Measurement of Nitrogen and Nitrogen Isotopic Ratios Using Reduction Pyrolysis Coupled with Mass Spectrometry

Abstract

An analytical system for measuring total nitrogen and its isotopic abundance in a variety of environmental samples has been developed. A reductive pyrolysis system and a directional focusing 6-inch gas mass spectrometer were combined into the analytical system. In the reductive part of the system, nitrogen species are converted to ammonia with an atmosphere of hydrogen in the presence of a heated nickel catalyst. Five percent of the gas stream is split away for measuring total nitrogen by a conductivity detector. The ammonia is removed from the gas stream employing a cold finger reaction vessel.

The hydrogen-free ammonia is decomposed thermally to nitrogen and hydrogen at 1000°C, employing a hot rhenium filament. The N₂ produced from the decomposition is used for measuring the abundance of masses 28 and 29 by mass spectrometry. From this ratio, the ¹⁵N atom fraction is calculated.

Standard samples of N₂, ammonia, orchard leaves and urea have been successfully analyzed to determine isotopic compositions. Samples containing as little as 20 μg of total nitrogen can be analyzed by this system. By the addition of multi-reaction vessels, three samples may be completed per hour.     

Effect of excitation states of nitrogen on the surface nitridation of iron and steel in d.c. glow discharge plasma

The plasma nitriding phenomena that occur on the surfaces of iron and steel were investigated. In particular, the correlation between the kinds of nitrogen radicals and the surface nitriding reaction was investigated using a glow‐discharge apparatus. To control the excitation of nitrogen radicals, noble gas mixtures were used for the plasma gas. The highly populated metastables of noble gases selectively produce excited nitrogen molecules (N₂*) or nitrogen molecule ions (N₂⁺). The optical emission spectra suggested that the formation of N₂*‐rich or N₂⁺‐rich plasma was successfully controlled by introducing different kinds of noble gases. Auger electron spectroscopy and XPS were used to characterize the depth profile of the elements and chemical species on the nitrided surface. The nitride layer formed by a N₂⁺‐rich plasma had a much higher nitrogen concentration than that by a N₂*‐rich plasma, likely due to the larger chemical activity of the N₂⁺ species as well as the N₂⁺ sputtering bombardment to the cathode surface. The strong reactivity of the N₂⁺ species was also confirmed from the chemical shift of N 1s spectra for iron nitrides. An iron nitride formed by the N₂⁺‐rich plasma has higher stoichiometric quantity of nitrogen than that formed by the N₂*‐rich plasma. Besides the effect of nitrogen radicals on surface nitridation, the contribution of the chromium in steel to the nitriding reaction was also examined. This chromium can promote a nitriding reaction at the surface, which results in an increase in the nitrogen concentration and the formation of nitride with high nitrogen coordination. Copyright © 2010 John Wiley & Sons, Ltd.     

New apparatus for measuring surface areas as low as 0.1 cm2

An apparatus is described which permits accurate determination of gas adsorption of solids having lower adsorptive capacity. Conventional measurement of gas uptake by solids having surface areas less than 1 m² is not generally feasible at subatmospheric pressure. The small pressure drop due to adsorption is easily lost in the noise created by ambient temperature fluctuations. Possible experimental errors in the volumetric measurement of gas uptake due to temperature fluctuations were greatly reduced by using an apparatus whose reference and sample adsorption cells are disposed in a lateral symmetry. The apparatus consists of a differential micromanometer whose two arms are connected in a lateral symmetry to a pair of sample and reference burets of nearly equal volume for dosing gas into sample and reference cells also of nearly equal volume. When the two cells are immersed in the same temperature bath this design greatly reduces measurement uncertainties due to fluctuations in the temperature. Surface areas as small as 300, 1 and 0.1 cm² were measured through volumetric gas uptakes of nitrogen at 78 K, krypton at 78 K and xenon at 90 K, respectively.     

Determination of carbon monoxide with palladium chloride

The gas sample transfer apparatus, as shown in Fig. 1, was designed to facilitate the analysis of several carbon monoxide samples during a normal work day. With this apparatus, gas samples were accurately transferred from the Tedlar bags leak free through the system to the reaction flask. Changing from one gas concentration to another was greatly simplified by using the quick-connect fittings; gas volumes were readily calculated from direct readings taken from the differential pressure gauge.Tests were conducted to assess the performance of the PdCl₂ method and the sampling system with various gas mixtures. The proposed PdCl₂ method utilizing this particular sampling scheme is capable of successfully measuring carbon monoxide in nitrogen cylinder gas at levels of 0.1, 0.05, and 0.01% with expected average recoveries of 96, 96, and 93%, respectively.     

Environmentally friendly method for determination of ammonia nitrogen in fertilisers and wastewaters based on flow injection-spectrophotometric detection using natural reagent from orchid flower

Crude aqueous extract from the orchid ‘Dendrobium Sonia earsakul’ was utilised as a natural product reagent in flow injection analysis (FIA) incorporating a gas diffusion unit (GD) for the determination of ammonia nitrogen. Sample solution was injected into a NaOH donor stream to generate ammonia gas (NH₃). In the GD unit, NH₃ diffused across a PTFE gas-permeable membrane into the acceptor stream of the orchid extract. As the result, the aqueous orchid reagent became more alkaline and its colour changed from purple to green. The change in the colour of orchid acceptor correlated with the concentration of ammonia nitrogen in the sample and its absorbance monitored by a spectrophotometer at 600 nm. Ammonia nitrogen in chemical fertiliser samples and wastewater samples from agricultural fields were determined and reported as %N (w/w) and mg N L^?1, respectively. For chemical fertilisers which contained high content of ammonia nitrogen, a flow rate of 1.0 mL min^?1 and injection volume of 100 µL were used with a linear range of 5–40 mmol L^?1 and detection limit of 2.12 mmol L^?1. However, a higher sensitivity was required for wastewater samples having low ammonia nitrogen content. The flow rate was reduced to 0.3 mL min^?1 and the injection volume increased to 1000 µL. As a result, detection limit of 0.76 mmol L^?1 was achieved with linear range of 1–5 mmol L^?1. The results of our method agreed well with that using the OPA method employing fluorescence detection.     

Limitations in detection of 15N incorporation by mass spectrometry in protein-based stable isotope probing (protein-SIP)

The method of protein-based stable isotope probing (protein-SIP) has previously been shown to allow the modeling of carbon fluxes in microbial communities, thus tackling one of the key questions in microbial ecology. The method allows the analysis of stable isotope distribution in peptides, revealing metabolic activities of the species present in an ecosystem. Besides carbon, an application of protein-SIP with nitrogen is of interest for resolving the nitrogen fluxes in microbial communities. Thus, the sensitivity and reliability of a protein-SIP approach employing ¹⁵N was analyzed. For this, cultivations of Pseudomonas fluorescens ATCC 17483 with different ratios of ¹⁴N/¹⁵N were performed, from 10 % down to 0.1 % ¹⁵N. After incubation leading to complete labeling of biomass, proteins were extracted and separated by one-dimensional gel electrophoresis, followed by tryptic digest and UPLC Orbitrap MS/MS analysis. ¹⁵N relative isotope abundance (RIA) was calculated based on isotopic patterns from identified peptides in mass spectra. Proteomics data have been deposited to ProteomeXchange with identifier PXD000127. The distribution of ¹⁵N RIA values among peptides was analyzed in samples with different ¹⁵N amount, and potential causes for variations within individual samples of either technical or biological origin were investigated. Using a number of 50 peptides, significant differences (p?≤?0.05) in ¹⁵N incorporation were found between samples of different ¹⁵N RIA down to 0.1 %. The study demonstrates that protein-SIP using ¹⁵N is sufficiently sensitive for quantitative investigation of microbial activity in nitrogen cycling processes.     

Etude de la reaction 14N(d.n)15O a basse energie et application au dosage de l'azote dans le zirconium

Study of the ¹⁴N(d,n)¹⁵O reaction at low energy and application to the determination of nitrogen in zirconiumThe thick target yield for the ¹⁴N(d,n)¹⁵O reaction weis determined between 1 MeV and 5.5 MeV; the “average energy” and the sensitivity of the analysis for nitrogen are calculated. Nitrogen can be determined in zirconium samples at the level of 35 μg g^?1, by irradiating at 4 MeV and 5.5 MeV in a deuteron beam from a Van de Graaff accelerator. After irradiation, the samples are subjected to reducing fusion in a graphite crucible, to separate ¹⁵O. The results are compared with those obtained by other methods on identical samples; there is a difference of 3–17%.     

Double indication in catalytic-kinetic analysis: simultaneous photometric and thermometric indication of the iodine-azide reaction in closed and flow systems

The iodine—azide reaction catalyzed by sulphur-containing compounds is followed simultaneously by optical and thermometric measurements in closed and flowing systems. In the closed system, thiosulphate can be determined in the range 32.4–324 μg ml^-1, by observing the turbidity caused by the nitrogen formed during the reaction and the temperature changes. With the flow apparatus, thiosulphate can be determined in the range 112–1120 μg ml^-1 by continuously mixing the sample and reagent solutions. H₂S in nitrogen 5–100 ppm) is measured by sweeping the gas into the reaction Cuvette. In a third flow procedure, H₂S is liberated continuously from sodium sulphide solutions (0.1–10 μg S^2- ml^-1) by ascorbic acid, and swept to the measuring cuvette with nitrogen.     

Sequential injection system for on-line analysis of total nitrogen with UV-mineralization

An automatic method for the determination of total nitrogen in wastewater by sequential injection analysis and mineralization with UV radiation has been developed. The method is based on the mineralization of the samples with sodium persulphate in basic medium under UV radiation. Small volumes of sample and reagents are firstly aspirated into a single channel and then propelled by flow reversal to the UV reactor and then to the detector. The organic and inorganic nitrogen compounds are oxidized to nitrate that is then measured at 226 nm. The sequential injection procedure has been optimized and the factors affecting the efficiency of the oxidation have been studied with a number of test substances with different chemical structures and properties. Solutions in the concentration range 1-56 g l⁻¹ of nitrogen can be analyzed with the described procedure. The sample rate is of 30-40 samples h⁻¹. The LOD is 0.6 mg l⁻¹ N and the reproducibility is 1.8% (28 mg l⁻¹ N). Organic carbon in the form of glucose was added to a number of test solutions to study the potential interference of organic matter.The method was compared with the Kjeldahl digestion method by analyzing 15 wastewater samples with both methods. The nitrate and nitrite content of the non-oxidized samples were subtracted from the corresponding nitrogen content determined after photo-oxidation and the value compared with the Kjeldahl nitrogen content.     

Surface modification of polyester by oxygen‐ and nitrogen‐plasma treatment

In this paper, we present a study on the surface modification of polyethyleneterephthalate (PET) polymer by plasma treatment. The samples were treated by nitrogen and oxygen plasma for different time periods between 3 and 90 s. The plasma was created by a radio frequency (RF) generator. The gas pressure was fixed at 75 Pa and the discharge power was set to 200 W. The samples were treated in the glow region, where the electrons temperature was about 4 eV, the positive ions density was about 2 × 10¹⁵ m^?3, and the neutral atom density was about 4 × 10²¹ m^?3 for oxygen and 1 × 10²¹ m^?3 for nitrogen. The changes in surface morphology were observed by using atomic force microscopy (AFM). Surface wettability was determined by water contact angle measurements while the chemical composition of the surface was analyzed using XPS. The stability of functional groups on the polymer surface treated with plasma was monitored by XPS and wettability measurements in different time intervals. The oxygen‐plasma‐treated samples showed much more pronounced changes in the surface topography compared to those treated by nitrogen plasma. The contact angle of a water drop decreased from 75° for the untreated sample to 20° for oxygen and 25° for nitrogen‐plasma‐treated samples for 3 s. It kept decreasing with treatment time for both plasmas and reached about 10° for nitrogen plasma after 1 min of plasma treatment. For oxygen plasma, however, the contact angle kept decreasing even after a minute of plasma treatment and eventually fell below a few degrees. We found that the water contact angle increased linearly with the O/C ratio or N/C ratio in the case of oxygen or nitrogen plasma, respectively. Ageing effects of the plasma‐treated surface were more pronounced in the first 3 days; however, the surface hydrophilicity was rather stable later. Copyright © 2008 John Wiley & Sons, Ltd.     

Determination of nitrogen in boron carbide by instrumental photon activation analysis

Boron carbide is widely used as industrial material, because of its extreme hardness, and as a neutron absorber. As part of a round-robin exercise leading to certification of a new reference material (ERM-ED102) which was demanded by the industry we analysed nitrogen in boron carbide by inert gas fusion analysis (GFA) and instrumental photon activation analysis (IPAA) using the ¹⁴N(γ,n)¹³N nuclear reaction. The latter approach is the only non-destructive method among all the methods applied. By using photons with energy below the threshold of the ¹²C(γ,n)¹¹C reaction, we hindered activation of matrix and other impurities. A recently installed beam with a very low lateral activating flux gradient enabled us to homogeneously activate sample masses of approximately 1 g. Taking extra precautions, i.e. self-absorption correction and deconvolution of the complex decay curves, we calculated a nitrogen concentration of 2260 ± 100 μg g⁻¹, which is in good agreement with our GFA value of 2303 ± 64 μg g⁻¹. The values are the second and third highest of a rather atypical (non-S-shape) distribution of data of 14 round-robin participants. It is of utmost importance for the certification process that our IPAA value is the only one not produced by inert gas fusion analysis and, therefore, the only one which is not affected by a possible incomplete release of nitrogen from high-melting boron carbide. Figure Twin-Detector system for analyzing spatially extended samples     

Measurement of surface area of polymer shavings

A modified BET apparatus has been developed to measure the surface area of polymer samples in the range 0.2–3.0 m². The sample holder is designed to permit easy entry for bulky samples, such as polymer shavings, which cannot be loaded through a capillary or exposed to the high temperature required to seal glass. Other apparatus modifications, which allow the accurate measurement of surface area, include the addition of a dibutyl phthalate differential manometer in conjunction with a pressure reservoir. Corrections for the dead volume, nonideality of nitrogen, and the thermal gradient in the sample holder are developed. Calibrations of the equipment with inorganic samples of known areas from 1.81 to 11.8 m² gave highly accurate values of 1.74 to 11.6 m². Results on high-density polyethylene shavings showed precision in determinations and very close agreement in the value of 49.7 for C in the BET equation with the published value, 47.7.     

Peculiarities of gas chromatographic analysis of 6N4 volatile hydrides using a helium ionization detector operated in the ionization amplification mode

A pneumovacuum circuit of a Tsvet-800-2 gas chromatograph with a helium ionization detector (HID) in the ionization amplification mode is proposed for controlling limiting trace impurities in ammonia, arsine, monosilane, and phosphine of grade 6N4. The conditions for the preparation and reliable operation of the chromatograph with the HID are systematized. The effect of an additive of (0.0–17.6) × 10^–4 mol % of hydrogen in the carrier gas of the detector on the polarity of its signal on the impurities of oxygen and nitrogen at a level of (0.5–20.0) × 10^–5 mol % is studied. Methods are proposed for eliminating systematic errors in the determination of the concentration of oxygen and nitrogen in the range from 0.5 × 10^–5 to 1 × 10^–3 mol % in 6N4 volatile hydrides, caused by “counter” impurities in the carrier gas, the mechanism of the heteropolar sensitivity of the HID, and irreversible chemical reactions of oxygen traces with phosphine and products of its decomposition on a CaA–ShM zeolite in a separating column of a chromatograph. Gas chromatography analysis of ammonia, arsine, monosilane, and phosphine of grade 6N4 was carried out.     

Nitrogen distribution in polymers determined by proton track counting and 14 MeV neutron activation analysis

Bulk and surface nitrogen levels of compression-molded samples of polyacrylonitrile, poly (styrene-co-acrylonitrile), poly(methacrylonitrile), polycaprolactam, polyimide, and a cured epoxy resin (EPON 828 — JEFFAMINE T-403, 100:50) were determined, respectively, by 14 MeV NAA (¹⁴N(n,2n)¹³N) and by analysis of proton tracks registered in cellulose nitrate detectors (CN85) as a result of their exposure to protons from the termal neutron induced reaction,¹⁴N(n,p)¹⁴C. Except for a few polymers, agreement of results obtained for nitrogen levels in the bulk and surface regions is within expected limits. Intrinsic limitations of the method for practical determinations of nitrogen are covered. The spatial homogeneity of materials consisting of nitrogen and non-nitrogen containing segments is easily determined by image analysis of proton track densities. These data are then used to construct nitrogen distribution plots and topographical maps. The potential ability of the proton track image analysis technique for distinguishing phase separation or immiscibility of nitrogen containing polymers is discussed.     

Determination of nitrogen in hydrolyzed protein formulations by continuous vapour phase FTIR

An on-line system with vapour generation (VG) and Fourier transform infrared (FTIR) spectrometric detection has been developed for the determination of free ammonium and organic nitrogen in agrochemical formulations containing hydrolyzed proteins. Commercial samples were digested, in batch mode, with sulphuric acid and the obtained solution was alkalinized on-line to transform the NH₄⁺ to NH₃ that was continuously monitored by FTIR. Free ammonium was determined in the same system after simple dilution of undigested samples with water. Different gas phase separators were assayed in order to introduce gaseous NH₃ into a home made IR gas cell of 10 cm pathlength, where the corresponding FTIR spectra were acquired by accumulating 10 scans per spectrum. The 967.0 cm⁻¹ band was used for the quantification of ammonia. The figures of merit of the proposed method involve a linear range up to 100 mg L⁻¹, a limit of detection (3σ) of 1.4 mg L⁻¹ of N, a limit of quantification (10σ) of 4.8 mg L⁻¹ of N, a precision (R.S.D.) of 3.0% for 10 replicate determinations of a 10.0 mg L⁻¹ of N and a sample measurement frequency of 60 h⁻¹. The method was successfully applied to the determination of free ammonium and total N in commercial amino acid formulations and results compare well with those obtained by the Kjeldhal method.     

Calculation of the 14N and 2H quadrupole coupling tensor in trimeric imidazole

The ¹⁴N and ²H quadrupole coupling tensors at both nitrogen and at all hydrogen sites, have been calculated by ab initio SCF MO methods, in a trimeric imidazole molecule having the geometry found in the solid state. The ¹⁴N changes produced at N(1)H by hydrogen bonding are correctly reproduced to a noteworthy degree of precision, as is the ²H quadrupole interaction at the same site. The changes in the values of the three principal components of the tensor, which have been inferred from NQR measurements when imidazole passes from the gas to the solid state, are supported by the calculations, both for N(1)H and N(3).     

Counting Radio-Krypton Atoms with a Laser

Because of their unique chemical and physical properties, long-lived rare krypton radioisotopes, ⁸⁵Kr and ⁸¹Kr, are ideal tracers for environmental samples, including air, groundwater and ice. Atom trap trace analysis (ATTA) is a new laser-based method for counting both ⁸⁵Kr and ⁸¹Kr atoms with the abundance as low as 10^-14 with micro-liters (STP) krypton gas. The entire system for rare radio-krypton measurement built at Hefei is presented, including the atom trap trace analysis instrument and sampling apparatus of gas extraction from water and krypton purification. Atmospheric⁸⁵Kr concentrations at different places in China were measured, showing a range of 1.3-1.6 Bq/m³, consistent with the northern hemispheric baseline. As a demonstration of the system, some shallow and deep groundwater samples in north and south China were sampled and dated.     

Coupled N,C and S stable isotope measurements using a dual‐column gas chromatography system

Many laboratories routinely analyze plant, animal and soil samples with elemental analyzers coupled to isotope ratio mass spectrometers, obtaining rapid results for nitrogen (%N, δ¹⁵N) and carbon (%C, δ¹³C) from the same sample. The coupled N and C measurements are possible because of a gas chromatography (GC) separation of N₂ and CO₂ gases produced in elemental analysis. Adding a second GC column allows additional measurement of sulfur (%S, δ³⁴S) from the same sample, so that combined N, C and S information is obtained routinely. Samples are 1–15 mg, and replicates generally differ by less than 0.1‰ for δ¹⁵N, δ¹³C or δ³⁴S. An example application shows that the N, C, and S measurement system allows a three‐dimensional view of element dynamics in estuarine systems that are undergoing pollution inputs from upstream watersheds. Extension of these GC principles should allow coupled H, C, N, and S isotope measurements in future work. Copyright © 2007 John Wiley & Sons, Ltd.     

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.     

Ammonia-filled discharge tubes for optical emission spectrometric determination of 15N : 14N isotope ratios

A procedure is described for filling a discharge tube, permanently attached to a vacuum line, with ammonia in the pressure range 1.5–5 Torr, without any carrier gas. The ¹⁵N : ¹⁴N isotope ratio is determined from the N₂ spectrum emitted when the tube is excited by a 2450-MHz microwave source. Cooling one end of the discharge tube to —60°C enables the N₂ (1,0) bandheads at 316 nm to be used for ¹⁵N contents down to about 0.04 atom-%. Unidentified interfering bands and emission from NH, OH, H₂ and H are discussed. Samples containing 1–30 mg of nitrogen can be analysed with an accuracy and precision suitable for most soil—plant studies employing ¹⁵N as a tracer.