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(15)N-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(2)O 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(15)N=+6.27‰), and the average δ(15)N 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(15)N-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(15)N in NO(3)(-) than original techniques.     

Removal of nitrite with sulfamic acid for nitrate N and O isotope analysis with the denitrifier method

In environmental water samples that contain both nitrate (NO) and nitrite (NO), isotopic analysis of nitrate alone by all currently available methods requires pretreatment to remove nitrite. Sulfamic acid addition, used previously for this purpose (Wu JP, Calvert SE, Wong CS. Deep‐Sea Research Part I – Oceanographic Research Papers 1997; 44: 287), is shown here to be compatible with the denitrifier method for both N and O isotope analysis of nitrate. Sulfamic acid at a pH of ～1.7 reduces nitrite to N₂. Samples are then neutralized with base prior to isotope analysis, to alleviate the buffering demands of the bacterial media and as a precaution to prevent modification of nitrate during storage with the residual sulfamic acid at low pH. Under appropriate reaction conditions, nitrite is completely removed within minutes. Sulfamic acid treatment does not compromise the completeness of the conversion of nitrate into N₂O or the precision and accuracy of N and O isotope measurements by the denitrifier method. Nitrite concentrations upwards of 7 times the ambient nitrate can be removed without affecting the isotope composition of nitrate. The method is applied to analyses of the coupled N and O isotopes of nitrate and nitrite in waters of the Mexican Margin, to illustrate its efficacy and utility when employed either in the field upon sample collection or in the lab after months of frozen sample storage. Copyright © 2009 John Wiley & Sons, Ltd.     

Extraction-visible spectrophotometric method for determination of nitrate : Application to water analysis

An extraction-visible spectrophotometric method for determination of nitrate is described. The method is based on the extraction of nitrate with tetraphenylphosphonium chloride and the exchange of the nitrate in the colorless Ph₄P-NO₃ extract by intensely colored vanadium(V)-4-(2-pyridylazo)resorcinol complex. The color intensity of this extract is very stable and reproducible. Absorption maximum appears at 560 nm with molar absorptivity 3.6 · 10⁴ 1 mol⁻¹cm⁻¹. The application of the method to water analysis was investigated and the procedure for determination of nitrate in drinking water is developed.     

A new isolation procedure of nitrate from freshwater for nitrogen and oxygen isotope analysis

The nitrogen (δ¹⁵N) and oxygen isotope (δ¹⁸O) analysis of nitrate (NO₃^–) from aqueous samples can be used to determine nitrate sources and to study N transformation processes. For these purposes, several methods have been developed; however, none of them allows an accurate, fast and inexpensive analysis. Here, we present a new simple method for the isolation of nitrate, which is based on the different solubilities of inorganic salts in an acetone/hexane/water mixture. In this solvent, all major nitrate salts are soluble, whereas all other oxygen‐bearing compounds such as most inorganic carbonates, sulfates, and phosphates are not. Nitrate is first concentrated by freeze‐drying, dissolved in the ternary solvent and separated from insoluble compounds by centrifugation. Anhydrous barium nitrate is then precipitated in the supernatant solution by adding barium iodide. For δ¹⁸O analysis, dried Ba(NO₃)₂ samples are directly reduced in a high‐temperature conversion system to CO and measured on‐line using isotope ratio mass spectrometry (IRMS). For δ¹⁵N analysis, samples are combusted in an elemental analyzer (EA) coupled to an IRMS system. The method has been tested down to 20 µmol NO₃^– with a reproducibility (1SD) of 0.1‰ for nitrogen and 0.2–0.4‰ for oxygen isotopes. For nitrogen we observed a small consistent ¹⁵N enrichment of +0.2‰, probably due to an incomplete precipitation process and, for oxygen, a correction for the incorporation of water in the precipitated Ba(NO₃)₂ has to be applied. Apart from being robust, this method is highly efficient and low in cost. Copyright © 2011 John Wiley & Sons, Ltd.     

Hg stable isotope analysis by the double-spike method

Recent publications suggest great potential for analysis of Hg stable isotope abundances to elucidate sources and/or chemical processes that control the environmental impact of mercury. We have developed a new MC-ICP-MS method for analysis of mercury isotope ratios using the double-spike approach, in which a solution containing enriched ¹⁹⁶Hg and ²⁰⁴Hg is mixed with samples and provides a means to correct for instrumental mass bias and most isotopic fractionation that may occur during sample preparation and introduction into the instrument. Large amounts of isotopic fractionation induced by sample preparation and introduction into the instrument (e.g., by batch reactors) are corrected for. This may greatly enhance various Hg pre-concentration methods by correcting for minor fractionation that may occur during preparation and removing the need to demonstrate 100% recovery. Current precision, when ratios are normalized to the daily average, is 0.06‰, 0.06‰, 0.05‰, and 0.05‰ (2σ) for ²⁰²Hg/¹⁹⁸Hg, ²⁰¹Hg/¹⁹⁸Hg, ²⁰⁰Hg/¹⁹⁸Hg, and ¹⁹⁹Hg/¹⁹⁸Hg, respectively. This is slightly better than previously published methods. Additionally, this precision was attained despite the presence of large amounts of other Hg isotopes (e.g., 5.0% atom percent ¹⁹⁸Hg) in the spike solution; substantially better precision could be achieved if purer ¹⁹⁶Hg were used.     

A simple and reliable method reducing sulfate to sulfide for multiple sulfur isotope analysis

Rationale

Precise analysis of four sulfur isotopes of sulfate in geological and environmental samples provides the means to extract unique information in wide geological contexts. Reduction of sulfate to sulfide is the first step to access such information. The conventional reduction method suffers from a cumbersome distillation system, long reaction time and large volume of the reducing solution. We present a new and simple method enabling the process of multiple samples at one time with a much reduced volume of reducing solution.

Methods

One mL of reducing solution made of HI and NaH₂PO₂ was added to a septum glass tube with dry sulfate. The tube was heated at 124°C and the produced H₂S was purged with inert gas (He or N₂) through gas‐washing tubes and then collected by NaOH solution. The collected H₂S was converted into Ag₂S by adding AgNO₃ solution and the co‐precipitated Ag₂O was removed by adding a few drops of concentrated HNO₃.

Results

Within 2–3 h, a 100% yield was observed for samples with 0.2–2.5 μmol Na₂SO₄. The reduction rate was much slower for BaSO₄ and a complete reduction was not observed. International sulfur reference materials, NBS‐127, SO‐5 and SO‐6, were processed with this method, and the measured against accepted δ³⁴S values yielded a linear regression line which had a slope of 0.99 ± 0.01 and a R² value of 0.998.

Conclusions

The new methodology is easy to handle and allows us to process multiple samples at a time. It has also demonstrated good reproducibility in terms of H₂S yield and for further isotope analysis. It is thus a good alternative to the conventional manual method, especially when processing samples with limited amount of sulfate available.

     

Contribution of atmospheric nitrate to stream‐water nitrate in Japanese coniferous forests revealed by the oxygen isotope ratio of nitrate

Evaluation of the openness of the nitrogen (N) cycle in forest ecosystems is important in efforts to improve forest management because the N supply often limits primary production. The use of the oxygen isotope ratio (δ¹⁸O) of nitrate is a promising approach to determine how effectively atmospheric nitrate can be retained in a forest ecosystem. We investigated the δ¹⁸O of nitrate in stream water in order to estimate the contribution of atmospheric NO in stream‐water NO (f_atm) from 26 watersheds with different stand ages (1–87 years) in Japan. The stream‐water nitrate concentrations were high in young forests whereas, in contrast, old forests discharged low‐nitrate stream water. These results implied a low f_atm and a closed N cycle in older forests. However, the δ¹⁸O values of nitrate in stream water revealed that f_atm values were higher in older forests than in younger forests. These results indicated that even in old forests, where the discharged N loss was small, atmospheric nitrate was not retained effectively. The steep slopes of the studied watersheds (>40°) which hinder the capturing of atmospheric nitrate by plants and microbes might be responsible for the inefficient utilization of atmospheric nitrate. Moreover, the unprocessed fraction of atmospheric nitrate in the stream‐water nitrate in the forest (f_unprocessed) was high in the young forest (78%), although f_unprocessed was stable and low for other forests (5–13%). This high f_unprocessed of the young forest indicated that the young forest retained neither atmospheric NO nor soil NO effectively, engendering high stream‐water NO concentrations. Copyright © 2010 John Wiley & Sons, Ltd.     

An automated method for the analysis of stable isotope labeling data in proteomics

An algorithm is presented for the generation of a reliable peptide component peak table from liquid chromatography-mass spectrometry (LC-MS) and subsequent quantitative analysis of stable isotope coded peptide samples. The method uses chemical noise filtering, charge state fitting, and deisotoping toward improved analysis of complex peptide samples. Overlapping peptide signals in mass spectra were deconvoluted by correlation with modeled peptide isotopic peak profiles. Isotopic peak profiles for peptides were generated in silico from a protein database producing reference model distributions. Doublets of heavy and light labeled peak clusters were identified and compared to provide differential quantification of pairs of stable isotope coded peptides. Algorithms were evaluated using peptides from digests of a single protein and a seven-protein mixture that had been differentially coded with stable isotope labeling agents and mixed in known ratios. The experimental results correlated well with known mixing ratios.     

Error assessment of nitrogen and oxygen isotope ratios of nitrate as determined via the bacterial denitrification method

Currently, bacterial denitrification is becoming the accepted method for δ¹⁵N‐ and δ¹⁸O‐NO determination. However, proper correction methods with international references (USGS32, USGS34 and USGS35) are needed. As a consequence, it is important to realize that the corrected isotope values are derived from a combination of several other measurements with associated uncertainties. Therefore, it is necessary to consider the propagated uncertainty on the final isotope value. This study demonstrates how to correctly estimate the uncertainty on corrected δ¹⁵N‐ and δ¹⁸O‐NO values using a first‐order Taylor series approximation. The bacterial denitrification method errors from 33 batches of 561 surface water samples varied from 0.2 to 2.1‰ for δ¹⁵N‐NO and from 0.7 to 2.3‰ for δ¹⁸O‐NO, which is slightly wider than the machine error, which varied from 0.2 to 0.6‰ for δ¹⁵N‐N₂O and from 0.4 to 1.0‰ for δ¹⁸O‐N₂O. The overall uncertainties, which are composed of the machine error and the method error, for the 33 batches ranged from 0.3 to 2.2‰ for δ¹⁵N‐NO and from 0.8 to 2.5‰ for δ¹⁸O‐NO. In addition, the mean corrected δ¹⁵N and δ¹⁸O values of 132 KNO₃‐IWS (internal working standard) measurements were computed as 8.4 ± 1.0‰ and 25.1 ± 2.0‰, which is a slight underestimation for δ¹⁵N and overestimation for δ¹⁸O compared with the accepted values (δ¹⁵N = 9.9 ± 0.3‰ and δ¹⁸O = 24.0 ± 0.3‰). The overall uncertainty of the bacterial denitrification method allows the use of this method for source identification of NO. Copyright © 2010 John Wiley & Sons, Ltd.     

Simplified version of the sodium salicylate method for analysis of nitrate in drinking waters

The salicylate method for determination of nitrate in drinking waters has been simplified by using the calibration solutions for 54 days, and therefore, reducing the work time. The results obtained can be considered satisfactory with regard to limit of determination (0.10 mg l⁻¹ nitrate as nitrogen (NO₃⁻-N)), precision (relative standard deviations <4% for five determinations were found) and accuracy (recoveries from 88 to 106% for nitrate in spiked drinking water were obtained).     

High accuracy method for the application of isotope dilution to gas chromatography/mass spectrometric analysis of gases

A new isotope dilution mass spectrometry (IDMS) method for high-accuracy quantitative analysis of gases has been developed and validated by the analysis of standard mixtures of carbon dioxide in nitrogen. The method does not require certified isotopic reference materials and does not require direct measurements of the highly enriched spike. The relative uncertainty of the method is shown to be 0.2%. Copyright Crown copyright 2003. Reproduced with the permission of Her Majesty's Stationery Office.     

A new approach to determine method detection limits for compound-specific isotope analysis of volatile organic compounds

Compound-specific isotope analysis (CSIA) has been established as a useful tool in the field of environmental science, in particular in the assessment of contaminated sites. What limits the use of gas chromatography/isotope ratio mass spectrometry (GC/IRMS) is the low sensitivity of the method compared with GC/MS analysis; however, the development of suitable extraction and enrichment techniques for important groundwater contaminants will extend the fields of application for GC/IRMS. So far, purge and trap (P&T) is the most effective, known preconcentration technique for on-line CSIA with the lowest reported method detection limits (MDLs in the low microg/L range). With the goal of improving the sensitivity of a fully automated GC/IRMS analysis method, a commercially available P&T system was modified. The method was evaluated for ten monoaromatic compounds (benzene, toluene, para-xylene, ethylbenzene, propylbenzene, isopropylbenzene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, fluorobenzene) and ten halogenated volatile organic compounds (VOCs) (dichloromethane, cis-1,2-dichloroethene, trans-1,2-dichloroethene, carbon tetrachloride, chloroform, 1,2-dichloroethane, trichloroethene, tetrachlorethene, 1,2-dibromoethane, bromoform). The influence of method parameters, including purge gas flow rates and purge times, on delta13C values of target compounds was evaluated. The P&T method showed good reproducibility, high linearity and small isotopic fractionation. MDLs were determined by consecutive calculation of the delta13C mean values. The last concentration for which the delta13C value was within this iterative interval and for which the standard deviation was lower than +/-0.5 per thousand for triplicate measurements was defined as the MDL. MDLs for monoaromatic compounds between 0.07 and 0.35 microg/L are the lowest values reported so far for continuous-flow isotope ratio measurements using an automated system. MDLs for halogenated hydrocarbons were between 0.76 and 27 microg/L. The environmental applicability of the P&T-GC/IRMS method in the low-microg/L range was demonstrated in a case study on groundwater samples from a former military air field contaminated with VOCs.     

Hydropyrolysis as a preparative method for the compound-specific carbon isotope analysis of fatty acids

Compound-specific stable carbon isotope analysis by gas chromatography/combustion/isotope ratio mass spectrometry is an effective and risk-free means of investigating fatty acid metabolism. Straightforward analysis, however, leads to poor chromatographic resolution, while derivatization adds carbon thereby corrupting the starting stable isotopic composition. Hydropyrolysis is a new approach which defunctionalizes fatty acids to yield the corresponding n-alkanes thus retaining the carbon skeleton intact and improving chromatography, allowing the faithful measurement of carbon isotope ratios.     

Magnetic isotope effect in the photochemical reduction of uranyl nitrate

A magnetic isotope effect was found in the photochemical reduction of uranyl nitrate by p-methoxyphenol. The initial²³⁸U/²³⁵U ratio equal to 8.93±0.008 was altered during the reaction to 8.945±0.008-8.970±0.005 in the reaction product, namely, uranium tetrafluoride.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1902–1903, August, 1990.