On-line technique for the determination of the delta37Cl of inorganic and total organic Cl in environmental samples

Here we describe an on-line method for measuring delta(37)Cl values of chloride bearing salts, waters, and organic materials using multicollector continuous-flow isotope ratio mass spectrometry (CF-IRMS). Pure AgCl quantitatively derived from total Cl in water, inorganic Cl salts, and biological samples was reacted with iodomethane in evacuated 10-mL stopper sealed glass vials to produce methyl chloride gas. A GV Instruments Multicollector CF-IRMS with CH(3)Cl optimized collector geometry was modified to accommodate a headspace single-sample gas injection port prior to a GC column. The GC column was a 2-m Porapak-Q packed column held at 160 degrees C. The resolved sample CH(3)Cl was introduced to the IRMS source in a helium stream via an open split. delta(37)Cl values were calculated by measurement of CH(3)Cl at m/z 52/50 and by comparison to a reference pulse of CH(3)Cl calibrated to standard mean ocean chloride. Sample CH(3)Cl analysis time was approximately 6 min. Injections of 40 microL of pure CH(3)Cl gas yielded a repeatability (+/-SD) of +/-0.06 per thousand for delta(37)Cl (n = 10). Combined GC and IRMS source linearity for CH(3)Cl was <0.2 per thousand/nA (V) peak height. External repeatability, based on processing of seawater and NaCl reference solutions, was better than +/-0.08 per thousand. The smallest sample for delta(37)Cl analysis by this method was approximately 0.2 micromol of Cl. Selected results from a river basin and biological samples study illustrate the potential of on-line chlorine isotope assays in environmental pollution studies.     

Compound-specific chlorine isotope analysis: a comparison of gas chromatography/isotope ratio mass spectrometry and gas chromatography/quadrupole mass spectrometry methods in an interlaboratory study

Chlorine isotope analysis of chlorinated hydrocarbons like trichloroethylene (TCE) is of emerging demand because these species are important environmental pollutants. Continuous flow analysis of noncombusted TCE molecules, either by gas chromatography/isotope ratio mass spectrometry (GC/IRMS) or by GC/quadrupole mass spectrometry (GC/qMS), was recently brought forward as innovative analytical solution. Despite early implementations, a benchmark for routine applications has been missing. This study systematically compared the performance of GC/qMS versus GC/IRMS in six laboratories involving eight different instruments (GC/IRMS, Isoprime and Thermo MAT-253; GC/qMS, Agilent 5973N, two Agilent 5975C, two Thermo DSQII, and one Thermo DSQI). Calibrations of (37)Cl/(35)Cl instrument data against the international SMOC scale (Standard Mean Ocean Chloride) deviated between instruments and over time. Therefore, at least two calibration standards are required to obtain true differences between samples. Amount dependency of δ(37)Cl was pronounced for some instruments, but could be eliminated by corrections, or by adjusting amplitudes of standards and samples. Precision decreased in the order GC/IRMS (1σ ≈ 0.1‰), to GC/qMS (1σ ≈ 0.2-0.5‰ for Agilent GC/qMS and 1σ ≈ 0.2-0.9‰ for Thermo GC/qMS). Nonetheless, δ(37)Cl values between laboratories showed good agreement when the same external standards were used. These results lend confidence to the methods and may serve as a benchmark for future applications.     

Chlorine stable isotopes: a comparison of dual inlet and thermal ionization mass spectrometric measurements

Chlorine stable isotope ratios, 37Cl/35Cl, currently are measured using dual-inlet and thermal-ionization mass spectrometry. These two different analytical techniques, however, have never been cross calibrated. A set of samples with chlorine stable isotope delta values ranging from -4.4 to +0.3 % relative to standard mean ocean water chloride has been analyzed using both of these techniques. Our data show that both techniques can yield similar results within analytical uncertainty. CsCl thermal ionization data are extremely sensitive to the amount of chlorine being measured and cannot be used to determine absolute ratios without an independent means of correcting for machine-induced mass fractionation. As long as standards and samples are of equivalent size, however, the differences between samples measured by thermal ionization remain constant Dual inlet stable isotope mass spectrometry is suited best for samples of > 10 micromol Cl, yielding chlorine stable isotope data with < or =0.1% reproducibilities (2sigma). Thermal ionization mass spectrometry easily accommodates samples of approximately0.1-0.3 micromol Cl, with achievable uncertainties of < or =0.2% (2sigma).     

Perchlorate isotope forensics

Perchlorate has been detected recently in a variety of soils, waters, plants, and food products at levels that may be detrimental to human health. These discoveries have generated considerable interest in perchlorate source identification. In this study, comprehensive stable isotope analyses (37Cl/35Cl and 18O/17O/16O) of perchlorate from known synthetic and natural sources reveal systematic differences in isotopic characteristics that are related to the formation mechanisms. In addition, isotopic analyses of perchlorate extracted from groundwater and surface water demonstrate the feasibility of identifying perchlorate sources in contaminated environments on the basis of this technique. Both natural and synthetic sources of perchlorate have been identified in water samples from some perchlorate occurrences in the United States by the isotopic method.     

A technique for the determination of 18O/16O and 17O/16O isotopic ratios in water from small liquid and solid samples

We have developed a new technique in which a solid reagent, cobalt(III) fluoride, is used to prepare oxygen gas for isotope ratio measurement from water derived either from direct injection or from the pyrolysis of solid samples. The technique uses continuous flow, isotope ratio monitoring, gas chromatography/mass spectrometry (irmGC/MS) to measure the delta18O and delta17O of the oxygen gas. Water from appropriate samples is evolved by a procedure of stepped pyrolysis (0-1000 degrees C, typically in 50 degrees C increments) under a flowing stream of helium carrier gas. The method has considerable advantages over others used for water analysis in that it is quick; requires only small samples, typically 1-50 mg of whole rock samples (corresponding to approximately 0.2 micromol of H2O); and the reagent is easy and safe to handle. Reproducibility in isotope ratio measurement obtained from pyrolysis of samples of a terrestrial solid standard are delta18O +/- 0.54, delta17O +/- 0.33, and delta17O +/- 0.10/1000, 1sigma in all cases. The technique was developed primarily for the analysis of meteorites, and the efficiency of the method is illustrated herein by results from water standards, solid reference materials, and a sample of the Murchison CM2 meteorite.     

Analysis of perchlorate in water by reversed-phase LC/ESI-MS/MS using an internal standard technique

A new method was developed for the analysis of perchlorate in water by using reversed-phase liquid chromatograhy/electrospray ionization-mass spectrometry/mass spectrometry (LC/ESI-MS/MS) in the negative ESI mode. Selective and sensitive perchlorate detection was obtained by monitoring the 35ClO4- --> 35ClO3- and 37ClO4- --> 37ClO3- mass transitions. The 35ClO4- --> 35ClO3- transition was quantitated against the internal standard oxygen-labeled sodium perchlorate (NaCl18O4). Sample pretreatment for the removal of major common anions and dissolved metal ions along with internal standard quantitation sufficiently compensated for ion suppression caused by the matrix. The 37ClO4- --> 37ClO3- transition was examined to provide additional specificity. The method sensitivity, accuracy, and precision were investigated by analyzing fortified blank samples, field samples, and performance evaluation samples. The results (1.01-13.5 microg/L) for the proficiency evaluation samples differed from the certified values (1.04-14.1 microg/L) by 3-18%. The developed reversed-phase LC/ESI-MS/MS method was rapid, accurate, and reproducible. The calculated method detection limits were 0.007 microg/L for deionized reagent water and 0.009 microg/L for synthesized reagent water, respectively. The minimum reporting limit was conservatively set to 0.05 microg/L.     

Analysis of perchlorate in water and soil by electrospray LC/MS/MS

A method has been developed for the determination of perchlorate in water and soil matrixes using electrospray liquid chromatography/mass spectrometry/mass spectrometry. Perchlorate is quantitated by monitoring the ion signal from mass 83, which is formed by a loss of an oxygen atom from the perchlorate molecular ion. The method was developed to be effective and economical in production laboratory analysis of perchlorate in environmental water and soil samples. Data were gathered to define method sensitivity, performance, selectivity, and robustness. Analyte stability, method susceptibility to interferences, and the reliability of the chlorine isotope ratio as an identification tool were examined. The aqueous method detection limit (MDL) is 0.05 microg/L and was determined using an actual groundwater matrix. The soil MDL is 0.5 microg/kg and was determined using Ottawa sand. The stability study was performed by spiking water samples at 0.25, 10, and 20 microg/L and analyzing them 50 days later. Acceptable recoveries were obtained for all samples. The relative standard deviation (RSD) for the replicate analyses in the stability study indicates that the method is capable of RSD values less than 5% in a relatively clean groundwater matrix. The ionization suppression study was performed by spiking water samples containing 1000 mg/L carbonate, chloride, and sulfate with 0.05 and 0.5 microg/L perchlorate and then measuring the recovery of the spike. The results indicate that the procedure does not have significant suppression effects at the high salt levels tested. Calibration, quality control sample, field sample, and suppression study data were combined to examine isotope ratio reliability. The results of that work show that chlorine isotope ratios can be used to define statistical process control limits for use as an additional analyte identification tool.     

Gas-phase ion association provides increased selectivity and sensitivity for measuring perchlorate by mass spectrometry

Perchlorate (ClO4-) competitively inhibits the uptake of iodide by the thyroid gland. Trace quantities of perchlorate are being increasingly detected in food and environmental samples. There is great concern that perchlorate contamination may be far more widespread than believed until now. Increasingly sensitive and unambiguous methods are needed for measuring perchlorate. We report here an ion chromatography-ion association-electrospray ionization-mass spectrometry (IC/IA-ESI-MS) method of substantially greater selectivity and sensitivity than other available single-stage MS approaches. A long chain dipositive cationic agent (D2+) is added postcolumn in low concentration. This ion associates with perchlorate, even in the gas phase. Perchlorate is, thus, detected as DClO4+ in the positive ion mode at an m/z value between 300 and 400 (depending on the choice of D2+). This results in much better S/N and selectivity, as compared to detecting 35ClO4- at m/z 99, where H34SO4- also responds. We show results for various dicationic agents which vary in their selectivity and affinity for ClO4-, typically being at least 1 order of magnitude more selective for ClO4- over HSO4-. For a 100-microL injected standard, limits of detection (LOD, S/N = 3) are as good as 25 ng/L on a single quadrupole mass spectrometer. Calibration for concentrations up to 100 microg/L displays an r2 value of > or =0.9993. We show applicability to various real samples. A number of the studied reagents are suitable for such applications.     

Isotopic variations of Zn in biological materials

Variations in the isotopic composition of Zn present in various biological materials were determined using high-resolution multicollector inductively coupled plasma mass spectrometry (MC-ICPMS), following digestion and purification by anion exchange chromatography. To correct for differences in instrumental mass discrimination effects between samples and standards, Cu was employed as an elemental spike. Complementary analyses of Zn separates by sector field ICPMS instruments revealed that the concentrations of the majority of potentially interfering elements were reduced to negligible levels. Residual spectral interferences resulting from (35)Cl(16)O(2)(+), (40)Ar(14)N(2)(+), and (40)Ar(14)N(16)O(+) could be instrumentally resolved from the (67)Zn, (68)Zn, and (70)Zn ion beams, respectively, during measurement by MC-ICPMS. The only other observed interference in the Cu and Zn mass range that could not be effectively eliminated by high-resolution multicollection resulted from (35)Cl(2)(+), necessitating modification of the sample preparation procedure to allow accurate (70)Zn detection. Complete duplication of the entire analytical procedure for human whole blood and hair, as well as bovine liver and muscle, provided an external reproducibility of 0.05-0.12 per thousand (2sigma) for measured delta(66/64)Zn, delta(67/64)Zn, and delta(68/64)Zn values, demonstrating the utility of the method for the precise isotopic analysis of Zn in biological materials. Relative to the selected Zn isotopic standard, delta(66/64)Zn values for biological samples varied from -0.60 per thousand in human hair to +0.56 per thousand in human whole blood, identifying the former material as the isotopically lightest Zn source found in nature to date.     

Quantitative determination of absolute organohalogen concentrations in environmental samples by X-ray absorption spectroscopy

An in situ procedure for quantifying total organic and inorganic Cl concentrations in environmental samples based on X-ray absorption near-edge structure (XANES) spectroscopy has been developed. Cl 1s XANES spectra reflect contributions from all Cl species present in a sample, providing a definitive measure of total Cl concentration in chemically heterogeneous samples. Spectral features near the Cl K-absorption edge provide detailed information about the bonding state of Cl, whereas the absolute fluorescence intensity of the spectra is directly proportional to total Cl concentration, allowing for simultaneous determination of Cl speciation and concentration in plant, soil, and natural water samples. Absolute Cl concentrations are obtained from Cl 1s XANES spectra using a series of Cl standards in a matrix of uniform bulk density. With the high sensitivity of synchrotron-based X-ray absorption spectroscopy, Cl concentration can be reliably measured down to the 5-10 ppm range in solid and liquid samples. Referencing the characteristic near-edge features of Cl in various model compounds, we can distinguish between inorganic chloride (Cl(inorg)) and organochlorine (Cl(org)), as well as between aliphatic Cl(org) and aromatic Cl(org), with uncertainties in the range of approximately 6%. In addition, total organic and inorganic Br concentrations in sediment samples are quantified using a combination of Br 1s XANES and X-ray fluorescence (XRF) spectroscopy. Br concentration is detected down to approximately 1 ppm by XRF, and Br 1s XANES spectra allow quantification of the Br(inorg) and Br(org) fractions. These procedures provide nondestructive, element-specific techniques for quantification of Cl and Br concentrations that preclude extensive sample preparation.     

Major factors affecting the isotopic measurement of chlorine based on the Cs2Cl+ ion by thermal ionization mass spectrometry

The factors that affect isotopic measurement of chlorine based on Cs2Cl+ ion by thermal ionization mass spectrometry were studied. Graphite is essential for the emission of Cs2Cl+ ion from CsCl. No Cs2Cl+ ions are detected in the absence of graphite on the filament. The emission of Cs2Cl+ ion and the measured 37Cl/35Cl ratio were affected by different varieties of graphite, the pH value of the sample solution, and impurities in the solution. High-precision isotopic measurement of chlorine based on Cs2Cl+ ion is achieved only by using graphite with a perfect crystal structure. The measured 37Cl/35Cl ratios were much higher, and even the emission of Cs2Cl+ ion becomes impossible when pH values of the sample solution were higher than 6, corresponding to the presence of excess Cs. The measured 37Cl/35Cl ratios were higher when SO4(2-) and NO3- anions were present. The results show that the measured 37Cl/35Cl ratios are weakly related to the amount of chlorine on filament in a range from 1 to 500 microg.     

Continuous extraction of trapped air from bubble ice or water for on-line determination of isotope ratios

We describe a new continuous extraction system for trapped air from bubble ice or water for on-line determination of the isotopic composition of the main air components nitrogen and oxygen (delta15N, delta18O, and delta17O). Studies of the composition of air from bubbles trapped in polar ice are providing fundamental information about ancient atmospheric composition and, therefore, are an important tool to learn more about Earth's climate. The new system proved to work reliably for standard air admixed and subsequently removed from a water stream. The precision (1 SD) of standard measurements is approximately 0.04/1000 for delta15N, approximately 0.1/1000 for delta18O, and approximately 0.15/1000 for delta17O. Ice measurements with the new on-line system are promising. Continuous measurements of nitrogen as well as oxygen isotope ratios can be performed with a spatial resolution of approximately 3 cm and nearly the same precision as for the standards. However, the measured delta values of ice are generally lower, as compared to ice measured with conventional techniques, as a result of a time-dependent dissolution process of air in water associated with kinetic fractionation, which affects standard and sample differently. By modeling the dynamics of the this dissolution process, we found a reason for the lack of accuracy and propose an improvement of the system that will lead to a better accuracy of the ice measurements.     

A method for the extraction of ammonium from freshwaters for nitrogen isotope analysis

The measurement of delta15N values of inorganic nitrogen species is an important analytical tool to trace nitrogen species in order to understand nitrogen cycling in aquatic systems. Nitrogen isotope analysis of freshwater ammonium has, however, been hindered by the lack of a simple and reliable technique to measure delta15N values at natural abundance levels. We present a simple and rapid method to concentrate ammonium from freshwater samples for on-line N-isotope ratio determination. Ammonium is collected by adsorption on N-free cation exchange resins. The dried N-loaded exchange resin is then directly combusted to produce N2 gas for subsequent delta15N analysis. The method was evaluated with simulated freshwater solutions containing varying amounts of standard NH4+-N (delta15N = 2.1 per thousand) and potentially interfering inorganic and organic compounds. In general, the cation exchange resin method gives accurate and reproducible delta15N values (sigma1 < 0.3 per thousand; n = 10). Because of adsorption interference, high concentrations of cations in solution may cause ammonium loss but do not result in measurable isotope fractionation. Replicate extractions of the ammonium standard added to water collected from four Swiss lakes demonstrate the good performance of this method when applied to low ionic strength natural water samples with modest concentrations of dissolved organic nitrogen.     

Attenuated total reflectance FTIR detection and quantification of low concentrations of aqueous polyatomic anions

Development of a new quantitative method for determining low concentrations of aqueous polyatomic anions using attenuated total reflectance (ATR) FTIR spectroscopy is described. Evaporated thin-film coatings of anion-selective tetraalkylated ferrocenium salts were applied to the surface of ATR crystals, which enabled anion detection limits to be lowered up to 23 000-fold below those achieved using the commercially available spectrometer with identical uncoated ATR crystals. Linear calibration curves based on d(absorbance)/dt, which is related to the rate of anion exchange in the thin film, were established in the 0.04-30 microM range. Limits of detection (10-min analyses) for perchlorate, chlorate, trifluoromethanesulfonate, perfluoro-n-butanesulfonate, perfluoro-n-octanesulfonate, tetrafluoroborate, hexafluorophosphate, and pinacolylmethylphosphonate in aqueous solution were 0.03, 0.2, 0.05, 0.07, 0.06, 0.06, 0.6, and 0.7 microM, respectively, using the thin-film coatings. This simple detection/quantification method afforded good reproducibility with relatively fast detection times.     

Behaviour of metals under the conditions of roasting MSW incinerator fly ash with chlorinating agents.

A total elemental analysis was performed on a municipal solid waste (MSW) fly ash sample, before and after it was treated at 1000 degrees C, to reveal the metal distribution between the volatile matter and the ash residue. Metals such as Pb, Zn, Cd, and to a lesser degree, Cr, Mn and Ni, were volatilized. Addition of chlorinating agents generally increased the volatility of certain elements. More acid resistant compounds were formed in the ash residue after the heat treatment using CaCl2 as a chlorinating agent. The efficiencies of volatilization of the metals, using Cl2 as a chlorinating agent, were generally higher compared with using CaCl2. However, CaCl2 was found to be a more selective chlorinating agent for volatilizing the heavy metals of concern, i.e., Pb, Cd, Zn and Cu. The efficiencies of volatilization of the recovered metals were approximately proportional to their standard free-energy changes (delta G(o) for the corresponding chlorination reactions.     

GC/multiple collector-ICPMS method for chlorine stable isotope analysis of chlorinated aliphatic hydrocarbons

Stable isotopic characterization of chlorine in chlorinated aliphatic pollution is potentially very valuable for risk assessment and monitoring remediation or natural attenuation. The approach has been underused because of the complexity of analysis and the time it takes. We have developed a new method that eliminates sample preparation. Gas chromatography produces individually eluted sample peaks for analysis. The He carrier gas is mixed with Ar and introduced directly into the torch of a multicollector ICPMS. The MC-ICPMS is run at a high mass resolution of >/=10 000 to eliminate interference of mass 37 ArH with Cl. The standardization approach is similar to that for continuous flow stable isotope analysis in which sample and reference materials are measured successively. We have measured PCE relative to a laboratory TCE standard mixed with the sample. Solvent samples of 200 nmol to 1.3 micromol (24-165 microg of Cl) were measured. The PCE gave the same value relative to the TCE as measured by the conventional method with a precision of 0.12 per thousand (2x standard error) but poorer precision for the smaller samples.     

Determination of the total oxygen isotopic composition of nitrate and the calibration of a delta 17O nitrate reference material

A thermal decomposition method was developed and tested for the simultaneous determination of delta 18O and delta 17O in nitrate. The thermal decomposition of AgNO3 allows for the rapid and accurate determination of 18O/ 16O and 17O/16O isotopic ratios with a precision of +/- 1.5 per thousand for delta 18O and +/- 0.11 per thousand for delta 17O (delta 17O = delta 17O - 0.52 x delta 18O). The international nitrate isotope reference material IAEA-NO3 yielded a delta 18O value of +23.6 per thousand and delta 17O of -0.2 per thousand, consistent with normal terrestrial mass-dependent isotopic ratios. In contrast, a large sample of NaNO3 from the Atacama Desert, Chile, was found to have delta 17O = 21.56 +/- 0.11 per thousand and delta 18O = 54.9 +/- 1.5 per thousand, demonstrating a substantial mass-independent isotopic composition consistent with the proposed atmospheric origin of the desert nitrate. It is suggested that this sample (designated USGS-35) can be used to generate other gases (CO2, CO, N2O, O2) with the same delta 17O to serve as measurement references for a variety of applications involving mass-independent isotopic compositions in environmental studies.     

Estimating octanol-air partition coefficients of nonpolar semivolatile organic compounds from gas chromatographic retention times

Relative gas chromatographic retention times on a non-polar stationary phase can be used to determine the octanol-air partition coefficient (K(OA)) and the energy of phase transfer between octanol and the gas phase (delta(OA)U) for semivolatile, nonpolar organic compounds. The only prerequisites are knowledge of the temperature-dependent K(OA) of a standard reference compound and directly measured K(OA) values at one temperature for a sufficient number of calibration compounds. It is shown that the technique is capable of predicting the K(OA) of polychlorinated benzenes, biphenyls and naphthalenes as well as polybrominated diphenyl ethers within the environmentally relevant temperature range with an average deviation from directly measured values of <0.2 log units.     

Determination of perchlorate anion in foods by ion chromatography-tandem mass spectrometry

A rapid, sensitive, and specific method was developed for determining perchlorate anion in lettuce, cantaloupe, bottled water, and milk. A test portion of chopped crop homogenate was extracted with diluted nitric acid and filtered. Milk proteins were precipitated with acetonitrile, and the supernatant, after centrifugation, was cleaned up on a graphitized carbon solid-phase extraction column. Water samples were analyzed directly. All test solutions were syringe filtered and mixed with an 18O4-labeled perchlorate internal standard before ion chromatography-tandem mass spectrometry. A strong anion exchange column eluted with 100 mM ammonium acetate in 50:50 (v/v) acetonitrile/water was interfaced via electrospray ionization to a triple stage quadrupole mass spectrometer operated in the negative ion mode. The labeled internal standard corrected for any sample matrix effects on measured signals. Four parent-to-product ion transitions, for loss of oxygen, were monitored for native and 18O4-labeled perchlorate anion, respectively: 35Cl-perchlorate, m/z 99 --> 83 and 107 --> 89; 37Cl-perchlorate, m/z 101 --> 85 and 109 --> 91. The limit of quantitation was 1.0 microg/kg in lettuce, 2.0 microg/kg in cantaloupe, 0.50 microg/L in bottled water, and 3.0 microg/L in milk. Native perchlorate was recovered from fortified test portions in the range 93-107% for lettuce, 107-114% for cantaloupe, 100-115% for bottled water, and 99-101% for milk.     

Microbial community analysis of perchlorate-reducing cultures growing on zero-valent iron

Anaerobic microbial mixed cultures demonstrated its ability to completely remove perchlorate in the presence of zero-valent iron. In order to understand the major microbial reaction in the iron-supported culture, community analysis comprising of microbial fatty acids and polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) techniques was performed for perchlorate reducing cultures. Analysis of fatty acid methyl esters (FAMEs) and subsequent principal component analysis (PCA) showed clear distinctions not only between iron-supported perchlorate reducing culture and seed bacteria, but also among perchlorate-reducing cultures receiving different electron donors. The DGGE pattern targeting the chlorite dismutase (cld) gene showed that iron-supported perchlorate reducing culture is similar to hydrogen-fed cultures as compared to acetate-fed culture. The phylogenetic tree suggested that the dominant microbial reaction may be a combination of the autotrophic and heterotrophic reduction of perchlorate. Both molecular and chemotaxonomic experimental results support further understanding in the function of zero-valent iron as an adequate electron source for enhancing the microbial perchlorate reduction in natural and engineered systems.