Determination Of Selenite Research Articles

Nanozymes with reductase-like activities: antioxidant properties and electrochemical behavior.

Nanozymes (NZs) as stable cost-effective mimics of natural enzymes may be promising catalysts in food and environmental biotechnology, biosensors, alternative energy and medicine. The majority of known NZs are mimetics of oxidoreductases, although there are only limited data regarding mimetics of reductases. In the present research, a number of metal-based NZs were synthesized via chemical methods and screened for their antioxidant ability in solution. The most effective reductase-like Zn/Cd/Cu NZ was characterized in detail. Its antioxidant properties in comparison with several food products and Trolox, as well as substrate specificity, size and composition were studied. Zn/Cd/Cu NZ was shown to mimic preferentially selenite reductase. The amperometric sensor was constructed possessing a high sensitivity (1700 A M−1 m−2) and a broad linear range (16–1000 μM) for selenite ions. The possibility to apply the fabricated sensor for selenite determination in commercial mineral water has been demonstrated.

Determination of selenite and selenomethionine in kefir grains by reversed-phase high-performance liquid chromatography-inductively coupled plasma-optical emission spectrometry.

A new and efficient reversed-phase high-performance liquid chromatography-inductively coupled plasma-optical emission spectrometry method was developed for the simultaneous separation and determination of SeO3 2- and seleno-dl-methionine in kefir grains. For the system, limits of detection and quantitation values for SeO3 2- and seleno-dl-methionine were calculated as 0.52/1.73mg/kg (as Se) and 0.26/0.87mg/kg (as Se), respectively. After performing the system analytical performance, recovery experiment was done for kefir grains and percent recovery results for SeO3 2- and seleno-dl-methionine were calculated as 98.4 ± 0.8% and 93.6 ± 1.0%, respectively. It followed by the feeding studies that the kefir grains were exposed to three different concentrations of SeO3 2- (20, 30, and 50mg/kg) for approximately 4 days at room temperature to investigate the conversion/non-conversion of SeO3 2- to seleno-dl-methionine. Next, the fed grains were extracted with tetramethylammonium hydroxide pentahydrate solution (20%, w/w) and then sent to the developed system. There was no detectable seleno-dl-methionine found in fed kefir grains at different concentrations of SeO3 2- while inorganic or elemental selenium in the fed kefir grains was determined between 1579.5 - 3116.0mg/kg (as Se). Selenium species in the kefir grains samples was found in the form of SeO3 2- proved by using an anion exchange column.

Determination of selenocyanate, selenate, and selenite in mining wastewater by GC–MS using sequential derivatization and extraction

Selenium speciation analysis is usually carried out using complex hyphenated analytical systems such as LC-ICP-MS. Here we present a novel selenium speciation approach based on a sequential derivatization and extraction combined with gas chromatography mass spectrometry for the simultaneous determination of selenite, selenate, and selenocyanate in aqueous mine wastewater samples. Selenocyanate was derivatized with triethyloxonium tetrafluoroborate to ethylselenocyanate, which was extracted into chloroform, following which the sample was split into two aliquots. One aliquot was acidified and 3,5-bis(trifluoromethyl)-o-phenylenediamine was used for the novel derivatization of selenite to 4,6-bis(trifluoromethyl)-2,1,3-benzoselenadiazole, for the determination of selenite. For the second aliquot, concentrated hydrochloric acid was added along with 4-nitro-o-phenylenediamine to simultaneously reduce selenate to selenite and derivatize the combined “selenite + selenate” fraction to 5-nitro-2,1,3-benzoselenadiazole. The benzoselenadiazoles were extracted with chloroform and all extracts were combined for GC–MS analysis. Low ng g−1 detection limits were reported for all three species. The method is unhindered by concentrations of chloride and sulphate up to 3%, as well as nitrate concentrations up to 3% for selenocyanate and selenite analysis, with minor losses in sensitivity for selenate up to 100 ppm nitrate, making the method particularly suitable for aqueous mine waste characterization. Quantitative trace selenium speciation was achieved using cost-effective materials and apparatus on a simple-to-operate benchtop instrument. The novel methodology was tested on gold mine wastewater samples; comparing to total selenium, a 63–149% recovery as the sum of species was observed. Additionally, this novel speciation approach was compared to LC-ICP-MS based selenium speciation and a reasonable agreement was found in the species distribution.

An optical sensor for selenite determination in aqueous samples

BackgroundIn the present work, an optical sensor has been developed for the determination of selenium (IV) in aqueous samples. The optical sensor is a cellulose triacetate plasticized membrane with tri-octyl methyl ammonium chloride (Al-336) immobilized in it as an ion exchanger.MethodsThe sensor works in two steps. In the first step, a 1 × 2 cm piece of the membrane is allowed to equilibrate with a known amount of selenium (IV) solution and potassium iodide in acidic medium. During this equilibration, the iodometric reaction takes place and, I3− (tri-iodide anion) is formed which gets sorbed into the membrane due to which the colorless film changes to yellow color showing an absorption maxima at 355 nm which is a characteristic of I3−. In the second step the I3− loaded membrane is allowed to equilibrate with a solution of variamine blue dye in presence of sodium acetate. In this step, oxidation of the dye takes place and the membrane shows a color change from yellow to violet and a shift in the absorption maxima from 355 to 550 nm.ResultsThe method is used to study the quantitative determination of selenium in aqueous samples, where the amount of selenium is varied and the corresponding absorbance at 550 nm is measured. A good linear curve is obtained in the concentration range of 0.2 to 3 μg mL−1 of selenium.ConclusionsThe proposed method can be successfully applied to determination of selenium (IV) in various pharmaceutical and environmental samples.

Construction of a novel "Off-On" fluorescence sensor for highly selective sensing of selenite based on europium ions induced crosslinking of nitrogen-doped carbon dots

A highly blue luminescent nitrogen-doped carbon dots (N-CDs) was prepared, using citric acid and l-lysine via one pot hydrothermal treatment, and applied to the determination of selenite. The synthesized N-CDs exhibited a high photostability, excellent optical features with a high quantum yield (21.5%) and average size of 3nm. The method is based on the competition between oxygen donor atoms in selenite oxyanion with those from carboxylate and hydroxyl groups at the surface of N-carbon dots for europium ions binding. N-CDs could be readily quenched upon the addition of europium ions owing to high affinity of carboxylate and hydroxyl groups at the surface of carbon dots to Eu3+ leading to aggregation of N-CDs (state OFF). After selenite addition, disruption of the aggregated N-CDs takes place, leading to restoration of the quenched fluorescence (State ON). The repeatability was less than 3.2% for selenite in both standard and real samples (n = 3). The method provides a simple procedure enabled selective detection of selenite with a linear range of 0.078–21.4µgmL−1 and a limit of detection of 53.0ngmL−1 (S/N = 3). The accuracy and precision were evaluated based on the detection of selenite in health care products with satisfactory results.

Ionic liquid-assisted separation and determination of selenium species in food and beverage samples by liquid chromatography coupled to hydride generation atomic fluorescence spectrometry

Different ionic liquids (ILs) were assayed as mobile phase modifiers for the separation and determination of selenite [Se(IV)], selenate [Se(VI)], selenomethionine (SeMet) and Se-methylselenocysteine (SeMeSeCys) by reversed-phase high-performance liquid chromatography coupled to hydride generation atomic fluorescence spectrometry (RP-HPLC-HG-AFS). The use of several ILs: 1-butyl-3-methylimidazolium chloride, 1-hexyl-3-methylimidazolium chloride ([C6mim]Cl), 1-octyl-3-methylimidazolium chloride, 1-dodecyl-3-methylimidazolium bromide, 1-hexadecyl-3-methylimidazolium bromide and tributyl(methyl)phosphonium methylsulphate was evaluated. Also, the effect of pH, buffer type and IL concentration on the separation of Se species was studied. Complete separation was attained within 12min using a C8 column and a gradient performed with a mobile phase containing 0.1% (v/v) [C6mim]Cl at pH 6.0. The proposed method allows the separation of inorganic and organic Se species in a single chromatographic run, adding further benefits over already reported methods based on RP-HPLC. In addition, the influence of ILs on the AFS signals of each Se species was evaluated and a multivariate methodology was used for optimization of AFS sensitivity. The limits of detection were 0.92, 0.86, 1.41 and 1.19μgL−1 for Se(IV), Se(VI), SeMet and SeMeSeCys, respectively. The method was successfully applied for speciation analysis of Se in complex samples, such as wine, beer, yeast and garlic.

A simplified method for inorganic selenium and selenoaminoacids speciation based on HPLC–TR–HG–AFS

A simplified speciation method for the determination of selenite, selenate and three selenoaminoacids (selenocystine, selenomethylselenocysteine and selenomethionine) has being developed, based on the coupling of high performance liquid chromatography (HPLC), thermoreduction (TR), hydride generation (HG) and atomic fluorescence spectrometry (AFS). Most of the existing methods based on AFS detection employ a two step procedure to reduce selenate to selenite before HG: (i) Ultraviolet radiation followed by (ii) heating, to produce volatile hydrides of the selenium compounds. The proposed simplified method HPLC–TR–HG–AFS does not require ultraviolet radiation. Instead, KBr dissolved in a HCl solution is added during the heating step (thermoreduction), resulting in an effective hydride generation of the selenium species. Different variables (temperature, HCl and NaBH4 concentrations) have been optimized, using both univariant and multivariant experimental designs. The proposed method is therefore less complex and allows limits of detection, reproducibility and repeatability values similar or better than the existing AFS detection methods described in the literature. A Certified Reference Material (SELM-1 with certified selenomethionine content) and a Se-enriched algae sample have been successfully analyzed with the proposed method. The results were also compared to an alternative technique (GC–MS) that provided similar results.

Analysis Of Selenium Species Using Cathodic Stripping Voltammetry

M respectively when an accumulation potential of 100 mV was applied for 90 seconds. Simultaneous determination of selenite and dimethyl diselenide was also carried out. The effect of selenite on the determination of dimethyl diselenide and vice versa was studied. The influence of metal ions such as lead, copper, cadmium and zinc on the reduction signals for selenite and dimethyl diselenide was also investigated. Key words: Cathodic stripping voltammetry, Se(IV), dimethyldiselenide

A New and Simple Method for the Determination of Trace Fe(III), Cu(II), Ni(II),Zn(II) and Se(IV) in Human Hair by Differential Pulse Polarography

A new and simple differential pulse polarographic (DPP) method for the trace element determination in hair has been established. While Ni in hair was determined in ammonia buffer, four elements (Fe, Cu, Zn, Se) could be determined simultaneously in acetate buffer with only one sample addition. For this purpose human hair taken from a healthy person is first digested with acids and then the polarogram is taken at various electrolytes and pH values. In acetate buffer at pH 4 zinc could be determined. The same electrolyte was used in the presence of EDTA for the determination of both copper and iron. For selenite determination, hydrogen catalytic peak at about -1.1 V in acetate buffer (pH= 3.3) was used in the presence of 50 - 100 times more molybdenum, Mo(VI). In this medium the limit of detection was 10–9M for selenite. Ni peak could be separated from zinc peak in ammonia buffer at pH 9.8 and was determined in this medium. The quantities of trace elements were found as Zn 201 μg/g, Cu 20 μg/g, Ni 1.2 μg/g, Se 0.9 μg/g and Fe 15 μg/g. With the proposed method the reference hair was also analyzed and good agreement was obtained.

Headspace-solid phase microextraction of selenium(IV) from human blood and water samples using polypyrrole film and analysis with ion mobility spectrometry

Headspace-solid phase microextraction (HS-SPME) with a polypyrrole (PPy)-coated fiber was applied as a sample preparation method for determination of selenite [Se(IV)] following derivatization with 1,2-diaminobenzene to convert into the piaselenol form and analysis by ion mobility spectrometry (IMS). The HS-SPME–IMS method presents good repeatability (RSDs <6%), simplicity, good sensitivity and short analysis times. The influence of the various analytical parameters on microextraction procedure, i.e. ligand concentration, pH, ionic strength, equilibrium time and temperature has been evaluated and optimized. The calibration graph was linear in the range of 20–320 ng mL −1 with a detection limit of 12 ng mL −1. The method was applied for determination of selenium in human serum and environmental surface water samples with satisfactory recovery.

New selenite ion-selective electrodes based on 5,10,15,20-tetrakis-(4-methoxyphenyl)-21H,23H-porphyrin-Co(II)

New polymeric membrane (PME), modified carbon paste (MCPE), and coated wire (CWE) selenite ion-selective electrodes based on 5,10,15,20-tetrakis-(4-methoxyphenyl)-21H,23H-porphyrin-Co(II) (CoTMeOPP) are reported. The best composition was the electrode containing 2% CoTMeOPP as the active material and 49% TCP as plasticizer. The electrodes reveal a Nernstian behavior over a concentration range of 5.5 × 10 −5 to 1.1 × 10 −2 M for PME, 5.2 × 10 −5 to 1.2 × 10 −2 M for MCPE and 1.2 × 10 −4 to 4.4 × 10 −3 M for CWE. The potentiometric response is pH dependent, since selenous acid is a diprotic acid. The slope of the selenite PVC electrode was −57.0 mV for the monovalent anion at pH 6.47, and −26.0 mV for the divalent anion at pH 11.00. The detection limits were 3.4 × 10 −5 and 4.7 × 10 −5 M at pH values 6.47 and 11.00, respectively. The electrodes manifest advantages of low resistance, very short response time (15 s), and most importantly good selectivities relative to a wide variety of other anions. In fact, the proposed selenite ion-selective electrodes show a great improvement compared to previously reported electrodes for selenite ion. The electrode was used for the determination of selenite in selenite/selenate mixture, in sodium selenite raw material powder, and in VitaFit Selenium ACE antioxidant tablets with recovery ranges of 90.0–103.3%.

Solid phase extraction – Multisyringe flow injection system for the spectrophotometric determination of selenium with 2,3-diaminonaphthalene

In the present work, a solid phase extraction (SPE) is hyphenated with an automatic MSFIA system to improve the selenite determination based on the reaction of selenite with aromatic o-diamines (such as 2,3-diaminonaphthalene (DAN)) to form the piazselenol complex. This reaction is greatly influenced by acid concentration, temperature, the time needed for colour development, and presence of foreign ions. For these reasons a thermostatic bath, glycine, and Na(2)-EDTA are used as heater, buffer, and masking agent, respectively. The principle of the determination is based on the sorption of the piazselenol onto a C(18) membrane disk, followed by its elution by acetonitrile. The piazselenol can then be detected by absorptiometry or fluorometry, both detection techniques being tested in our system. The best detection limit (1.7 microg L(-1)) and RSD (3.04%) are obtained by absorptiometry at 380 nm. Environmental samples were spiked and analyzed, with recoveries close to 100%.

Comparison of tetraethylborate and tetraphenylborate for selenite determination in human urine by gas chromatography mass spectrometry, after headspace solid phase microextraction

Two different derivatizing reagents were tested for the development of a fast and sensitive method for the determination of selenites (Se IV) in human urine. The reagents were sodium tetraethylborate (NaBEt 4) and tetraphenylborate (NaBPh 4), respectively, and the procedure is based on in situ derivatization of selenites in aqueous medium. Selenite ions are converted to diethylselenide (DESe) or diphenylselenide (DPhSe) and subsequently collected from the headspace by solid phase microextraction using a silica fiber coated with polydimethylsiloxane (HS-SPME). Finally, they are quantitated by GC/MS in SIM mode. Ethylation over phenylation was proved preferable for the headspace extraction because of the higher volatility of the diethyl-derivative of selenites. The optimization of the HS-SPME conditions was performed both in aqueous and urinary solutions. Under the optimum conditions for HS-SPME, the gas chromatographic conditions were also optimized. Between the two alkylation reagents tetraethylborate was proved more efficient and the quantitation was satisfactory. Aqueous certified reference materials were analyzed to evaluate the accuracy of the method. The precision of the method was 4.2% and the calculated detection limit was 0.05 μg L −1 for human urine.

Headspace and direct immersion solid phase microextraction procedures for selenite determination in urine, saliva and milk by gas chromatography mass spectrometry

Two solid phase microextraction modes were investigated and compared for their performance on the determination of selenites in various biological liquids like human urine and saliva and various types of milk. Using sodium tetraethylborate (NaBEt 4) as ethylating reagent, selenites are converted in situ to volatile diethylselenides (DESe) in aqueous medium. The derivative is collected in situ by solid phase microextraction (SPME) using a silica fiber coated with poly(dimethylsiloxane) (PDMS) either from the headspace (HS-SPME) or directly from the liquid phase (LP-SPME) and finally determined by capillary GC/MS. Under optimum conditions of SPME, the GC separation was also optimized. Between the two examined microextraction techniques, direct immersion of the PDMS fiber in the liquid phase was proved less satisfactory. In contrast, the headspace procedure appears to be more efficient. The quantification of selenites was achieved in SIM mode with good analytical performance. A non-fat milk powder certified reference material was analyzed to evaluate the accuracy of the method. The overall precision of the method was ranged between 6.2% and 9.7%. Detection limits achieved were 0.05 μg L −1 for human urine, 0.08 μg L −1 for saliva and 0.03–0.06 μg L −1 in various milk matrices.

CdS Quantum Dots as Fluorescence Probes for the Detection of Selenite

Water-soluble cadmium sulfide (CdS) quantum dots (QD) capped by mercaptoacetic acid were synthesized by aqueous-phase arrested precipitation and characterized by transmission electron microscopy, a spectrofluorometer, and an ultraviolet visible (UV-Vis) spectrophotometer. Based on the fluorescence quenching of CdS QD by selenite in the presence of glutathione (GSH), a simple, rapid, sensitive, and selective detection method for selenite was proposed. Under the optimum conditions, the calibration graph was linear in the range of 0.05 µmol L−1 to 11.2 µmol L−1. The limit of detection is 0.03 µmol L−1. The usefulness of the proposed method was evaluated for the determination of selenite in sodium selenite tablet and sodium selenite and vitamin E injection, and the results agreed with the labeled values. In addition, the effect of foreign ions (common anions and biologically relevant cations) on the fluorescence of the CdS QD was examined to evaluate the selectivity. The quenching mechanism is also described.

Highly sensitive redox-photometric determination of selenite and iodide ions in mineral waters

The use of two-phase reactions involving the antimony(V) hexachloride complex was proposed for the redox-photometric determination of selenite and iodide ions in some potable mineral waters with detection limits of 5 × 10−5 and 3 × 10−5 mg/L, respectively. The procedures exhibit a high selectivity and reproducibility and do not require lacking reagents and expensive instrumentation. The relative standard deviation is no higher than 20%.

Comparison of two derivatizing agents for the simultaneous determination of selenite and organoselenium species by gas chromatography and atomic emission detection after preconcentration using solid-phase microextraction

Two methods for the simultaneous determination of selenite and two organoselenium compounds, dimethylselenide (DMSe) and dimethyldiselenide (DMDSe), are proposed. Both methods involve sample preconcentration by solid-phase microextraction (SPME) and capillary gas chromatography coupled to atomic emission detection (GC-AED). The main difference between the methods is the derivatizing agent used to complex the inorganic species: sodium tetraethylborate and 4,5-dichloro-1,2-phenylenediamine. The parameters affecting the derivatization and preconcentration steps, chromatographic separation as well as detection of the compounds were optimized. Direct immersion (DI) mode and a relatively long extraction time were selected for the method involving the formation of the piazselenol complex, better sensitivity being achieved for the three analytes under study. In this case, detection limits ranged between 3 and 25 ng L −1, depending on the compound. Headspace mode (HS) and extraction times of 20 min were selected for the method involving tetraalkylborate, and detection limits of between 7.3 and 55 ng L −1 were obtained. DMSe and Se(IV) were found in several of the water samples analyzed at concentrations of 0.07–1.0 ng mL −1.

Optimization of a Chemical Modifier in the Determination of Selenium by Graphite Furnace Atomic Absorption Spectrometry and Its Application to Wheat and Wheat Flour Analysis

A method for the determination of total selenium in wheat and wheat flour using graphite furnace atomic absorption spectrometry (GFAAS) with palladium/ascorbic acid as a chemical modifier was studied. The effects of nickel nitrate, palladium/ascorbic acid, and palladium/magnesium nitrate as chemical modifiers on the sensitivity in the determination of selenite, selenate and selenomethionine by GFAAS were compared. The palladium/ascorbic acid modifier was used for the determination of total selenium in wheat and wheat flour, because the oxidation states of the selenium ion are not important in the determination. The detection limit was estimated to be 1 microg L(-1) (calculated as 3sigma of the blank); the calibration curve was linear for the concentration range 5 - 50 microg L(-1) and the recovery range was 96.66 - 101.80%. The optimal ashing and atomizing temperatures were 1300 degrees C and 2250 degrees C, respectively. The proposed method was successfully applied to the determination of total selenium in wheat and wheat flour.

Solid-phase microextraction-capillary gas chromatography combined with microwave-induced plasma atomic-emission spectrometry for selenite determination.

The use of solid-phase microextraction (SPME) with gas chromatography coupled to microwave-induced plasma atomic-emission detection (GC-MIP-AED) is described for selenite [Se(IV)] speciation. Aqueous standards were derivatised with sodium tetraethyl- or tetrapropylborate and extracted by SPME. Headspace extraction of the ethyl and propyl derivatives was studied. Relevant experimental conditions were optimised, including conditions for derivatisation and extraction and those of gas chromatographic analysis. The limits of detection achieved for headspace sampling of derivatised Se(IV) were in the low ng mL(-1) range for both ethylation and propylation. When the method was applied to analysis of selenite in selenised yeast reference material results were in good agreement with the indicated values.

Determination of selenite and selenate in drinking water: a fully automatic on-line separation/pre-concentration system coupled to electrothermal atomic spectrometry with permanent chemical modifiers

A time-based flow injection (FI) separation pre-concentration system coupled to an electrothermal atomic absorption spectrometer (graphite furnace) has been developed for the direct ultra-trace determination of selenite and selenate in drinking water. The pre-concentration of both forms of selenium is carried out onto a micro-column packed with an anionic resin (Dowex 1X8) that is placed in the robotic arm of the autosampling device. Selenite and selenate are sequentially eluted with HCl 0.1 M and HCl 4 M, respectively. The interference of large quantities of chloride during selenium atomisation is prevented by using iridium as a “permanent” chemical modifier. The features of the pre-concentration separation system for both species are: 53% efficiency of retention and an enhancement factor of 82 for a pre-concentration time of 180 s (sample flow rate=3 ml min −1) with HCl elution volumes of 100 μl. The detection limit (3 s) is 10 ng l −1 for the two species and the relative standard deviation ( n=10) at the 200 ng l −1 level is 3.5% for selenite and 5.6% for selenate. The addition of selenite and selenate stock standard solutions to tap water samples yields a 97–103% recovery of both species.