Rapid speciation of Se(IV) and Se(VI) by flow injection–capillary electrophoresis system with contactless conductivity detection

A flow injection–capillary electrophoresis system with contactless conductivity detection and hydrostatic-pressure-generated flow was used for the fast and sensitive speciation of Se(IV) and Se(VI). The sample throughput was 25 samples per hour using a background electrolyte solution containing 8.75 mM l-histidine (His) adjusted to pH 4.00 with acetic acid. The repeatability of peak areas (n=8) was better than 1.41% and the limits of detection were 190 g L^–1 and 7.5 g L^–1 for Se(IV) and Se(VI), respectively. The interference from carbonate, typically present in water samples, was eliminated by using a low-pH electrolyte in which carbonate is uncharged and migrates at the EOF front. The method was applied to the analysis of Se(IV) and Se(VI) in soil samples that were spiked with both selenium species and the results for recovery of both selenium species were in good agreement with their introduced concentrations.     

Simultaneous determination of arsenic and selenium in biological samples by HG-AFS

A new method is proposed for simultaneous determination of traces of arsenic (As) and selenium (Se) in biological samples by hydride-generation double-channel non-dispersive atomic-fluorescence spectrometry (HG-AFS) from tartaric acid media. The effects of analytical conditions on fluorescence signal intensity were investigated and optimized. Interferences from coexisting ions were evaluated. Under optimum conditions linear response ranges above 20 g L^–1 for As and 32 g L^–1 for Se were obtained with detection limits of 0.13 and 0.12 g L^–1, respectively. The precision for elevenfold determination of As at the 4 g L^–1 level and of Se at the 8 g L^–1 level were 2.7 and 1.9% (RSD), respectively. Recoveries of 92.5–95.5% for As and 101.2–108.4% for Se were obtained for four biological samples and two certified biological reference materials. The proposed method has the advantages of simple operation, high sensitivity, and high efficiency; it was successfully used for simultaneous determination of As and Se in biological samples.     

Ceria nanoparticles deposited on graphene nanosheets for adsorption of copper(II) and lead(II) ions and of anionic species of arsenic and selenium

A nanocomposite prepared from graphene nanosheets and cerium nanoparticles (G/CeO₂) was applied to the extraction of Se(IV), As(V), As(III), Cu(II) and Pb(II). The structure of G/CeO₂ was investigated by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The optimal pH values for extraction are 4.0 for As(V), 3.0 for Se(IV), and 6.0 for both Cu(II) and Pb(II). The maximum adsorption capacity of G/CeO₂ (expressed as mg·g^?1) were calculated by the Langmuir model and are found to be 8.4 for As(V), 14.1 for Se(IV), 50.0 for Cu(II) and 75.6 for Pb(II). The sorbent was applied to dispersive solid phase microextraction prior to direct quantitation by energy-dispersive X-ray fluorescence spectrometry without the need for prior elution. The limits of detection (in ng·mL^?1 units) are 0.10 for As(V), 0.11 for Se(IV), 0.19 for Cu(II) and 0.21 for Pb(II). The precisions (RSDs) are <4.5%. The accuracy of the method (1 - 4%) was verified by analysis of the certified reference material (CRM 1640a - natural water). The method was successfully applied in ultratrace element determination and to the speciation of selenium in environmental waters.

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Graphical abstract The method gives possibility of simultaneous preconcentration and determination in environmental waters of both anionic (As(V) and Se(IV)) and cationic (Cu(II) and Pb(II)) forms of selected metals using graphene nanosheets and cerium nanoparticles. Se(IV) can be selective determined in the presence of Se(VI).

     

A nanocomposite consisting of MIL-101(Cr) and functionalized magnetite nanoparticles for extraction and determination of selenium(IV) and selenium(VI)

A metal-organic framework nanocomposite was synthesized and applied to speciation analysis of Se(IV) and Se(VI). The sorbent is composed of MIL-101(Cr) and magnetite nanoparticles modified with dithiocarbamate. It is capably of selectively extracting Se(IV) at pH?=?1.85, while Se(VI) remains in solution. The total amount of selenium can then be determined by reducing Se(VI) to Se(IV) and also extracting it. The extraction parameters were optimized by employing design-of-experiments methodology. Selenium was then quantified by electrothermal AAS. Figures of merit include (a) a 10 ng·L^?1 limit of detection, (b) a linear response in the 30 ng·L^?1 to 10 μg·L^?1 concentration range, and (a) a relative standard deviation of <11.5% for Se(IV). The method was validated by analyzing certified reference materials (water and tomato leaves). It was also applied to the speciation analysis of Se(IV) and Se(VI) in (spiked) water samples and of total selenium in agricultural samples.

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Graphical abstract Schematic of the synthesis of a metal-organic framework nanocomposite for speciation analysis of Se(IV) and Se(VI). The sorbent is composed of MIL-101(Cr) and magnetite nanoparticles modified with dithiocarbamate. Selenium can be quantified by electrothermal AAS with a 10 ng L^?1 detection limit.

     

Determination of selenite,selenate and selenomethionine by ion chromatography,microwave digestion and HGAAS

An on-line system for the simultaneous determination of Se(IV), Se(VI) and selenomethionine (Se-Met) in aqueous samples was developed, consisting of separation by ion chromatography, microwave digestion and detection by hydride generation atomic absorption spectrometry. 8.3 mmol/l Na₂HPO₄ (pH 9.2) was used as mobile phase for the ion chromatography, with a flow-rate of 1.5 ml/min. After the separation the sample was mixed with concentrated KBr-HCl solution and heated with microwave energy to digest Se-Met and reduce Se(VI) to Se(IV). The detection limits were 15 g/l, 12 g/l and 103 g/l for Se(IV), Se(VI) and Se-Met, respectively.     

Gas-phase chemiluminescent determination of inorganic selenium in water samples following flow injection hydride generation and cryotrapping

We describe a method for the determination of inorganic selenium in water samples via gas-phase chemiluminescence (GPCL). Se(IV) was first derivatized with 4-nitro-o-phenylenediamine to form 5-nitropiazselenol. The latter was decomposed by persulfate through photocatalytic oxidation to give Se(VI), which was reduced to Se(IV). Selenium hydride was generated from Se(IV) through reduction with sodium borohydride and then preconcentrated using cryotrapping. The cryotrapped hydride was evaporated and carried to a reaction chamber by a stream of helium, where it produced GPCL as a result of ozonation. The method exhibits a wide linear calibration range (from 0.5?μg?L^?1 to 1.0?mg?L^?1) with a detection limit of 0.12?μg?L^?1 (for n?=?11), and a relative standard deviation of 3.90?% (at n?=?11) at 5.0?μg?L^?1 level of selenium. The method was applied to the determination of inorganic selenium in water samples and gave satisfactory results.

Hemilability and regioselectivity in palladium and platinum complexes of dppm(E) [E = O, S, Se] ligands

Hemilability and nonrigidity in a series of mixed P^PE donor ligands where E = O, S, or Se have been studied in palladium and platinum complexes of the type [M{κ²-(dimethylamino)ethylnaphthyl-C,N)}(P^PE)][SbF₆] where P^PE = Ph₂PCH₂P(E)Ph₂.The role of the donor in hemilability, regioselectivity and the binding preferences of particular donors trans to the metallated carbon atom were also investigated. NMR parameters including couplings to ¹⁹⁵Pt and ⁷⁷Se were investigated for cis and trans isomers. The magnitude of ²J¹³C-⁷⁷Se couplings can readily distinguish the cis and trans isomers. A large through-space ¹³C-⁷⁷Se ³J coupling was observed in one of the amino methyl groups of the dppm(Se) complexes.     

Simultaneous speciation of selenoproteins and selenometabolites in plasma and serum by dual size exclusion-affinity chromatography with online isotope dilution inductively coupled plasma mass spectrometry

A method for the simultaneous speciation of selenoproteins and selenometabolites in mouse plasma has been developed based on in series two-dimensional size exclusion and affinity high-performance liquid chromatography (2D/SE-AF-HPLC), using two columns of each type, and hyphenation to inductively coupled plasma-(quadrupole) mass spectrometry (ICP-QMS). The method allows the quantitative determination of selenoprotein P (SeP), extracellular glutathione peroxidase (eGPx), selenoalbumin (SeAlb), and selenometabolites in mouse plasma using species-unspecific isotope dilution (SUID). The 2D chromatographic separation is proposed to remove typical spectral interferences in plasma from chloride and bromide on ⁷⁷Se (⁴⁰Ar³⁷Cl) and ⁸²Se (⁸¹Br¹H). In addition, the approach increases chromatographic resolution allowing the separation of eGPx from Se metabolites of low molecular mass. The method is robust, reliable, and fast with a typical chromatographic runtime less than 20 min. Precision in terms of relative standard deviation (n?=?5) is in the order of 4 %, and detection limits are in the range of 0.2 to 1.0 ng Se g^?1. Method accuracy for determination of total protein bound to Se was assessed by analyzing human serum reference material (BCR-637) certified for total Se content, and latterly applied to mouse plasma (Mus musculus). In summary, a reliable speciation method for the analysis of eGPx, selenometabolites, SeP, and SeAlb in plasma/serum samples is proposed for the first time and is applicable to the evaluation of Se status in human in clinical studies and other mammals for environmental or toxicological assessment.

Development and application of a simple routine method for the determination of selenium in serum by octopole reaction system ICPMS

The aim of the study was to develop an inductively coupled plasma mass spectrometry (ICPMS) method for robust and simple routine determination of selenium in serum. Polyatomic interferences on ⁷⁶Se, ⁷⁷Se, and ⁷⁸Se were removed by applying an octopole reaction system ICPMS with the reaction cell pressurized with H₂ gas. We developed a novel simple optimization routine for the H₂ gas flow based on a signal-to-noise ratio (SNR) calculation of the selenium signal measured in a single selenium standard. The optimum H₂ flow was 2.9 mL min^–1. The selenium content in serum was determined after a 50-fold dilution with 0.16 M HNO₃ and quantified by using addition calibration and gallium as an internal standard. The method detection limit was 0.10 g L^–1 for ⁷⁶Se and ⁷⁸Se and 0.13 g L^–1 for ⁷⁷Se. Human serum samples from a case-control study investigating if selenium was associated with risk of colorectal adenoma were analyzed. The average selenium concentration for the control group (n=768) was 137.1 g L^–1 and the range was 73.4–305.5 g L^–1. The within-batch repeatability (a batch is ten samples) estimated from 182 replicate analyses was 6.3% coefficient of variation (CV), whereas the between-batch repeatability was 7.4% CV estimated from 361 replicates between batches. The method accuracy was evaluated by analysis of a human serum certified reference material (Seronorm Serum level II, Sero A/S, Norway). There was a fairly good agreement between the measured average of 145±3 g L^–1 (n=36) and the certified value of 136±9 g L^–1. In addition the method was successfully applied for analysis of zinc serum concentrations without further optimization. For the Seronorm certified reference material a value of 911±75 g L^–1 (n=31) for zinc was obtained, which corresponds well to the certified zinc value of 920±60 g L^–1.     

Determination of Total Selenium Content in Cereals and Bakery Products by Flow Injection Hydride Generation Graphite Furnace Atomic Absorption Spectrometry Applying in-situ Trapping on Iridium-Treated Graphite Platforms

A flow injection hydride generation graphite furnace atomic absorption spectrometric (FI-HG-GFAAS) method was applied to the determination of Se in Se-doped and undoped cereals and bakery products. For the purpose of doping, the soils used for the cultivation of the cereals were dosed with Se-doped foliar fertilizers. The samples were dissolved in a mixture of HNO₃ and H₂O₂ solutions using microwave-assisted digestion. The decomposition of H₂Se generated from the sample solutions and the trapping of elemental Se were performed at a temperature of 300°C on an Ir-pretreated integrated graphite platform of a transversally heated graphite atomizer (THGA). For release of the trapped Se within a fairly short atomization time (5s), an atomization temperature of 2200°C was observed to be optimal. The overall efficiency of hydride generation, transport and trapping was 86%.The upper limit of the linear dynamic range of calibration was 10µgL^–1, which corresponds to 0.5µgg^–1 for solid samples. Recovery of the samples spiked with Se^VI solutions was found to be 93±6% on average. The relative standard deviation of the determinations was less than 8%. The limit of detection was found to be 0.06µgL^–1, corresponding to 3ngg^–1 for solid samples. The accuracy of the method was verified with the use of IAEA-155 (whey powder) certified reference material. End-capped THGA tubes resulted in an extension of the linear calibration range compared to that of standard THGAs.The Se content in bakery products made of undoped cereals ranged from 7.7 to 68ngg^–1 (wet weight) in 18 samples, whereas the Se content of the corresponding cereals was found to be below 100ngg^–1 (wet weight). The Se level of cereals grown on soils treated with Se-doped fertilizers ranged from 128 to 1046ngg^–1 (wet weight), and it depended linearly on the Se concentration of the corresponding foliar fertilizer.     

Hydride generation-headspace single-drop microextraction-electrothermal atomic absorption spectrometry method for determination of selenium in waters after photoassisted prereduction

A headspace single-drop microextraction (SDME) method has been developed in combination with electrothermal atomic absorption spectrometry for determination of total inorganic Se and Se(IV). SeH₂ is generated in a 40 mL volume closed-vial and trapped onto a Pd(II)-aqueous containing drop that is supported at the needle tip of a high-precision chromatographic syringe. Sample pre-treatment by UV irradiation prior to hydride generation allowed converting Se(VI) into Se(IV), hence, facilitating total Se determination. A fractional factorial design was employed for characterising the effect of relevant variables over SeH₂ trapping. The variables showing the most significant effect were Pd(II) concentration in the drop and extraction time. A preconcentration factor of about 25 is achieved. The limit of detection of Se was 0.15 ng/mL using trapping onto a 3-μL drop and the precision expressed as relative standard deviation was about 3%. The limit of detection could be improved further using repeated sampling of the headspace.     

Intramolecular Interactions in Anisole,Thioanisole, and Selenoanisole. Application of Natural Bond Orbitals Method

Calculations of molecules PhXMe (X = O, S, Se) in MP2(f)/6-31G(d) approximation were performed. The stationary points on the potential energy surface were determined and identified. The anisole molecule is planar with a barrier to rotation H 7.78 kJ mol^-1. In thioanisole and selenoanisole (H 3.08 and 10.25 kJ mol^-1 respectively) the energy minimum corresponds to an orthogonal form. Analysis of relation between intramolecular interactions and conformational structure of the molecules in question was performed by the method of natural bond orbitals. In X atoms one lone electron pair is a hybrid orbital with the following fraction of s-component: 38-45% (O), 66-68% (S), and 73-74% (Se). The second lone electron pair is virtually pure pz-AO.     

Determination of selenium(IV) and selenium(VI) in natural water samples by neutron activation analysis after chemical pre-collection

Neutron activation analysis using shortlived^77mSe (T=17.5 s) preceded by chemical pre-collection of Se on activated carbon (AC) has been applied to the fractional determination of Se(IV) and Se(VI) in natural water samples such as river water and sea water. Two chemical procedures were adopted: adsorption of Se(IV)/Bismuthiol-II complex on AC and adsorption of Se(VI)/Bismuthiol-II on AC after reduction of Se(VI) to Se(IV). It was shown that Se(IV) and Se(VI) can be determined at a <g l^–1 (ppb) level in natural waters by the present method.     

Inductively coupled plasma mass spectrometry for stable isotope tracer investigations

Inductively coupled plasma mass spectrometry (ICP-MS) has now been developed for application to stable isotope tracer investigations of several minerals/trace elements. Use of this method for such purposes requires an understanding of a number of fundamental issues: analytical chemistry performance of the method of isotopic analysis, relationship of the level of enriched isotope administered to the subject with background level of the isotope already present, the issues of cost, and finally the specific details of the biological issues to be explored.In this paper, a brief discussion of these issues is presented. As an example, the discussion is presented in relation to selected aspects of metabolism of selenium, employing the three stable isotopes⁷⁴Se,⁷⁷Se, and⁸²Se in the rat as the biological model.Analytical performance of hydride generation/ICP-MS is discussed for the required analyses of selenium isotopes. It is shown that for solutions containing 10 ng/ml Se of natural isotopic composition, optimized signal/background ratios greater than 40/1 can be obtained, resulting in worst-case detection limits (ng Se) of 2 (⁷⁴Se), and 0.6 (^77,82Se). The precision and accuracy of isotope ratio measurements for the method used routinely in biological studies is 1%. The accuracy of the method for quantitative isotopic analysis is compared with hydride generation/atomic absorption spectrophotometry (HG/AAS). The following results are given (g Se/g or ml; mean + 1 SD,n = 3–5; first HG/ICP-MS, second HG/AAS): SRM 1577a [bovine liver] 0.697 ± 0.002 versus 0.69 ± 0.01; human blood plasma 0.098 ± 0.001 versus 0.135 ± 0.008; human red cells 0.211 ± 0.002 versus 0.216 ± 0.012; and human urine 0.0473 ± 0.0003 versus 0.0489 ± 0.0003.An experiment is described with the rat to show the feasibility of the method for studies of selenium metabolism. Rats were placed on Se-free diet for eight weeks, given their Se requirements in the drinking water in the form of⁷⁶SeO ₃ ^2– and a single-day (day 3) replacement of their water with that containing highly enriched⁷⁴SeO ₃ ^2– . Isotopic analysis of carcass and selected organs revealed a high degree of isotopic enrichment with respect to⁷⁴Se during the entire eight weeks of the experiment, indicating the feasibility of this approach for detailed investigations of selenium metabolism in the rat.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria     

Rapid determination of selenium in various marine species by instrumental neutron activation analysis

Instrumental neutron activation analysis was used to determine selenium concentrations in several marine organisms including two certified reference materials /NRCC lobster hepatopancreas, NBS oyster tissue/ and one uncertified material /IAEA fish homogenate/. The⁷⁶Se/n, /^77mSe/T=17.4 s/ reaction was successfully employed to achieve an overall precision between 3–10% and detection limits between 0.3–0.6 g/g. The accuracy of the results, as compared to the certified values, was in excellent agreement with the NBS material and only slightly lower /9%/ for the NRCC material.     

Determination of Arsenic in Food Samples by Hydride Generation – Atomic Absorption Spectrometry

A method for the determination of trace amounts of arsenic in food samples using flow injection analysis and atomic absorption spectrometry with hydride generation (FI-HG AAS) was developed. The parameters of the flow injection system and the hydride generation were optimized with respect to reagent concentrations, atomization temperature, injection volume, reaction coil length and carrier flow rate. The limits of detection and quantification were 0.34µgL^–1 and 1.2µgL^–:1, respectively, and the analytical curve is linear up to 30.0µgL^–1 arsenic. The relative standard deviation for 12 replicates varies between 5% for 4.0µgL^–1 As and 1.8% for 30.0µgL^–1 As, with an injection frequency of up to 135h^–1. Interferences from Ni(II), Cu(II), Fe(III), Cr(III), Mo(II), Bi(III), Se(IV), Se(VI), Sb(III) and Sb(V) could be masked with a mixture of ascorbic acid-KI in a 5.0molL^–1 HCl solution. The accuracy of the proposed method was evaluated by using certified reference materials of biological samples, and the method was used to determine the content of arsenic in fish and coffee beans.     

Comparison of methods for the determination of total selenium in plasma by magnetic sector inductively coupled plasma mass spectrometry

Three sample preparation methods (dilution, microwave digestion and ethanol addition) were evaluated for the determination of Se in human plasma using magnetic sector inductively coupled plasma mass spectrometry (ICP-MS). A number of instrumental parameters were also considered, namely the choice of internal standard (Sc, Rh, In) and pre-defined spectral resolution (m/Δm ∼300 (low resolution, LR) and ∼7500 (high resolution, HR)). The isotopes and were selected for analysis avoiding the major and unresolvable Ar dimer interferences associated with other Se isotopes.Ethanol addition was found to be the most suitable and reliable sample preparation method. The optimum ethanol concentration for a 1+9 dilution of the plasma sample was 0.5% (v/v). Indium or Sc were selected over Rh as internal standards as Seronorm™ Level 1 target values were achieved at lower concentrations of ethanol modifier. Representative ICP-MS detection limits for this method using (HR) and (HR), were 0.1 and 0.2 μg l⁻¹.Accurate analysis of Se in Seronorm™ Level 1 (MI0181) was found using either or and HR (86±5 and 83±5 μg l⁻¹ respectively, n=5, In internal standard, target value=83±6 μg l⁻¹). Although offering improved precision (e.g. LR: 0-3% versus HR: 1-6%), accurate results were only obtained with LR for the Seronorm™ Level 1 sample when using (86±1 μg l⁻¹). The major interference precluded accurate Se analysis using and LR for all samples considered. Reliable Se concentrations were only found for a “real” pooled sample when using (HR), in part a result of non-negligible and unresolved interference when using LR. Consistently elevated Se concentrations were found under all conditions when Seronorm™ Level 2 (NO0371) was analysed. These results were confirmed by independent GF-AAS analysis.Magnetic sector ICP-MS with (HR), in combination with the ethanol addition sample preparation method, was used for the analysis of Se in human plasma samples as part of a small pilot study. Average measured selenium concentrations were 102±18 μg l⁻¹ (n=18).     

Determination of selenium in sediment by isotope-dilution inductively coupled plasma mass spectrometry with an octapole reaction cell

A method is described for determination of selenium in sediment by isotope-dilution inductively coupled plasma mass spectrometry with an octapole reaction cell (ID–ICP–ORCMS). Sediment samples were digested with HNO₃, HClO₄, and HF, and the digestion included an elaborate evaporation process to remove bromine from the digested solution. Simple strong cation-exchange disk filtration was used to remove rare earth elements (REE) from the digested solution, because REE²⁺ seriously interfere with Se isotopes (i.e. ¹⁵⁶Gd²⁺ with ⁷⁸Se⁺, ¹⁶⁰Gd²⁺ with ⁸⁰Se⁺). Addition of acetic acid to the filtrate was examined to improve the sensitivity of ICP–ORCMS measurement of Se⁺ by means of a carbon-enhancement effect. The interfering for selenium isotopes were almost eliminated by use of H₂ as reaction gas. Interference from BrH⁺ formed in the reaction cell was negligible because the Br was removed in the evaporation process. Approximately 99.5% of REE were removed by cation-exchange disk filtration yet more than 99% of Se remained in the filtrate solution. The intensity for Se⁺ was enhanced approximately fourfold by addition of 5% (v/v) of acetic acid whereas that for was barely enhanced. Measured ⁸⁰Se/⁷⁸Se ratios in unspiked digested solutions of the sample were in good agreement with that for an Se standard solution. The analytical results for Se in the certified reference materials MESS-3 and PACS-1 were in good agreement with their certified values, with small uncertainties.