Determination of the natural abundance δ15N of nortropane alkaloids by gas chromatography–isotope ratio mass spectrometry of their ethylcarbamate esters

An important route for the detoxification of tropane alkaloids involves N-demethylation to the nor-compounds followed by further degradation. In order to study the mechanisms of the pertinent reactions, a suitable means to determine the isotope ratios of the substrates and products is required. However, the polarity and functionality of the nortropane compounds makes their analysis as free bases difficult. A method is described which allows both the quantification of nortropane alkaloids and the determination of their natural abundance δ¹⁵N values. The protocol exploits the derivatisation of the alkaloids by reaction with ethyl chloroformate in aqueous medium and the quantitative extraction of the ensuing ethylcarbamate esters. The improved chromatographic properties of these derivatives gives ample separation of the isomeric nortropine and norpseudotropine for measurement of their δ¹⁵N (‰) values by isotope ratio mass spectrometry interfaced to gas chromatography. Adequate separation could not be achieved with the underivatised compounds. Repeatability and precision are sufficient to allow differences in the δ¹⁵N values (∆δ¹⁵N) > 0.8‰ to be measured, with a standard deviation routinely ∼0.3‰. The methodology has been tested by determining the changes in the δ¹⁵N values of nortropine and norpseudotropine during degradation by cell suspension cultures of a Pseudomonas strain expressing a specific capacity for tropine catabolism. The precision and reproducibility are shown sufficient to allow the evolution of the δ¹⁵N values to be followed during the fermentation.     

Stable carbon isotope ratios of methyl-branched fatty acids are different to those of straight-chain fatty acids in dairy products

Methyl-branched fatty acids (MBFAs) are the dominant form of fatty acid found in many bacteria. They are also found at low levels in a range of foodstuffs, where their presence has been linked to bacterial sources. In this study we evaluated the potential of compound-specific isotope analysis to obtain insights into the stable carbon isotope ratios (δ¹³C values in ‰) of individual MBFAs and to compare them to the stable carbon isotope ratios of straight-chain fatty acids in food. Due to their low abundance in foodstuffs, the MBFAs were enriched prior to gas chromatography coupled to isotope ratio mass spectrometric (GC–IRMS) analysis. After transesterification, urea complexation was used to suppress the 16:0 and 18:0 methyl esters that were dominant in the samples. Following that, silver-ion high performance liquid chromatography was used to separate the saturated from the unsaturated fatty acids. The resulting solutions of saturated fatty acids obtained from suet, goat’s milk, butter, and human milk were studied by GC–IRMS. The δ¹³C values of fatty acids with 12–17 carbons ranged from −25.4‰ to −37.6‰. In all samples, MBFAs were most depleted in carbon-13, followed by the odd-chain fatty acids 15:0 and 17:0. 14:0 and 16:0 contained the highest proportions of carbon-13. The results from this study illustrate that MBFAs have distinctive δ¹³C values and must originate from other sources and/or from very different substrates. These measurements support the initial hypothesis that δ¹³C values can be used to attribute MBFAs to particular sources.     

Follow-up of stable isotope analysis of organic versus conventional milk

Analysis of the stable isotope ratio of carbon (δ ¹³C) and α-linolenic acid (C18:3ω3) content in milk fat is a useful indicator of organic milk production. Referring to corresponding measurements, further analyses of stable isotope ratios were performed in 120 samples of conventionally and organically produced whole milk collected from German retailers during a period of 18 months. Conventional milk predominantly exhibited higher δ ¹⁵N values than organic milk, the latter of which never exceeded a maximum δ ¹⁵N threshold value of 5.50‰. Measurements of δ ³⁴S did not differ significantly between organic and conventional milk. Because δ ¹³C, in general, is related to maize consumption, δ ¹³C in milk protein and δ ¹³C in milk fat were equally suited for authentication of organic milk. Thus, a high correlation (r = 0.99) was established between δ ¹³C in milk protein and lipids. Although occurring on different levels in organic and conventional milk, the relatively constant fractionation of carbon isotopes between protein and fat will allow for the advanced detection of adulteration in processed milk products, such as fraudulent combinations of organic milk fat and conventional skim milk. In addition to the strong correlation between C18:3ω3 and δ ¹³C_protein (r = −0.91), a mutual dependence was identified between both δ ¹³C_protein and δ ¹⁵N (r = 0.66) and C18:3ω3 and δ ¹⁵N (r = −0.61). Thus, multi-variable analyses are useful to increase robustness and reduce the number of exceptions in organic milk authentication. Future work involving multivariate statistical analysis can possibly further improve milk authentication in various respects including differentiating between brands of retail milk.     

Determination of compound-specific Hg isotope ratios from transient signals using gas chromatography coupled to multicollector inductively coupled plasma mass spectrometry (MC-ICP/MS)

MeHg and inorganic Hg compounds were measured in aqueous media for isotope ratio analysis using aqueous phase derivatization, followed by purge-and-trap preconcentration. Compound-specific isotope ratio measurements were performed by gas chromatography interfaced to MC-ICP/MS. Several methods of calculating isotope ratios were evaluated for their precision and accuracy and compared with conventional continuous flow cold vapor measurements. An apparent fractionation of Hg isotopes was observed during the GC elution process for all isotope pairs, which necessitated integration of signals prior to the isotope ratio calculation. A newly developed average peak ratio method yielded the most accurate isotope ratio in relation to values obtained by a continuous flow technique and the best reproducibility. Compound-specific isotope ratios obtained after GC separation were statistically not different from ratios measured by continuous flow cold vapor measurements. Typical external uncertainties were 0.16‰ RSD (n = 8) for the ²⁰²Hg^/198Hg ratio of MeHg and 0.18‰ RSD for the same ratio in inorganic Hg using the optimized operating conditions. Using a newly developed reference standard addition method, the isotopic composition of inorganic Hg and MeHg synthesized from this inorganic Hg was measured in the same run, obtaining a value of δ ²⁰²Hg = −1.49 ± 0.47 (2SD; n = 10). For optimum performance a minimum mass of 2 ng per Hg species should be introduced onto the column.     

Identification of ground water contaminations by landfills using precise boron isotope ratio measurements with negative thermal ionization mass spectrometry

 Precise boron isotope ratio measurements with negative thermal ionization mass spectrometry were used for the identification of ground water contaminations by leakages of landfills. BO^- ₂thermal ions were produced to determine the ¹¹B/¹⁰B isotope ratio, which was expressed as δ¹¹B value in ‰ normalized to the standard reference material NIST SRM 951. For example, household waste influences the boron isotope ratio by specific components such as washing powder. In the case of one investigated landfill low δ¹¹B values correlate well with high boron concentrations in contaminated seepage water samples and vice versa for uncontaminated ground water samples. Possible boron contributions of rainwater were taken into account, determining a boron content of 2.3 μg/L and a δ¹¹B value of 13.1‰ for a representative sample. Such low boron concentrations were determined by isotope dilution mass spectrometry (detection limit 0.3 μg/L) whereas higher contents were also analyzed by a spectrophotometric method. However, different sources of contamination could only be identified by the isotope ratio and not by the concentration of boron. Received: 9 December 1996/Accepted: 18 February 1997     

Mg isotope fractionation in biogenic carbonates of deep-sea coral, benthic foraminifera, and hermatypic coral

High-precision Mg isotope measurements by multiple collector inductively coupled plasma mass spectrometry were applied for determinations of magnesium isotopic fractionation of biogenic calcium carbonates from seawater with a rapid Mg purification technique. The mean δ²⁶Mg values of scleractinian corals, giant clam, benthic foraminifera, and calcite deep-sea corals were −0.87‰, −2.57‰, −2.34‰, and −2.43‰, suggesting preferential precipitation of light Mg isotopes to produce carbonate skeleton in biomineralization. Mg isotope fractionation in deep-sea coral, which has high Mg calcite skeleton, showed a clear temperature (T) dependence from 2.5 °C to 19.5 °C: 1,000 × ln(α) = −2.63 (±0.076) + 0.0138 (±0.0051) × T(R ² = 0.82, p < 0.01). The δ²⁶Mg values of large benthic foraminifera, which are also composed of a high-Mg calcite skeleton, can be plotted on the same regression line as that for deep-sea coral. Since the precipitation rates of deep-sea coral and benthic foraminifera are several orders of magnitude different, the results suggest that kinetic isotope fractionation may not be a major controlling factor for high-Mg calcite. The Mg isotope fractionation factors and the slope of temperature dependence from deep-sea corals and benthic foraminifera are similar to that for an inorganically precipitated calcite speleothem. Taking into account element partitioning and the calcification rate of biogenic CaCO₃, the similarity among inorganic minerals, deep-sea corals, and benthic foraminiferas may indicate a strong mineralogical control on Mg isotope fractionation for high-Mg calcite. On the other hand, δ²⁶Mg in hermatypic corals composed of aragonite has been comparable with previous data on biogenic aragonite of coral, sclerosponges, and scaphopad, regardless of species differences of samples.     

High-precision measurement of mercury isotope ratios in sediments using cold-vapor generation multi-collector inductively coupled plasma mass spectrometry

An on-line Hg reduction technique using stannous chloride as the reductant was applied for accurate and precise mercury isotope ratio determinations by multi-collector (MC)-ICP/MS. Special attention has been paid to ensure optimal conditions (such as acquisition time and mercury concentration) allowing precision measurements good enough to be able to significantly detect the anticipated small differences in Hg isotope ratios in nature. Typically, internal precision was better than 0.002% (1 RSE) on all Hg ratios investigated as long as approximately 20 ng of Hg was measured with a 10-min acquisition time. Introducing higher amounts of mercury (50 ng Hg) improved the internal precision to <0.001%. Instrumental mass bias was corrected using ²⁰⁵Tl/²⁰³Tl correction coupled to a standard-sample bracketing approach. The large number of data acquired allowed us to validate the consistency of our measurements over a one-year period. On average, the short-term uncertainty determined by repeated runs of NIST SRM 1641d Hg standard during a single day was <0.006% (1 RSD) for all isotope pairs investigated (²⁰²Hg/¹⁹⁸Hg, ²⁰²Hg/¹⁹⁹Hg, ²⁰²Hg/²⁰⁰Hg, and ²⁰²Hg/²⁰¹Hg). The precision fell to <0.01% if the long-term reproducibility, taken over 11 months (over 100 measurements), was considered. The extent of fractionation has been investigated in a series of sediments subject to various Hg sources from different locations worldwide. The ratio ²⁰²Hg/¹⁹⁸Hg expressed as δ values (per mil deviations relative to NIST SRM 1641d Hg standard solution) displayed differences from +0.74 to −4.00‰. The magnitude of the Hg fractionation per amu was constant within one type of sample and did not exceed 1.00‰. Considering all results (the reproducibility of Hg standard solutions, reference sediment samples, and the examination of natural samples), the analytical error of our δ values for the overall method was within ±0.28‰ (1 SD), which was an order of magnitude lower than the extent of fractionation (4.74‰) observed in sediments. This study confirmed that analytical techniques have reached a level of long-term precision and accuracy that is sufficiently sensitive to detect even small differences in Hg isotope ratios that occur within one type of samples (e.g., between different sediments) and so far have unequivocally shown that Hg isotope ratios in sediments vary within approximately 5‰.     

Impact of the deuterium isotope effect on the accuracy of 13C NMR measurements of site-specific isotope ratios at natural abundance in glucose

The application of isotope ratio methods in authenticity and traceability relies on the accuracy and robustness of the methodology employed. An unexpected source of error has now been identified, which can introduce major and variable inaccuracies into the determination of site-specific isotope ratio measurement by quantitative ¹³C NMR spectrometry if not correctly controlled. This is the isotope chemical shift effect, which comes into play when hydrogen atoms in the target molecule enter into exchange with deuterated water present at trace levels in the deuterated solvent used as the frequency lock. Even at a level of contamination as low as 0.02%, an error of 5‰ can be introduced, fivefold the required accuracy of 1‰. How to avoid this source of error is discussed.     

Determination of theophylline in serum by high performance liquid chromatography/mass spectrometry with atmospheric pressure chemical-ionization (APCI HPLC/MS)

 A method for the determination of theophylline (TH), without derivatization, in serum by isotope dilution mass spectrometry using labelled [1, 3-¹⁵N₂-2-¹³C]theophylline (LTH) as internal standard is described. After deproteinization, the analyte is directly injected into a high performance liquid chromatography – mass spectrometer operating with atmospheric-pressure chemical-ionization (APCI HPLC/MS). The concentrations of TH in sera measured by APCI HPLC/MS are compared with results from gas chromatography – isotope dilution mass spectrometry (GC-ID/MS), high performance liquid chromatography (HPLC) and fluorescence polarization immunoassay (FPIA). The accuracy, precision and recovery of the APCI HPLC/MS and GC-ID/MS methods are discussed. The coefficient of variation (CV) determined from duplicate samples was less than 2%. The detection limit was 10 ng/ml at a signal-to-noise ratio of 3:1. Received: 17 January 1996/Revised: 26 March 1996/Accepted: 5 April 1996     

Determination of nitrogen‐15 isotope fractionation in tropine: evaluation of extraction protocols for isotope ratio measurement by isotope ratio mass spectrometry

N‐Demethylation of tropine is an important step in the degradation of this compound and related metabolites. With the purpose of understanding the reaction mechanism(s) involved, it is desirable to measure the ¹⁵N kinetic isotope effects (KIEs), which can be accessed through the ¹⁵N isotope shift (Δδ¹⁵N) during the reaction. To measure the isotope fractionation in ¹⁵N during tropine degradation necessitates the extraction of the residual substrate from dilute aqueous solution without introducing artefactual isotope fractionation. Three protocols have been compared for the extraction and measurement of the ¹⁵N/¹⁴N ratio of tropine from aqueous medium, involving liquid‐liquid phase partitioning or silica‐C18 solid‐phase extraction. Quantification was by gas chromatography (GC) on the recovered organic phase and δ¹⁵N values were obtained by isotope ratio measurement mass spectrometry (irm‐MS). Although all the protocols used can provide satisfactory data and both irm‐EA‐MS and irm‐GC‐MS can be used to obtain the δ¹⁵N values, the most convenient method is liquid‐liquid extraction from a reduced aqueous volume combined with irm‐GC‐MS. The protocols are applied to the measurement of ¹⁵N isotope shifts during growth of a Pseudomonas strain that uses tropane alkaloids as sole source of carbon and nitrogen. The accuracy of the determination of the ¹⁵N/¹⁴N ratio is sufficient to be used for the determination of ¹⁵N‐KIEs. Copyright © 2009 John Wiley & Sons, Ltd.     

Specific Interaction Between Negative Atmospheric Ions and Organic Compounds in Atmospheric Pressure Corona Discharge Ionization Mass Spectrometry

The interaction between negative atmospheric ions and various types of organic compounds were investigated using atmospheric pressure corona discharge ionization (APCDI) mass spectrometry. Atmospheric negative ions such as O₂^–, HCO₃^–, COO^–(COOH), NO₂^–, NO₃^–, and NO₃^–(HNO₃) having different proton affinities served as the reactant ions for analyte ionization in APCDI in negative-ion mode. The individual atmospheric ions specifically ionized aliphatic and aromatic compounds with various functional groups as atmospheric ion adducts and deprotonated analytes. The formation of the atmospheric ion adducts under certain discharge conditions is most likely attributable to the affinity between the analyte and atmospheric ion and the concentration of the atmospheric ion produced under these conditions. The deprotonated analytes, in contrast, were generated from the adducts of the atmospheric ions with higher proton affinity attributable to efficient proton abstraction from the analyte by the atmospheric ion.     

Differentiation of organically and conventionally produced milk by stable isotope and fatty acid analysis

Increasing sales of organic milk mean intensified tests for authenticity are required. In addition to comprehensive documentation, analytical methods to identify organic milk, and thus to differentiate it from conventional milk, are needed for consumer protection. Because the composition of milk is fundamentally dependent on the feeding of the cows, thirty-five samples from both production systems in Germany, including farm and retail milk, were collected within 12 months, to reflect seasonal variation, and appropriate properties were analysed. Fatty acid analysis enabled organic and conventional milk to be completely distinguished, because of the higher α-linolenic acid (C18:3ω3) and eicosapentaenoic acid (C20:5ω3) content of the former. Organic milk fat contained at least 0.56% C18:3ω3 whereas the maximum in conventional milk was 0.53%. Because of the parallel seasonal course of the C18:3ω3 content of organic and conventional retail samples, however, time-resolved comparison at the five sampling dates resulted in a clearer difference of 0.34 ± 0.06% on average. Analysis of stable carbon isotopes (δ¹³C) also enabled complete distinction of both types of milk; this can be explained by the different amounts of maize in the feed. For conventional milk fat δ¹³C values were −26.6‰ or higher whereas for organic milk fat values were always lower, with a maximum of −28.0‰. The time-resolved average difference was 4.5 ± 1.0‰. A strong negative correlation (r = −0.92) was found between C18:3ω3 and δ¹³C. Analysis of a larger number of samples is required to check the preliminary variation ranges obtained in this pilot study and, probably, to adjust the limits. Stable isotopes of nitrogen (δ¹⁵N) or sulfur (δ³⁴S) did not enable assignment of the origin of the milk; in cases of ambiguity, however, some trends observed might be useful in combination with other properties. Figure Correlation (r = −0.92) between δ¹³C value and C18:3ω3 content of milk fat from different production systems     

C and N stable isotope variation in urine and milk of cattle depending on the diet

The stable carbon and nitrogen isotopic composition of urine and milk samples from cattle under different feeding regimes were analysed over a period of six months. The isotope ratios were measured with isotope ratio mass spectrometry (IRMS). The δ ¹³C values of milk and urine were dependent on different feeding regimes based on C₃ or C₄ plants. The δ ¹³C values are more negative under grass feeding than under maize feeding. The δ ¹³C values of milk are more negative compared to urine and independent of the feeding regime. Under grass feeding the analysed milk and urine samples are enriched in ¹³C relative to the feed, whereas under maize feeding the ¹³C/¹²C ratio of urine is in the same range and milk is depleted in ¹³C relative to the diet. The difference between the ¹⁵N/¹⁴N ratios for the two feeding regimes is less pronounced than the ¹³C/¹²C ratios. The δ ¹⁵N values in urine require more time to reach the new equilibrium, whereas the milk samples show no significant differences between the two feeding regimes.     

Enantiomeric resolution of persistent compounds of technical toxaphene (CTTs) on t-butyldimethylsilylated β-cyclodextrin phases

Summary Eight of the most important single compounds of technical toxaphene were separated on t-butyldimethylsilylated β-cyclodextrin (β-BSCD) diluted in a medium polar phase using gas chromatography with electron capture detectors (GC-ECD). The enantiomeric resolution of all compounds was obtained in one GC run. The β-BSCD phase also separated the enantiomers of oxychlordane, cis- and trans-chlordane as well as α-HCH. Problems in the enantioselective determination of CTTs in biological samples are discussed. Finally, the enantioselective determination of the two most recalcitrant CTTs in biological samples was achieved using electron capture negative ionization mass spectrometry (GC-ECNI-MS) in the single ion monitoring (SIM) mode. Presented at the 21^st ISC held in Stutgart, Germany, 15^th–20^th September, 1996.     

Characterization of ganoderma spore lipid by stable carbon isotope analysis: implications for authentication

The ratios of stable carbon isotopes (¹³C/¹²C) of ganoderma fruiting body, ganoderma spore, ganoderma spore lipid (GSL) and individual fatty acids in GSL were determined by gas chromatography–stable isotope ratio mass spectrometry and elemental analysis–stable isotope ratio mass spectrometry. These values fall into a range from −26.9 to −23.3‰, suggesting that the cut log as the Ganoderma-cultivated substrate in Fujian, China, may belong to C3 plants. Eighteen fatty acids were identified and their abundances measured by gas chromatography–mass spectrometry in the six GSL samples with C_16:0, C_18:0, C_18:1 and C_18:2 as major constituents, and C_16:1 is evidently enriched compared with the other edible vegetable oils. On the basis of the compositions of fatty acids and stable carbon isotopes in GSL, we have developed a novel method to detect the adulteration of GSL products with cheaper edible vegetable oils. An example of ideal blending between GSL and C4 or C3 vegetable oil is further provided to expound the discrimination procedures and corresponding sensitive indicators. Simultaneously, the carbon isotope fractionation in the biosynthesis of individual fatty acids was observed, revealing that the formation of C_18:0 from C_16:0 in ganodema spores had no conspicuous ¹³C enrichment of +0.4‰ for Ganoderma sinensis spore and +0.1‰ for G. lucidum spore; the desaturation of C_18:0 to C_18:1 resulted in a distinct ¹³C depletion of −1.4‰ for G. sinensis spore and −0.9‰ for G. lucidum spore; and the next desaturation from C_18:1 to C_18:2 displayed no evident ¹³C fractionation of −0.1‰ for G. sinensis spore and −0.2‰ for G. lucidum spore. Figure Ganoderma lucidum has been widely used in traditional Chinese medicines. Ganoderma spore lipid (GSL) extracted from the spores of G. lucidum has been approved as a health food supplement. However, because of rarity, GSL has become a target for adulteration with cheaper vegetable oils.     

15N isotope-selective infrared multiphoton dissociation of nitromethane by a free electron laser

Successful experiments on the isotope-selective infrared multiphoton dissociation (IR MPD) of nitromethane molecules in the region of stretching vibrations of the NO₂ group have been performed for the first time under IR free electron laser (FEL) irradiation in a mixture with the natural content of the¹⁵N isotope of 0.4%. The content of the¹⁵N isotope in the products of NO dissociation varies within 0.1–1.6% as a function of the laser radiation frequency. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 739–742, April, 1998.     

Mass spectrometric method for the absolute calibration of the intramolecular nitrogen isotope distribution in nitrous oxide

A mass spectrometric method to determine the absolute intramolecular (position-dependent) nitrogen isotope ratios of nitrous oxide (N₂O) has been developed. It is based on the addition of different amounts of doubly labeled ¹⁵N₂O to an N₂O sample of the isotope ratio mass spectrometer reference gas, and subsequent measurement of the relative ion current ratios of species with mass 30, 31, 44, 45, and 46. All relevant quantities are measured by isotope ratio mass spectrometers, which means that the machines inherent high precision of the order of 10^–5 can be fully exploited. External determination of dilution factors with generally lower precision is avoided. The method itself can be implemented within a day, but a calibration of the oxygen and average nitrogen isotope ratios relative to a primary isotopic reference material of known absolute isotopic composition has to be performed separately. The underlying theoretical framework is explored in depth. The effect of interferences due to ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O in the ¹⁵N₂O sample and due to ¹⁵N ₂ ⁺ formation are fully accounted for in the calculation of the final position-dependent nitrogen isotope ratios. Considering all known statistical uncertainties of measured quantities and absolute isotope ratios of primary isotopic reference materials, we achieve an overall uncertainty of 0.9 (1). Using tropospheric N₂O as common reference point for intercomparison purposes, we find a substantially higher relative enrichment of ¹⁵N at the central nitrogen atom over ¹⁵N at the terminal nitrogen atom than measured previously for tropospheric N₂O based on a chemical conversion method: 46.3±1.4 as opposed to 18.7±2.2. However, our method depends critically on the absolute isotope ratios of the primary isotopic reference materials air–N₂ and VSMOW. If they are systematically wrong, our estimates will also necessarily be incorrect.     

Determining the isotopic compositions of uranium and fission products in radioactive environmental microsamples using laser ablation ICP–MS with multiple ion counters

This paper presents the application of a multicollector inductively coupled plasma mass spectrometer (MC–ICP–MS)—a Nu Plasma HR—equipped with three ion-counting multipliers and coupled to a laser ablation system (LA) for the rapid and sensitive determination of the ²³⁵U/²³⁸U, ²³⁶U/²³⁸U, ¹⁴⁵Nd/¹⁴³Nd, ¹⁴⁶Nd/¹⁴³Nd, ¹⁰¹Ru/(⁹⁹Ru+⁹⁹Tc) and ¹⁰²Ru/(⁹⁹Ru+⁹⁹Tc) isotope ratios in microsamples collected in the vicinity of Chernobyl. Microsamples with dimensions ranging from a hundred μm to about 1 mm and with surface alpha activities of 3–38 mBq were first identified using nuclear track radiography. U, Nd and Ru isotope systems were then measured sequentially for the same microsample by LA–MC–ICP–MS. The application of a zoom ion optic for aligning the ion beams into the ion counters allows fast switching between different isotope systems, which enables all of the abovementioned isotope ratios to be measured for the same microsample within a total analysis time of 15–20 min (excluding MC–ICP–MS optimization and calibration). The ¹⁰¹Ru/(⁹⁹Ru+⁹⁹Tc) and ¹⁰²Ru/(⁹⁹Ru+⁹⁹Tc) isotope ratios were measured for four microsamples and were found to be significantly lower than the natural ratios, indicating that the microsamples were contaminated with the corresponding fission products (Ru and Tc). A slight depletion in ¹⁴⁶Nd of about 3–5% was observed in the contaminated samples, but the Nd isotopic ratios measured in the contaminated samples coincided with natural isotopic composition within the measurement uncertainty, as most of the Nd in the analyzed samples originates from the natural soil load of this element. The ²³⁵U/²³⁸U and ²³⁶U/²³⁸U isotope ratios were the most sensitive indicators of irradiated uranium. The present work yielded a significant variation in uranium isotope ratios in microsamples, in contrast with previously published results from the bulk analysis of contaminated samples originating from the vicinity of Chernobyl. Thus, the ²³⁵U/²³⁸U ratios measured in ten microsamples varied in the range from 0.0073 (corresponding to the natural uranium isotopic composition) to 0.023 (corresponding to initial ²³⁵U enrichment in reactor fuel). An inverse correlation was observed between the ²³⁶U/²³⁸U and ²³⁵U/²³⁸U isotope ratios, except in the case of one sample with natural uranium. The heterogeneity of the uranium isotope composition is attributed to the different burn-up grades of uranium in the fuel rods from which the microsamples originated. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The use of stable isotopes ratios for authentication of fruit juices

The determination of the content of stable isotopes, ¹⁸O and ²H, respectively, in juice water facilitates the distinction between authentic juices and juices made from concentrates by redilution with tap water. At the same time, the detection of C₄ cane or corn-derived sugar syrups in fruit juices which are produced from C₃ fruit types is thus facilitated by the characteristic differences in ¹³C/¹²C, expressed as δ ¹³C (‰) values due to photosynthetic CO₂ assimilation via the C₃₋, C₄₋, and crassulacean acid metabolism pathways. In this study, the quantitative determination of water added to an authentic juice, on the basis of δ ¹⁸O, and δ ²H values, respectively, was successfully performed. Also, the δ ¹⁸O, and δ ²H of juice water and δ ¹³C of the whole juice in 18 samples were also determined. The results obtained provided us with the possibility of distinguishing between authentic fruit juices and those obtained by redilution of concentrated fruit juices and the detection of C⁴ type added sugar.     

Determination of ultra-trace levels of priority PBDEs in water samples by isotope dilution GC(ECNI)MS using 81Br-labelled standards

A gas chromatography electron capture negative ionization mass spectrometry (GC(ECNI)MS) procedure for the determination of priority polybrominated diphenyl ethers (PBDEs; congeners 28, 47, 99, 100, 153 and 154) in water samples at regulatory EU levels has been developed. The method is based on the use of ⁸¹Br-labelled PBDEs for isotope dilution analysis and the measurement of ⁷⁹Br/⁸¹Br isotope ratios in gas chromatography peaks with the electron capture negative ionization technique. The suitability of this ion source for the precise and accurate measurement of bromine isotope ratios has been demonstrated. The general ECNI-IDMS procedure was evaluated by the analysis of NIST SRM 1947 (Lake Michigan fish tissue) with satisfactory results. For the analysis of water samples, 500 mL of the samples were spiked with the labelled PBDEs and extracted with 10 mL isooctane for 30 min. The extract was evaporated down to ca. 100 μL and injected in the GC(ECNI)MS. Detection limits ranged from 0.014 ⁻¹ to 0.089 pg mL⁻¹ depending on the congener. Recoveries from real water samples, spiked at a level of 0.5 pg mL⁻¹, ranged from 77% to 102%.