Analytical methods for the speciation of selenium compounds: a review

In vitro study on the competitive reactions between arsenite and selenite with glutathione

Exposure to arsenic can cause various biological effects by increasing the production of reactive oxygen species (ROS). Selenium acts as a beneficial element by regulating ROS and limiting heavy metal uptake and translocation. There are studies on the interactive effects of As and Se in plants, but the antagonistic and synergistic effects of these elements based on their binding to glutathione (GSH) molecules have not been studied yet. In this study, we aimed to investigate the antagonistic or synergistic effects of As and Se on the binding mechanism of Se and As with GSH at pH 3.0, 5.0, or 6.5. The interaction of As and Se in Se(SG)2 + As(III) or As(SG)3 + Se(IV) binary systems and As(III) + Se(IV) + GSH ternary system were examined depending on their ratios via liquid chromatography diode array detector/electrospray mass spectrometry (LC-DAD/MS) and liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS). The results showed that the formation of As(GS)3 was not detected in the As(III) + Se(SG)2 binary system, indicating that As(III) did not affect the stability of Se(SG)2 complex antagonistically. However, in the Se(IV) + As(SG)3 binary system, the addition of Se(IV) to As(SG)3 affected the stability of As(SG)3 antagonistically. Se(IV) reacted with GSH, disrupting the As(SG)3 complex, and consequently, Se(SG)2 formation was measured using LC-MS/DAD. In the Se(IV) + GSH + As(III) ternary system, Se(SG)2 formation was detected upon mixing As(III), Se(IV), and GSH. The increase in the concentration of As(III) did not influence the stability of the Se(SG)2 complex. Additionally, Se(IV) has a higher affinity than As(III) to the GSH, regardless of the pH of the solution. In both binary and ternary systems, the formation of the by-product glutathione trisulfide (GSSSG) was detected using LC-ESI-MS/MS.

Speciation of inorganic selenium in natural water by in situ solid-phase extraction using functionalized silica

Functionalized adsorbents with poly-(4,9-dioxododecane-1,12-guanidine) (SiO₂-PDDG) and mercaptophenyl groups (MPhS) were used for the separation of Se(VI) and Se(IV) for the first time. Fixation of PDDG was characterized by capillary electrophoresis and TGA/DSC. The quantitative extraction of Se(VI) proceeded due to anion exchange at pH 3-7. The adsorption capacity of SiO₂-PDDG for Se(VI) was 28 μmol g^-1. Silicas with mercaptophenyl groups were used for the extraction of Se(IV) from solutions in the range of 2 M HCl - pH 6.5. The adsorption capacity of MPhS was 35 μmol g^-1. A system of columns containing synthesized adsorbents was proposed for the separation of Se(VI) and Se(IV) and their subsequent determination by ICP-MS. Optimal parameters of adsorption include a flow rate of 1 mL min^-1, pH of 5, and sample volume of 200 mL. Se(IV) was desorbed with 5 mL of 0.25 M 2,3-dimercapto-1-propanesulphonic acid and Se(VI) with 5 mL of 1 M HNO₃. The preconcentration factor was 40. The limits of detection (3s) were 0.75 and 1.25 ng L^-1 for Se(VI) and Se(IV), respectively. The proposed method (SPE-ICPMS) was used to determine selenium species in natural water and certified reference materials. The separation was carried out directly at the sampling site.

Improving selenium accumulation in broilers using Escherichia coli Nissle 1917 with surface-displayed selenite reductase SerV01

Selenium levels have a critical impact on livestock and poultry, and selenium nanoparticles (SeNPs) have shown significant efficiency in supplementation. This study identified a high-efficiency selenite reductase, SerV01, in Staphylococcus aureus LZ-01, which can convert Se₂O₃^2- to SeNPs. Subsequently, SerV01 was introduced into the intestines of the broilers using the surface display-engineered E. coli Nissle 1917 (EcN). The results showed that the engineered bacteria (EcN-IS) significantly increased the selenium content by 0.87 mg kg^-1, 0.52 mg kg^-1, and 6.10 mg L^-1 in the liver, breast muscle, and serum, respectively. With SeNPs + EcN-IS treatment, glutathione peroxidase and thioredoxin reductase levels reached 0.7536 ± 0.03176 U μL^-1 protein and 2.463 ± 0.1685 U μL^-1 protein, respectively. With the modified probiotics, the proportion of beneficial intestinal flora increased, with Lactobacillus and Propionibacterium accounting for 75.85% and 0.19%. This technology provides a novel idea to facilitate the exploitation of selenium in broiler diets and improve antioxidant capability.

Estimation of daily selenium intake by 3- to 5-year-old Japanese children based on selenium excretion in 24-h urine samples

To evaluate the daily Se intake of 3- to 5-year-old Japanese children, we used seventy-two urine samples collected from fifty-three children (twenty-seven male and twenty-six female) from two cities in Miyagi prefecture, Japan. For measuring low Se concentrations with high precision, accuracy and rapidity in the 24-h urine samples, we developed an instrumental neutron activation analysis (INAA) method, that is without any chemical separation, using the short-lived ^77mSe (t_1/2 = 17·4 s) nuclide. The estimated Se intake of the fifty-three children was 51·5 (sd 30·2) µg/d (geometric mean: 42·7 µg/d). Ten subjects (three male and seven female), successfully provided 24-h urine samples over two or three consecutive days; their Se intake was 37·4 (sd 5·9) µg/d. Based on the logarithmically transformed data of these ten subjects, the ratio of intra-/inter-individual variances of usual Se intake was 16·7 (28·0/1·7) and geometric mean was 27·7 µg/d. The 5th to 95th percentile of usual Se intake of these ten subjects was 17·5 to 40·4 µg/d, which ranged between the recommended dietary allowance and tolerable upper intake level of Se by the Dietary Reference Intakes for Japanese (2015).

Quantification of volatile-alkylated selenium and sulfur in complex aqueous media using solid-phase microextraction

Biologically produced volatile-alkylated Se and S compounds play an important role in the global biogeochemical Se and S cycles, are important constituents of odorous industrial emissions, and contribute to (off-)flavors in food and beverages. This study presents a fully automated direct-immersion solid-phase microextraction (DI-SPME) method coupled with capillary gas chromatography-mass spectrometry (GC/MS) for the simultaneous quantification of 10 volatile-alkylated Se and S compounds in complex aqueous media. Instrumental parameters of the SPME procedure were optimized to yield extraction efficiencies of up to 96% from complex aqueous matrices. The effects of sample matrix composition and analyte transformation during sample storage were critically assessed. With the use of internal standards and procedural calibrations, the DI-SPME-GC/MS method allows for trace-level quantification of volatile Se and S compounds in the ng/L range (e.g. down to 30 ng/L dimethyl sulfide and 75 ng/L dimethyl selenide). The applicability and robustness of the presented method demonstrate that the method may be used to quantify volatile Se and S compounds in complex aqueous samples, such as industrial effluents or food and beverage samples.

Effect of selenium enrichment on antioxidant activities and chemical composition of Lentinula edodes (Berk.) Pegl. mycelial extracts

Preparations derived from Lentinula edodes (Berk.) Pegl. mycelium are worldwide used as dietary supplements containing compounds active as immune system enhancers, demonstrating chemopreventive and anticancer activity. L. edodes mycelium enriched with organic forms of selenium like selenized yeast possess putative, higher cancer preventive properties. The objective of this study was to test the effect of enrichment in selenium on antioxidant, reducing and free radical scavenging activity of water and alcohol extracts from mycelium of L. edodes (Berk.). To elucidate the cause of enhanced antioxidant activity of extracts, a preliminary selenium speciation by specific oxido-reduction reaction was performed. Se-enrichment enhanced antioxidant activity, reducing power and free radical scavenging effect of mycelial extracts by almost 100-400%. Increase of activity was particularly high for diluted extracts (concentrations 0.1-0.5 mg/ml). The chemical composition of extracts from both Se-enriched and non-enriched mycelium was compared by determination of polyphenols, proteins, carbohydrates and lipids. Results showed that Se-enrichment enhanced antioxidant activities of mycelial extracts, likely by high amounts of organic Se-compounds (-II oxidation state) and elemental red selenium, and by increased polyphenols content. Our results suggest that Se-enrichment is a good method for enhancement of important activities of human dietary supplements, including Shiitake preparations.

Selenium speciation analysis using inductively coupled plasma-mass spectrometry

Selenium exists in several oxidation states and a variety of inorganic and organic compounds, and the chemistry of selenium is complex in both the environment and living systems. Selenium is an essential element at trace levels and toxic at greater levels. Interest in speciation analysis for selenium has grown rapidly in this last decade, especially in the use of chromatographic separation coupled with inductively coupled plasma-mass spectrometry (ICP-MS). Complete characterization of selenium compounds is necessary to understand selenium's significance in metabolic processes, clinical chemistry, biology, toxicology, nutrition and the environment. This review describes some of the essential background of selenium, and more importantly, some of the currently used separation methodologies, both chromatographic and electrophoretic, with emphasis on applications of selenium speciation analysis using ICP-MS detection.

Microchip capillary electrophoresis with amperometric detection for rapid separation and detection of seleno amino acids

This article describes an effective microchip capillary-electrophoresis protocol for rapid and effective measurements of food-related seleno amino acids, including Se-methionine (Se-Met), Se-ethionine (Se-Eth), Se-methyl cysteine (Se-Cys), utilizing o-phtaldialdeyde/2-mercaptoethanol (OPA/2-ME) derivatization. Relevant parameters of the chip separation and amperometric detection are examined and optimized using a response surface methodology (RSM). Under optimum conditions, the analytes could be separated and detected in a 30 mM borate buffer (pH 9.3, with 28 mM sodium dodecul sulfate) within 300 s using a separation voltage of 2000V and a detection voltage of +0.9 V. Linear calibration plots are observed for micromolar concentrations of the Se-amino acid compounds. The negligible sample volumes used in the microchip procedure obviates surface fouling common to amperometric measurements of selenoamino-acid compounds. The new microchip protocol offers great promise for a wide range of food applications requiring fast measurements and negligible sample consumption.

Determination of dimethylselenide and dimethyldiselenide by gas chromatography–photoionization detection

A simple method for the determination of volatile selenium compounds employing a gas chromatograph equipped with a photoionization detector is described. The method involves the direct injection of dimethylselenide (DMS) or dimethyldiselenide (DMDS) into the gas chromatograph; no derivatization of the sample was required. The photoionization detector was capable of detecting 60 pg (0.55 pmol) of DMS and 150pg (0.80pmol) DMDS. Sensitivity was 10-50 times greater with DMS and 4-20 times greater with DMDS when the photoionization detector was employed than when the flame ionization detector was employed.

Stable isotope-enriched selenite and selenate tracers for human metabolic studies: a fast and accurate method for their preparation from elemental selenium and their identification and quantification using hydride generation atomic absorption spectrometry

Stable isotope tracers are safe and nutritionally relevant tools for the investigation of mineral metabolism in man. Increased research into the functional role of selenium has resulted in a need for well-characterised, isotopically enriched solutions of the element in order to determine the nutritional relevance of selenium fortification of foods. A simple method for the conversion of isotopically enriched elemental selenium (2.5-10 mg) into selenite and selenate, and their accurate characterisation and quantification is described. Analysis of selenite and selenate tracers using continuous-flow hydride generation-atomic absorption spectrometry technique was based on the specificity of the selenium hydride reaction and allowed their precise (RSD<2.5%) and accurate determination in aqueous solutions. The detection and determination limits were at 0.13 and 0.36 microg Se/l, respectively. Isotopically enriched elemental selenium was converted into selenite and selenate by a nitric acid and a combined nitric acid/hydrogen peroxide oxidation, respectively. The conversion was quantitative (>95%) and specific for both inorganic selenocompounds. Selenite and selenate labels were stable in 0.1 mol/l nitric acid for at least 18 months, i.e. making them ideally suitable for use in long-term metabolic studies. An overview of data relating to the absorption and retention of selenium by humans obtained using the two, well-characterised, tracers is presented and indicates that selenite and selenate are equally well retained in adult men and infants, despite differences in their absorption and urinary excretion characteristics.

Determination of selenium compounds in urine by high-performance liquid chromatography--inductively coupled plasma mass spectrometry

Selenium species, selenite, selenate, selenomethionine (Semet), seneloethionine (Seet) and trimethylselenonium ion (TmSe) were separated in aqueous solution using a gel-permeation (polyvinyl alcohol-based resin) GS-220 column by eluting with 25 mM tetramethylammonium hydroxide and 25 mM malonic acid at pH 7.9. The GS-220 column coupled with inductively coupled plasma mass spectrometry was used for the separation, identification, and quantification of selenium compounds present in certified reference material (CRM) No. 18 human urine from the National Institute for Environmental Studies in Japan (NIES). Spiking of the authentic standard to the urine and use of a silica-based LC-SCX cation-exchange column validated the peak of selenium compounds. High concentrations of chloride and bromide in the urine eluted from the GS-220 column formed molecular ions 40Ar37Cl+ and 81Br1H+ in the plasma, and these molecular ions created additional peaks in the chromatograms when 77Se and 82Se isotopes were monitored respectively. Thus, both the isotopes were selected concurrently for signal monitoring to eliminate the interfering signals. On the LC-SCX column, chloride and bromide were eluted with selenate and complicated its determination, but the peak of TmSe was baseline separated from rest of the Se compounds. Two unknown Se compounds were detected in both the columns. An additional Se compound having the same retention time as that of Semet was detected on the LC-SCX column. Peaks of selenite, selenate, TmSe and unknown selenium compounds in the urine were baseline separated on the GS-220 column, and were free from interferences. Therefore, the GS-220 column was used for the determination of selenium compounds in NIES CRM No. 18. Unknown Se compounds were the predominant selenium species followed by selenite, TmSe and selenate. The estimated value of TmSe as Se, by the standard additions method using the GS-220 column, was 3.42 +/- 0.17 microg l(-1) and was in good agreement with the LC-SCX value [3.38 +/- 0.21 (n=5) microg l(-1)].

Solid-phase extraction for the simultaneous preconcentration of organic (selenocystine) and inorganic [Se(IV), Se(VI)] selenium in natural waters

This paper describes the combined use of a new preconcentration method using the Amberlite IRA-743 resin and high-performance liquid chromatography coupled to inductively coupled plasma mass spectrometry to determine simultaneously inorganic and organic selenium species in aquatic systems. The developed enrichment procedure, whose accuracy has been checked by recovery tests, is suitable for selenium speciation at environmental levels of 10 ng (Se) l(-1). The method has been applied to mineral and freshwater samples.

X-ray fluorescence determination of the loss of chosen electrolytes in the urine of children with a congenital cyanotic heart defect and after heart transplantation

In this paper results of the analysis of urine samples of healthy children and children with a congenital cyanotic heart defect and after heart transplantation are presented. The analysis of urine samples was carried out by X-ray fluorescence spectrometry with wavelength dispersion and using CRM urine Seronorm as a reference. It was found that for patients with a congenital cyanotic heart defect the loss of electrolytes like Na, Cl and K was increased. Moreover, urine samples of children from areas of different degree of environmental pollution were analysed. We observed (as expected) higher concentrations of heavy metals in the urine of children from ecological polluted areas.

Identification of volatile selenium compounds produced in the hydride generation system from organoselenium compounds

We report a novel aqueous derivatization of selenomethionine (Semet), selenoethionine (Seet) and trimethylselenonium ion (TmSe) by NaBH4 and HCI to volatile selenium species, namely, diethyldiselenide (DeDSe), dimethyldiselenide (DMDSe), dimethylselenide (DmSe) and ethylhydrogenselenide (ESeH), in the hydride generation (HG) system. The volatile selenium compounds produced in the HG system were on-line trapped and concentrated in a U-tube that was immersed in the liquid nitrogen trap. The trapped volatile Se compounds were volatilized at 80 degrees C in a water bath, and 50-500 microL of volatile gas was injected into the GC/AED and GC/MS, respectively. It has been established that DmSe, DmDSe, and DeDSe are the predominant Se compounds that are produced in the HG system from TmSe, Semet, and Seet, respectively, followed by ESeH from Seet. Analytical methods previously employed have stated that these compounds are inactive in the HG system. Prior decomposition of Semet, Seet, and TmSe to selenous acid is essential before HG. To the best of our knowledge, current findings for the production and identification of volatile selenium compounds in the HG system are new and different from existing reports; hence, direct estimation of Semet, Seet, and TmSe is possible when coupling with a HG system using a suitable Se-specific detector.

Selenium speciation in animal tissues after enzymatic digestion by high-performance liquid chromatography coupled to inductively coupled plasma mass spectrometry

A procedure is described for the enzymatic digestion of tuna and mussel samples that allows the determination of selenium species by high-performance liquid chromatography in conjunction with inductively coupled plasma mass spectrometry. The species were extracted by two-step enzymatic hydrolysis with a non-specific protease (subtilisin). The selenium species were separated on a Spherisorb 5 ODS/AMINO column using two different chromatographic conditions, namely phosphate buffers at pH 2.8 and pH 6.0 as mobile phases. The method determines organic (trimethylselenonium, selenocystine, selenomethionine and selenoethionine) and inorganic selenium species (selenite and selenate), but only organic selenium species were found in the samples. The sum of identified selenium species in the sample was about 30% of the total selenium present in the enzymatic extract despite the fact that recoveries of total hydrolysed selenium were 93-102%. Trimethylselenonium ion and selenomethionine were found in both tuna and mussel samples and an unknown selenium species was also found in tuna samples.

High-performance liquid chromatography of selenium compounds utilizing perfluorinated carboxylic acid ion-pairing agents and inductively coupled plasma and electrospray ionization mass spectrometric detection

Increasing speciation demands in clinical chemistry, toxicology and nutrition have made the determination of the total elements in a sample inadequate; the amount of an element and the chemical forms in which it is present need to be known. Inductively coupled plasma mass spectrometry (ICP-MS) was used after high-performance liquid chromatographic (HPLC) separation, as was electrospray ionization mass spectrometry (ESI-MS). The effect of variation of the number of carbon atoms in perfluorinated carboxylic acids used as ion-pairing agents for the separation of selenium compounds was examined. Trifluoroacetic acid (0.1%), pentafluoropropanoic acid (0.1%) or heptafluorobutanoic acid (0.1%; HFBA) were alternatively used as additives to methanol-water (1:99, v/v) solutions as mobile phases. Reversed-phase HPLC-ICP-MS with 0.1% HFBA in the mobile phase allowed more than 20 selenium compounds to be separated in 70 min in an isocratic elution mode; the separation of natural selenium-enriched sample extracts was examined and explained. The pH of the 0.1% HFBA solution was modified with hydrochloric acid or ammonia and the pH of the sample extracts before injection was modified in order to overcome unwanted double peak formation in the chromatograms of sample extracts. Oxidations of standard gamma-glutamyl-Se-methylselenocysteine and Se-methylselenocysteine were carried out using 30% H2O2 solution and identifications of selenium-containing oxidation products were made using HPLC-ICP-MS and HPLC-ESI-MS. The principal organic oxidation product in both cases was methaneseleninic acid (MeSeO2H).

On-line coupling flow injection microcolumn separation and preconcentration to electrothermal atomic absorption spectrometry for determination of (ultra)trace selenite and selenate in water

A flow injection manifold with an air-segmented and air-transported operational sequence for on-line coupling of microcolumn separation and preconcentration to electro-thermal atomic absorption spectrometry (ETAAS) was developed for the determination of (ultra)trace selenite and selenate in water. The determination of selenite was achieved by selective reaction with pyrrolidine dithiocarbamate (PDC), sorption of the resultant Se-PDC compound onto a conical microcolumn (10.2 microL) packed with RP C18 sorbent, elution with ethanol, and detection by ETAAS. The concentration of selenate was obtained as the difference between the concentrations of selenite after and before prereduction of selenate to selenite. With the developed manifold and operation sequence,the dispersion during elution and eluate transport and the eluent volume required for complete elution of the sorbed analyte were minimized. As a result, the sorbed analyte was quantitatively eluted from the column with only 26 microL of ethanol, and all the eluate was automatically introduced into the graphite tube by an air flow without the need of preheating the graphite tube or precise timing. Pretreatment of the graphite tube with iridium as a long-term "permanent" modifier effectively prevented analyte loss arising from the high volatility of the Se-PDC compound and greatly improved the precision, sensitivity, and detection limit. One thermal pretreatment of the graphite tube with injection of 150 microgram of iridium made possible at least 200 repetitive atomization cycles. With a preconcentration time of 180 s and a sample flow rate of 1.4 mL min(-1), an enhancement factor of 112 was achieved in comparison with direct injection of 30 microL of aqueous solution. The detection limit (3s) was 4.5 ng L(-1)Se. The RSD (n = 7) was 3.8% at 20 ng L(-1)Se. The concentrations of selenite and selenate determined in synthetic aqueous mixtures were in good agreement with the expected values. The recoveries for selenite from spiked seawater samples ranged from 98 to 102%. The concentrations of selenite in several seawater reference materials obtained with simple aqueous standard solutions for calibration agreed well with the certified and information values, respectively. In addition, the developed method was successfully applied to the certification of selenite and selenate in water.

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 Na(2)HPO(4) (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 microg/l, 12 microg/l and 103 microg/l for Se(IV), Se(VI) and Se-Met, respectively.