W. Robert Kelly

Revised δ34S Reference Values for IAEA Sulfur Isotope Reference Materials S-2 and S-3

Revised delta(34)S reference values with associated expanded uncertainties (95% confidence interval (C.I.)) are presented for the sulfur isotope reference materials IAEA-S-2 (22.62 +/- 0.16 per thousand) and IAEA-S-3 (-32.49 +/- 0.16 per thousand). These revised values are determined using two relative-difference measurement techniques, gas source isotope ratio mass spectrometry (GIRMS) and double-spike multi-collector thermal ionization mass spectrometry (MC-TIMS). Gas analyses have traditionally been considered the most robust for relative isotopic difference measurements of sulfur. The double-spike MC-TIMS technique provides an independent method for value-assignment validation and produces revised values that are both unbiased and more precise than previous value assignments. Unbiased delta(34)S values are required to anchor the positive and negative end members of the sulfur delta (delta) scale because they are the basis for reporting both delta(34)S values and the derived mass-independent Delta(33)S and Delta(36)S values.

Direct determination of mercury at picomole L−1 levels in bottled water by isotope dilution cold-vapor generation inductively coupled plasma mass spectrometry

The mercury concentration in 17 commercially available bottled waters (artesian, distilled, carbonated, and spring) from 7 different countries determined by isotope dilution cold-vapor inductively coupled plasma mass spectrometry (ID-CV-ICP-MS) ranged from the blank limited detection limit of <0.10 ng L−1 to 2.32 ng L−1 (ppt). Highly enriched 201Hg isotopic spike is added to approximately 20 mL water and thoroughly mixed. The Hg+2 in the sample is reduced on line with tin(II) chloride and the elemental Hg vapor is separated in a “liquid-matrix” separator and introduced directly into a quadrupole ICP-MS where the Hg isotope ratios (201Hg/202Hg) are measured in time-resolved analysis mode. The primary advantages of this method are (1) high sensitivity, the instrument detection limit is less than 0.05 ng L−1, (3σ), (2) very low chemical blank, the average blank (n = 3) is 0.17 ng L−1 ± 0.03 ng L−1, and (3) high accuracy of isotope dilution—the accuracy is limited by blank and counting statistics. All waters tested, including the major sellers in Europe and the United States, were approximately 1000 times lower than both the U.S. Food and Drug Administration (FDA) limit of 2 µg L−1 Hg (ppb) and the international World Health Organization (WHO) limit of 1 µg L−1 Hg.

Gravimetric approach to the standard addition method in instrumental analysis. 1.

A mathematical formulation for a gravimetric approach to the univariate standard addition method (SAM) is presented that has general applicability for both liquids and solids. Using gravimetry rather than volumetry reduces the preparation time, increases design flexibility, and makes increased accuracy possible. SAM has most often been used with analytes in aqueous solutions that are aspirated into flames or plasmas and determined by absorption, emission, or mass spectrometric techniques. The formulation presented here shows that the method can also be applied to complex matrixes, such as distillate and residual fuel oils, using techniques such as X-ray fluorescence (XRF) or combustion combined with atomic fluorescence or absorption. These techniques, which can be subject to matrix-induced interferences, could realize the same benefits that have been demonstrated for dilute aqueous solutions.

Origin and early history of Die Methode des Eichzusatzes or The Method of Standard Addition with primary emphasis on its origin, early design, dissemination, and usage of terms

The Method of Standard Additions or The Standard Addition Method, often referred to by its acronym as just SAM, is a proverbial workhorse in both inorganic and organic quantitative analytical chemistry and in related disciplines such as geochemistry. Its advantage in mitigating the effects of matrix interferences compared with the calibration curve approach is well known and is one of its major benefits. It is presented in virtually all standard textbooks on analytical chemistry in varying degrees of complexity. Yet the story of how it originated, and by whom, is not well known. It is generally believed that it originated in the early 1950s, introduced by spectroscopists. We have determined that the priority of its use and discovery apparently belongs exclusively to Hans Hohn (1906– 1978), a mining chemist, dating from the exposition of the method in his 1937 book on polarography, Chemische Analysen mit dem Polarographen. How the method became established in other disciplines quite different from polarography is not completely clear. It is likely that it was rediscovered independently in at least two separate disciplines, X-ray fluorescence (XRF) and atomic spectroscopy, almost simultaneously in 1953, 16 years after Hohn’s original description.

Determination of Cr in certified reference material HISS-1, marine sediment, by cold plasma isotope dilution ICP-MS and INAA: comparison of microwave versus closed (Carius) tube digestion

The recovery of Cr from National Research Council Canada (NRCC) certified reference material HISS-1 sediment was investigated by non-destructive instrumental neutron activation analysis (INAA) and isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) after two different chemical treatments: (1) microwave dissolution using HNO3, HF, and HClO4, followed by reflux with concentrated HClO4, and (2) a closed system method using sealed Carius tube digestion with HNO3 and HCl at high temperature (240 °C) and high pressure (1 × 107 Pa, ∼100 atm). The first chemical treatment gave recoveries of only ∼50% in agreement with observations of Yang et al. of NRCC. Samples digested in Carius tubes gave recoveries that ranged from 79 to 110% which may reflect heterogeneity in the samples rather than differing recoveries. This is supported by the excellent agreement of the mean value determined by INAA of 31.4 ± 4.5 µg g−1, and the mean value of 28.1 ± 3.9 µg g−1 by Carius tube digestion and the observation that these standard deviations are essentially equal and much larger than the measurement precisions. The results from this study are also in excellent agreement with the certified value of 30.0 ± 6.8 µg g−1. This preliminary study demonstrates that chemical treatment of this sediment with HNO3 and HCl acids in closed system Carius tubes is an efficient and capable preparation method for the determination of Cr in silicate materials which typically contain refractory Cr-bearing minerals. This study also demonstrates the unique advantage of using two independent methods for the determination of Cr in a complex matrix which has a high potential for low chemical recovery.

A theoretical comparison between intentional elemental and isotopic atmospheric tracers

Abstract A theoretical comparison has been made between intentional elemental and intentional isotopic tracers for the study of the fate of emissions into the atmosphere. The use of an elemental tracer requires a very large signal to background ratio because the latter cannot be determined while the tracer experiment is in progress. It is shown that the variation in the ambient background is the major source of uncertainty for the elemental tracer. The use of a stable isotopic tracer is definitive because the isotopic composition of the background is constant and can be measured in real time during the isotopic analysis of the sample. Simple error analysis suggests that the isotopic tracer is intrinsically definitive and more accurate compared to the elemental tracer. This is because elemental tracers are subject to inherent biases of unknown magnitude that do not exist for isotopic tracers. If it is assumed that the background can vary by a factor of two during a tracer experiment, then for a given confidence in the detection of the tracer signal more than 10 4 times more elemental tracer is required than isotopic tracer. It is concluded that stable isotopic tracers are inherently superior to elemental tracers; however; this conclusion now needs to be demonstrated in an actual field experiment.

Final report on key comparison CCQM-K35: Determination of sulfur in diesel fuel

The CCQM-K35 key comparison was organized by the Inorganic Analysis Working Group (IAWG) of the CCQM to test the capabilities of national metrological institutes (NMIs) to measure the sulfur content of diesel fuel at the 40 µg/g level. Four NMIs participated in the key comparison: the Federal Institute for Materials Research and Testing (BAM), the Institute for Reference Materials and Measurements (IRMM), the Laboratory of the Government Chemist (LGC) and the National Institute of Standards and Technology (NIST). NIST designed and coordinated the study. All four laboratories used isotope dilution mass spectrometric techniques, but two laboratories (BAM and NIST) used thermal ionization mass spectrometry (TIMS) and two laboratories (IRMM and LGC) used inductively coupled plasma mass spectrometry (ICP-MS). Both techniques require spiking and combustion of the sample prior to the instrumental determination to quantify the amount of sulfur in the fuels. The agreement among the laboratories was good as evidenced by the small values for both the equivalence statements (Di < 1 µg/g) and the associated uncertainties (ui < 2.3 µg/g). A pilot study (P26.1) was performed concurrently on a kerosene sample at the 8 µg/g level and on the same diesel sample used in this K35 study by laboratories preferring to participate in the pilot study. The results of the pilot study are reported separately. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Certification of sulfur in SRM 2724 diesel fuel oil by isotope dilution thermal ionization mass spectrometry and X-ray fluorescence

Abstract SRM 2724 is a no. 2-D diesel fuel oil that has been certified for total sulfur concentration at 425 ± 4 μ g g −1 (95% confidence interval) by isotope dilution thermal ionization mass spectrometry, which is the most accurate and precise method for the determination of sulfur in many different matrices. The sulfur concentration in this standard has also been measured by X-ray fluorescence (XRF) using a procedure derived from ASTM method D 2622-87. The mean of the XRF measurements was 423 ± 2 (1s), which demonstrates that XRF is capable of measuring sulfur accurately and precisely at the 500 μg g −1 level when it is properly calibrated using accurately certified standards of a similar matrix. SRM 2724 will be useful in validating the measurement process for the determination of sulfur by XRF in on-road diesel fuel at the 500 μg g −1 limit as mandated by EPA, effective from October 1993.

Tagging diesel and residential oil furnace emissions in Roanoke, Virginia, with enriched isotopes of samarium

Stable isotopic tracers were used in Roanoke, Virginia, to tag particulate emissions from diesel trucks and residential oil furnaces, two sources of soot and PAHs which cannot be differentiated on the basis of known constituents. Approximately 1.6 g of enriched 149Sm were used to tag 264 m3 of diesel fuel burned by the city bus and truck fleets; 0.39 g of 150Sm were used to tag 106 m3 of residential heating oil. Picogram amounts of the tracers were determined simultaneously by thermal-ionization mass spectrometry in fine particles collected within the city at signal-to-noise ratios as large as 6000. These results demonstrate the feasibility of tracing particles from multiple combustion sources with stable, separated isotopes.

Determination of low-level (sub-microgram) sulfur concentrations by isotope dilution multi-collector inductively couple plasma mass spectrometry using a 33S spike and internal normalization for mass bias correction.

RATIONALE The certification of sulfur (S) in Standard Reference Materials™ by the National Institute of Standards and Technology (NIST) has been exclusively performed using isotope dilution thermal ionization mass spectrometry (ID-TIMS). The ID-TIMS measurement method is limited in its capability for low concentration measurements (<1 µg/g) due to the blank associated with the chemical reduction procedure (≈0.2 S µg). Newly developed materials and applications, such as biofuels made from soy and nanomedicine, pose a challenge to the ID-TIMS technique because of their very low concentrations (<<1 µg/g) of S. As described here, a measurement technique with low S blanks is essential for low-level S measurements. METHODS An isotope dilution (ID) multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) method combined with an internal normalization approach for mass bias correction has been used to determine low-level S concentrations in synthetically prepared mixtures using a (33)S-enriched spike material. Dilute sulfur solutions (1 µg S/g) were prepared from NIST SRM 3154 (Sulfate Spectrometric Solution) to test the capability of this technique for measuring very low-level S concentrations. RESULTS The concentration results for each solution were 0.983 ± 0.004 µg/g (95% CI, n = 2, k = 2), 1.006 ± 0.005 µg/g (95% CI, n = 2, k = 2), and 0.999 ± 0.003 µg/g (95% CI, n = 2, k = 2), in excellent agreement with the gravimetric determination, deviating less than 0.35% and suggesting the technique can yield unbiased and accurate results. The blanks averaged 13 ± 0.0017 ng S (1s). CONCLUSIONS The data results provide a clear indication that the ID-MC-ICP-MS method for the determination of low-level S concentrations is feasible. The more than one order of magnitude reduction of the blanks suggests that it is a better alternative to the ID-TIMS method for very low S materials such as are encountered in biofuels and some biochemical species. Published 2012. This article is a US Government work and is in the public domain in the USA.