Massic Activity Ratios of the NBS/NIST Tritiated-Water Standards Issued Between 1954 and 1999

As part of the preparation and calibration of three new National Institute of Standards and Technology (NIST) tritiated-water radioactivity Standard Reference Materials (SRMs), the massic-activity (activity of the sample divided by the mass of the sample) ratios of all of the available NBS/NIST tritiated-water SRMs issued between 1954 and 1999 were measured using liquid-scintillation (LS) counting. Four of the tritiated-water standards (SRMs 4361, 4926B, 4927C, and 4927D) were not available for measurement. All of the other tritiated-water standards (SRMs 4361B, 4361C, 4926, 4926C, 4926D, 4926E, 4927, 4927B, 4927E, and 4927F) were available, having been stored in flame-sealed glass ampoules. Where possible, massic-activity ratios measured by liquid-scintillation counting are compared with massic-activity ratios calculated from gravimetric dilution factors. The agreement is well within the stated uncertainties. For two of the tritiated-water standards that were not available for measurement (SRMs 4361 and 4926B), massic-activity ratios calculated from gravimetric dilution factors are available.     

Fast and Precise Method for Pb Isotope Ratio Determination in Complex Matrices using GC-MC-ICPMS: Application to Crude Oil, Kerogen, and Asphaltene Samples

A new method to determine Pb isotope ratio without ion-exchange-matrix separation is proposed. After acid digestion, Pb was ethylated to Et(4)Pb, separated from the digested solution (black shale, asphaltene, crude oil and kerogen) by extraction in isooctane, and then injected into a gas chromatograph coupled to a multicollector inductively coupled plasma mass spectrometer. Seven isotopes ((202)Hg, (203)Tl, (204)Pb, (205)Tl, (206)Pb, (207)Pb, (208)Pb) were monitored simultaneously with peak duration of 23 s. GC elution was operated under wet plasma conditions where a thallium standard solution was introduced to the mass spectrometer for mass bias correction. The total time of the procedure (sample preparation and analysis, after acid digestion) was reduced by a factor of 15 compared to conventional-continuous sample introduction. Data treatment was carried out using the linear regression slope method. Mass bias was corrected using the double correction method (first thallium normalization followed by classical bracketing). For the (208/206)Pb and (207/206)Pb ratios, precision (2RSD(EXT), n = 21) was 49 and 69 ppm, and the bias between experimental results and reference values was better than 0.0033 and 0.0007 ‰, when injecting 1.2 ng of ethylated Pb SRM NIST 981 solution. Results obtained by this method were validated by comparison with those obtained via conventional-continuous sample introduction. The applicability of this approach was demonstrated with the analysis of black shale, asphaltene, crude oil and kerogen samples.     

Development of isotope dilution cold vapor inductively coupled plasma mass spectrometry and its application to the certification of mercury in NIST standard reference materials

An isotope dilution cold vapor inductively coupled plasma mass spectrometry (ID-CV-ICPMS) method featuring gaseous introduction of mercury via tin chloride reduction has been developed and applied to the quantification and certification of mercury in various NIST standard reference materials: SRM 966 Toxic Metals in Bovine Blood (30 ng x mL(-1)); SRM 1641d Mercury in Water (1.6 microg x mL(-1)); and SRM 1946 Lake Superior Fish Tissue (436 ng x g(-1)). Complementary mercury data were generated for SRMs and NIST quality control standards using cold vapor atomic absorption spectroscopy (CVAAS). Certification results for the determination of mercury in SRM 1641d using two independent methods (ID-CV-ICPMS and CVAAS) showed a degree of agreement of 0.3% between the methods. Gaseous introduction of mercury into the ICPMS resulted in a single isotope sensitivity of 2 x 10(6) counts x s(-1)/ng x g(-1) for 201Hg and significantly reduced the memory and washout effects traditionally encountered in solution nebulization ICPMS. Figures of merit for isotope ratio accuracy and precision were evaluated at dwell times of 10, 20, 40, 80, and 160 ms using SRM 3133 Mercury Spectrometric Solution. The optimum dwell time of 80 ms yielded a measured 201Hg/202Hg isotope ratio within 0.13% of the theoretical natural value and a measurement precision of 0.34%, on the basis of three replicate injections of SRM 3133.     

Standard Reference Materials (SRMs) for the Calibration and Validation of Analytical Methods for PCBs (as Aroclor Mixtures)

Six Standard Reference Materials (SRMs®) have been prepared by the National Institute of Standards and Technology (NIST) for the determination of PCBs as different Aroclor mixtures in methanol. Six additional SRMs of the same Aroclors in transformer oil have also been prepared. Specifically, solutions of Aroclors 1016, 1232, 1242, 1254, and 1260 have been gravimetrically prepared (individually) in methanol and transformer oil, mixed, and transferred to amber glass ampoules in approximately 1.2 mL aliquots. Gas chromatography with electron capture detection (GC-ECD) has been used to verify the gravimetric data for each solution and transformer oil SRM. Liquid chromatography was used for the isolation of the Aroclors from the transformer oil SRMs prior to GC-ECD analysis. Separate calibration solutions and oils were prepared with Aroclor levels similar to those in each methanol solution and transformer oil SRM and were processed alongside the samples. The GC-ECD response of each Aroclor was monitored relative to internal standards that were added to the complex mixtures for quantification. The gravimetric concentrations of Aroclors 1242 and 1254 in methanol were also examined by the same method of analysis (GC-ECD) using several different sources of Aroclors and two different capillary GC columns: a 5 % phenyl methylpolysiloxane phase and a relatively non-polar phase. The preparation of the materials, the gas chromatographic results, and the certified concentration values for each Aroclor SRM are described in this paper.     

Lithium isotope composition of basalt glass reference material

We present data on the lithium isotope compositions of glass reference materials from the United States Geological Survey (USGS) and the National Institute of Standards and Technology (NIST) determined by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS), thermal ionization mass spectrometry (TIMS), and secondary ionization mass spectrometry (SIMS). Our data on the USGS basaltic glass standards agree within 2 per thousand, independent of the sample matrix or Li concentration. For SIMS analysis, we propose use of the USGS glasses GSD-1G (delta(7)Li 31.14 +/- 0.8 per thousand, 2sigma) and BCR-2G (delta(7)Li 4.08 +/- 1.0 per thousand, 2sigma) as suitable standards that cover a wide range of Li isotope compositions. Lithium isotope measurements on the silica-rich NIST 600 glass series by MC-ICPMS and TIMS agree within 0.8 per thousand, but SIMS analyses show systematic isotopic differences. Our results suggest that SIMS Li isotope analyses have a significant matrix bias in high-silica materials. Our data are intended to serve as a reference for both microanalytical and bulk analytical techniques and to improve comparisons between Li isotope data produced by different methodologies.     

Use of sol-gels as solid matrixes for trace analysis by UV laser ablation and laser-enhanced ionization detection

Zirconium and silicon sol-gels were investigated as solid materials for trace elemental analysis of pelletized solid samples by laser ablation and laser-enhanced ionization. The highly homogeneous dispersion of an internal standard spiked in the solid material obtained with the sol-gel formation process leads to a significant improvement in signal repeatability and to an increase in the precision of measurements through better correction of variations in the laser ablation rate. Signal repeatability values of 5-8% RSD were obtained for Pb in NIST 1632c Bituminous Coal sample pellets prepared using both sol-gels, as compared to 9-21% for graphite-based sample pellets. Furthermore, the zirconium sol-gel was shown to offer better resilience to signal bias due to preferential ablation and a more accurate correction of ablation rate using the internal standardization method.     

Determination of polycyclic aromatic hydrocarbons with molecular weight 300 and 302 in environmental-matrix standard reference materials by gas chromatography/mass spectrometry

An analytical approach based on gas chromatography/ mass spectrometry (GC/MS) is presented for the measurement of polycyclic aromatic hydrocarbons with molecular weight (MW) 300 and 302 in environmental samples. Three different GC stationary phases [5% and 50% phenyl methylpolysiloxane and dimethyl (50% liquid crystalline) polysiloxane] were compared, and retention indexes (RI) are given for 23 individual MW 302 isomers. Identification of MW 300 and 302 isomers in four environmental-matrix Standard Reference Materials (SRMs) (SRM 1597, coal tar extract; SRM 1648 and SRM 1649a, air particulate matter; and SRM 1941, marine sediment) was based on the comparison of RI data and mass spectra from authentic standards. Dibenzo[a,l]pyrene, which is of considerable interest because of its high carcinogenicity, was identified and quantified in the four environmental-matrix SRMs. A total of 23 isomers of MW 302 and four isomers of MW 300 were quantified in four different environmental-matrix SRMs, and the results are compared to previously reported results based on liquid chromatography with fluorescence detection.     

Relative intensity correction of Raman spectrometers: NIST SRMs 2241 through 2243 for 785 nm, 532 nm, and 488 nm/514.5 nm excitation

Standard Reference Materials SRMs 2241 through 2243 are certified spectroscopic standards intended for the correction of the relative intensity of Raman spectra obtained with instruments employing laser excitation wavelengths of 785 nm, 532 nm, or 488 nm/514.5 nm. These SRMs each consist of an optical glass that emits a broadband luminescence spectrum when illuminated with the Raman excitation laser. The shape of the luminescence spectrum is described by a polynomial expression that relates the relative spectral intensity to the Raman shift with units in wavenumber (cm(-1)). This polynomial, together with a measurement of the luminescence spectrum of the standard, can be used to determine the spectral intensity-response correction, which is unique to each Raman system. The resulting instrument intensity-response correction may then be used to obtain Raman spectra that are corrected for a number of, but not all, instrument-dependent artifacts. Peak area ratios of the intensity-corrected Raman spectrum of cyclohexane are presented as an example of a methodology to validate the spectral intensity calibration process and to illustrate variations that can occur in this measurement.     

Use of sol-gels as solid matrixes for simultaneous multielement determination by radio frequency glow discharge optical emission spectrometry: determinations of suspended particulate matter

A new approach for the analysis of particulate matter by radio frequency glow discharge optical emission spectrometry (rf-GD-OES) is described. Dispersion of the particles in a sol-gel sample matrix provides a convenient means of generating a thin film suitable for sputter-sampling into the discharge. Acid-catalyzed sol-gel glasses synthesized from tetramethyl orthosilicate were prepared and spun-cast on glass substrates. The resultant thin films on glass substrates were analyzed to determine the discharge operating conditions and resultant sputtering characteristics while a number of optical emission lines of the film components were monitored. Slurries of powdered standard reference materials NIST SRM 1884a (Portland Cement) and NIST SRM 2690 (Coal Fly Ash) dispersed in the sols were cast into films in the same manner. Use of the sol-gels as sample matrixes allows for background subtraction through the use of analytical blanks and may facilitate the generation of calibration curves via readily synthesized, matrix-matched analytical standards in solids analysis. Detection limits were determined for minor elements via the RSDB method to be in the range of 1-10 microg/g in Portland Cement and Coal Fly Ash samples for the elements Al, Fe, Mg, S, and Si. Values for Ca were in the range of 15-35 microg/g. This preliminary study demonstrates the possibility of incorporating various insoluble species, including ceramics and geological specimens in powder form, into a solid matrix for further analysis by either rf-GD-OES or MS.     

Ultratrace uranium fingerprinting with isotope selective laser ionization spectrometry

Uranium isotope ratios can provide source information for tracking uranium contamination in a variety of fields, ranging from occupational bioassay to monitoring aftereffects of nuclear accidents. We describe the development of isotope selective laser ionization spectrometry for ultratrace measurement of the minor isotopes (234)U, (235)U, and (236)U with respect to (238)U. The inherent isotopic selectivity of three-step excitation with single-mode continuous wave lasers results in measurement of the minor isotopes at relative abundances below 1 ppm and is not limited by isobaric interferences such as (235)UH(+) during measurement of (236)U. This relative abundance limit is attained without mass spectrometric analysis of the laser-created ions. Uranyl nitrate standards from an international blind comparison were used to test analytical performance for different isotopic compositions and with quantities ranging from 11 ng to 10 microg total uranium. Isotopic ratio determination was demonstrated over a linear dynamic range of 7 orders of magnitude with a few percent relative precision and detection limits below 500 fg for the minor isotopes.     

A Monte Carlo model for studying the microheterogeneity of trace elements in reference materials by means of synchrotron microscopic X-ray fluorescence

Synchrotron micro-XRF, a trace-level microanalytical method, allows quantitative study of the nature and degree of heterogeneity of inorganic trace constituents in solid materials with a homogeneous matrix. In this work, the standard reference materials NIST SRM 613, Trace Elements in 1 mm Glass Wafers, and NIST SRM 1577a, Trace Elements in Bovine Liver, are examined at the 10100-ng mass level using X-ray beams of 5-150 microm in diameter. A procedure based on a large number of repeated analyses of small absolute amounts of the SRMs allows calculation of the minimal representative mass of the standard. The microheterogeneity of both NIST SRM 613 and NIST SRM 1577a was investigated with the aim of evaluating their suitability as reference materials for trace-level microanalytical techniques. A Monte Carlo simulation model was constructed for both homogeneous and heterogeneous materials to elucidate the dependence of the calculated minimal representative mass on the total analyzed mass in the case of materials that showstrongly heterogeneous features at the microscopic level.     

Preparation of Reference Material 8504, Transformer Oil

A new reference material (RM), RM 8504, has been prepared for use as a diluent oil with Aroclors in transformer oil Standard Reference Materials (SRMs) 3075 to 3080 and SRM 3090 when developing and validating methods for the determination of polychlorinated biphenyls (PCBs) as Aroclors in transformer oil or similar matrices. SRMs 3075-3080 and SRM 3090 consist of individual Aroclors in the same transformer oil that was used to prepare RM 8504. A unit of RM 8504 consists of one bottle containing approximately 100 mL of transformer oil. No additional constituents have been added to the oil.     

Speciation of elements in NIST particulate matter SRMs 1648 and 1650

X-ray absorption fine structure (XAFS) spectra for S, Cl, V, Cr, Mn, Cu, Zn, As, Br, Cd and Pb and Mossbauer spectra for Fe have been obtained for two National Institute of Standards and Technology (NIST) particulate matter (PM) standard reference materials (SRMs): urban PM (SRM 1648) and diesel PM (SRM 1650). The spectral data, complemented by information on elemental concentrations from proton-induced X-ray-emission (PIXE) spectroscopy, were used to interpret the speciation of these elements in these complex materials. It appears that all the metallic elements investigated occur in oxidized forms, principally as sulfates in the diesel PM SRM and as sulfates, oxides, and possibly other forms (e.g. clays?) in the urban PM. A minor fraction of the sulfur and major fractions of the halogens, Cl and Br, occur as organosulfide (thiophene) and organohalide occurrences, respectively, that must be associated with the abundant carbonaceous matter that constitutes the major component of the two PM SRMs. Most of the sulfur, however, occurs as sulfate in the urban PM and as bisulfate in the diesel PM. In addition, elemental oxidation states have been determined directly by the spectroscopic techniques. Such information is often the key parameter in determining the toxicity and solubility of specific elements in PM, both of which are important in understanding the threat that such elements may pose to human health. For the two HAP elements, Cr and As, for which the toxicity depends greatly on oxidation state, the XAFS data showed that both elements are present in both SRMs predominantly in the less toxic oxidation states, Cr(III) and As(V). The potential of the XAFS spectra for use as source apportionment signatures is illustrated by reference to chromium, which exists in these two PM SRMs in very different forms.     

Chemically assisted laser ablation ICP mass spectrometry

A new laser ablation technique combined with a chemical evaporation reaction has been developed for elemental ratio analysis of solid samples using an inductively coupled plasma mass spectrometer (ICPMS). Using a chemically assisted laser ablation (CIA) technique developed in this study, analytical repeatability of the elemental ratio measurement was successively improved. To evaluate the reliability of the CLA-ICPMS technique, Pb/U isotopic ratios were determined for zircon samples that have previously been analyzed by other techniques. Conventional laser ablation for Pb/U shows a serious elemental fractionation during ablation mainly due to the large difference in elemental volatility between Pb and U. In the case of Pb/U ratio measurement, a Freon R-134a gas (1,1,1,2-tetrafluoroethane) was introduced into the laser cell as a fluorination reactant. The Freon gas introduced into the laser cell reacts with the ablated sample U, and refractory U compounds are converted to a volatile U fluoride compound (UF6) under the high-temperature condition at the ablation site. This avoids the redeposition of U around the ablation pits. Although not all the U is reacted with Freon, formation of volatile UF compounds improves the transmission efficiency of U. Typical precision of the 206Pb/238U ratio measurement is 3-5% (2sigma) for NIST SRM 610 and Nancy 91500 zircon standard, and the U-Pb age data obtained here show good agreement within analytical uncertainties with the previously reported values. Since the observed Pb/U ratio for solid samples is relatively insensitive to laser power and ablation time, optimization of ablation conditions or acquisition parameters no longer needs to be performed on a sample-to-sample basis.     

Determinations of rare earth element abundance and U-Pb age of zircons using multispot laser ablation-inductively coupled plasma mass spectrometry

We have developed a new calibration technique for multielement determination and U-Pb dating of zircon samples using laser ablation-inductively coupled plasma mass spectrometry (ICPMS) coupled with galvanometric optics. With the galvanometric optics, laser ablation of two or more sample materials could be achieved in very short time intervals (~10 ms). The resulting sample aerosols released from different ablation pits or different solid samples were mixed and homogenized within the sample cell and then transported into the ICP ion source. Multiple spot laser ablation enables spiking of analytes or internal standard elements directly into the solid samples, and therefore the standard addition calibration method can be applied for the determination of trace elements in solid samples. In this study, we have measured the rare earth element (REE) abundances of two zircon samples (Nancy 91500 and Pre?ovice) based on the standard addition technique, using a direct spiking of analytes through a multispot laser ablation of the glass standard material (NIST SRM612). The resulting REE abundance data show good agreement with previously reported values within analytical uncertainties achieved in this study (10% for most elements). Our experiments demonstrated that nonspectroscopic interferences on 14 REEs could be significantly reduced by the standard addition technique employed here. Another advantage of galvanometric devices is the accumulation of sample aerosol released from multiple spots. In this study we have measured the U-Pb age of a zircon sample (LMR) using an accumulation of sample aerosols released from 10 separate ablation pits of low diameters (~8 μm). The resulting (238)U-(206)Pb age data for the LMR zircons was 369 ± 64 Ma, which is in good agreement with previously reported age data (367.6 ± 1.5 Ma). (1) The data obtained here clearly demonstrate that the multiple spot laser ablation-ICPMS technique can become a powerful approach for elemental and isotopic ratio measurements in solid materials.     

Summarizing the effectiveness of supercritical fluid extraction of polycyclic aromatic hydrocarbons from natural matrix environmental samples

A summary of the supercritical fluid extraction (SFE) of polycyclic aromatic hydrocarbons (PAHs) from four natural matrix Standard Reference Materials (SRMs) is presented. The work involved the investigation of the effects of extraction fluid [carbon dioxide (CO(2)), chlorodifluoromethane (R22), and 1,1,1,2-tetrafluoroethane (HFC134a)], fluid modifier (dichloromethane and aniline), temperature (60, 150, and 200 °C) and added water on the SFE recoveries of PAHs compared to certified results from Soxhlet extractions. For SRM 1649a (Urban Dust/Organics), R22 yielded excellent recoveries (>90% of certified concentrations) of all PAHs measured, while results for the same SRM using HFC134a as the fluid were typically <80% of the certified concentrations for most of the PAHs measured. For SRM 1941a and 1944, both aquatic sediments with similar physical and chemical compositions, extractions of the wet materials with dichloromethane-modified CO(2) (10%, v/v) yielded quantitative recoveries of all PAHs for SRM 1944 but an obvious trend of lower recoveries for higher molecular weight PAHs (≥228 amu) for SRM 1941a. Results of SFEs of SRM 1650 (Diesel Particulate Matter) showed that this material is the most refractory of the SRMs investigated in this study, with recoveries of indeno[1,2,3-cd]pyrene and benzo[ghi]perylene at <20% of the Soxhlet results.     

Determination of serum cholesterol by a modification of the isotope dilution mass spectrometric definitive method

An isotope dilution mass spectrometric (ID/MS) method for cholesterol is described that uses capillary gas chromatography with cholesterol-13C3 as the labeled internal standard. Labeled and unlabeled cholesterol are converted to the trimethylsilyl ether. Combined capillary column gas chromatography and electron impact mass spectrometry are used to obtain the abundance ratio of the unlabeled and labeled [M+.] ions from the derivative. Quantitation is achieved by measurement of each sample between measurements of two standards whose unlabeled/labeled ratios bracket that of the sample. Seven pools were analyzed by this method: standard reference material (SRM) 1951, which consists of three frozen serum pools with low, medium, and high levels of cholesterol; SRM 1952, which consists of three freeze-dried serum pools with low, medium, and high levels of cholesterol; and SRM 909, a freeze-dried serum pool. The method is a modification of our original definitive method for cholesterol. The modified method uses much better chromatographic separations to assure specificity and a new method of implementing selected ion monitoring on a magnetic mass spectrometer to obtain high-precision measurements of ion intensity ratios on narrow gas chromatographic peaks. The modified method has a coefficient of variation (CV) of 0.22%, which is an improvement over the original method's CV of 0.36%. The measurements were found to be free of interference. The high precision and absence of bias qualify this method as a candidate definitive method.     

Simultaneous determination of mono-, di-, and tributyltin in sediments by isotope dilution analysis using gas chromatography--ICPMS

A mixed spike containing 119Sn-enriched monobutyltin (MBT), dibutyltin (DBT), and tributyltin (TBT) was prepared by direct butylation of 119Sn-enriched tin metal using a 1:3 molar excess of butyl chloride with iodide and triethylamine as catalysts. The isotopic composition of the different tin species in the spike solution was determined by gas chromatography- ICPMS after aqueous ethylation using sodium tetraethylborate. Reverse isotope dilution analysis was used for the characterization of the spike by means of natural MBT, DBT, and TBT standards. No species transformation was evident during derivatization from the reverse isotope dilution experiments based on the measured isotope ratios both before and after spiking. The mixed spike was applied to the simultaneous analysis of MBT, DBT, and TBT in certified reference materials, PACS-2 and CRM 646, with satisfactory results.     

Development of an on-line flow injection Sr/matrix separation method for accurate, high-throughput determination of Sr isotope ratios by multiple collector-inductively coupled plasma-mass spectrometry

This work introduces a newly developed on-line flow injection (FI) Sr/Rb separation method as an alternative to the common, manual Sr/matrix batch separation procedure, since total analysis time is often limited by sample preparation despite the fast rate of data acquisition possible by inductively coupled plasma-mass spectrometers (ICPMS). Separation columns containing approximately 100 muL of Sr-specific resin were used for on-line FI Sr/matrix separation with subsequent determination of (87)Sr/(86)Sr isotope ratios by multiple collector ICPMS. The occurrence of memory effects exhibited by the Sr-specific resin, a major restriction to the repetitive use of this costly material, could successfully be overcome. The method was fully validated by means of certified reference materials. A set of two biological and six geological Sr- and Rb-bearing samples was successfully characterized for its (87)Sr/(86)Sr isotope ratios with precisions of 0.01-0.04% 2 RSD (n = 5-10). Based on our measurements we suggest (87)Sr/(86)Sr isotope ratios of 0.713 15 +/- 0.000 16 (2 SD) and 0.709 31 +/- 0.000 06 (2 SD) for the NIST SRM 1400 bone ash and the NIST SRM 1486 bone meal, respectively. Measured (87)Sr/(86)Sr isotope ratios for five basalt samples are in excellent agreement with published data with deviations from the published value ranging from 0 to 0.03%. A mica sample with a Rb/Sr ratio of approximately 1 was successfully characterized for its (87)Sr/(86)Sr isotope signature to be 0.718 24 +/- 0.000 29 (2 SD) by the proposed method. Synthetic samples with Rb/Sr ratios of up to 10/1 could successfully be measured without significant interferences on mass 87, which would otherwise bias the accuracy and uncertainty of the obtained data.