Elemental Analyzer/isotope Ratio Mass Spectrometry Research Articles

A new approach to δ15N compound‐specific amino acid trophic position measurements: preparative high pressure liquid chromatography technique for purifying underivatized amino acids for stable isotope analysis

Compound specific isotope analysis of amino acids (CSI‐AA) has emerged as an important new method for investigating trophic dynamics in both aquatic and terrestrial systems. Multiple studies have shown that δ15N values of glutamic acid (Glu) and phenylalanine (Phe) can be coupled to provide precise estimates of trophic position (TP), while simultaneously decoupling baseline δ15N values from the effects of trophic transfer. However, the current standard gas chromatography‐combustion‐isotope ratio mass spectrometry (GC‐C‐IRMS) approach is limited by high expense, limited availability, and relatively low precision. We present a new method for making TP estimates in biological samples by CSI‐AA (TPCSIA), based on a high‐pressure liquid chromatography (HPLC) purification of underivatized amino acids, followed by offline elemental analysis‐isotope ratio mass spectrometry (EA‐IRMS). We compare results from our new HPLC/EA‐IRMS method versus GC‐C‐IRMS in both standard and natural materials. Nitrogen isotopic values of purified Glu and Phe standards were identical within error for both methods. In five widely different marine organisms, the δ15N values of Glu and Phe were also indistinguishable within error between the two approaches; however, the δ15N values produced by the HPLC/EA‐IRMS approach had higher average precision (average SD = 0.3 ± 0.2 ‰) than the GC‐C‐IRMS measurements (average SD = 0.45 ± 0.15 ‰). The resulting TPCSIA estimates were statistically indistinguishable (t < 1.2, df = 6, P > 0.3) between the two methods for all organisms examined. Our HPLC/EA‐IRMS method may therefore allow significant expansion of TPCSIA applications, requiring only commonly available instrumentation to produce high precision TPCSIA values.

A novel high-temperature combustion based system for stable isotope analysis of dissolved organic carbon in aqueous samples. I: development and validation.

Traditionally, dissolved organic carbon (DOC) stable isotope analysis (SIA) is performed using either offline sample preparation followed by elemental analyzer/isotope ratio mass spectrometry (EA/IRMS) or a wet chemical oxidation (WCO)-based device coupled to an isotope ratio mass spectrometer. The first method is time-consuming and laborious. The second involves the risks of underestimation of DOC concentration and isotopic fractionation due to incomplete oxidation. The development of an analytical method for accurate and sensitive DOC SIA is described in this study. A high-temperature combustion (HTC) system improves upon traditional methods. A novel total organic carbon (TOC) system, specially designed for SIA, was coupled to an isotope ratio mass spectrometer. An integrated purge and trap technique (peak focusing), flexible injection volume (0.05-3 mL), favorable carrier gas flow, modified ash crucible, new design of combustion tube and optimized drying system were used to achieve the necessary performance. The system can reliably measure concentrations up to 1000 mgC/L. Compounds resistant to oxidation, such as barbituric acid, melamine and humic acid, were analyzed with recovery rates of 100 ± 1% proving complete oxidation. In this initial testing, the δ(13) C values of these compounds were determined with precision and trueness of ≤ 0.2‰ even with 3.5% salinity. Further tests with samples with low DOC concentrations resulted in LOQSIA method values of 0.5 mgC/L and 0.2 mgC/L for LOQSIA instr , considering an accuracy of ± 0.5‰ as acceptable. The novel HTC system coupled to an isotope ratio mass spectrometer resulted in significantly improved sensitivity. The system is suitable for salt-containing liquids and compounds that are resistant to oxidation, and it offers a large concentration range. A second paper (which follows this one in this issue) will present a more comprehensive assessment of the analytical performance with a broad set of solutions and real samples. This highly efficient TOC stable isotopic analyzer will probably open up new possibilities in biogeochemical carbon cycle research.

Calibration strategies for the determination of stable carbon absolute isotope ratios in a glycine candidate reference material by elemental analyser-isotope ratio mass spectrometry.

We report a methodology for the determination of the stable carbon absolute isotope ratio of a glycine candidate reference material with natural carbon isotopic composition using EA-IRMS. For the first time, stable carbon absolute isotope ratios have been reported using continuous flow rather than dual inlet isotope ratio mass spectrometry. Also for the first time, a calibration strategy based on the use of synthetic mixtures gravimetrically prepared from well characterised, highly (13)C-enriched and (13)C-depleted glycines was developed for EA-IRMS calibration and generation of absolute carbon isotope ratio values traceable to the SI through calibration standards of known purity. A second calibration strategy based on converting the more typically determined delta values on the Vienna PeeDee Belemnite (VPDB) scale using literature values for the absolute carbon isotope ratio of VPDB itself was used for comparison. Both calibration approaches provided results consistent with those previously reported for the same natural glycine using MC-ICP-MS; absolute carbon ratios of 10,649 × 10(-6) with an expanded uncertainty (k = 2) of 24 × 10(-6) and 10,646 × 10(-6) with an expanded uncertainty (k = 2) of 88 × 10(-6) were obtained, respectively. The absolute carbon isotope ratio of the VPDB standard was found to be 11,115 × 10(-6) with an expanded uncertainty (k = 2) of 27 × 10(-6), which is in excellent agreement with previously published values.

δ13C values and phytanic acid diastereomer ratios: combined evaluation of two markers suggested for authentication of organic milk and dairy products

Authentication of organic milk by suitable markers is currently attracting more and more interests in food control. In this study, we aimed to compare the efficiency of the markers stable carbon isotope ratio (δ13C value) with the SRR/RRR phytanic acid diastereomer ratio (SRR/RRR) of milk fat for distinguishing different feeding systems. For stable carbon isotope ratio analysis by elemental analysis–isotope ratio mass spectrometry (EA-IRMS), we first developed a simple sample preparation method based on milk fat extracts allowed to evaporate the solvent in tin capsules for liquid samples. The δ13C values and reproducibility measured with this alternative sample pre-treatment method excellently matched those obtained with the current standard method. Applied to milk samples, the results of the EA-IRMS analysis were linked to the SRR/RRR, and both markers allowed to distinguish milk from cows fed with hay (δ13C value > −28.0 ‰, SRR/RRR <1.5) from feed used in conventional milk production which contained maize silage (C4-plants) (p < 0.001). Milk fat of organic retail cheese samples was also highly depleted in 13C (δ13C value −30.0 ‰ ± 1.1), and the SRR/RRR was low (<1.5). However, seven cheese samples showed inconsistent δ13C values and SRR/RRR, most likely due to the feeding of grass silage. Both parameters (δ13C values and SRR/RRR), together with the phytanic acid content of milk fat, also allowed distinguishing between the feeding of high amounts of pasture, hay, and/or grass silage.

Interrogating trees for isotopic archives of atmospheric sulphur deposition and comparison to speleothem records

Palaeorecords which depict changes in sulphur dynamics form an invaluable resource for recording atmospheric pollution. Tree rings constitute an archive that are ubiquitously available and can be absolutely dated, providing the potential to explore local- to regional-scale trends in sulphur availability. Rapid isotopic analysis by a novel “on-line” method using elemental analyser isotope ratio mass spectrometry (EA-IRMS) is developed, achieving sample precision of <0.4‰ using sample sizes of 40 mg wood powder. Tree cores from NE Italy show trends in pollution, evidenced through increasing concentrations of sulphur towards the youngest growth, and inverse trends in sulphur isotopes differentiating modern growth with light sulphur isotopes (+0.7‰) from pre-industrial growth (+7.5‰) influenced by bedrock composition. Comparison with speleothem records from the same location demonstrate replication, albeit offset in isotopic value due to groundwater storage. Using EA-IRMS, tree ring archives form a valuable resource for understanding local- to regional-scale sulphur pollution dynamics.

Stable Carbon and Nitrogen Isotopic Compositions of Allyl Isothiocyanate: Implications for the Discrimination of Natural and Artificial Products

Allyl isothiocyanate (AITC) has been widely used in the food industry for its specific pungent odor and antibacterial effect. In this study, we measured stable carbon and nitrogen isotopic compositions of AITC by elemental analysis/isotope ratio mass spectrometry (EA/IRMS) in order to evaluate their potential as a conventional tool for discrimination between natural (i.e., plant-derived) and artificial products. A significant difference in the isotopic compositions for carbon (2–4‰) and nitrogen (22–30‰) was obtained between natural (N = 4) and artificial products (N = 2), which implies that isotopic analysis is potentially useful to discrimination natural and artificial AITCs, even though the number of examined samples is still small at this moment. We also demonstrated the carbon isotope analysis of AITC painted on plastic sheets by gas chromatography/isotope ratio mass spectrometry (GC/IRMS), which may suggest that isotope analysis is applicable to not only pure AITCs, but also to those found in complex mixtures as various industrial products.

High‐precision measurement of phenylalanine δ15N values for environmental samples: A new approach coupling high‐pressure liquid chromatography purification and elemental analyzer isotope ratio mass spectrometry

Compound-specific isotope analysis of individual amino acids (CSI-AA) is a powerful new tool for tracing nitrogen (N) source and transformation in biogeochemical cycles. Specifically, the δ(15)N value of phenylalanine (δ(15)N(Phe)) represents an increasingly used proxy for source δ(15)N signatures, with particular promise for paleoceanographic applications. However, current derivatization/gas chromatography methods require expensive and relatively uncommon instrumentation, and have relatively low precision, making many potential applications impractical. A new offline approach has been developed for high-precision δ(15)N measurements of amino acids (δ(15)N(AA)), optimized for δ(15)N(Phe) values. Amino acids (AAs) are first purified via high-pressure liquid chromatography (HPLC), using a mixed-phase column and automated fraction collection. The δ(15)N values are determined via offline elemental analyzer-isotope ratio mass spectrometry (EA-IRMS). The combined HPLC/EA-IRMS method separated most protein AAs with sufficient resolution to obtain accurate δ(15)N values, despite significant intra-peak isotopic fractionation. For δ(15)N(Phe) values, the precision was ±0.16‰ for standards, 4× better than gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS; ±0.64‰). We also compared a δ(15)N(Phe) paleo-record from a deep-sea bamboo coral from Monterey Bay, CA, USA, using our method versus GC/C/IRMS. The two methods produced equivalent δ(15)N(Phe) values within error; however, the δ(15)N(Phe) values from HPLC/EA-IRMS had approximately twice the precision of GC/C/IRMS (average stdev of 0.27‰ ± 0.14‰ vs 0.60‰ ± 0.20‰, respectively). These results demonstrate that offline HPLC represents a viable alternative to traditional GC/C/IMRS for δ(15)N(AA) measurement. HPLC/EA-IRMS is more precise and widely available, and therefore useful in applications requiring increased precision for data interpretation (e.g. δ(15)N paleoproxies).

Gas chromatography combined with mass spectrometry, flame ionization detection and elemental analyzer/isotope ratio mass spectrometry for characterizing and detecting the authenticity of commercial essential oils of Rosa damascena Mill.

The essential oil of Rosa damascena Mill. is known for its fine perfumery application, use in cosmetic preparations and for several pharmacological activities. Due to its high value, it can be easily adulterated with flavors or cheaper oils. This study is aimed at a detailed phytochemical characterization of commercial samples of R. damascena essential oil and at their authenticity assessment. Nineteen commercial samples of R. damascena essential oil of different geographic origin and an additional authentic one, directly extracted by hydro-distillation from fresh flowers, were considered. GC/MS and GC/FID techniques were applied for the phytochemical analysis of the samples. EA/IRMS (Elemental Analyzer/Isotope Ratio Mass Spectrometry) and GC/C (Combustion)/IRMS were used to determine the δ(13)C composition of bulk samples and of some specific components. Citronellol (28.7-55.3%), geraniol (13.5-27.3%) and nonadecane (2.6-18.9%) were the main constituents of Bulgarian and Turkish essential oils, while those from Iran were characterized by a high level of aliphatic hydrocarbons (nonadecane: 3.7-23.2%). The δ(13)C values of bulk samples were between -28.1 and -26.9‰, typical for C3 plants. The δ(13)C values of specific components were in the usual range for natural aromatic substances from C3 plants, except for geranyl acetate, which displayed higher values (up to -18‰). These unusual δ(13)C values were explained by the addition of a natural cheaper oil from a C4 plant (Cymbopogon martinii, palmarosa), which was found to occur in most of the essential oils. GC/C/IRMS, in combination with GC/MS and GC/FID, can be considered as an effective and reliable tool for the authenticity control of R. damascena essential oil.

Quantification, morphology and source of humic acid, kerogen and black carbon in offshore marine sediments from Xiamen Gulf, China

Three types of macromolecular organic matters (MOMs), i.e. humic acid (HA), kerogen+black carbon (KB), and black carbon (BC) were extracted from marine sediments of Xiamen Gulf, southeast of China. The chemical composition, morphological property and source of the three extractions were characterized by elemental analyzer/isotope ratio mass spectrometry (EA/IRMS) and scanning electron microscope (SEM). The results showed that KB was the predominant fraction in MOMs, which accounted for 61.79%-89.15% of the total organic content (TOC), while HA consisted less than 5%. The relative high contents of kerogen and BC, and low contents of HA in the samples indicated that anthropogenic input might be the major source of organic matter in marine sediments near the industrial regions. The characterization of SEM, not only revealed morphological properties of the three fractions, but also allowed a better understanding of the source of MOMs. The δ13C values of the three fractions suggested that materials from terrestrial C3 plants were predominant. Furthermore, the anthropogenic activities, such as the discharge of sewage, coal and biomass combustion from industry nearby and agricultural practices within drainage basin of the Jiulong River, were remarkably contributed to the variations in δ13C values of MOMs in the offshore marine sediments.

Differentiation of Lard, Chicken Fat, Beef Fat and Mutton Fat by GCMS and EA-IRMS Techniques

A study was conducted to differentiate lard, chicken fat, beef fat and mutton fat using Gas Chromatography Mass Spectrometry (GC-MS) and Elemental Analyzer-Isotope Ratio Mass Spectrometry (EA-IRMS). The comparison of overall fatty acid data showed that lard and chicken fat share common characteristics by having palmitic, oleic and linoleic acid as major fatty acids while beef and mutton fats shared common characteristics by possessing palmitic, stearic and oleic acid as major fatty acids. The direct comparisons among the fatty acid data, therefore, may not be suitable for discrimination of different animal fats. When the fatty acid distributional data was subjected to Principle Component Analysis (PCA), it was demonstrated that stearic, oleic and linoleic acids as the most discriminating parameters in the clustering of animal fats into four subclasses. The bulk carbon analysis of animal fats using EA-IRMS showed that determination of the carbon isotope ratios (δ¹³C) would be a good indicator for discriminating lard, chicken fat, beef fat and mutton fat. This would lead to a faster and more efficient method to ascertain the source of origin of fats used in food products.

Effect of 13C enrichment and sugar type on analysis of sugars by gas chromatography/combustion/isotope ratio mass spectrometry

The objective of this investigation was to test gas-chromatographic compound-specific analysis for studies on the isotopic composition of (13)C-enriched sugar molecules. The effects of (13)C enrichment and type of sugar (C5, C6) will provide valuable information on isotopic correction for future studies employing (13)C-enriched sugars. Five sugar solutions of xylose, mannose and glucose with (13)C enrichments ranging between 1.1 and 1.5 atom-% were prepared. The (13)C enrichments of the initial sugars were measured by elemental analyser/isotope ratio mass spectrometry (EA/IRMS); (13)C enrichments for derivatised sugars were obtained by gas chromatography/combustion/IRMS (GC/C/IRMS). The linear relationships between the (13)C enrichments of the initial sugars and the values for the derivatised sugars were sugar-type dependent. Corrections for GC/C/IRMS values took into account the kinetic isotope effect (KIE) of the derivatising agent associated with the coefficient (K(d)) and a newly introduced second coefficient (K(c)) associated with the KIE of the sugar. While K(d) was constant, K(c) varied with sugar type. During derivatisation acetate groups with (12)C and sugars with more (13)C reacted faster. Coefficients for the specific ranges of (13)C enrichments under study have to be assessed and the reactions of different sugar types have to be taken into account to avoid underestimation of (13)C enrichment of up to 9% (C5) or overestimation of up to 4% (C6).

Discrimination of stable isotopes in fin whale tissues and application to diet assessment in cetaceans

In stable isotope research, the use of accurate, species-specific diet-tissue discrimination factors (i.e., Δ(13)C and Δ(15)N) is central to the estimation of trophic position relative to primary consumers and to the identification of the dietary sources of an individual. Previous research suggested that the diet of fin whales from the waters off northwestern Spain is overwhelmingly based on krill, thus permitting reliable calculation of discrimination values in this wild population. After confirming that the stable isotope ratios (δ(13)C and δ(15)N values) in muscle from 65 aged fin whales remained constant through age classes (4-65 years), the signatures were determined in muscle, bone protein, skin, liver, kidney, baleen plates and brain, as well as food (krill), from a subset of individuals to calculate discrimination factors. Signatures were determined by means of elemental analysis isotope ratio mass spectrometry (EA-IRMS) using a ThermoFinnigan Flash 1112. The isotopic values remained constant regardless of age. The mean Δ(15)N values between krill and whale tissues ranged from 2.04 in bone protein to 4.27‰ in brain, and those of Δ(13)C ranged from 1.28 in skin to 3.11‰ in bone protein. This variation was consistent with that found in other groups of mammals, and is attributed to variation in tissue composition and physiology. Because discrimination factors are relatively constant between taxonomically close species, the results here obtained may be reliably extrapolated to other cetaceans to improve dietary reconstructions. The skin discrimination factors are of particular relevance to monitoring diet through biopsies or other non-destructive sampling methods. The large difference in bone protein discrimination factors from those of other tissues should be taken into consideration when bone collagen is used to determine trophic level or to assess diet in paleodietary isotopic reconstructions.

Development of a methodology using gas chromatography‐combustion‐isotope ratio mass spectrometry for the determination of the carbon isotope ratio of caffeine extracted from tea leaves (Camellia sinensis)

Compound-specific isotope analysis (CSIA) of the extracted caffeine can be used to determine the authenticity of the origin of tea. Elemental analysis-isotope ratio mass spectrometry (EA-IRMS), which is widely used to measure the carbon isotope ratio of caffeine, has a strict requirement for the purity of the extracted caffeine. To obtain high-purity caffeine from tea leaves, the conventional extraction process has to be repeated and usually takes about 5-6 h. To improve the measurement of the carbon isotope ratio of caffeine, a more rapid and accurate measuring method is needed. An analytical protocol was developed for the determination of the carbon isotope ratio of caffeine from tea leaves using gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) combined with our extraction process. The procedure to extract caffeine and determine its carbon isotope ratio takes around 1.5 h. The standard deviation of the method is less than 0.1‰ (1σ). The measured carbon isotope ratios were not influenced by the amount of caffeine injected (0.08-0.62 µg) or by the extraction yield of caffeine from the tea leaves. The carbon isotope ratios of caffeine from eight tea cultivars were determined using the protocol.

Stable Carbon Isotope Ratio Analysis of Anhydrosugars in Biomass Burning Aerosol Particles from Source Samples

A new method for stable carbon isotope ratio analysis of anhydrosugars from biomass burning aerosol particle source filter samples was developed by employing Thermal Desorption--2 Dimensional Gas Chromatography--Isotope Ratio Mass Spectrometry (TD-2DGC-IRMS). Compound specific isotopic measurements of levoglucosan, mannosan, and galactosan performed by TD-2DGC-IRMS in a standard mixture show good agreement with isotopic measurements of the bulk anhydrosugars, carried out by Elemental Analyzer--Isotope Ratio Mass Spectrometry (EA-IRMS). The established method was applied to determine the isotope ratios of levoglucosan, mannosan, and galactosan from source samples collected during combustion of hard wood, softwood, and crop residues. δ(13)C values of levoglucosan were found to vary between -25.6 and -22.2‰, being higher in the case of softwood. Mannosan and galactosan were detected only in the softwood samples showing isotope ratios of -23.5‰ (mannosan) and -25.7‰ (galactosan). The isotopic composition of holocellulose in the plant material used for combustion experiments was determined with δ(13)C values between -28.5 and -23.7‰. The difference in δ(13)C of levoglucosan in biomass burning aerosol particles compared to the parent fuel holocellulose was found to be -1.89 (±0.37)‰ for the investigated biomass fuels. Compound specific δ(13)C measurements of anhydrosugars should contribute to an improved source apportionment.

Incorporation of nitrogen from N2 fixation into amino acids of zooplankton

Eurytemora affinis (Copepoda) were fed 15N‐labeled Rhodomonas salina (Cryptophyta) or 15N‐labeled Nodularia spumigena (Cyanobacteria) in excess under controlled laboratory conditions. Zooplankton collected from the Baltic Sea were fed natural phytoplankton amended with 15N‐labeled N. spumigena. We quantified the direct incorporation of 15N tracer from N2‐fixing N. spumigena (diazotroph nitrogen) and ammonium‐utilizing R. salina into the amino acid nitrogen (AA‐N) of zooplankton using complementary gas chromatography‐combustion‐isotope ratio mass spectrometry, gas chromatography‐mass spectrometry, and elemental analysis‐isotope ratio mass spectrometry approaches. Specific and mass‐specific TN and AA‐N incorporation rates of the 15N tracers were calculated for zooplankton. Highest incorporation of 15N was found in field zooplankton relying on N. spumigena and in E. affinis relying on R. salina. Lowest incorporation was found in E. affinis relying on N. spumigena. Decreasing specific and mass‐specific rates during field experiments possibly were due to food shortage, whereas decreasing rates in E. affinis grazing on R. salina were more likely due to satiation. Specific and mass‐specific rates were consistently low in E. affinis when exposed to N. spumigena, suggesting that these animals were reluctant to feed on N. spumigena. Essential isoleucine received most of the diazotroph nitrogen in field zooplankton, while nonessential amino acids received most 15N tracer in E. affinis. N. spumigena was clearly an important amino acid nitrogen source for Baltic Sea zooplankton.

Stable carbon isotope composition of perirenal adipose tissue fatty acids from Engadine sheep grazing either mountain or lowland pasture1

To provide further insights into ruminant lipid digestion and metabolism, and into cis-9,trans-11 18:2 synthesis, 12 growing Engadine lambs grazing either mountain pasture (2,250 m above sea level; n = 6) or lowland pasture (400 m above sea level; n = 6) were studied. Both pastures consisted exclusively of C(3) plants. Before the experiment, all animals grazed a common pasture for 6 wk. Grasses and perirenal adipose tissues of the sheep were analyzed for fatty acids by gas chromatography. Stable C-isotope ratios (δ(13)C values in ‰ vs. the Vienna Pee Dee Belemnite standard) were determined in the composite samples by elemental analysis-isotope ratio mass spectrometry. The δ(13)C of the individual fatty acids were measured by gas chromatography-combustion-isotope ratio mass spectrometry. The δ(13)C value of the entire mountain pasture grass was -27.5‰ (SD 0.31), whereas that of the lowland pasture grass was -30.0‰ (SD 0.07). This difference was reflected in the perirenal adipose tissues of the corresponding sheep (P < 0.05), even though the δ(13)C values were less in the animals than in the grass. The δ(13)C values for cis-9 16:1 and cis-9 18:1 in perirenal fat differed between mountain and lowland lambs (P < 0.05). The 16:0 in the adipose tissue was enriched in (13)C by 5‰ compared with the dietary 16:0, likely as a result of partly endogenous synthesis. The δ(13)C values of cis-9,trans-11 18:2 (cis-9,trans-11 CLA) in the adipose tissue were smaller than those of its dietary precursors, cis-9,cis-12 18:2 and cis-9,cis-12,cis-15 18:3; conversely, the δ(13)C values of trans-11 18:1 were not, suggesting that large proportions of perirenal cis-9,trans-11 18:2 were of endogenous origin and discrimination against (13)C occurred during Δ(9)-desaturation. The same discrimination was indicated by the isotopic shift between 16:0 and cis-9 16:1 in the mountain grazing group. Furthermore, the δ(13)C values of cis-9,trans-11 18:2 were smaller relative to the precursor fatty acids in the mountain lambs compared with the lowland group. This result suggests a reduced extent of biohydrogenation in lambs grazing on mountain grass in comparison with those grazing on lowland grass. This was supported by the smaller cis-9,trans-11 18:2 concentrations in total fatty acids found in the adipose tissues of the lowland lambs (P < 0.001). The results of this study demonstrate that natural differences between δ(13)C values of swards from different pastures and the adipose tissue fatty acids could be used as tracers in studies of lipid metabolism in ruminants.

Carbonate removal by acid fumigation for measuring the δ13C of soil organic carbon

Ramnarine, R., Voroney, R. P., Wagner-Riddle, C. and Dunfield. K. E. 2011. Carbonate removal by acid fumigation for measuring the δ 13 C of soil organic carbon. Can. J. Soil Sci. 91: 247–250. Complete removal of carbonates from calcareous soil samples is critical for accurate measurement of the quantity and isotopic signature (δ13C) of soil organic carbon (SOC). Carbonates confound SOC and δ13C measurements because they have δ13C values ranging from −10‰ to +2‰, whereas those of soil organic carbon range from −27‰ to −13‰, depending on the source of plant residues. Commonly used methods for removing carbonates involve treatment with acid followed by repeated water washings; however, these methods are time consuming, labour-intensive and lead to losses of acid- and water-soluble organic carbon. Fumigation of soil samples with HCl was evaluated as an alternative method, and the time required for complete carbonate removal was determined in this study. Moistened soil samples, taken from 0- to 10-cm and 30- to 50-cm depths, were exposed to HCl vapours for periods of 0, 6, 12, 24, 48, 72, and 96 h, followed by measurements of total C and δ13C using coupled elemental analyzer-isotope ratio mass spectrometry. The minimum time required to remove all carbonates was ca. 30 h and 56 h for surface and subsurface soils containing 0.80 and 1.94% inorganic C, respectively. Therefore, the fumigation period required is dependent on the total carbonate content of the sample and the nature of the carbonate (pedogenic vs lithogenic). In our study, the rate of removal of inorganic carbon was 0.08–0.10 mg h−1 for soil samples sizes with 2.4 to 5.8 mg of carbonate-C, a rate similar to previous studies on acid fumigation. A “correction factor” was used to account for a change in sample mass due to fumigation and is necessary for accurate determination of SOC concentration using our proposed methodology.

Development of an enantiomer‐specific stable carbon isotope analysis (ESIA) method for assessing the fate of α‐hexachlorocyclo‐hexane in the environment

α-Hexachlorocyclohexane (α-HCH) is the only chiral isomer of the eight 1,2,3,4,5,6-HCHs and we have developed an enantiomer-specific stable carbon isotope analysis (ESIA) method for the evaluation of its fate in the environment. The carbon isotope ratios of the α-HCH enantiomers were determined for a commercially available α-HCH sample using a gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) system equipped with a chiral column. The GC-C-IRMS measurements revealed δ-values of -32.5 ± 0.8‰ and -32.3 ± 0.5‰ for (-) α-HCH and (+) α-HCH, respectively. The isotope ratio of bulk α-HCH was estimated to be -32.4 ± 0.6‰ which was in accordance with the δ-values obtained by GC-C-IRMS (-32.7 ± 0.2‰) and elemental analyzer-isotope ratio mass spectrometry (EA-IRMS) of the bulk α-HCH (-32.1 ± 0.1‰). The similarity of the isotope ratio measurements of bulk α-HCH by EA-IRMS and GC-C-IRMS indicates the accuracy of the chiral GC-C-IRMS method. The linearity of the α-HCH ESIA method shows that carbon isotope ratios can be obtained for a signal size above 100 mV. The ESIA measurements exhibited standard deviations (2σ) that were mostly < ± 0.5‰. In order to test the chiral GC-C-IRMS method, the isotope compositions of individual enantiomers in biodegradation experiments of α-HCH with Clostridium pasteurianum and samples from a contaminated field site were determined. The isotopic compositions of the α-HCH enantiomers show a range of enantiomeric and isotope patterns, suggesting that enantiomeric and isotope fractionation can serve as an indicator for biodegradation and source characterization of α-HCH in the environment.

An improved method of ion exchange for nitrogen isotope analysis of water nitrate

Nitrate nitrogen and oxygen isotopes have been widely used to trace the nitrogen biogeochemical cycle by identifying NO 3 − sources. An improved method of anion exchange was developed to measure δ 15N-NO 3 − in fresh water by continuous-flow elemental analyzer/isotope ratio mass spectrometry (EA-IRMS). We used a custom-built exchange resin column, a peristaltic pump and the oven-drying method in our experiments. Consequently, the amount of Ag 2O used as a neutralizer was reduced, time was saved, and operation became simpler than before. Meanwhile, analytical precision remained identical to previous studies. KNO 3 solutions were prepared at 0.2, 5 and 25 mg-N L −1 from KNO 3 standard salt (δ 15N = +6.27‰), and the average δ 15N values of the solutions after having been absorbed on and subsequently stripped from anion columns were +6.62 ± 0.22‰ ( n = 6), +6.38 ± 0.09‰ ( n = 6), and +6.26 ± 0.07‰ ( n = 6), respectively. In addition, the “natural” water sample δ 15N-NO 3 − showed consistency in comparison to standards, and the mean standard deviation by the different approaches was 0.08‰. Accordingly, by these improvements the anion exchange resin technique is demonstrated to be more suitable for measuring δ 15N in NO 3 − than original techniques.

Interrelationship of TOC, As, Fe, Mn, Al and Si in shallow alluvial aquifers in Chapai-Nawabganj, Northwestern Bangladesh: implication for potential source of organic carbon

Two boreholes and ten piezometers in the Ganges flood plain were drilled and installed for collecting As-rich sediments and groundwater. Groundwater samples from the Ganges flood plain were collected for the analysis of cations (Ca2+, Mg2+, K+, Na+), anions (Cl−, NO3−, SO42−), total organic carbon (TOC), and trace elements (As, Mn, Fe, Sr, Se, Ni, Co, Cu, Mo, Sb, Pb). X-ray powder diffraction was performed to characterize the major mineral contents of aquifer sediments and X-ray fluorescence (XRF) to analyze the major chemical composition of alluvial sediments. Results of XRF analysis clearly show that fine-grained sediments contain higher amounts of trace element because of their high surface area for adsorption. Relative fluorescence index (15–38 QSU) of humic substance in groundwater was measured using spectrofluorometer, the results revealed that groundwater in the Ganges flood plain contains less organic matter (OM). Arsenic concentration in water ranges from 2.8 to 170 μg/L (mean 50 μg/L) in the Ganges flood plain. Arsenic content in sediments ranges from 2.1 to 14 mg/kg (mean 4.58 mg/kg) in the flood plains. TOC ranges from 0.49 to 3.53 g/kg (mean 1.64 g/kg) in the Ganges flood plain. Arsenic is positively correlated with TOC (R2 = 0.55) in sediments of this plain. Humic substances were extracted from the sediments from the Ganges flood plain. Fourier transform infrared analysis of the sediments revealed that the plain contains less humic substances. The source of organic carbon was assigned from δ13C values obtained using elemental analysis-isotope ratio mass spectrometry (EA-IRMS); the values (−10 to −29.44‰) strongly support the hypothesis that the OM of the Ganges flood plain is of terrestrial origin.