Stability of Se species in plant extracts rich in phenolic substances

Since there is growing awareness of the strong dependence of the antioxidative function of selenium (Se) upon its chemical form, the stability of Se species during sample preparation is an important factor in obtaining qualitative and quantitative results. Many plant samples are rich in phenolic compounds (antioxidants), but data about their effect on specific Se species in extracts of plant samples are scarce. Therefore, the aim of this study was to investigate the effect of the most common phenolic substances in plant parts, namely tannin and the flavonoid rutin, on the concentration and/or transformation of several Se species (SeMet, SeCys₂, SeMeSeCys, Se(VI) and Se(IV)) during sample preparation (24 h incubation at 37 °C) and storage (4 days at 4 °C). Moreover, the effect observed was then studied in a real sample, buckwheat, because this plant is known as a rich source of phenolics, especially tannin and rutin. Se speciation was carried out by on-line coupling of ion-exchange HPLC-ICP-MS after water and enzymatic (protease) hydrolysis. The results showed that the ratio between the two antioxidants has an important role. When the antioxidants were present together, the response for Se(IV) was observed to start to decrease only at a ratio of rutin to tannin of 1:100 (w/w), indicating the ratio between antioxidants in buckwheat seeds. After water extraction, only 40% and after enzymatic extraction 80% of Se(IV) remained, but no other Se compound was detected with the system used. Furthermore, the extracts were not stable during storage at 4 °C. Signals for other Se species were stable. The results obtained for buckwheat seeds showed a decrease in Se(IV) response during sample preparation and storage, comparable to the one obtained with the experiments performed in vitro. However, Se species in extracts of other buckwheat parts (leaves, stems and sprouts) were stable. These results indicate that reactions in the extraction process and during storage may affect Se speciation and may result in misidentifications and inaccurate values.     

Speciation analysis of selenium enriched green onions (Allium fistulosum) by HPLC-ICP-MS

Green onions (Allium fistulosum) enriched with 10 or 100 μg mL^− 1 Se(IV) or SeMet were analyzed for total selenium and species distribution. Anion and cation exchange chromatographies were applied for the separation of selenium species with mass spectrometric detection. Two different sample preparation methods (NaOH and enzymatic) were compared from the Se extraction efficiency point of view. Total selenium concentration accumulated by the onions reached the 200 μg g^− 1 level expressed for dry weight when applying SeMet at a concentration of 100 μg mL^− 1 as the source of Se. Speciation studies revealed that both in onion bulbs and leaves the predominant form of organic selenium is Se-methyl-selenocysteine (MeSeCys). When Se(IV) was applied for Se-enrichment at a concentration level of 100 μg mL^− 1 both onion leaf and bulb contained a significant amount of inorganic selenium. An unknown compound was also detected.     

Multielemental speciation of As, Se, Sb and Te by HPLC-ICP-MS

An anion exchange HPLC-ICP-MS procedure allowing the simultaneous multielemental speciation analysis of arsenic, selenium, antimony and tellurium has been developed. Four arsenic species (As^III, As^V, monomethylarsonic acid and dimethylarsinic acid), two selenium species (Se^IV and Se^VI) may be determined in a single run as well as one antimony (Sb^V) and one tellurium species (Te^VI). Alternatively Sb and/or Te may be used as internal standards for As and Se speciation studies. Optimisation of ICP-MS conditions led to satisfactory relative (0.01 (Sb^V) to 1.8 (Se^VI) ng ml⁻¹) and absolute detection limits (1–180 pg). Reproducibility ranged from 3.1 to 5.6% and the linearity was verified in the 0–200 ng ml⁻¹ range.     

Selenium species determination in selenium-enriched pumpkin (Cucurbita pepo L.) seeds by HPLC-UV-HG-AFS.

Pumpkins were treated by spraying the leaves in the flowering period with a water solution containing 1.5 mg Se per liter in the form of Na2SeO4. The average total selenium content of seeds was found to be 0.19 microg g(-1) in nontreated pumpkins and 1.1 microg g(-1) in exposed ones. For speciation analysis, enzymatic hydrolysis with different amounts of Protease XIV was carried out. Under optimal conditions of enzymatic hydrolysis, 90% of the total selenium was found in soluble forms. Separation of species was performed using HPLC on anion and cation exchange columns and for detection UVHG-AFS was applied. In enzymatic hydrolysis extracts, the main fraction of selenium was bound as selenomethionine (SeMet), representing on average of 81 +/- 8% of the total Se content in the sample.     

Speciation analysis of selenium in rice samples by using capillary electrophoresis-inductively coupled plasma mass spectrometry

An enzyme-assisted extraction used to extract all species of selenium in rice sample and a sensitive analytical method for the determination of ultratrace Se(VI), Se(IV), SeCys₂ (selenocystine) and SeMet (selenomethionine) with capillary electrophoresis-inductively coupled plasma mass spectrometry were firstly described in this study. The extraction method is simple, effective and can be used to extract trace selenium compounds in rice with high extraction efficiency and no altering its species. The analytical method has a detection limit of 0.1-0.9 ng Se/mL, and can be used to determine trace Se(VI), Se(IV), SeCys₂ and SeMet in rice directly without any derivatization and pre-concentration. With the help of above methods, we have successfully determined Se(VI), Se(IV), SeCys₂ and SeMet in selenium-enriched rice within 18 min with a recovery of 90-103% and a RSD (relative standard deviation, n = 6) of 3-7%. Our results indicated that selenium-enriched rice contained only one species of selenium, SeMet, and its concentration is in range of 0.136-0.143 μg Se/g dried weight. The proposed method providing a realistic approach for the nutritional and toxical evaluation of different selenium compounds in nutritional supplements.     

Capabilities of mixed-mode liquid chromatography coupled to inductively coupled plasma mass spectrometry for the simultaneous speciation analysis of inorganic and organically-bound selenium

This work investigates for the first time the potential of mixed-mode (anion-exchange with reversed-phase) high performance liquid chromatography coupled to inductively coupled plasma mass spectrometry (ICP-MS) for the simultaneous retention and selective separation of a range of inorganic and organically-bound selenium (Se) species. Baseline separation and detection of selenocystine (SeCys₂), Se-methyl-selenocysteine (SeMC), selenomethionine (SeMet), methylseleninic acid (MSA), selenite, γ-glutamyl-methyl-selenocysteine (γ-glutamyl-SeMC), and selenate in a Se standard mixture by mixed-mode HPLC-ICP-MS was achieved by switching between two citrate mobile phases of different pH and ionic strength within a single chromatographic run of 20 min. Limits of detection obtained for these Se species ranged from 80 ng kg^?1 (for SeMC) to 123 ng kg^?1 (for selenate). Using this approach as developed for selenium speciation, an adequate separation of inorganic and organic As compounds was also achieved. These include arsenite, arsenate, arsenobetaine (AsB) and dimethylarsenic acid (DMA), which may coexist with Se species in biological samples. Application of the newly proposed methodology to the investigation of the elemental species distribution in watercress (used as the model sample) after enzymatic hydrolysis or leaching in water by accelerated solvent extraction (ASE) was addressed. Only SeMet, SeMC and selenate could be tentatively identified in watercress extracts by mixed-mode HPLC-ICP-MS and retention time matching with standards. Recoveries (n = 3) of these Se species from samples spiked with standards averaged 102% (for SeMC), 94.9% (for SeMet) and 98.3% (for selenate). Verification of the presence of SeMet and SeMC in an enzymatic watercress extract was achieved by on-line HPLC-ESI MS/MS in selected reaction monitoring (SRM) mode.     

Total selenium and selenomethionine in pharmaceutical yeast tablets: assessment of the state of the art of measurement capabilities through international intercomparison CCQM-P86

Results of an international intercomparison study (CCQM-P86) to assess the analytical capabilities of national metrology institutes (NMIs) and selected expert laboratories worldwide to accurately quantitate the mass fraction of selenomethionine (SeMet) and total Se in pharmaceutical tablets of selenised-yeast supplements (produced by Pharma Nord, Denmark) are presented. The study, jointly coordinated by LGC Ltd., UK, and the Institute for National Measurement Standards, National Research Council of Canada (NRCC), was conducted under the auspices of the Comité Consultatif pour la Quantité de Matière (CCQM) Inorganic Analysis Working Group and involved 15 laboratories (from 12 countries), of which ten were NMIs. Apart from a protocol for determination of moisture content and the provision of the certified reference material (CRM) SELM-1 to be used as the quality control sample, no sample preparation/extraction method was prescribed. A variety of approaches was thus used, including single-step and multiple-step enzymatic hydrolysis, enzymatic probe sonication and hydrolysis with methanesulfonic acid for SeMet, as well as microwave-assisted acid digestion and enzymatic probe sonication for total Se. For total Se, detection techniques included inductively coupled plasma (ICP) mass spectrometry (MS) with external calibration, standard additions or isotope dilution MS (IDMS), inductively coupled plasma optical emission spectrometry , flame atomic absorption spectrometry and instrumental neutron activation analysis. For determination of SeMet in the tablets, five NMIs and three academic/institute laboratories (of a total of five) relied upon measurements using IDMS. For species-specific IDMS measurements, an isotopically enriched standard of SeMet (⁷⁶Se-enriched SeMet) was made available. A novel aspect of this study relies on the approach used to distinguish any errors which arise during analysis of a SeMet calibration solution from those which occur during analysis of the matrix. To help those participants undertaking SeMet analysis to do this, a blind sample in the form of a standard solution of natural abundance SeMet in 0.1 M HCl (with an expected value of 956 mg kg⁻¹ SeMet) was provided. Both high-performance liquid chromatography (HPLC)–ICP-MS or gas chromatography (GC)–ICP-MS and GC-MS techniques were used for quantitation of SeMet. Several advances in analytical methods for determination of SeMet were identified, including the combined use of double IDMS with HPLC-ICP-MS following extraction with methanesulfonic acid and simplified two-step enzymatic hydrolysis with protease/lipase/driselase followed by HPLC-ICP-IDMS, both using a species-specific IDMS approach. Overall, satisfactory agreement amongst participants was achieved; results averaged 337.6 mg kg⁻¹ (n = 13, with a standard deviation of 9.7 mg kg⁻¹) and 561.5 mg kg⁻¹(n = 11, with a standard deviation of 44.3 mg kg⁻¹) with median values of 337.6 and 575.0 mg kg⁻¹ for total Se and SeMet, respectively. Recovery of SeMet from SELM-1 averaged 95.0% (n = 9). The ability of NMIs and expert laboratories worldwide to deliver accurate results for total Se and SeMet in such materials (selensied-yeast tablets containing approximately 300 mg kg⁻¹ Se) with 10% expanded uncertainty was demonstrated. The problems addressed in achieving accurate quantitation of SeMet in this product are representative of those encountered with a wide range of organometallic species in a number of common matrices. Figure Looking into the quantitative speciation of selenium in pharmaceutical supplements Photo courtesy of LGC.     

Electrochemical vapor generation of selenium species after online photolysis and reduction by UV-irradiation under nano TiO<Subscript>2</Subscript> photocatalysis and its application to selenium speciation by HPLC coupled with atomic fluorescence spectrometry

An online UV photolysis and UV/TiO₂ photocatalysis reduction device (UV–UV/TiO₂ PCRD) and an electrochemical vapor generation (ECVG) cell have been used for the first time as an interface between high-performance liquid chromatography (HPLC) and atomic fluorescence spectrometry (AFS) for selenium speciation. The newly designed ECVG cell of approximately 115 L dead volume consists of a carbon fiber cathode and a platinum loop anode; the atomic hydrogen generated on the cathode was used to reduce selenium to vapor species for AFS determination. The noise was greatly reduced compared with that obtained by use of the UV–UV/TiO₂ PCRD–KBH₄–acid interface. The detection limits obtained for seleno-DL-cystine (SeCys), selenite (Se^IV), seleno-DL-methionine (SeMet), and selenate (Se^VI) were 2.1, 2.9, 4.3, and 3.5 ng mL^–1, respectively. The proposed method was successfully applied to the speciation of selenium in water-soluble extracts of garlic shoots cultured with different selenium species. The results obtained suggested that UV–UV/TiO₂ PCRD–ECVG should be an effective interface between HPLC and AFS for the speciation of elements amenable to vapor generation, and is superior to methods involving KBH₄.     

Selenium speciation in dill (Anethum graveolens L.) by ion pairing reversed phase and cation exchange HPLC with ICP-MS detection

In the present work, speciation of selenium in dill (Anethum graveolens L.), supplemented with sodium selenite during its growth, was performed using ion pairing reversed phase and cation exchange chromatography. Heptafluorobutyric acid (HFBA) was used as the ion-pairing agent in reversed phase chromatography. In cation exchange chromatography, two different gradient programs were employed for the identification of selenospecies using pyridinium formate as the mobile phase. Low molecular weight selenocompounds were extracted from root, stem and dill leaf with 0.1 M HCl. Enzymatic digestion was used for the extraction of selenospecies related to high molecular weight compounds. The chromatograms obtained from different parts of the plant revealed major differences in the type of selenospecies as well as their concentrations. The major selenospecies found in different parts of the plant is Se-methyl-selenocysteine (MeSeCys). Another major Se species identified is Se-methyl-selenomethionine (MeSeMet), which has the highest relative concentration in the root indicating possible Se volatilization from that part of the plant. Selenomethionine (SeMet) is present in minor quantities in all parts of the plant.     

Cs6Nb4Se22 and K12Nb6Se35.3: two new compounds containing the M4Q22 building block

The title compounds, namely hexacaesium tetraniobium docosaselenide and dodecapotassium hexaniobium pentatriacontaselenide, were formed from their respective alkali chalcogenide reactive flux and niobium metal. Both compounds fall into the larger family of solid‐state compounds that contain the M₂Q₁₁ building block (M = Nb, Ta; Q = Se, S), where the metal chalcogenide forms dimers of face‐shared pentagonal bipyramids. Cs₆Nb₄Se₂₂ contains two Nb₂Se₁₁ building blocks linked by an Se—Se bond to form isolated Nb₄Se₂₂ tetrameric building blocks surrounded by caesium ions. K₁₂Nb₆Se_35.3 contains similar Nb₄Se₂₂ tetramers that are linked by an Se—Se—Se unit to an Nb₂Se₁₁ dimer to form one‐dimensional anionic chains surrounded by potassium ions. Further crystallographic studies of K₁₂Nb₆Se_35.3 demonstrate a new M₂Se₁₂ building block because of disorder between an Se²⁻ site (85%) and an (Se—Se)²⁻ unit (15%). The subtle differences between the structures are discussed.     

Sequential photocatalyst-assisted digestion and vapor generation device coupled with anion exchange chromatography and inductively coupled plasma mass spectrometry for speciation analysis of selenium species in biological samples

We have developed an on-line sequential photocatalyst-assisted digestion and vaporization device (SPADVD), which operates through the nano-TiO₂-catalyzed photo-oxidation and reduction of selenium (Se) species, for coupling between anion exchange chromatography (LC) and inductively coupled plasma mass spectrometry (ICP-MS) systems to provide a simple and sensitive hyphenated method for the speciation analysis of Se species without the need for conventional chemical digestion and vaporization techniques. Because our proposed on-line SPADVD allows both organic and inorganic Se species in the column effluent to be converted on-line into volatile Se products, which are then measured directly through ICP-MS, the complexity of the procedure and the probability of contamination arising from the use of additional chemicals are both low. Under the optimized conditions for SPADVD – using 1 g of nano-TiO₂ per liter, at pH 3, and illuminating for 80 s – we found that Se(IV), Se(VI), and selenomethionine (SeMet) were all converted quantitatively into volatile Se products. In addition, because the digestion and vaporization efficiencies of all the tested selenicals were improved when using our proposed on-line LC/SPADVD/ICP-MS system, the detection limits for Se(IV), Se(VI), and SeMet were all in the nanogram-per-liter range (based on 3σ). A series of validation experiments – analysis of neat and spiked extracted samples – indicated that our proposed methods could be applied satisfactorily to the speciation analysis of organic and inorganic Se species in the extracts of Se-enriched supplements.     

TCF selenium preconcentration in geological materials for determination at sub-mugg(-1) with INAA (Se/TCF-INAA)

In geological samples, Se concentration ranges from 1 × 10⁻⁹ g g⁻¹ up to 1 × 10⁻³ g g⁻¹. The analytical difficulty at low concentration (<1 μg g⁻¹), is one of the main reasons why the geological cycle of Se is poorly known. The analytical method that consisted of preconcentration of Se with thiol cotton fiber (TCF) followed by graphite furnace atomic absorption spectrometry (GFAAS) has been modified by finishing with instrumental neutron activation analysis (INAA). The modified technique involves sample dissolution (HF-HNO₃-H₂O₂) and evaporation to dryness at low temperature (55-60 °C) to avoid selenium volatilization. Se^VI is converted to Se^IV by adding 6 M HCl to the dry residuum and the solution is then heated in a covered boiling bath (95-100 °C). The solution is diluted to obtain 0.6 M HCl and then collected on TCF. The TCF is placed in a polyethylene vial for irradiation in the SLOWPOKE II reactor (Montréal) for 30 s at a neutron flux of 10¹⁵ m⁻² s⁻¹. The 162 keV peak of ^77mSe (half-life 17.36 s) is read for 20 s after a decay of 7 s. The amount of sample to be dissolved is controlled by two competing effects. To obtain low detection limits, a larger amount of sample should be dissolved. On the other hand, the TCF could become saturated with chalcophile elements when large sample is used. Sulfur is a good indicator of the amount of Se and chalcophile elements present. In S poor sample (<100 μg g⁻¹) 3.0 g of sample was used and the L_D was ∼2 ng g⁻¹. In S high samples (>1.5% S) 0.05 g of sample was used and the L_D was ∼120 ng g⁻¹. The present work also includes suggested Se concentration for eight international geological reference materials (IGRM) that compare favorably with literature values.     

Neue Rheniumkomplexe mit Trichalkogenido- und Tetrachalkogenido-Chelatliganden

New Rhenium Complexes Containing Trichalcogenido and Tetrachalcogenido Chelate Ligands The reactions of Cp*ReCl₄ with polychalcogenide salts such as Na₂S₄ or (NEt₄)₂Se₆ lead initially to the violet trichalcogenido chelate complexes Cp*ReCl₂(E₃) (E = S ( 3a ), Se ( 3b )) which, due to their functional chloro ligands, can be used as intermediates for further reactions. Upon hydrolysis in moist solvents or aminolysis with tert. butylamine 3a, b are converted into the tetrachalcogenido chelate complexes Cp*Re(O)(E₄) (E = S ( 4a ), Se ( 4b )) and Cp*Re(N^tBu)(E₄) (E = S ( 5a ), Se ( 5b )), respectively. X-Ray structure analyses were carried out for the three mononuclear cyclo-oligoselenido compounds 3b–5b . It appears that the size of the Se^2?_n chelate ring (n = 3 or 4) essentially depends on steric factors within the coordination sphere of rhenium.     

Selenium speciation in soil extracts using LC-ICP-MS

Selenium (Se) speciation in soil affects its bioavailability to crops. An analytical procedure for the determination of inorganic Se species (selenite and selenate) in soil extracts by anion-exchange liquid chromatography (LC) with ICP-MS detection has been developed, with 10-fold higher sensitivity than existing HGAAS-based soil Se measurements. A comparison of phosphate extraction solutions on agricultural soils amended with 20?µg?kg^–1 selenate or selenite was carried out, and a 0.016?M?KH₂PO₄ extraction solution is recommended. Recovery of selenate was >91%; however, selenite recovery ranged between 18.5% and 46.1%, due to rapid binding to the soil. Soil preparation did not have a significant (p?>?0.05) effect on the extractability of the selenate or selenite amendments. The stability of Se species in the phosphate extracts was variable, depending on temperature and storage time. Therefore, immediate (<1?h) analysis of the soil extracts is preferable. The method developed was applied to the determination of extractable Se from six arable soils in the UK. Extractable Se levels in these soils ranged between 6 and 13?µg?kg^–1 consisting of selenite and some soluble organic Se.     

Bis(diethoxyselenophosphinoyl) triselenide and bis(diisopropoxyselenophosphinoyl) diselenide

The title compounds, bis(diethoxyselenophosphinoyl) triselenide, [P(OEt)₂Se]Se₃[P(OEt)₂Se], and bis­(diisopropoxy­selenophosphinoyl) diselenide, [P(OⁱPr)₂Se]Se₂[P(OⁱPr)₂Se], comprise an Se₃ chain or an Se₂ chain bridging two (RO)₂PSe groups.     

Selenium uptake onto natural pyrite

⁷⁹Se is a major dose-determining redox-sensitive nuclide in safety analysis of radioactive waste disposal sites. In aqueous solutions, selenium forms soluble anionic species (Se^IVO₃ ^2? and Se^VIO₄ ^2?) that hardly sorb on negatively charged surfaces of common host-rock minerals. However, Se is known to have a strong affinity with sulphides and interacts with pyrite, a common minor mineral of argillaceous rocks being considered as host formations for radioactive waste repositories. In this study, we present micro- and bulk X-ray spectroscopy data (μ-XRF, μ-XANES, and EXAFS) showing that, under nearly anoxic conditions, dissolved SeO₃ ^2? and SeO₄ ^2? sorb directly onto the pyrite surface and are subsequently reduced to Se⁰ with increasing ageing time (up to 8 months). These results suggest that the mobility of ⁷⁹Se^IV released from radioactive waste could greatly decrease through uptake on the pyrite surface followed by transformation into a sparingly soluble reduced form.     

Separation and determination of seleno amino acids using gas chromatography hyphenated with inductively coupled plasma mass spectrometry after hollow fiber liquid phase microextraction

A new derivatization–extraction method for preconcentration of seleno amino acids using hollow fiber liquid phase microextraction (HF‐LPME) was developed for the separation and determination of seleno amino acids in biological samples by gas chromatography–inductively coupled plasma mass spectrometry (GC–ICP‐MS). Derivatization was performed with ethyl chloroformate (ECF) to improve the volatility of seleno amino acids. Parameters influencing microextraction, including extraction solvent, pH of sample solution, extraction time, stirring speed, and inorganic salt concentration have been investigated. Under the optimal conditions, the limits of detection (LODs) obtained for Se‐methyl‐selenocysteine (SeMeCys), selenomethionine (SeMet), and selenoethionine (SeEth) were 23, 15, and 11 ng Se l⁻¹, respectively. The relative standard deviations (RSDs) were 14.6%, 16.4%, and 19.4% for SeMeCys, SeMet, and SeEth (c = 1.0 ng ml⁻¹, n = 7), respectively, and the RSDs for SeMeCys, SeMet could be improved obviously if SeEth was utilized as the internal standard. The proposed method was applied for the determination of seleno amino acids in extracts of garlic, cabbage, and mushroom samples, and the recoveries for the spiked samples were in the range of 96.8–108% and 93.4–115% with and without the use of SeEth as internal standard. The developed method was also applied to the analysis of SeMet in a certified reference material of SELM‐1 yeast and the determined value is in good agreement with the certified value. Copyright © 2008 John Wiley & Sons, Ltd.     

Speciation of selenomethionine and selenocystine using online micro-column containing Cu(II) loaded nanometer-sized Al2O3 coupled with ICP-MS detection

A flow injection online speciation procedure by using micro-column packed with Cu(II) loaded nanometer-sized Al₂O₃ coupled to inductively coupled plasma mass spectrometry (ICP-MS) for the separation and determination of selenomethionine (SeMet) and selenocystine (SeCys₂) has been developed. The main factors affecting the separation and preconcentration of SeMet and SeCys₂ including pH value, sample flow rate, eluent concentration, eluent volume and flow rate, and interfering ions have been investigated. It was found that SeCys₂ could be selectively retained by micro-column packed with Cu(II) loaded nanometer-sized Al₂O₃ at pH 4.0, and the retained SeCys₂ could be eluted by 1.0 mol L⁻¹ HNO₃, while SeMet was not retained and passed through the micro-column directly at this pH. Both SeMet and SeCys₂ could be quantitatively adsorbed by the micro-column at pH 9.0, and the retained SeMet and SeCys₂ could be easily eluted with 1.0 mol L⁻¹ HNO₃. The content of SeMet was obtained by subtracting the SeCys₂ from the total content of seleno amino acids. With the enrichment factor of 7.8 and 7.7, the limits of detection (LODs) for SeMet and SeCys₂ were found to be 24 pg Se mL⁻¹ and 21 pg Se mL⁻¹, respectively. The relative standard deviations (RSDs) for SeCys₂ and SeMet with seven replicate determinations of 1.0 ng mL⁻¹ SeMet and SeCys₂, were 2.1% and 1.6%, respectively, the sampling frequency of 8 h⁻¹ was obtained. The proposed method was applied to the speciation of SeMet and SeCys₂ in selenized yeast, human urine and serum with satisfactory results.     

Phase study of the system Li2Se-In2Se3

The binary system Li₂Se-In₂Se₃ was investigated in the range of 40 to 100 mol% In₂Se₃ by thermoanalytical and X-ray methods. The system is characterized by two eutectic points. Beside the two binary components and the known ternary compound LiInSe2 another ternary compound crystallizes in this binary system at 83.3 mol% In₂Se₃. This compound was identified as LiIn₅Se₈. In contrast to (Cu, Ag)^IB₅ ^IIIC₈ ^VI compounds such as CuIn₅S₈ [1] it does not crystallize in the spinel structure. LiIn₅Se₈ shows a stratified structure. The melting point was determined to be at 810°C. Starting from room temperature up to the melting point no phase transitions were observed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Certification of a new selenized yeast reference material (SELM-1) for methionine, selenomethinone and total selenium content and its use in an intercomparison exercise for quantifying these analytes

A new selenized yeast reference material (SELM-1) produced by the Institute for National Measurement Standards, National Research Council of Canada (INMS, NRC) certified for total selenium (2,059±64 mg kg⁻¹), methionine (Met, 5,758±277 mg kg⁻¹) and selenomethionine (SeMet, 3,431±157 mg kg⁻¹) content is described. The ±value represents an expanded uncertainty with a coverage factor of 2. SeMet and Met amount contents were established following a methanesulfonic acid digestion of the yeast using GC-MS and LC-MS quantitation. Isotope dilution (ID) calibration was used for both compounds, using ¹³C-labelled SeMet and Met. Total Se was determined after complete microwave acid digestion based on ID ICP-MS using a ⁸²Se spike or ICP-OES spectrometry using external calibration. An international intercomparison exercise was piloted by NRC to assess the state-of-the-art of measurement of selenomethione in SELM-1. Determination of total Se and methionine was also attempted. Seven laboratories submitted results (2 National Metrology Institutes (NMIs) and 5 university/government laboratories). For SeMet, ten independent mean values were generated. Various acid digestion and enzymatic procedures followed by LC ICP-MS, LC AFS or GC-MS quantitation were used. Four values were based on species-specific ID calibration, one on non-species-specific ID with the remainder using standard addition (SA) or external calibration (EC). For total selenium, laboratories employed various acid digestion procedures followed by ICP-MS, AFS or GC-MS quantitation. Four laboratories employed ID calibration, the remaining used SA or EC. A total of seven independent results were submitted. Results for methionine were reported by only three laboratories, all of which used various acid digestion protocols combined with determination by GC-MS and LC UV. The majority of participants submitted values within the certified range for SeMet and total Se, whereas the intercomparison was judged unsuccessful for Met because only two external laboratories provided values, both of which were outside the certified range.