A new analytical technique is described for the determination of d 34 S that is comparable to or better than modern gas source mass spectrometry in precision and accuracy, but requires about a factor of 10 less sample. The technique is based on the production of singularly charged arsenic sulfide molecular ions (AsS þ ) by thermal ionization using silica gel as an emitter and combines multiple-collector thermal ionization mass spectrometry (MC-TIMS) with a 33 S-
36S double spike to correct instrumental fractionation. Three international sulfur standards (IAEA-S-1, IAEA-S-2, and IAEA-S-3) were measured to evaluate the precision and accuracy of the new technique and to evaluate the consensus values for these standards. Two different double spike preparations were used. Thermal ionization mass spectrometry (TIMS) has been used since WWII in the geological community and to a lesser extent in the analytical chemistry community to measure trace element concentrations with high accuracy (<0.5%). Multiple-collector instruments that are capable of isotopic ratio measurements approaching 1 ppm precision for many elements are now quite common in the geochemical community. The use of TIMS for isotopic composition measurements for sulfur is relatively recent but has been limited by precisions of about 1 to 2% resulting from lack of instrumental fractionation control. 2 Changes in instrumental fractionation during data collection that are not always reproducible among samples are the fundamental limitation to the accuracy and precision of isotopic measurements by TIMS. Therefore, the measured ratio at the detector differs from the true ratio and changes during the measurement. It is almost universally observed that during thermal vaporization, the vapor is enriched in the light isotopes but becomes progressively heavier as the reservoir is depleted. This fractionation process is frequently modeled using Rayleigh-like fractionation. Thorough discussions can be found in Russell et al. 3 and Esat 4 as applied to calcium and magnesium. A Rayleigh-generated vapor curve and the associated reservoir composition curve for the 32 S/ 34 S ratio as a function of reservoir depletion are shown in Fig. 1(a). It is assumed that the species evaporating is arsenic sulfide (AsS). Figure 1(b) shows the vapor curve for the 32 S/ 34 S ratio recast in d 34 S
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.
A material containing single-wall carbon nanotubes (SWCNTs) with other carbon species, catalyst residues, and trace element contaminants has been prepared by the National Institute of Standards and Technology for characterization and distribution as Standard Reference Material SRM 2483 Carbon Nanotube Soot. Neutron activation analysis (NAA) and inductively coupled plasma mass spectrometry (ICP-MS) were selected to characterize the elemental composition. Catalyst residues at percentage mass fraction level were determined with independent NAA procedures and a number of trace elements, including selected rare earth elements, were determined with NAA and ICP-MS procedures. The results of the investigated materials agreed well among the NAA and ICP-MS procedures and good agreement of measured values with certified values was found in selected SRMs included in the analyses. Based on this work mass fraction values for catalyst and trace elements were assigned to the candidate SRM.
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