Determination Of Total Selenium Research Articles

Evaluation of ammonia as diluent for serum sample preparation and determination of selenium by graphite furnace atomic absorption spectrometry

A method for the determination of total selenium in serum samples by graphite furnace atomic absorption spectrometry was evaluated. The method involved direct introduction of 1:5 diluted serum samples (1% v/v NH 4OH+0.05% w/v Triton X-100 ®) into transversely heated graphite tubes, and the use of 10 μg Pd+3 μg Mg(NO 3) 2 as chemical modifier. Optimization of the modifier mass and the atomization temperature was conducted by simultaneously varying such parameters and evaluating both the integrated absorbance and the peak height/peak area ratio. The latter allowed the selection of compromise conditions rendering good sensitivity and adequate analyte peak profiles. A characteristic mass of 49 pg and a detection limit (3s) of 6 μg 1 −1 Se, corresponding to 30 μg l −1 Se in the serum sample, were obtained. The analyte addition technique was used for calibration. The accuracy was assessed by the determination of total selenium in Seronorm™ Trace Elements Serum Batch 116 (Nycomed Pharma AS). The method was applied for the determination of total selenium in ten serum samples taken from individuals with no known physical affection. The selenium concentration ranged between 79 and 147 μg l −1, with a mean value of 114±22 μg l −1.

DETERMINATION OF SELENIUM IN DRINKING WATER WITH A SIMPLE FIELD DEVICE

ABSTRACT A new method for the determination of total selenium in drinking water in the low ppb range was developed and optimized. The method use field device Supralab FD which was previously designed for the determination of arsenic in drinking water. The method relies on the formation of selenium hydride and its reaction with sodium perborate impregnated on a support to form a colored product, elemental selenium, which is optically detected. As a reductant, sodium borohydride is used. Thus, only Se(IV) reacts whereas Se(VI) cannot be reduced directly requiring pre-reduction with hydrochloric acid. The limit of detection of method (3σ) is 0.7 µg/dm3, the limit of quantification is 2 µg/dm3. The total reaction time for concentration up to 10 µg/dm3 is 5 min; higher concentrations require up to 15 min.

HPLC-ICP-MS determination of selenium distribution and speciation in different types of nut.

In addition to determination of total selenium in nuts, the element distribution among different fractions (lipid extract, low molecular weight, and protein fractions), and speciation analysis were studied. Improved precision for total selenium determination was observed after elimination of lipids. Because selenium was not detected in any of the lipid extracts obtained from the different types of nuts (ICP-MS), in each determination and/or speciation procedure used in this work lipids were extracted (chloroform-methanol, 2:1) and discarded before analysis. In agreement with previously reported data, high selenium levels were found in Brazil nuts (those purchased without shells contained approximately a quarter the content than those purchased with shells) and significantly lower levels in walnuts, cashews, and pecans nuts. Low-molecular-weight compounds were extracted with perchloric acid (0.4 mol L(-1)) to furnish a fraction containing 3 to 15% of the total selenium in different types of nuts. The proteins were isolated from nut samples by dissolution in 0.1 mol L(-1) sodium hydroxide and subsequent precipitation with acetone. They were then dissolved in phosphate buffer pH 7.5. Analysis of protein fractions focused on selenium in two possible states - weakly and firmly bound to proteins. Results obtained for Brazil nuts by size-exclusion chromatography with on-line ICP-MS detection, in the absence and in the presence of beta-mercaptoethanol, showed that approximately 12% of total selenium was weakly bound to proteins. To obtain information about firmly bound selenium, the protein extracts were hydrolyzed enzymatically with proteinase K. Speciation was performed by means of ion-pairing HPLC-ICP-MS. The primary species found in all types of nuts was Se-methionine (19-25% of total selenium for different types of nuts).

Kinetic–spectrophotometric determination of selenium in natural water after preconcentration of elemental selenium on activated carbon

A simple, accurate, sensitive and selective method is described for rapid determination of ultra-trace quantities of selenium. Selenium (IV) was collected on activated carbon after reduction to elemental Se by l-ascorbic acid. The collected selenium was then determined based on its accelerating effect on the oxidation reaction of methyl orange with bromate in acidic media. Total amount of Se(IV) and Se(VI) were collected on AC after their reduction by hydrazine. Se(IV), Se(VI) and total selenium could be determined by the method. Selenium in the range 10–10 000 ng could be determined by the method. The method was used to the determination of Se(IV), Se(VI) and total selenium in natural water with satisfactory results.

Ultrasonic extraction combined with fast furnace analysis as an improved methodology for total selenium determination in seafood by electrothermal-atomic absorption spectrometry

Based on the analytical minimalism concept outlined by Halls [J. Anal. Atomic Spectromet. 10 (1995) 169], a fast methodology for determination of Se in seafood by electrothermal-atomic absorption spectrometry (ETAAS) is proposed. The following features are considered in order to meet with such a concept: (i) rapid sample preparation based on accelerated extraction of Se by means of ultrasound; (ii) shortening of the thermal program by omission of the pyrolysis stage; (iii) use of simple aqueous standards for calibration. Addition of a Pd matrix modifier was seen to be needed for quantitative recovery of Se. The method was validated against two certified reference materials and subsequently applied to determination of Se in six seafood samples. Contents ranged from 1 to 3 μg g −1. The limit of detection was 0.16 μg g −1 and the precision, expressed as relative standard deviation, was in the range 3–12%. Re-adsorption of Se was seen to be a source of error which could be overcome by working with low-acidity media.

On-line copper and iron removal and selenium(VI) pre-reduction for the determination of total selenium by flow-injection hydride generation-inductively coupled plasma optical emission spectrometry

Selenium was determined in samples with high copper and iron contents by hydride generation-inductively coupled plasma optical emission spectrometry (HG-ICP-OES) after flow-injection (FI) on-line copper and iron removal and selenium(VI) reduction. A Dowex 1X-8 anion-exchange microcolumn was used for the separation of selenium from copper and iron as their chloro-complexes. Se(VI) was then reduced on-line by heating a PTFE coiled reactor (150 cm long, 0.7 mm i.d.) in a 100°C water bath. After reduction of Se(VI), a 900-μl sample was injected into the carrier stream containing hydrochloric acid and sodium tetrahydroborate to generate the hydride. A limit of detection of 0.4 μg l −1 (RSD 2.3% for 20 μg l −1 selenium) was obtained. The application of the method to Geochemical Standard Reference Samples and copper metal reference materials (MBH) demonstrated that results were in good statistical accordance with certified values.

Selenium-enriched sprouts. A raw material for fortified cereal-based diets.

The selenium supply in almost all European countries, including Austria and Germany, is below the recommended daily intake. In these countries, selenium fortification of foods and the use of selenium supplements are quite popular to compensate for low Se intake from diets. In general, wheat (Triticum aestivum) is known to be a good source for bioavailable selenium, and many studies have been performed to enrich selenium in wheat by selenium fertilization of the soil. In the present work, the process of sprouting was investigated as an alternative to enrich selenium in wheat. Sprouting was chosen because it additionally improves the nutritional value of seeds, for example, by a higher vitamin content, a better quality of protein, and some other parameters. Wheat, alfalfa (Medicago sativa), and sunflower (Helianthus annuus) seeds were germinated for 5 and 7 days in solutions containing selenate. The selenium sensitivity of the sprouts was tested by measuring visible germination levels and seedling development. Uptake rates were studied by determination of total selenium using inductively coupled plasma mass spectrometry (ICP-MS). Metabolism of the absorbed selenium was analyzed by determination of selenium species in extracts of the sprouts using anion exchange HPLC coupled to ICP-MS. It was shown that sunflower sprouts were the most resistant and had the highest uptake rates (up to 900 mg/kg), but almost 100% of the selenium was extracted with water and found to be nonmetabolized selenate. Wheat and alfalfa were less resistant and enriched selenium up to concentrations of 100 and 150 mg of Se/kg of dry mass, respectively. The metabolism of the selenate was inversely related to the total uptake rates. At low Se enrichment (approximately 1-2 mg of Se/kg), <20% of the total selenium content within the sprouts remained as inorganic selenium, indicating a high metabolism rate. With increasing uptake the amount of selenate increased to approximately 40-50%. However, with the method used it is possible to produce sprouts containing certain amounts of selenium, which might provide substantial proportions of bioavailable selenium. In combination with the generally high nutritional value of sprouts, they might serve for production of improved cereal-based diets.

Selenium Determination in Biological Fluids Using Zeeman Background Correction Electrothermal Atomic Absorption Spectrometry

Procedures for the direct determination of total selenium in urine, serum, and blood using electrothermal atomic absorption spectrometry are presented. In the selected experimental conditions, Zeeman correction is mandatory to compensate for the high background signals. The sample diluted and containing 0.1% (w/v) Triton X-100 is introduced directly into the electrothermal atomizer. A solution containing 15% (w/v) hydrogen peroxide, 0.65% (w/v) nitric acid, and 0.5% (w/v) nickel is injected separately into the atomizer. Calibration is carried out using the standard additions method. The detection limit is 30 pg selenium. If palladium, instead of nickel, is used as the chemical modifier, calibration can be carried out against aqueous standards, and the detection limit is 45 pg. In this case, three separate injections are required to prevent precipitation problems in the automatic injector. The reliability of the procedures is checked by analyzing three certified reference materials and by recovery studies. Mean recoveries are 99.7% for serum, 99.4% for urine, and 100.8% for blood samples. Relative standard deviation values are ±4.0% for serum, ±3.9% for urine, and ±4.5% for blood.

Analysis of Interlaboratory Performance in the Determination of Total Selenium in Water

This study explored the performance of experienced laboratories in the analysis for total selenium in water by a variety of analytical methods. The goal of the study was to examine intra- and interlaboratory variability. Replicates (n = 7) of 7 sample types that included a reference material of known Se concentration, natural waters, and treated wastewaters were submitted to 7 laboratories with prequalified Se analytical experience. Results of the study indicated wide ranges in minimum and maximum results, distinct differences in laboratory precision, and routine reporting of numerical results below statistical limits of quantitation. Hydride generation as a sample introduction technique demonstrated superior performance. In general, the study supports a caution advisory about using low-level Se data, especially results lower than about 10 micrograms Se/L, without quantitating the statistical uncertainty of the data. Because this study used data from samples that were submitted in bulk to participating laboratories prequalified for Se analytical expertise and experience, it can be considered a best-case demonstration of performance.

Ultra-trace determination of Se in sediments by electrothermal vaporizer-inductively coupled plasma-mass spectroscopy: use of the ETV as a thermochemical reactor

The performance of electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS) was evaluated for the ultra-trace determination of total selenium in sediment reference materials. Citric acid, when combined with an appropriate thermal program, promoted the early release of Se in a molecular form from the graphite surface, effecting a separation of the analyte from the concomitant matrix, thereby demonstrating the use of the ETV as a thermochemical reactor. No special sample pretreatment is needed and an absolute detection limit of 10 pg was achieved. Concentrations of Se in different sediment CRMs were determined and results obtained by both isotope dilution (ID) and standard addition (SA) methodologies were compared and evaluated. Mass bias effects prevented accurate application of ID techniques.

Redoxspeciation of selenium in water samples by cathodic stripping voltammetry using an automated flow system

A method for the determination of Se(IV) and Se(VI) species in water samples is described based on the cathodic stripping voltammetry of copper selenide at the mercury electrode using an automated flow system. The use of a voltammetric flow-through cell facilitates the automation of the measurements. It is shown that the applied electrolyte exchange is reducing the matrix effect during the voltammetric determination step. The system was optimized systematically with respect to the electrochemical and hydrodynamic parameters. The prior reduction of Se(VI) to Se(IV) was carried out after 1 h of UV-irradiation in alkaline medium (pH 11.0). The content of Se(VI) was calculated from the difference between the content of Setotal and Se(IV), previously determined. The linear range for the measurements was from 1.0 to 150.0 µg l–1 with a detection limit calculated as 0.35 µg l–1 and a RSD of 1.4% (n = 5) for 5.0 µg l–1 of Se(IV) measured. During the UV-irradiation of water samples not only the reduction of Se(VI) takes place but also the decomposition of the organic matter present. The combination of both enables the speciation of Se(IV) and Se(VI) as well as the determination of total selenium in water samples with a high content of organic interferences. A mechanism for both photolytic processes, the reduction of selenium and decomposition of the organic matter is proposed, showing that both phenomena occur under the conditions used. The interferences of some organic compounds and inorganic ions were tested. Recoveries between 93 and 109% were obtained using the standard addition method for spiked samples of purified water, model water, drinking water and lake water. Additional measurements were carried out on a certified water sample.

Determination of total selenium in serum, whole blood and erythrocytes by ICP-MS

A simple method for measuring selenium in serum by ICP-MS using a 1+14 dilution of a 100 µl sample in a diluent containing 1.0% v/v butan-1-ol to eliminate interferences at 78 Se from argon adduct ions has been shown to be accurate and precise in a 24 month evaluation for routine clinical assays and has been developed to allow analysis of whole blood and erythrocytes. Excellent performances were achieved in two External Quality Control Assessment programmes for the measurement of selenium in serum: Centre du Toxicologie de Quebec and Trace Element Quality Assessment Scheme from the University of Surrey (Guildford). The regression equation for our ICP-MS data versus target values for analysis of Se in 109 sera from November 1996 to March 1998 was: ICP-MS=0.969x Target+0.031 µmol l –l , r=0.996. For serum analysis the detection limit was 0.02 µmol l –1 , and the RSDs at 0.3-2.0 µmol l –1 were 2.0-4.0% within-run and 4.3-10.0% between-run. Initial applications of this method to measuring selenium in whole blood (1+14 dilution) and red blood cells (1+29 dilution) gave detection limits of 0.02 µmol l –1 ; RSDs of 2.1-5.8% within-run, and of 5.0-8.1% between-run, at concentrations of 0.3-2.0 µmol l –1 . Analysis of a whole blood reference sample (Seronorm 404107), with a target concentration of 1.01 µmol l –1 and acceptable range of 1.01-1.15 µmol l –1 , gave 1.06±0.06 µmol l –1 (mean±s).

Benzene extraction and cobalt(iii) oxide powder collection for the determination of total selenium in food and biological samples by electrothermal atomic absorption spectrometry using a tungsten furnace

Benzene extraction and cobalt(III) oxide powder collection were studied for the determination of total selenium in food and biological samples by electrothermal atomic absorption spectrometry using a tungsten furnace. Selenium was extracted into benzene using hydrochloric acid with the addition of potassium iodide, and cobalt(III) oxide powder was added to the benzene to collect the extracted selenium. The cobalt(III) oxide powder was separated from benzene by vacuum filtration and then suspended in 1 ml of water. Part of the slurry thus obtained was introduced into the tungsten furnace and the determination of selenium performed. As a result, 100% recoveries of selenium(IV) and selenium(VI) were obtained for 8 M hydrochloric acid containing 1.0 M potassium iodide. The relative standard deviation using the slurry at the 0.20 mg l–1 selenium level with the proposed method was 3.4% (n = 6). The calibration curve was linear up to 0.50 mg l–1 (5.0 ng per 10 µl) and the detection limit (3σ) was 16.0 µg l–1 (0.16 ng per 10 µl). The proposed method was applied to food and biological samples.

Analytical selenoamino acid studies by chromatography with interfaced atomic mass spectrometry and atomic emission spectral detection

Consumption of selenium enriched plants or yeast-based nutritional supplements has been reported to provide anticarcinogenic benefits which are selenium compound dependent. Separation and identification of these selenium compounds is critical to understand the activity. Plants and yeast convert inorganic selenium in the soil or growth media into organoselenium compounds, probably following a route similar to the sulfur assimilatory pathway. Non-volatile selenium compounds produced include selenoamino acids, some of which have shown anticarcinogenic activity. Volatile compounds produced by chemical reaction of involatile precursors have also been found. An ion pair chromatographic method with ICP-MS detection for the separation of selenoamino acid standards potentially present in real samples is given. The method allows separation of selenoamino acids including such analytes as the cis-trans isomers of Se-1-propenyl-dl-selenocysteine. The method also provides the capability of determining the presence of selenoxides and possibly selenones, and tracking of other functionalities and reactions by selective derivatization. Alternatively, selenoamino acids are treated with ethylchloroformate to produce stable volatile derivatives which are amenable to GC separation with element specific atomic emission detection (GC-AED). Results of total selenium determination and speciation of selenium enriched yeast-based nutritional supplements, selenium enriched allium vegetables and bioremediation samples are presented.

Determination of selenium stable isotopes by gas chromatography–mass spectrometry with negative chemical ionisation

A gas chromatography mass spectrometric method using negative chemical ionisation was developed for the determination of stable isotopes of selenium for evaluation of selenium absorption and retention from foods in humans. The method involves an acid digestion to convert all selenium into selenite, which subsequently reacts with 4-nitro- o-phenylene-diamine to form a volatile piazselenole. The piazselenole, after extraction into an organic solvent, was analysed for its isotopic selenium composition by gas chromatography mass spectrometry. Negative chemical ionisation is reported for the first time for the determination of selenium stable isotopes and its analytical characteristics were compared to those of electron impact mass spectrometric ionisation, classically used for the determination of selenium. The negative chemical ionisation technique allowed accurate determination of total selenium by isotope dilution and of selenium isotope ratios in biological samples. The repeatability for total selenium and for stable isotope ratios was good (R.S.D.≤10%) within the range of 50 to 250 ng selenium. The detection limit for the investigated selenium isotopes was approximately 1 pg (signal to noise ratio at 3). The applicability of the developed stable isotope methodology was demonstrated by the determination of the selenium absorption and retention from foods in a pilot study using one human adult.

Determination of selenium in soil by hydride generation AAS

Hydride generation AAS was applied for the determination of total selenium in soil. The influence of various anions and cations present in the sample solution on the generation of selenium hydride was investigated. Special attention was paid to the wet oxidation procedure for the sample dissolution. The proposed procedure involves microwave sample preparation by using a mixture of H2O2/HNO3/H2SO4. The accuracy of the procedure was checked by the analysis of the standard reference material Buffalo River Sediment (SRM-2704, NIST). Satisfactory agreement of the results obtained with the values reported was achieved. The detection limit of the procedure was 25 ng Se/g of soil. The relative standard deviation of the measurements varied from 5.5% for SRM-2704 up to 10% for the real soil samples.

Influence of iron on the determination of total selenium in sewage sludge by electrothermal atomic absorption spectrometry

The determination of total selenium by ETAAS in complex matrices such as sewage sludge has always been a challenge owing to the problems encountered with the solubilization of selenium species and the interferences observed during their determination. The use of a microwave oven in order to shorten the digestion step can lead to the solubilization of larger amounts of elements that can behave as interferents in the determination of Se by ETAAS. This study has shown the particular influence of Fe on the determination of Se by ETAAS. A correction procedure was developed and applied successfully to different digested samples of sewage sludge.

Anodic stripping voltammetric determination of total lead, copper and selenium in whole blood and blood serum

A two-step procedure including appropriate wet-digestions, separation of selenium from interfering ions such as heavy metal ions with pentyl alcohol and anodic stripping voltammetric (ASV) determination of Pb2+, Cu2+ and SeO32– is developed. The elements in digested whole blood and serum sample solutions were determined by using a standard addition method. 1 × 10–9 mol/L SeO2–3, Cu2+ and Pb2+ were successfully determined with relative standard deviations of approximately 1–2% (n = 6–8).

Evaluation of high-pressure microwave digestion methods for hydride generation atomic absorption spectrometric determination of total selenium and arsenic in sediment

Five closed-vessel microwave digestion methods were compared for the accurate determination of arsenic and selenium in NIST SRM 1645 River Sediment by flow-injection hydride-generation atomic absorption spectrometric methods. The digestion methods using five different acid mixtures (HNO3/ H2SO4, HNO3/HCl04, HNO3/HCl, HNO3/HCl/HF, HNO3/H2SO4/HClO4) were all found to be reliable for the determination of the analytes. Taking into consideration the safety and suitability for the analysis of other metals, the methods based on the use ofaqua regia are recommended for closed vessel microwave digestion with pressure control. Using the quick digestion program, the presence of up to 10% organic content in soil samples did not adversely affect the closed vessel digestion and did not cause the loss of volatile analytes. After digestion, opening the vessel under an inner pressure of below 345 kPa (50 psi) had no effect on the accuracy of the results. The recommended digestion methods (HNO3/HCl and HNO3/ HCl/HF) for the reliable determination of arsenic and selenium in different sediment samples were demonstrated. The calculated detection limits (3σ b ) were less than 0.030 μg/g and 0.033 μg/g for arsenic and selenium, respectively. All analytical results for arsenic and selenium in SRM 1645 River sediment, NRCC BCSS-1 Marine Sediment and NIES CRM Pond Sediment were within or near the certified and reported ranges, with the exception of selenium in NIES CRM No. 2 Pond Sediment.

On-line focused microwave digestion-hydride generation of inorganic and organic selenium: Total determination and inorganic selenium speciation by atomic absorption spectrometry

An on-line FIA pretreatment with HBr/KBrO 3, assisted by on-line focused microwave-induced digestion, has been coupled with hydride generation-atomic absorption spectrometry (HG-AAS) for final detection for total selenium determination. This total selenium determination is virtually independent of the different Se species investigated (selenite, selenate, selenomethionine, selenoethionine and selenocystine). Detection limits of 0.8 μg l −1 of Se can be achieved by AAS with precisions better than 5%. This continuous flow system for selenium determination allows a high sample throughput (about 30 samples h −1 can be analyzed) in which high automation can be achieved and constitutes a convenient real-time continuous detector for the different selenocompounds tested. Direct non-chromatographic speciation of inorganic selenium (selenite and selenate in their mixtures) is demonstrated by simple on-off operation of the focused microwaves connected in the flow system. Validation of this simple on-line FIA system has been carried out by analyzing total Se recovered from spiked tap waters and by speciating mixtures of Se(IV) and Se(VI) spiked to the same samples. The fast conversion of Se compounds into volatile selenium could be considered as a sort of specific “general” detector for Se compounds which can be extremely useful for Se speciation by hybrid chromatographic techniques.