSD, with a correlation coefficient of ?0.76. This trend indicates that possible chemical heterogeneities for most of these elements are smaller than the analytical uncertainty. Our results for Be, B, Ge, Sb and W are generally in good agreement with their reference values. In contrast, other elements in many of the reference glasses have only information values, upper limits or even no values, which restrict any detailed evaluation of the accuracy of the determined values. However, concentrations from multiple isotopes of one element analysed in this study showed excellent agreement, which guarantee the quality of our data to a certain extent.     

Boron and Oxygen Isotope Composition of Certified Reference Materials NIST SRM 610/612 and Reference Materials JB-2 and JR-2

We present data on the concentration, the isotope composition and the homogeneity of boron in NIST silicate glass reference materials SRM 610 and SRM 612, and in powders and glasses of geological reference materials JB-2 (basalt) and JR-2 (rhyolite). Our data are intended to serve as references for both microanalytical and wet-chemical techniques. The δ¹¹ B compositions determined by N-TIMS and P-TIMS agree within 0.5% and compare with SIMS data within 2.5%. SIMS profiles demonstrate boron isotope homogeneity to better than δ¹¹ B = 2% for both NIST glasses, however a slight boron depletion was detected towards the outermost 200 μm of the rim of each sample wafer. The boron isotope compositions of SRM 610 and SRM 612 were indistinguishable. Glasses produced in this study by fusing JB-2 and JR-2 powder also showed good boron isotope homogeneity, both within and between different glass fragments. Their major element abundance as well as boron isotope compositions and concentrations were identical to those of the starting composition. Hence, reference materials (glasses) for the in situ measurement of boron isotopes can be produced from already well-studied volcanic samples without significant isotope fractionation. Oxygen isotope ratios, both within and between wafers, of NIST reference glasses SRM 610 and SRM 612 are uniform. In contrast to boron, significant differences in oxygen isotope compositions were found between the two glasses, which may be due to the different amounts of trace element oxides added at ten-fold different concentration levels to the silicate matrix.     

δ98/95Mo values and Molybdenum Concentration Data for NIST SRM 610, 612 and 3134: Towards a Common Protocol for Reporting Mo Data

Molybdenum concentration and δ^98/95Mo values for NIST SRM 610 and 612 (solid glass), NIST SRM 3134 (lot 891307; liquid) and IAPSO seawater reference material are presented based on comparative measurements by MC‐ICP‐MS performed in laboratories at the Universities of Bern and Oxford. NIST SRM 3134 and NIST SRM 610 and 612 were found to have identical and homogeneous ⁹⁸Mo/⁹⁵Mo ratios at a test portion mass of 0.02 g. We suggest, therefore, that NIST SRM 3134 should be used as reference for the δ–Mo notation and to employ NIST SRM 610 or 612 as solid silicate secondary measurement standards, in the absence of an isotopically homogeneous solid geological reference material for Mo. The δ^98/95Mo_{JMC Bern} composition (Johnson Matthey ICP standard solution, lot 602332B as reference) of NIST SRM 3134 was 0.25 ± 0.09‰ (2s). Based on five new values, we determined more precisely the mean open ocean δ^98/95Mo_{SRM 3134} value of 2.09 ± 0.07‰, which equals the value of δ^98/95Mo_{JMC Bern} of 2.34 ± 0.07‰. We also refined the Mo concentration data for NIST SRM 610 to 412 ± 9 μg g^?1 (2s) and NIST SRM 612 to 6.4 ± 0.7 μg g^?1 by isotope dilution. We propose these concentration data as new working values, which allow for more accurate in situ Mo determination using laser ablation ICP‐MS or SIMS.     

Determination of Selenium Concentrations in NIST SRM 610, 612, 614 and Geological Glass Reference Materials Using the Electron Probe, LA-ICP-MS and SHRIMP II

A combination of EMPA, sensitive high resolution ion microprobe (SHRIMP II) and/or LA-ICP-MS techniques was used to measure the concentration of selenium (Se) in NIST SRM 610, 612, 614 and a range of reference materials. Our new compiled value for the concentration of Se in NIST SRM 610 is 112 ± 2 μg g⁻¹. The concentration of Se in NIST SRM 612, using NIST SRM 610 for calibration, determined using LA-ICP-MS (confirmed using SHRIMP II) was 15.2 ± 0.2 μg g⁻¹. The concentration of Se in NIST SRM 614, using LA-ICP-MS was 0.394 ± 0.012 μg g⁻¹. LA-ICP-MS determination of Se in synthetic geological glasses BCR-2G, BIR-1G, TB-1G and the MPI-DING glasses showed a range in concentrations from 0.062 to 0.168 μg g⁻¹. Selenium in the natural glass, VG2, was 0.204 ± 0.028 μg g⁻¹.     

Fitness-for-Purpose of Reference Material Reference Values in Relation to Traceability of Measurement, as Illustrated by USGS BCR-1, NIST SRM 610 and IAEA NBS28

As in all fields of sample analysis, reference materials play a large role in supporting measurements in the geosciences. While a rather large number of materials are in distribution (> 380), not all are equally effective or fit-for-purpose in supporting laboratory data quality and thereby assuring the desired comparability of measurements between laboratories. Equally important, reference values that are not fit-for-purpose cannot be used effectively to establish traceability links between laboratory measurements and national and international standards. The needed fitness-for-purpose is not achieved for reference values either when more than one reference value has been proposed and a consensus does not exist among users as to which should be used by all, or when reference value uncertainties are too large in comparison to those of routine laboratory measurements. The focus of this review will be, first to outline the current reality, and second to suggest ways in which certifications of RMs can be improved to provide reference values that are universally accepted and more fit-for-purpose in general laboratory use. The discussion will be illustrated largely by current uses of USGS BCR-1, NIST SRM 610 and IAEA NBS28, as these three materials are those for which the largest body of newly published data exists, according to recent bibliographies of the geoanalytical literature published annually in Geostandards Newsletter: The Journal of Geostandards and Geoanalysis.     

Investigation of Thulium and Other Rare Earth Element Mass Fractions in NIST SRM 1632a Bituminous Coal Reference Material by Quadrupole ICP‐MS

Inductively coupled plasma‐mass spectrometry after lithium metaborate fusion and digestion was used to measure the rare earth element (REE) mass fractions of several reference materials including NIST SRM 1632a, a historical bituminous Pennsylvania seam coal. While most of the REE mass fractions measured in this study were consistent with the published consensus data, the measured mass fraction of thulium for NIST SRM 1632a was consistently lower compared with the published data. Chondrite normalisation of the published consensus data for NIST SRM 1632a produced a positive thulium anomaly (Tm = 1.78), which is inconsistent with a terrestrial source of sediment. Normalisation of REE mass fractions collected in this study produced no significant Tm anomaly (Tm = 0.93), which agrees with the sedimentary depositional environment of coal. Therefore, a revised mass fraction of 0.16 mg kg^?1 Tm in NIST SRM 1632a is recommended.