New isotope ratio mass spectrometric method of precise δ37Cl determinations

The most precise method of chlorine isotope analysis described to date is based on the isotope ratio mass spectrometry (IRMS) of chlorine quantitatively converted into chloromethane, CH₃Cl. This gas can be produced from several chlorine‐containing compounds and analyzed by IRMS. However, the mass spectrum of chloromethane is rather complicated and the ratio of the most abundant ions (mass‐52/mass‐50) differs from the ³⁷Cl/³⁵Cl isotope ratio. This difference becomes significant when the δ exceeds 10‰. Moreover, the electron ionization source yields approximately 80% of all the ionic species at the useful masses 50 and 52. To overcome these drawbacks, we have devised a negative ion mass spectrometer which retains all the best features of IRMS, including a dual‐inlet system with changeover valve, dual collector assembly and CH₃Cl gas as analyte. In the modified ion source we have replaced the ionization chamber with an electron beam by a metal tube with a hot metal filament inside it. Within this tube the ³⁵Cl^? and ³⁷Cl^? ions are produced with an efficiency dependent on the filament material and its temperature. No other ionic species were found in the mass spectrum except of traces at masses 26 and 28 at ppm levels, probably due to the formation of CN^? and CO^?. The minimal amount of Cl used in our method is of the order of 5 µmol (3 mg AgCl) and the precision is better than 0.005‰ (1σ). Copyright © 2009 John Wiley & Sons, Ltd.     

Precise and accurate isotope ratio measurements by ICP-MS

The precise and accurate determination of isotope ratios by inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS (LA-ICP-MS) is important for quite different application fields (e.g. for isotope ratio measurements of stable isotopes in nature, especially for the investigation of isotope variation in nature or age dating, for determining isotope ratios of radiogenic elements in the nuclear industry, quality assurance of fuel material, for reprocessing plants, nuclear material accounting and radioactive waste control, for tracer experiments using stable isotopes or long-lived radionuclides in biological or medical studies). Thermal ionization mass spectrometry (TIMS), which used to be the dominant analytical technique for precise isotope ratio measurements, is being increasingly replaced for isotope ratio measurements by ICP-MS due to its excellent sensitivity, precision and good accuracy. Instrumental progress in ICP-MS was achieved by the introduction of the collision cell interface in order to dissociate many disturbing argon-based molecular ions, thermalize the ions and neutralize the disturbing argon ions of plasma gas (Ar+). The application of the collision cell in ICP-QMS results in a higher ion transmission, improved sensitivity and better precision of isotope ratio measurements compared to quadrupole ICP-MS without the collision cell [e.g., for 235U/238U approximately 1 (10 microg x L(-1) uranium) 0.07% relative standard deviation (RSD) vs. 0.2% RSD in short-term measurements (n = 5)]. A significant instrumental improvement for ICP-MS is the multicollector device (MC-ICP-MS) in order to obtain a better precision of isotope ratio measurements (with a precision of up to 0.002%, RSD). CE- and HPLC-ICP-MS are used for the separation of isobaric interferences of long-lived radionuclides and stable isotopes by determination of spallation nuclide abundances in an irradiated tantalum target.     

Osmium isotope ratio measurements by negative thermal ionization mass spectrometry (NTI-MS)

Summary A negative thermal ionization technique using a Faraday cup as detector is presented, which allows precise isotope ratio measurements of osmium. For the major isotopes the relative standard deviation is in the range of 0.004% to 0.07%. Sample amounts down to 1 ng were loaded as hexachloroosmic acid. From the results it is concluded that also sample amounts in the pg range will result in sufficient ion currents of OsO ₃ ^– , avoiding an electron multiplier as detector. H₂OsCl₆ was chosen because this compound is the most suitable chemical form after separation of osmium from geological samples. The introduction of oxygen or freons into the ion source enhances the emission of OsO ₃ ^– ions by a factor of up to more than ten. Ionization efficiencies of more than 30% are obtained by using oxygen. The stability of the ion current is better in the case of oxygen than in the case of freons. However, both compounds are stabilizing the high voltage potentials of the optical ion lenses in the ion source, a permanent problem in NTI-MS. The described isotope ratio measurement is an important improvement for Re/Os dating of terrestrial and extraterrestrial samples.     

Chlorid-spurenanalyse in silikatgesteinen durch massenspektrometrische isotopenverdünnungsanalyse

An analytical procedure for determining traces of chloride in silicate rocks, using mass spectrometric isotope dilution analysis with a ³⁷Cl-enriched spike, is described. The chlorine isotope ratios are measured with a double-filament thermal ion-source using the production of negative ions from a hot rhenium filament as the ionization method. The samples are decomposed with hydrofluoric acid in a Teflon bomb and chloride is separated from the acidic solution by selective precipitation as AgCl. The relative standard deviation for the chloride determination is 1–4% for chloride contents of 100–200 ppm. This precision is better than any described in the literature for chloride trace analysis. The problem of the blank in chloride trace determination in rock samples is discussed.     

Gas chromatography coupled to electron capture negative ion mass spectrometry with nitrogen as the reagent gas – an alternative method for the determination of polybrominated compounds

Gas chromatography in combination with electron capture negative ion mass spectrometry (GC/ECNI‐MS) is a sensitive method for the determination of polybrominated compounds in environmental and food samples via detection of the bromide ion isotopes m/z 79 and 81. The standard reagent gas for inducing chemical ionization in GC/ECNI‐MS is methane. However, the use of methane has some drawbacks as it promotes carbonization of the filament and ion source. In this study, we explored the suitability of nitrogen as reagent gas for the determination of brominated flame retardants (polybrominated diphenyl ethers (PBDEs), polybrominated biphenyls (PBBs), allyl‐2,4,6‐tribromophenyl ether (ATE) and 2,3‐dibromopropyl‐2,4,6‐tribromophenyl ether (DPTE)) and halogenated natural products (for instance, methoxylated tetrabrominated diphenylethers and polybrominated hexahydroxanthene derivatives). An ion source temperature of 250°C and a nitrogen pressure of 7 Torr in the ion source gave the highest response for m/z 79 and 81 of virtually all investigated polybrominated compounds. Using these conditions, nitrogen‐mediated GC/ECNI‐MS usually gave higher sensitivity than the method with methane previously used in our lab. In addition, the ion source was not contaminated to the same degree and the lifetime of the filament was significantly increased. Moreover, the response factors of the different polybrominated compounds with the exception of 2,4,6‐tribromophenol were more uniform than with methane. Nitrogen is available at very high purity at relatively low price. Copyright © 2009 John Wiley & Sons, Ltd.     

Detection of fast molecular beams by ion ejection from a surface

When superthermal beams of non-alkali-containing molecules strike a pre-treated, oxidized Re filament at 1200 K, positive ions (particularly those of alkali contaminants in the filament) are ejected from the surface, rendering the neutral beams detectable. Within the energy range studied (a few eV) the efficiency of ion production increases approximately exponentially with the translational energy of the incident neutral molecule. Maximum ionization efficiencies were in the range 0.1–10%, for compounds with ionization potentials (IP's) ? 10 eV. For those of IP > 10 eV, ionization efficiencies were unobservably low (? 10^?6), as were yields of negative ions (or electrons) for any compound.     

Isotopenverh?ltnismessung von Mo, V, Ti und Zr sowie deren Konzentrationsbestimmung in W?ssern mit einem Thermionen-Quadrupol-Massenspektrometer

Summary Negative MoO ₃ ^– and positive V⁺, Ti⁺, and Zr⁺ thermal ions are produced in a double-filament ion source to determine the isotope ratios of these elements in a quadrupole mass spectrometer. The average relative standard deviation for all isotope ratio measurements is 0.5%. The ratio Me⁺/MeO⁺ (Me=V, Ti, Zr) is followed dependent on the temperature of the ionization filament. A linear plot is obtained for log (Me⁺/MeO⁺) versus 1/T with increasing Me⁺/MeO⁺ ratios for higher temperatures using a single and a double-filament arrangement. An analytical procedure is developed, which allows the simultaneous measurement of Mo and V, and of Ti and Zr as well from one filament by a stepwise variation of the filament temperatures. Mo, V, Ti, and Zr traces in the ng/g and pg/g level of different water samples could be analysed with isotope dilution mass spectrometry using enriched isotopes of⁹⁷Mo,⁵⁰V,⁴⁷Ti, and⁹¹Zr. The precision lies usually in the range of 1 to 6% and the detection limits are 0.002 ng/g for Mo, 0.02 ng/g for V, 0.05 ng/g for Ti, and 0.01 ng/g for Zr using sample amounts of 250 g.     

Isotopic Analysis of tungsten using multiple collector-inductively coupled plasma-mass spectrometer coupled with electrothermal vaporization technique

We have developed a micro-electrothermal vaporization (μETV) device for the multiple collector-ICP-mass spectrometry (μETV-MC-ICPMS) to improve analytical precision in the ¹⁸²W/¹⁸³W and ¹⁸⁴W/¹⁸³W ratio measurements from nanogram quantities of W. The W solution was loaded onto the Re-filament, and the gradual evaporation of W was achieved by controlling the incident current onto the Re filament and D-Glucose. With the W evaporation under the Ar atmosphere, the measured W isotope ratios became erroneous mainly due to the contribution of signal spikes Ala-Arg-Gly-Phy-Tyr. In strike contrast, signal intensity profile became smooth when the He ambient/carrier gas was employed, and this resulted in better precision in the isotope ratio measurements. The measured UV–vis isotope ratio data obtained with present μETV technique were significantly deviated from the ratio data obtained with solution nebulization technique, mainly due to the contribution of the isotope fractionation effect through the evaporation process. Rigorous testing for the correction of the isotope fractionation processes pH–activity curve revealed that the Rayleigh fractionation law, rather than the conventional exponential law, provided the most reliable ratio data (1.851720 ± 0.000018 for ¹⁸²W/¹⁸³W and 2.141248 ± 0.000028 for ¹⁸⁴W/¹⁸³W ratios), which agreed well with the ratio data obtained through the conventional solution nebulization technique (1.851718 ± 0.000039 for ¹⁸²W/¹⁸³W and 2.141248 ± 0.000022 for ¹⁸⁴W/¹⁸³W). Moreover, mass dependency for the mass fractionation law suggested that W was evaporated as oxides (WO₃), rather than the metallic form (W), from the Re filament, and therefore, information concerning the chemical form of the analytes could also be derived by the ETV technique developed in this study. The data presented here demonstrate clearly that the ETV sample introduction technique has a potential to become a sensitive tool for the precise isotope analysis for the MC-ICPMS technique.     

Certification of the chlorine content of the isotopic reference materials IRMM-641 and IRMM-642

The Slovak Institute of Metrology and the Institute for Reference Materials and Measurements have collaborated in the certification of the two chlorine reference materials IRMM-641 and IRMM-642. Until now no isotopically enriched chlorine isotopic reference material certified for isotopic composition and content has been available commercially. The isotopic reference materials IRMM-641 and IRMM-642 described herein are certified for isotopic composition and for chlorine content. The chlorine content of the reference material IRMM-641 was certified by use of high-precision argentometric coulometric titration at the Slovak Institute of Metrology. The base material used for IRMM-641 is NIST Standard Reference Material 975. The chlorine content of the reference material IRMM-642 was measured by isotope dilution, using negative thermal ionization mass spectrometry at the Institute for Reference Materials and Measurements. Both standard reference materials were prepared by dissolving NaCl in water. The reference material IRMM-641 contains 0.025022 +/- 0.00011 mol kg(-1) chlorine of natural isotopic composition with an n(37Cl)/n(35Cl) ratio of 0.31977 +/- 0.00082. The reference material IRMM-642 contains 0.004458 +/- 0.000028 mol kg(-1) chlorine with an n(37Cl)/n(35Cl) ratio of 0.01914 +/- 0.00088.     

Negative Thermionen-Massenspektrometrie von Selen

Summary Negative thermal ionization is used to determine the selenium isotope ratios in a double-filament ion source. A thin film of barium hydroxide on the rhenium ionization filament is applied to increase the Se^– thermal ion current. The produced Se^– ion beam is by a factor of about four higher when selenious acid instead of barium selenite or sodium selenate is used. A strong dependence of the ion current on the temperature of the ionization filament is found showing the maximum ion intensity at temperatures of 970–1000 C. The different selenium isotope ratios of samples with natural isotopic abundance can be determined with relative standard deviations of 0.3–0.6%. This reproducibility is a good basis to improve the accuracy of the selenium atomic weight in the future by a calibrated measurement. An enriched ⁸²Se spike is used to analyse selenium traces in aquatic systems with isotope dilution mass spectrometry down to the pg/g level. In the concentration range of 4–23 ng/g the selenium content is determined with relative standard deviations of 0.1–5%. The results agree well with those obtained with a hydride generation atomic absorption system. It is shown that the described method of isotope dilution mass spectrometry analyses the sum of the inorganic species selenate, selenite and selenide, but not volatile organic selenium compounds.

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Herrn Prof. Dr. W. Fresenius zum 75. Geburtstag gewidmet     

Negative ion electrospray and tandem mass spectrometric analysis of platelet activating factor (PAF) (1-hexadecyl-2-acetyl-glycerophosphocholine)

The analysis of 1-hexadecyl-2-acetyl-glycerophosphocholine (platelet activating factor, PAF) by negative ion and normal-phase liquid chromatography/tandem mass spectrometry (LC/MS/MS) was investigated as an alternative technique to the currently used gas chromatography/MS and positive ion LC/MS/MS procedures. The positive ion [M + H]+ derived from PAF and generated by electrospray ionization is abundant, but the potential presence of isobaric 1-octadecanoyl-2-lyso-glycerophosphocholine (stearoyl-lyso-GPC) and 1-hexadecanoyl-2-formyl-glycerophosphocholine (PFPC) in biological samples limits the use of the most abundant collision-induced decomposition (CID) transition (formation of the phosphocholine ion, m/z 524-->184) if chromatographic separation is not achieved. Less abundant CID product ions, such as loss of the neutral ketene molecule derived from the respective fatty acyl groups, provide the requisite specificity, but the intensity of these transitions yields a signal-to-noise ratio that greatly diminishes the analytical sensitivity. With negative ion LC/MS/MS, however, the molecular anions [M - 15]- derived from PAF, stearoyl-lyso-GPC and PFPC decompose to the carboxylate anions at m/z 59, 283 and 255, respectively, permitting discrimination of these isobaric molecules even without chromatographic separation. In addition, the CID of [M - 15]- was favorable, yielding ion currents of sufficient intensity to permit the measurement of PAF when isolated from small quantities of biological material. With the use of a stable isotopically labeled variant of PAF and isotope dilution, negative ion LC/MS/MS was found to measure PAF reliably even in the presence of the isobaric stearoyl-lyso-GPC and permitted the use of non-chlorinated mobile phases for normal-phase high-performance LC.     

Electron capture negative ion and positive ion chemical ionization mass spectrometry of polychlorinated phenoxyanisoles

Several polychlorinated phenoxyphenols with three to nine chlorine atoms were examined as their methyl ethers by electron capture negative ion and positive ion chemical ionization and electron impact mass spectrometry. In chemical ionization studies methane, hydrogen, nitrogen, helium and argon were used as reagent gases. Selected compounds were also examined with deuteriomethane, ammonia and deuterioammonia as reagent gases. Utilization of chemical ionization spectra in conjuction with electron impact spectra provides substantial structural information about these compounds. Chemical ionization spectra provide information about chlorine atom substitution. The position of phenoxy substitution can be established from electron capture negative ion and positive ion spectra.     

Mass spectrometry of long-lived radionuclides

The capability of determining element concentrations at the trace and ultratrace level and isotope ratios is a main feature of inorganic mass spectrometry. The precise and accurate determination of isotope ratios of long-lived natural and artificial radionuclides is required, e.g. for their environmental monitoring and health control, for studying radionuclide migration, for age dating, for determining isotope ratios of radiogenic elements in the nuclear industry, for quality assurance and determination of the burn-up of fuel material in a nuclear power plant, for reprocessing plants, nuclear material accounting and radioactive waste control. Inorganic mass spectrometry, especially inductively coupled plasma mass spectrometry (ICP-MS) as the most important inorganic mass spectrometric technique today, possesses excellent sensitivity, precision and good accuracy for isotope ratio measurements and practically no restriction with respect to the ionization potential of the element investigated—therefore, thermal ionization mass spectrometry (TIMS), which has been used as the dominant analytical technique for precise isotope ratio measurements of long-lived radionuclides for many decades, is being replaced increasingly by ICP-MS. In the last few years instrumental progress in improving figures of merit for the determination of isotope ratio measurements of long-lived radionuclides in ICP-MS has been achieved by the application of a multiple ion collector device (MC-ICP-MS) and the introduction of the collision cell interface in order to dissociate disturbing argon-based molecular ions, to reduce the kinetic energy of ions and neutralize the disturbing noble gas ions (e.g. of ¹²⁹Xe⁺ for the determination of ¹²⁹I). The review describes the state of the art and the progress of different inorganic mass spectrometric techniques such as ICP-MS, laser ablation ICP-MS vs. TIMS, glow discharge mass spectrometry, secondary ion mass spectrometry, resonance ionization mass spectrometry and accelerator mass spectrometry for the determination of long-lived radionuclides in quite different materials.     

Positive and negative‐mode laser desorption/ionization‐mass spectrometry (LDI‐MS) for the detection of indigoids in archaeological purple

Laser‐based ionization techniques have demonstrated to be a valuable analytical tool to study organic pigments by mass spectrometric analyses. Though laser‐based ionization techniques have identified several natural and synthetic organic dyes and pigments, they have never been used in the characterization of purple. In this work, positive and negative‐mode laser desorption/ionization mass spectrometry (LDI‐MS) was used for the first time to detect indigoids in shellfish purple. The method was used to study organic residues collected from archaeological ceramic fragments that were known to contain purple, as determined by a classical high‐performance liquid chromatography‐based procedure. LDI‐MS provides a mass spectral fingerprint of shellfish purple, and it was found to be a rapid and successful tool for the identification of purple. In addition, a comparison between positive and negative mode ionization highlighted the complementarity of the two ionization modes. On the one hand, the negative‐ion mode LDI‐MS showed a better selectivity and sensitivity to brominated molecules, such as 6,6'‐dibromoindigo, 6‐monobromoindigo, 6,6'‐dibromoindirubin, 6‐ and 6’‐monobromoindirubin, thanks to their electronegativity, and produced simpler mass spectra. On the other hand, negative‐ion mode LDI‐MS was found to have a lower sensitivity to non‐brominated compounds, such as indigo and indirubin, whose presence can be established in any case by collecting the complementary positive‐ion LDI mass spectrum. Copyright © 2013 John Wiley & Sons, Ltd.     

Gas chromatography negative ion chemical ionization mass spectrometry: application to the detection of alkyl nitrates and halocarbons in the atmosphere

Alkyl nitrates and very short-lived halocarbon species are important atmospheric trace gas species that are present in the low to sub parts per trillion concentration range. This presents an analytical challenge for their detection and quantification that requires instrumentation with high sensitivity and selectivity. In this paper, we present a new in situ gas chromatograph negative ion chemical ionization mass spectrometer (GC/NICI-MS) coupled to a non-cryogen sample pre-concentration system. This instrument, with detection limits of <0.01 ppt, is capable of detecting and quantifying a large suite of alkyl nitrate and halocarbon species with high sensitivity and precision. The effects of ion source temperature and reagent gas pressure on the ionization efficiency of the NICI mode are investigated and the results are used to optimize the sensitivity. The NICI mode is compared to the more frequently used electron impact (EI) ionization and the enhancements in sensitivity are presented for all the calibrated compounds.     

Free amino acids analysis by liquid chromatography with tandem mass spectrometry in several botanicals with antioxidant character

A novel method based on liquid chromatography with tandem mass spectrometry for the analysis of 19 amino acids in plant materials is described. For the analysis, the plant material is extracted with 0.1 N hydrochloric acid with internal standards present in the extraction solution. The filtered extracts are injected using no clean‐up into the liquid chromatographic system coupled with a triple‐quadrupole tandem mass spectrometer with an electrospray ionization source. The analytes are separated using ion pair chromatography on a reversed‐phase column. The detection is performed in multiple‐reaction monitoring positive‐ion mode. Quantitation is obtained using calibrations. The validated procedure has been applied for the analysis of amino acids in 18 samples of plant material including botanicals with antioxidant character. The analysis requires 16 min separation time, has excellent precision and accuracy allowing amino acid analysis in a wide range of concentrations.     

The direct total evaporation (DTE) method for TIMS analysis

The total evaporation (TE) method is an established analytical method for safeguards measurements of uranium and plutonium isotope-amount ratios using thermal ionization mass spectrometry. As fractionation effects are minimized in this analytical method, it is a method of choice in many practical applications that require high accuracy and precision isotope abundance ratio measurements. The speed of signal regulation is a critical parameter for a steady sample evaporation process. Standard TE methods use the data system to read the ion signal and its difference from the target intensity is used to determine the increment in which the filament is heated. The new, hardware-driven proprietary direct total evaporation method uses an analog regulator in the filament power supply with direct feedback of the detector intensity. Only target values are set by the data system initially. The filament heating and sample evaporation process is then carried out by the hardware. The data system just monitors, collects, and calculates the data. Due to the nature of electronic regulation the ion signal is kept stable for the duration of the run until the whole sample is consumed. For routine uranium isotopic analyses of the major isotope-amount ratio n(²³⁵U)/n(²³⁸U) using a modified MAT261 instrument with SPECTROMAT? hardware and software upgrades, precision (relative standard deviation, expressed as a percent) and accuracy (relative difference, expressed as a percent) of 0.05 % are obtained for low enriched and high enriched uranium certified reference materials.     

Compatibility of electron ionization and soft ionization methods in gas chromatography/orthogonal time‐of‐flight mass spectrometry

Orthogonal acceleration time‐of‐flight (oa‐TOF) mass spectrometry (MS) was coupled to gas chromatography (GC) to measure ion yields (ratio of ion counts to number of neutrals entering the ion source) and signal‐to‐noise (S/N) in the electron ionization (EI) mode (hard ionization) as well as in the soft ionization modes of chemical ionization (CI), electron capture negative ion chemical ionization (NICI) and field ionization (FI). Mass accuracies of the EI and FI modes were also investigated. Sixteen structurally diverse volatile organic compounds were chosen for this study. The oa‐TOF mass analyzer is highly suited for FI MS and provided an opportunity to compare the sensitivity of this ionization method to the more conventional ionization methods. Compared to the widely used quadrupole mass filter, the oa‐TOF platform offers significantly greater mass accuracy and therefore the possibility of determining the empirical formula of analytes. The findings of this study showed that, for the instrument used, EI generated the most ions with the exception of compounds able to form negative ions readily. Lower ion yields in the FI mode were generally observed but the chromatograms displayed greater S/N and in many cases gave spectra dominated by a molecular ion. Ion counts in CI are limited by the very small apertures required to maintain sufficiently high pressures in the ionization chamber. Mass accuracy for molecular and fragment ions was attainable at close to manufacturer's specifications, thus providing useful information on molecular ions and neutral losses. The data presented also suggests a potentially useful instrumental combination would result if EI and FI spectra could be collected simultaneously or in alternate scans during GC/MS. Copyright © 2009 John Wiley & Sons, Ltd.     

Determination of boron isotopic variations in aquatic systems with negative thermal ionization mass spectrometry as a tracer for anthropogenic influences

A technique for precise boron isotope ratio measurements with a high detection power has been developed by negative thermal ionization mass spectrometry (NTIMS). Relative standard deviations in the range of 0.03-0.3% have been obtained for the determination of the (11)B/(10)B isotope ratio using nanogram amounts of boron. Ba(OH)(2) has been applied as ionization promoter for the formation of negative thermal ions. By adding MgCl(2) better reproducibilities of the measurement have been achieved. A possible interference of BO(-)(2) ions at mass number 42 by CNO(-) could be excluded by the sample preparation technique used. Contrary to other NTI techniques no dependence of the measured isotope ratio on the boron amount used has been observed. Anthropogenic and natural saline influences in ground water have been successfully identified by boron isotope ratio determinations with this NTIMS method, due to the different isotopic composition of boron in natural and anthropogenic substances. In sewage, the boron isotope ratio is substantially influenced by washing powder, which contains low (11)B/(10)B ratios (expressed in delta(11)B values normalized to the standard reference material NIST SRM 951). In contaminated ground water, low delta(11)B values are normally correlated with high boron and high chloride concentrations. On the other hand, delta(11)B shifts to higher values in less contaminated samples. For ground water with saline influences, only the delta(11)B determination, and not the boron or chloride content, allowed the correct identification of this natural source of contamination.