The determination of selenium in urine

The selenium excreted in urine can be measured to assess the dietary status of selenium, an essential trace element in human nutrition. The objectives of this work were: 1) to develop a procedure, capable of high sample throughout, by which the major interferences can be reduced such that selenium concentrations can be measured in urine by Neutron Activation Analysis (NAA) using^77mSe (17.4 s; and 2) to apply the method to a human dietary selenium study in which several selenium monitors were compared. The method involves a pre-irradiation arsenic-coprecipitation separation of the selenium from urine in the presence of a high specific-activity⁷⁵Se tracer. The processed urine samples are analyzed using NAA. The procedure was applied to 58 urine specimens longitudinally collected from 12 subjects consuming three different levels of selenium. A dose-response relationship was observed in urine as well as a high correlations with both serum and whole blood selenium concentrations.     

Gas-chromatographic determination of selenium

Selenium(IV) is reacted with 2,3-diaminonaphthalene at pH 2 to form the well established complex, which is extracted into hexane. An aliquot of the hexane layer is analyzed gas chromatographically with an electron-capture detector. As little as 510^?10 g of selenium could be detected; 0.01μg of selenium could be determined in a sample by extracting int 0.1 ml of hexane and injecting a 5-μl aliquot of the extract. The method was applied to the determination of physiological amounts of selenium in human blood and urine. Averages of 0.38 p.p.m. and 0.007 p.p.m. selenium were found in blood and urine. respectively. River water samples were also analyzed. Complete analysis time for a single sample is less than 3 h. including time for digestion of the sample and 2 h for formation of the complex.     

Determination of selenium in human body fluids by hydride-generation atomic absorption spectrometry : Optimization of sample decomposition

The accuracy of the determination of selenium in human body fluids by hydride-generation a.a.s. depends critically upon the sample decomposition-method used. Digestion with HNO₃ alone gave low selenium recoveries, but with nitric, sulfuric and perchloric acids at a final temperature of 310°C gave results that agreed with those obtained by other techniques. The recovery of selenomethionine added to whole blood and of trimethylselenonium iodide added to urine was 97–104%. The average selenium values found for 6 healthy individuals were 88 μg l^?1 in whole blood, 75 μg l^?1 in blood plasma and 307 μg (kg Hb)^?1 in erythrocytes. A detection limit of 5 μg l^?1 Se in body fluids was found under routine conditions.     

Characteristics of selenium in australian marine biota

The occurrence, distribution and speciation of selenium in Australian marine biota is discussed. Biochemical pathways for the accumulation of selenium by marine organisms are also postulated. Comparison of the levels of selenium in macroalgae, fish, crustaceans and molluscs indicates that preferential accumulation of selenium by particular taxa does not occur. Phaeophyta have significantly lower selenium concentrations than Rhodophyta and Chlorophyta. Fish have lower selenium contents in muscle tissues than molluscs and crustaceans. Marine animals with different dietary intake (planktonic vs herbivorous vs carnivorous) are not observed to have significantly different levels of selenium (P>0.05). Selenium in all the organisms studied was predominantly associated with free amino-acids or protein residues and was not present as characterizable inorganic selenium species (SeO₃^2?, SeO₄^2?). These results indicate that selenium is probably only incorporated into biota for specific biochemical purposes with any exces selenium being excreted or eliminated.     

Methodological advances for selenium speciation analysis in yeast

Advances in analytical methodology for speciation of selenium in selenized-yeast food supplements were discussed on the basis of the recent developments in the authors’ laboratory. Particular attention was given to the sample preparation with regard to the fractionation of selenium into different classes of chemical species, the high resolution fractionation of selenium from yeast water extracts by size-exclusion chromatography and characterization of the water soluble protein fraction by combined matrix-assisted laser desorption ionization (MALDI)-time-of-flight mass spectrometry (TOF MS) and electrospray quadrupole-TOF tandem MS. The true speciation of protein-incorporated selenium (down to individual proteins characterized by a unique aminoacid sequence) was discussed using an example of a family of selenium-containing proteins formed in yeast by the substitution of methionine residues by selenomethionine in a salt stress-induced protein.     

Investigation of the Declared Value of Selenium in Food Supplements by HG-AFS

The method of hydride generation atomic fluorescence spectrometry (HG-AFS) was optimised for determination of selenium in food supplements. Due to the high and varied content of Cu, Mg and Zn in the samples, the standard addition method was found to be the most appropriate. The reliability of the method was checked by the independent method of radiochemical neutron activation analysis, and good agreement was found between the two methods. HG-AFS is simple and rapid for Se determination in food supplements based on minerals. Agreement between the selenium values found and declared was worse than 10% in 9 out of 13 supplements. Furthermore, 2 of the 14 supplements did not comply with the recommendations stated in the 27^th edition of the U.S. Pharmacopoeia, which states that minerals and vitamins in food supplements should be in the range of 90 and 200% of the declared value.     

Speciation and bioavailability of selenium in yeast-based intervention agents used in cancer chemoprevention studies

This study investigated the speciation and bioavailability of selenium in yeast-based intervention agents from multiple manufacturers from several time points. Sources of selenized yeast included Nutrition 21 (San Diego, CA), which supplied the Nutritional Prevention of Cancer (NPC) Trial from 1981-1996; Cypress Systems (Fresno, CA; 1997-1999); and Pharma Nord (Vejle, Denmark; 1999-2000), which supplied the Prevention of Cancer by Intervention by Selenium (PRECISE) Trial pilot studies. The low-molecular-selenium species were liberated from the samples by proteolytic hydrolysis followed by separation by ion exchange liquid chromatography and detection by inductively coupled plasma-mass spectrometry. The results for the NPC tablets showed that selenomethionine, together with 3 unidentified selenium compounds, were predominant in the sample hydrolysates. The relative amounts of the 4 selenium species varied (p < 0.05) among several of the 7 tablet batches used during the course of the NPC Trial. In comparison, 5 batches of more recently produced selenized yeasts, which were used as a source of selenium in the PRECISE and other trials, contained less of the unknown compounds and more selenomethionine at 54-60% of the total selenium in the yeasts. One batch of yeast, however (from 1985), which originated from the same producer as the yeast used in the NPC tablets, contained only 27% of selenium in the sample as selenomethionine. Human subjects receiving 200 microg selenium/day in the UK PRECISE Pilot Trial showed a higher concentration (p < 0.01) and higher increase from baseline in plasma selenium than did the same dosage used in the NPC Trial. Differences in intake, speciation, or bioavailability of selenium from the yeast-based supplements in the population groups studied may explain this. Furthermore, the selenium concentration in whole blood from the Danish PRECISE Pilot Trial was higher (p < 0.001) than that obtained with synthetic L-selenomethionine in a comparable group of Danes, both groups having been treated with 300 microg selenium/day.     

Investigation of the appearance of supplemental enriched Se-76 using the human nail as a dietary monitor

The principal objective of this study was to determine if the use of a stable enriched tracer of Se-76 could be used to determine the delay time between a dietary intake of selenium and its appearance in fingernails and toenails. Selenium is an essential trace element in human nutrition. It has been studied at the Missouri University Research Reactor (MURR) for the past 15 years using an Instrumental Neutron Activation Analysis (INAA) technique. The principal route of human exposure to selenium is through the diet. Selenium concentrations of nails, blood, hair, and urine have been used as indicators of dietary selenium intake. In this study, a cohort consisting of seven men and five women ingested three selenium supplements of 150 g each over a three day period. The selenium was enriched in Se-76 (96.48%) and ingested as selenite in orange juice following an overnight fast. Fingernails and toenails were collected prior to the selenium supplementation and for several months afterward to be used as biochemical indicators. The peak⁷⁶Se concentration in the fingernails and toenails occurred at 19–23 and 16–32 weeks after supplementation, respectively.     

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.     

Radiochemical separation of7 5Se with o-phenylenediamine for instrumental neutron activation analysis of selenium in biological materials

A new method of radiochemical separation for determination of selenium in biological materials has been developed. The method is based on the reaction of tetravalent selenium with the amino groups of aromatic ortho-diamines resulting in selenodiazoles, which can be easily extracted with organic solvents. The biological samples were irradiated in a reactor, mineralized with nitric and perchloric acids and MgCl₂. Selenium was transformed to teravalent form with hydrochloric acid and the resulting bezoselenodiazole formed in the reaction with o-phenylenediamine was extracted with toluene. The radioactivity of selenium^{7 5}Se was measured with a well-type NaI(Tl) scientillation detector. The accuracy of the method was verified by determining selenium levels in SRM NBS. The detection limit of the method was found to be 0.5 ng Se. The comparison of selenium levels in human blood determined by HG-AAS. non-destructive NAA and the described method is presented.     

Direct determination of selenium in whole blood by anodic stripping voltammetry

Trace quantities of selenium can be determined in the presence of iron, copper and lead using anodic stripping voltammetry, depositing at –0.60 V in 0.1M HClO₄ and stripping in the anodic direction. Two separate peaks are observed at –0.25 V and –0.10 V belonging to copper and selenium, respectively. Sometimes one peak may be observed for both copper and selenium. In this case one more stripping (without deposition) must be done to obtain separate peaks. After standard addition, two strippings have to be done also. With this proposed method, 10^–7 M selenium could be determined as (1.09±0.03) × 10^–7 M with a 90% confidence interval in blood samples without any separation.Presented at Xth National Chemistry Congress, Bursa, Turkey, September 19–21, 1994     

Novel selenium containing non-detergent sulphobetaines

The first selenium containing non-detergent sulphobetaines are described. They have been prepared in multi-gram quantities from readily available pyridyl precursors. Incorporation of selenium was achieved using nucleophilic organoselenide anions. The resulting products were shown by ⁷⁷Se NMR spectroscopy to be homogeneous with respect to selenium oxidation state.     

Electrothermal atomic absorption spectrometric determination of selenium in blood serum and seminal fluid after protein precipitation with trichloroacetic acid

The determination of selenium in body fluids by electrothermal atomic absorption spectrometry (e.a.a.s.) suffers from severe spectral interferences from phosphate which results in overcompensation when a continuum-source background corrector is used. The separation of selenium from phosphate by protein precipitation with trichloroacetic acid allows the determination of selenium in blood serum and seminal fluid by e.a.a.s. after thermal stabilization with silver, nickel or copper. The selenium concentration in seminal fluid from healthy, fertile Norwegian donors ranged from 0.09 to 1.30 μmol l^-1 with a group average of 0.44 μmol l^-1 (n = 15).     

Changes in the elemental content of blood components by selenium supplement in humans

A study was undertaken to determine the concentration of selenium in whole blood, erythrocytes and plasma in samples obtained from three groups (each group consisting of one male and one female subject) given as a supplement a commercial product containing the element, together with vitamins A, C and E. Blood samples were obtained on one-day-a-week basis from each group, over a period of 75 days, both in the morning and in the afternoon, as part of a gastric function investigation, in which the subjects were given orange juice after fasting. Results indicate that the level of selenium in whole blood and its components increases with dose and time and that concentrations of the element are maintained for at least 45 days following cessation of supplementation. The influence of the supplement on some electrolytes such as Na, Cl, Br and Rb was also investigated.     

Radiochemical determination of oxygen traces in selenium

A sensitive method for determination of oxygen in selenium is described. Oxygen is converted into the gaseous compound sulfur dioxide labelled with³⁵S. The β-radiation of³⁵S is measured by liquid scintillation counting. The detection limit is 0.02 ppm. The efficiency of vacuum distillation in the purification of selenium, and the influence of air, dry oxygen and water on the absorption of oxygen by selenium have been investigated.     

Kinetic determination of trace levels of selenium(IV) and total selenium by Nile Blue A/hydrogen peroxide method

A kinetic method for the determination of selenium(IV) traces is proposed, based on its inhibitory action on the oxidation of Nile Blue A by hydrogen peroxide in phosphate buffer (pH 10.5). A linear dependence was established between the rate of the proposed indicator reaction and selenium concentration in the range 9.5 × 10^–2-1.58 ng cm^–3. The experimental conditions of maximal selenium effect were established. Selenium, determined by the tangent method, was determined at concentrations over the range 0.22–1.26 ng cm^–3, with relative standard deviations up to 4.5%. The reaction rate was followed spectrophotometrically. The effect of foreign ions on the accuracy of this method was also investigated. The method was applied to the determination of selenium in pharmaceutical preparations and wheat flour.     

Determination of selenium in human tissues by atomic absorption spectrometry

A simple method is described for the determination of selenium in human tissues without the use of perchloric acid. Digestion with nitric and sulphuric acids is followed by hydride generation and atomic absorption spectrometry. Results for NBS bovine liver and IAEA horse kidney reference materials were in good agreement with assigned concentrations, as was also achieved with the perchloric acid digestion. Recovery of added selenium was >90%, and the relative standard deviation was 5.5% for within-batch and 6.9% for between-batch analyses. The values of selenium in heart tissue were 0.9–1.3 μg g^?1 dry weight.     

Internal correction of spectral interferences and mass bias for selenium metabolism studies using enriched stable isotopes in combination with multiple linear regression

The analytical methodology for the in vivo study of selenium metabolism using two enriched selenium isotopes has been modified, allowing for the internal correction of spectral interferences and mass bias both for total selenium and speciation analysis. The method is based on the combination of an already described dual-isotope procedure with a new data treatment strategy based on multiple linear regression. A metabolic enriched isotope (⁷⁷Se) is given orally to the test subject and a second isotope (⁷⁴Se) is employed for quantification. In our approach, all possible polyatomic interferences occurring in the measurement of the isotope composition of selenium by collision cell quadrupole ICP-MS are taken into account and their relative contribution calculated by multiple linear regression after minimisation of the residuals. As a result, all spectral interferences and mass bias are corrected internally allowing the fast and independent quantification of natural abundance selenium (^natSe) and enriched ⁷⁷Se. In this sense, the calculation of the tracer/tracee ratio in each sample is straightforward. The method has been applied to study the time-related tissue incorporation of ⁷⁷Se in male Wistar rats while maintaining the ^natSe steady-state conditions. Additionally, metabolically relevant information such as selenoprotein synthesis and selenium elimination in urine could be studied using the proposed methodology. In this case, serum proteins were separated by affinity chromatography while reverse phase was employed for urine metabolites. In both cases, ⁷⁴Se was used as a post-column isotope dilution spike. The application of multiple linear regression to the whole chromatogram allowed us to calculate the contribution of bromine hydride, selenium hydride, argon polyatomics and mass bias on the observed selenium isotope patterns. By minimising the square sum of residuals for the whole chromatogram, internal correction of spectral interferences and mass bias could be accomplished. As a result, the tracer/tracee ratio could be calculated for each selenium-containing species and a time relationship for synthesis and degradation established. Both selenite and selenized yeast labelled with ⁷⁷Se were employed for comparative purposes.     

Analytical determination of selenium in medical samples,staple food and dietary supplements by means of total reflection X-ray fluorescence spectroscopy

Selenium is essential for many aspects of human health and, thus, the object of intensive medical research. This demands the use of analytical techniques capable of analysing selenium at low concentrations with high accuracy in widespread matrices and sometimes smallest sample amounts.In connection with the increasing importance of selenium, there is a need for rapid and simple on-site (or near-to-site) selenium analysis in food basics like wheat at processing and production sites, as well as for the analysis of this element in dietary supplements. Common analytical techniques like electrothermal atomic absorption spectroscopy (ETAAS) and inductively-coupled plasma mass spectrometry (ICP-MS) are capable of analysing selenium in medical samples with detection limits in the range from 0.02 to 0.7 μg/l. Since in many cases less complicated and expensive analytical techniques are required, TXRF has been tested regarding its suitability for selenium analysis in different medical, food basics and dietary supplement samples applying most simple sample preparation techniques.The reported results indicate that the accurate analysis of selenium in all sample types is possible. The detection limits of TXRF are in the range from 7 to 12 μg/l for medical samples and 0.1 to 0.2 mg/kg for food basics and dietary supplements. Although this sensitivity is low compared to established techniques, it is sufficient for the physiological concentrations of selenium in the investigated samples.     

Selective photometric determination of low conentrations of selenium(IV) and selenium(VI) in bottled drinking water

A procedure is developed for the selective photometric determination of selenium(IV) in bottled drinking water by the oxidation of Methylene Blue in 1 M HCl to colorless decomposition products and of selenium(VI) by its interaction with the specified reagent at pH 5–6 with the formation of a colored ion pair. The limits of detection are 1 and 0.8 µg/L, respectively. At the concentration of selenium(IV) 2 µg/L, the admissible weight ratios are: SeO₄^2-, Br₃^- (1: 20); Br^– (1: 60); I^–, IO₃^- and IO₄^- (1: 100). At equal concentration of selenium(VI), the following species: SeO₄^2-(1: 20); Br₃^-, Br^–, I^–, IO₃^-, and IO₄^- (1: 100) do not interfere with the determination. Other anions and cations present in highly mineralized waters do not interfere with the determination. The relative error of determination is 8–10% in the concentration range 2–10 µg/L of selenium(IV) and selenium(VI) and does not exceed 5% in their concentration range of 10–100 µg/L.