Sensitivities of selenite, selenate, selenomethionine and trimethylselenonium ion in aqueous solution and in blood plasma-ETAAS compared with ICP-MS

Aqueous solutions and blood plasma spiked with selenite (Se(IV)), selenate (Se(VI)), selenomethionine (SeMet) or trimethylselenonium (TMSe) iodide were analyzed by Zeeman-corrected electrothermal atomic absorption spectrometry (ETAAS) using palladium as a chemical modifier, and by inductively coupled plasma mass spectrometry (ICP-MS). Using ETAAS, the sensitivities for Se(IV), SeMet and TMSe in aqueous solution were similar, whereas the sensitivity of Se(VI) was 63% of that value. In blood plasma, the ETAAS sensitivities of Se(IV) and Se(VI) were equal, whereas the sensitivities of SeMet and TMSe were 87 and 56%, respectively, of that value. In contrast, the ICP-MS sensitivities obtained for Se(VI), TMSe and SeMet were between 96 and 98% of that obtained for Se(IV) in aqueous solution and in blood plasma. It is concluded, that ICP-MS is superior to ETAAS as the problem of differences in sensitivity of the selenium species when using ETAAS are not prevalent when using the ICP-MS technique.     

Marked individual variability in the levels of trimethylselenonium ion in human urine determined by HPLC/ICPMS and HPLC/vapor generation/ICPMS

Selenium species were determined using HPLC/ICPMS and HPLC/vapor generation/ICPMS in the urine from seven human volunteers investigated at background selenium concentrations and at slightly elevated concentrations after ingestion of 200 μg Se as a selenite supplement. Trimethylselenonium ion (TMSe) was present, together with selenosugars, in the urine samples, a result that dispels recent doubts about its possible previous misidentification with a cationic selenosugar. Although TMSe was present as only a trace metabolite in urine from five of the seven volunteers (0.02–0.28 μg Se/L, equivalent to 1–5% of the sum of selenosugars and TMSe), it was a significant metabolite (up to 4.6 μg Se/L, 22%) in one volunteer, and it was the major identified metabolite (up to 15 μg Se/L, 53%) in another volunteer. This marked individual variability in the formation of TMSe was maintained in a duplicate investigation of urine from the same seven volunteers.     

Incorporation of selenium into selenoprotein P and extracellular glutathione peroxidase: HPLC-ICPMS data with enriched selenite

The metabolic turnover of selenoprotein P (Sel P) and extracellular glutathione peroxidase (eGPx) in plasma was examined by HPLC-ICPMS, with the use of enriched Se, [82Se]selenite. [82Se]selenite was injected intravenously into rats at a dose of 25 micrograms 82Se kg-1 body weight, and the concentrations of labeled 82Se and naturally occurring 77Se in the serum, liver and kidneys were determined in samples obtained 1, 3, 6, 9, 12, 18 and 24 h after the injection. The distributions of both exogenous (labeled) 82Se and endogenous (naturally occurring) 77Se in serum, and supernatant fractions of the liver and kidneys were determined on a gel filtration column by HPLC-ICPMS. This dose was shown not to affect the constitutive levels of cellular GPx (cGPx), eGPx and Sel P. The labeled Se in Sel P increased soon after the injection, peaked at 6-9 h and then decreased, whereas that in eGPx continued increasing after 6 h post-injection and then throughout the remaining observation period in the bloodstream. These observations demonstrated the rapid and efficient incorporation of Se into Sel P in the liver and excretion into the plasma followed by the slow and steady incorporation of Se into eGPx in the kidneys and excretion into the plasma, with a minimal response of cGPx to selenite injection.     

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.     

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)].     

Microwave digestion of fish tissue for selenium determination by differential pulse polarography

In KIO(3)NH(3)NH(4)Cl medium, the selenium complex Se(O)SO(2-)(3), resulted from the reaction of selenite and sulphite in acid solution, gave a catalytic wave, which was applied to the determination of selenium in fish by differential pulse polarography. The sample was decomposed using the HNO(3)/H(2)SO(4)/H(2)O(2) digestion mixture in a closed PTFE digestion vessel with microwave heating. The detection limit was 0.06 mug/dm(3). The calibration curve was linear up to 8 mug/dm(3). Selenate present was reduced with hot hydrochloric acid to selenite. The recoveries of the selenite and selenate in two spiked samples investigated ranged from 91 to 104%. The NIES CRM No. 6 mussel was analyzed and the results obtained agreed well with the reference value (reference value: 1.5 mug/g; found: 1.43 +/- 0.05 mug/g). The results obtained by differential pulse polarography were in good agreement with those found by hydride generation atomic absorption spectrometry.     

ASSIMILATION,DISTRIBUTION, AND METABOLISM OF (75Se)-SELENITE,SELENATE, AND SELENOAMINO ACIDS BY ESCHERICHIA COLI

Abstract

Assimilation of selenium (Se) by Escherichia coli as (⁷⁵Se)-selenite, selenate, selenomethionine, selenocystine and Se?CH₃-selenocystine revealed that (a) selenoamino acids from a culture media are more completely assimilated than selenite or selenate and (b) that the amount of selenite is assimilated three to four times selenate. Most (>95%) of the Se assimilated by E. coli could not be solubilized by sonication and ethanol extraction but much (28% to 70%) of the Se, except Se from selenomethionine, was removed by alkaline dialysis. Se from selenocystine and from Se?CH₃-selenocystine dialyzed from intact cells, whereas Se from selenite and selenate did not. Dialyzable Se is that Se probably present in selenotrisulfide (R?S?Se?S?R) bonds or bound nonspecifically. Analysis of the soluble Se metabolites from selenite, selenate, selenomethionine and selenocystine showed that E. coli produces at least one major metabolic product common to all substrates which upon chromatography appeared to be selenocysteic acid. In monogastric animals selenite and selenate Se does not enter the primary protein structure as amino acids yet metabolites of selenite, selenate and selenocystine produced by E. coli could enter the primary protein structure of animals in minute amounts.     

Determination of human and Sprague-Dawley rat trimethylselenonium ion and total selenium urine concentrations from endogenous body selenium pool by neutron activation analysis

This study determined trimethylselenonium ion [TMSe, (CH₃)₃Se⁺] and total organic selenium cationic species urinary excretion values for healthy human subjects and Sprague-Dawley rats fed regular diets. The only source of TMSe was from the endogenous selenium body pool. Total selenium concentration, in urine was determined by instrumental neutron activation analysis. TMSe and total selenium cationic species concentrations and percent of total selenium urine excretion were determined by chemical neutron activation analysis and coupled anion-cation exchange chromatography and anion-exchange chromatography, respectively. Within experimental error, mean values for TMSe and cationic species as percent selenium were comparable for both human subjects and Sprague-Dawley rats. This study suggested that TMSe excreted in urine by healthy human subjects and Sprague-Dawley rats fed a normal diet is not a minor but a general metabolite of selenium ingested in a normal diet.     

Study of the stability of selenium compounds in human urine and determination by mixed ion-pair reversed-phase chromatography with ICP-MS detection

The stability of five selenium compounds, selenate, Se(VI), selenourea, SeUr, trimethylselenonium ion, TMSe(+), selenomethionine, SeMet, and selenoethionine, SeEt, at concentrations from 30-60 micro g L(-1) in a pooled human urine, stored in dark at -20 degrees C, 4 degrees C, or ambient temperature (ca. 25 degrees C), without addition of any stabilizing reagent was evaluated. The investigated Se species were determined independently by mixed ion-pair reversed-phase liquid chromatography with inductively coupled plasma mass spectrometric (ICP-MS) detection. The general trend is the lower the temperature used for storage, the higher the stability of Se species, when other conditions such as light, acidity, and container material are kept constant. On the basis of these results it is considered that the storage of urine samples at -20 degrees C for a short-term (within one month) is safe for Se speciation analysis. Long-term storage of urine samples for speciation analysis should, however, be undertaken with caution.     

Affinity of the elements in group VI of the periodic table to tumors and organs]

In order to investigate the tumor affinity radioisotopes, chromium (51Cr), molybdenum (99Mo), tungsten (181W), selenium (75Se) and tellurium (127mTe)--the elements of group VI in the periodic table--were examined, using the rats which were subcutaneously transplanted with Yoshida sarcoma. Seven preprarations, sodium chromate (Na251CrO4), chromium chloride (51CrCl3), normal ammonium molybdate ((NH4)299MoO7), sodium tungstate (Na2181WO4), sodium selenate (Na275SeO4), sodium selenite (Na275SeO3) and tellurous acid (H2127mTeO3) were injected intravenously to each group of tumor bearing rats. These rats were sacrificed at various periods after injection of each preparation: 3 hours, 24 hours and 48 hours in all preparations. The radioactivities of the tumor, blood, muscle, liver, kidney and spleen were measured by a well-type scintillation counter, and retention values (in every tissue including the tumor) were calculated in percent of administered dose per g-tissue weight. All of seven preparations did not have any affinity for malignant tumor. Na251CrO4 and H2127mTeO3 had some affinity for the kidneys, and Na275SeO3 had some affinity for the liver. Na2181WO4 and (NH4)299MoO4 disappeared very rapidly from the blood and soft tissue, and about seventy-five percent of radioactivity was excreted in urine within first 3 hours.     

Speciation of selenium compounds with ion-pair reversed-phase liquid chromatography using inductively coupled plasma mass spectrometry as element-specific detection

For selenium speciation analysis, the hyphenation of chromatographic separation with element-specific detection has proved a useful technique. A powerful separation system, which is capable of resolving several biologically and environmentally important selenium compounds in a single column, is greatly needed. However, that has been difficult to achieve. In this paper eight selenium compounds, namely, selenite [Se(IV)], selenate [Se(VI)], selenocystine (SeCys), selenourea (SeUr), selenomethionine (SeMet), selenoethionine (SeEt), selenocystamine (SeCM) and trimethylselenonium ion (TMSe+), were separated by using mixed ion-pair reagents containing 2.5 mM sodium 1-butanesulfonate and 8 mM tetramethylammonium hydroxide as a mobile phase. The separation of these anionic, cationic and neutral organic selenium compounds on a LiChrosorb RP18 reversed-phase column took only 18 min at a flow-rate of 1.0 ml/min with isocratic elution, and baseline separation among the six organic Se compounds was achieved. Inductively coupled plasma mass spectrometry (ICP-MS) was employed as element-specific detection. A comparison of ICP-MS signal intensity obtained with a Barbington-type nebulizer and with an ultrasonic nebulizer (USN) was made. Different signal enhancement factors were observed for the various selenium compounds when a USN was used. The speciation technique was successfully applied to the study on chemical forms of selenium in a selenium nutritional supplement. Selenomethionine was found to be the predominant constituent of selenium in the supplement.     

Loss of selenium in water samples at natural levels during storage

Selenium losses in river, ground, snow-melt and tap water samples, and the recovery of selenite, selenate and selenomethionine added to purified water have been studied. In 1-litre high-density polyethylene bottles, tap, river and snow-melt water samples (at Se concentrations of 44.5–138 ng/l) could be stored at 4 °C for up to 15 days without Se losses. In similar samples stored at room temperature Se losses of 13–25% after 15 days were found, except for groundwater, which showed no Se losses during storage for 13 months at room temperature or at 4 °C. Selenite and selenate added to purified water were recovered without losses after 15 days at 4 °C, while 7.5% of selenomethionine was lost. The stability of different chemical forms of Se during storage followed the order: selenate > selenomethionine > selenite. It is recommended that unacidified water samples should not be kept in polyethylene bottles at room temperature for more than 1 week, nor stored at 4 °C for more than 2 weeks, before analysis for Se.     

Evaluation of methods for the separation of trimethylselenonium ion as a major,minor or general metabolite using neutron activation analysis

Trinethylselenonium (TMSe) ion was separated by dual-column ion-exchange chromatography and assayed by neutron activation analysis. The TMSe content of the selenium in human urine was found to be (14±2)%, consistent with literature values. An altemate, published, multi-step procedure employing Reineckate precipitation followed by decomposition/volatilization conversion of selenium into dimethylselenide, perchloric acid digestion into selenite and subsequent analysis employing inductively coupled plasma mass spectrometry (ICP-MS) was evaluated. A maximum recovery of TMSe was estimated at 32%. It has been suggested that losses in each step of a multi-step procedure yield low recoveries as reported in the literature.     

A study of the removal of selenite and selenate from aqueous solutions using a magnetic iron/manganese oxide nanomaterial and ICP-MS

Selenium (Se) is naturally occurring in the environment and is an essential nutrient in mammals. However, environmental Se can be increased to toxic levels through different industrial practices. The potential adsorption of the Se oxoanions, selenite and selenate, from aqueous solutions onto nanosynthesized MnFe₂O₄ was investigated using batch techniques and DRC-ICP-MS spectroscopy. The nanomaterial (NM) was laboratory synthesized through slow titration of a mixture of Fe²⁺ and Mn²⁺ ions. X-ray diffraction and Scherrer's equation were used to determine the phase of the material and crystallite size, respectively. The effects of pH, reaction time, competitive anions, and the adsorption capacity of the synthesized NM to bind selenite and selenate were investigated. The Langmuir isotherm was used to determine the binding capacity of the NM. Results showed that the phase of the nanomaterial was similar to Jacobsite with a size of 27.5 nm. Results also showed that the sorption of either 100 ppb of selenite or selenate was pH independent in the pH range 2 to 6 and occurred within 5 min of contact time. The introduction of Cl⁻ and NO₃⁻ anions individually added to solution had no significant effect on the sorption of either selenite or selenate. However, it was found that the addition of SO₄²⁻ had a competitive effect only on the sorption of selenate, first seen at 10 ppm and more pronounced at 100 ppm of SO₄²⁻. In the presence of 100 ppm of PO₄³⁻, the adsorption of selenate decreased to 87% while selenite sorption decreased to 20%. From the Langmuir isotherm equation it was determined that the nano-Jacobsite had a selenite and selenate binding capacity of 6573.76 and 769.23 mg Se/kg of NM, respectively.     

A study of the removal of selenite and selenate from aqueous solutions using a magnetic iron/manganese oxide nanomaterial and ICP-MS

Selenium (Se) is naturally occurring in the environment and is an essential nutrient in mammals. However, environmental Se can be increased to toxic levels through different industrial practices. The potential adsorption of the Se oxoanions, selenite and selenate, from aqueous solutions onto nanosynthesized MnFe₂O₄ was investigated using batch techniques and DRC-ICP-MS spectroscopy. The nanomaterial (NM) was laboratory synthesized through slow titration of a mixture of Fe²⁺ and Mn²⁺ ions. X-ray diffraction and Scherrer's equation were used to determine the phase of the material and crystallite size, respectively. The effects of pH, reaction time, competitive anions, and the adsorption capacity of the synthesized NM to bind selenite and selenate were investigated. The Langmuir isotherm was used to determine the binding capacity of the NM. Results showed that the phase of the nanomaterial was similar to Jacobsite with a size of 27.5 nm. Results also showed that the sorption of either 100 ppb of selenite or selenate was pH independent in the pH range 2 to 6 and occurred within 5 min of contact time. The introduction of Cl⁻ and NO₃⁻ anions individually added to solution had no significant effect on the sorption of either selenite or selenate. However, it was found that the addition of SO₄²⁻ had a competitive effect only on the sorption of selenate, first seen at 10 ppm and more pronounced at 100 ppm of SO₄²⁻. In the presence of 100 ppm of PO₄³⁻, the adsorption of selenate decreased to 87% while selenite sorption decreased to 20%. From the Langmuir isotherm equation it was determined that the nano-Jacobsite had a selenite and selenate binding capacity of 6573.76 and 769.23 mg Se/kg of NM, respectively.     

Column-switching system for selenium speciation by coupling reversed-phase and ion-exchange high-performance liquid chromatography with microwave-assisted digestion-hydride generation-atomic fluorescence spectrometry

Speciation of selenocysteine (SeCys), selenomethionine (SeMet), selenoethionine (SeET), selenite (Se(IV)) and selenate (Se(VI)) has been accomplished using high-performance liquid chromatography, with the aid of an anion exchange column and a reversed-phase column, both connected through a six-port switching valve. On-line microwave-assisted digestion and hydride generation steps were performed prior to the atomic fluorescence detection. The elution of the seleno amino acids was accomplished in the reversed-phased column using water as mobile phase. Selenite and selenate were separated in the anion exchange column, using gradient elution with an acetate buffer. The separation of the five selenium compounds took place in 15 min. The detection limits obtained ranged between 0.6 and 0.9 microg l(-1). Values of r>0.998 were obtained for linear fit graphs. A commercial available urine sample was analyzed, in which SeCys and Se(IV) were quantified.     

Negative Thermionen-Massenspektrometrie von Selen

Summary Selenite and selenate were determined in ground waters with isotope dilution mass spectrometry (IDMS). This species analysis was possible by the use of an⁸²Se enriched selenite and selenate spike and a Chromatographic separation of both species after the isotope dilution step. In a column filled with the DEAE cellulose anion exchanger selenite could be separated with 1 mol/l formic acid, whereas selenate was eluted with 0.1 mol/1 nitric acid. The mass spectrometric isotope ratio measurement was carried out in a thermal ionization instrument using the formation of negative Se^– thermal ions for detection. Selenite, selenate and total selenium in ground water samples were determined in the concentration range of 0.2–20 n/g with relative standard deviations of 0.5%–5%. The selenate concentration was approximately ten to eighty times higher than the corresponding selenite concentration. There was always a difference of about 8% between the sum of the selenite and selenate concentrations and the total selenium concentration which can possibly be attributed to water-soluble selenides and elementary selenium, respectively.

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Herrn Prof. Dr. R. Neeb zum 60. Geburtstag gewidmet     

Selective pre-concentration of selenite from aqueous samples using mercapto-silica

Silica gel modified with 3-mercaptopropyl-trimethoxysilane was used for the selective separation and pre-concentration of selenite (Se(IV)) from aqueous solutions containing Se(IV) and selenate (Se(VI)). Over a wide range of acidity, from 2 mol l⁻¹ HCl to pH 9.00, Se(IV) was taken up by the mercaptopropyl-silica with nearly 100% efficiency; Se(VI) however was unretained. Se(IV) content was determined by hydride generation atomic absorption spectrometry (HGAAS), following batch release of the selenium from the pre-concentration medium by acidic periodate. The overall pre-concentration efficiency, including both take-up and elution, in the range of 89-106%. The method was applied to spiked seawater samples containing as low as 800 ng l⁻¹ Se in selenite form. This solid-phase extraction system offers several major advantages over conventional solvent extraction procedures. It firstly exhibits high selectivity for Se(IV) over Se(VI). Using the solid-phase media, pre-concentration of Se(IV) in dilute water samples can be carried out in the field, stabilizing the selenite-selenium in a convenient form for transport and storage. In addition, selenium stored on silica is derived solely from Se(IV) overcoming problems of selenium redox speciation changes and loss during storage.     

DRC™ ICP-MS coupled with automated flow injection system with anion exchange minicolumns for determination of selenium compounds in water samples

A flow injection system with anion exchange resin minicolumns was coupled with dynamic reaction cell (DRC™) ICP-MS for the determination and speciation of selenite and selenate at sub μg L⁻¹ levels. The charged selenate and uncharged selenite were separated on the first resin column in which only selenate was retained. The unretained selenite was then deprotonated with alkaline solution, and the resulting anionic selenite species was collected on the second column serially connected downstream. By setting a sample loop, total selenium can be determined together with selenite and selenate. The selenium species was eluted by nitric acid and carried to DRC™ ICP-MS for their detection. Using ammonia as reaction gas, the detection of ⁷⁸Se was improved. The enrichment factor was 20 for 10 mL of sample. The standard deviations (n = 5) of peak heights were 4.9%, 4.1%, and 7.0% for a 5.0 × 10⁻² μg L⁻¹ selenite and selenate, and total Se, respectively. The calibration graphs were linear from 2.0 × 10⁻² to 1.0 μg L⁻¹ selenite and selenate. And, the linearity for total selenium was good in the range of 10.0 × 10⁻² to 1.0 μg L⁻¹. The proposed method has been demonstrated for the application to natural and bottled drinking water samples.     

Selenium and nutrition: The accuracy and variability of the selenium content in commercial supplements

Summary Selenium is a required trace-element that has been found to be protective against serious chronic diseases such as cancer and cardiovascular disease in some, but not all, epidemiological studies using both case-control and intervention designs. As a result, the fraction of the adult U.S. population now taking a daily selenium supplement is steadily increasing. In this study, we analyzed 10 or more replicate Se supplement tablets, from each of 15 different products representing 12 different brand names with most being sampled at two different times separated by approximately 30 months. Two chemical forms, seleno-yeast and selenate were tested in 50, 100 and 200 µg/tablet dosages (seleno-yeast) and 25 and 200 µg/tablet dosages (selenate). Variations in contemporary lots were evaluated at both sampling periods. The Se content provided on the product label is generally understated. One tablet contained 2.5 times more selenium than the stated dose. Selenate supplements are less accurately labeled and more highly variable compared to yeast supplements. One popular multivitamin, labeled at 200 µg/tablet, contained tablets in excess of 300 µg. Many subjects using this supplement will exceed the 400 µg/day tolerable upper limit of intake, recently established, for Se by the Institute of Medicine’s Food and Nutrition Board.