Accurate determination of selenium in biological materials without perchloric acid for digestion

Selenium (10–80 μg kg^?1) is determined by hydride-generation atomic absorption spectrometry in mixed diet, faecal and urine samples and in standard reference materials (bovine liver, rice flour, wheat flour and horse kidney) after two digestion procedures. No difference was found in digestion efficiency between a nitric/sulphuric acid mixture and a nitric/sulphuric/perchloric acid mixture. The results suggest that the digestion of most biological materials for the determination of selenium does not require the use of perchloric acid.     

Semi-automated fluorimetric determination of nanogram quantities of selenium in biological material

A semi-automated method, based on the solvent extraction and fluorescence of 4,5-benzopiazselenol, is described for the determination of selenium in biological material. Samples, in batches of up to 150, are digested in a mixture of nitric and perchloric acids in temperature-controlled aluminium blocks. Selenate formed in the perchloric acid is reduced to selenite with hydrochloric acid. Metal ion interferences, including hydroxide adsorption of selenite, are prevented by ethylenediaminetetraacetic acid. Sulphate above 50 mM precipitates the 2,3-diaminonaphthalene, so wool digests were diluted accordingly. The digests are analysed from their digestion tubes at the rate of 40 per hour by the semi-automated method. Results for blood and a variety of food samples compare favourably with those obtained by other methods including hydride/atomic absorption and instrumental neutron-activation analysis.     

Comparison of two digestion methods used in the determination of selenium in marine biological tissues by gas chromatography with electron-capture detection

The performance of two decomposition procedures, with (a) nitric/perchloric/sulphuric acid and (b) nitric acid/magnesium nitrate, in the determination of selenium in marine biological tissues by gas chromatography with electron-capture detection was compared. Both methods were found satisfactory and performed equally well for sample dissolution, but method (b) was judged to be more convenient.     

Sample digestion procedures for trace element determination

A survey is given of both dry and wet procedures for matrix dissolution for the purpose of trace element determination in all kinds of materials. Various methods are compared and evaluated. With wet digestion methods special attention is paid to mixtures of strong acids and oxidants. Emphasis is laid on the completeness of these digestions, for which the use of HF is a prerequisite.     

Critical comparison of two standard digestion procedures for the determination of total mercury in natural water samples by cold vapour atomic absorption spectrometry

Two pretreatment procedures for total mercury determinations in natural water samples were compared. The first, the Swedish Standard method (DP1), involves digestion of water in the presence of concentrated nitric acid at 120°C and under pressure for 30 min. In the West German Standard method (DP2), small volumes of nitric and sulphuric acids, permanganate and peroxodisulphate are added to the sample, and digestion proceeded at 50°C in an ultrasonic bath. Mercury was determined after both digestion procedures using a modified cold vapour atomic absorption spectrometric method, in which mercury generated on addition of a reducing agent is collected and subsequently atomized in a platinum-lined graphite furnace. The efficacy of the two digestion procedures was tested using various standard organic mercury compounds and it was found that only DP2 provided quantitative recoveries. Purification of the reagents required by DP2 was achieved using a mercury-selective ion-exchange resin, Chelite S, resulting in blank levels below 1.5 ng Hg l^?1. Both methods were applied to the determination of total mercury in an unpolluted marsh water sample, giving 2.0 ng Hg l^?1 (DP1) and 2.7 ng Hg l^?1 (DP2). The West German Standard digestion procedure (DP2) is recommended for the determination of total mercury in natural water samples.     

Comparison of two digestion procedures for the determination of lead in lichens by electrothermal atomic absorption spectrometry

The efficiency of two procedures for the digestion of lichen was investigated using a heating block and a microwave oven. In the open vessels, concentrated nitric acid was added to the samples, left for 1 h, and the addition of 30% (v / v) hydrogen peroxide completed the digestion. In the closed system, the complete digestion was performed using concentrated nitric acid and hydrogen peroxide, reducing the amount of chemicals, time and contamination risk. Both digestion methods gave comparable results, and recoveries were statistically not different. For a lichen sample spiked with 10 μg Pb, the recovery was 111% and 110% using microwave and heating block digestion, respectively, while it was 100% and 103% for a 100 μg Pb spike. For the determination by electrothermal atomic absorption spectrometry samples were diluted 20 times with water and a volume of 20 μL was injected into the graphite furnace without chemical modifier. Pyrolysis and atomization temperatures of 700 °C and 1500 °C, respectively, were used. The characteristic mass was 8.4 ± 0.6 pg for aqueous calibration solutions and 8.9 ± 0.8 pg for samples. Calibration was against matrix matched standards. The recovery test showed some contamination problem with the lowest concentrations in both procedures. The detection limits were 4.4 μg L^− 1 with microwave oven and 5.4 μg L^− 1 with the heating block in the undiluted blank.     

Low volume microwave digestion for the determination of selenium in marine biological tissues by graphite furnace atomic absorption spectroscopy

A microwave digestion method for the determination of marine biological tissues has been developed to allow determination of selenium in small sample sizes (< 0.1 g). The benefits of this technique include maintaining concentrates in extracts without the subsequent over dilution encountered when using larger vessels, increased sample throughput and reduced loss of volatile material. Freeze dried biological material (< 0.1 g) and nitric acid (1 ml) were placed into 7 ml screw top Teflon vessels which are completely sealed on capping. Two 7 ml vials were placed into larger 120 ml vessels fitted with a Teflon spacer and 10 ml of distilled water. The effects of microwave power and time, and sample mass on selenium recovery from three marine standard reference materials (NIST SRM 1566a Oyster Tissue, NRCC DORM-1 Dogfish Muscle and NRCC TORT-1 Lobster Hepatopancreas) were examined. The optimum conditions: 600 W, 2 min; 0 W, 2 min; 450 W, 45 min, allowed quantitative recoveries of selenium from these and three other standard reference materials (NRCC DOLT-1 Dogfish liver, NIST RM-50 Albacore tuna and IAEA MA-A-2 fish flesh). Studies on sample mass showed that the analysis of sample masses from 0.025 to 0.1 g gave selenium concentrations within the certified range. Six species of selenium: selenite, selenate, selenomethionine, selenocysteine, selenocystamine, and trimethyl selenonium were added to oyster, dogfish, and lobster tissues. Recoveries were near quantitative for all species (94–105%) except trimethyl selenonium (90–101%).     

Boron analysis in biological material: microwave digestion procedure and determination by different methods

Investigations were made on the determination of boron in biological material. A procedure using the microwave digestion technique was optimised. The analyses were performed by photometry with azomethine-H, fluorimetry with carminic acid, inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry. The matrix interferences, the interferences caused by iron and the detection limits were investigated for these methods. Determinations were performed for plant material, wheat, milk, meat and blood plasma. Recoveries and precision of all methods were studied. To test accuracy and precision, four NBS reference materials were analysed and the results compared with the values in literature. Good recoveries were found with all methods, except the fluorimetric technique. The boron content found in the NBS standard reference materials agreed well with data reported before. Best precision was obtained for the two ICP methods. The azomethine-H method gave good results when material of high boron content was analysed. With the carminic acid method no reliable results were achieved.     

A specific micromethod for the determination of phytic acid in biological material

A method for the quantitative determination of phytic acid in biological material is described. The method permits a determination of phytic acid in quantities below 0.1 mg even if the material contains closely related compounds includingmyo-inositol pentakisphosphate.

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Eine spezifische Mikromethode für die Bestimmung von Phytinsäure in biologischem Material

Zusammenfassung Es wird eine Methode zur quantitativen Bestimmung von Phytinsäure in biologischem Material beschrieben. Die Methode erlaubt die Bestimmung von Phytinsäure in Mengen von weniger als 0.1 mg, selbst wenn das Untersuchungsmaterial nahe verwandte Substanzen wie z. B.myo-Inositpentakisphosphat enthält.

     

A method for determination of selenium in organic tissues using microwave digestion and liquid chromatography

An existing laboratory procedure for selenium analysis using open-vessel wet digestion and liquid chromatographic fluorescence determination was modified for use with microwave digestion. The proposed microwave digestion method eliminated the hazards associated with the use of HClO4 while maintaining excellent recoveries of selenium. A 2-step HNO3/H2O2 digestion procedure was developed. Digested samples were derivatized with 2,3-diaminonaphthalene, and the resultant piazselenol complex was measured fluorometrically using a liquid chromatograph. Measured values were in agreement with 9 different certified reference materials. The detection limit for this method was 0.54 ng Se/g tissue (3 sigma), and the calibration curve remained linear (r2 = 0.9968) up to 2 microg Se/g.     

Low volume microwave digestion and direct determination of selenium in biological samples by hydride generation-atomic fluorescence spectrometry

A low volume microwave digestion (LVMWD) procedure has been developed to have all forms of selenium (Se) compounds in biological samples decomposed to Se (IV) and allow total Se be directly determined by hydride generation-atomic fluorescence spectrometry (HG-AFS), or voltammetrically. Between 0.3 and 0.4 mL of mixed digestion reagents consisting of concentrated (15.4 M) HNO₃-(18.5 M) H₂SO₄ (v:v = 10:1) and >5 to <40 mg sample were found ideal systems with an optimized MW digestion program. Total Se in five certified reference materials was accurately determined. The results obtained by the conventional and LVMWD techniques agreed well. By avoiding pre-reduction step, this method, suitable for a wide range of biological samples, fully takes the advantages of HG-AFS or voltammetric techniques for their high sensitivity, high tolerance to matrix-related interference and accessibility in instrumentation. LVMWD not only enhances the sample output by 3 times and reduces the operational cost and acid wastes, but also makes the small sample analysis possible for many environmental and medical related research objectives. The digestion pathways of Se containing organic samples are also discussed based on the experimental results.     

A method for quantitative determination of deuterium content in biological material

A method was developed for quantitative determination of deuterium incorporated into live organisms or biological macromolecules. The deuterated biological material was mixed with a bovine serum albumin (BSA) supporter to make a homogeneous sample for which the deltaD value (vs. VSMOW) was analyzed using a dual-inlet gas isotope mass spectrometer. The method is described in detail, and the equation for calculation of deuterium content is presented, i.e., CbioD=1/500 x k x RVSMOW x CBSAH x 10(6) ppm. Deuterated hepatitis A virus (HAV) RNA and BSA were systematically investigated. The results demonstrate that the method is capable of direct measurement of deuterium content, and is highly repeatable and reliable with a standard deviation of +/-3 per thousand. It is stressed that the quantity of deuterated sample required is extremely small as a result of using BSA as supporter. The method may be applied in many fields, and has the strengths of simplicity, relative cheapness, and robustness.     

A catalytic-kinetic method for the determination of traces of fluoride in biological material

A catalytic-kinetic method for the determination of nanogram amounts of fluoride, based on its inhibiting action on the catalyst in the zirconium-catalyzed reaction between perborate and iodide, is described. The rate measurements are accomplished very simply by using a Landolt reaction system with a biamperometric detection of the induction period. The standard deviation in the working range 19–190 ng of fluoride (1–10 nmol) is ± 2 ng. The method can be made specific by combining it with a separation of the fluoride by microdiffusion. This has been demonstrated by analyzing biological material and comparing the results with the fluoride contents obtained with other methods.     

Selection of reference methods for the determination of selenium in biological materials

Summary Over the period 1985–1989 the European Community Bureau of Reference (BCR) has certified 7 biological reference materials for selenium with concentrations ranging from 0.03 to 10 mg/kg. Certification analyses were carried out by the most experienced laboratories in the Member States, using Fluorimetry, Hydride formation, combined with ICP or AAS, Electrothermal AAS with or without Zeeman-background correction, Instrumental or radiochemical NAA. Results from these exercises are available in a form suitable for estimating the experimental precision of the various methods as a function of concentration. With the same number of replicates for all analyses the logarithm of the observed standard deviations have constant variance, independent of concentration; for each method the absolute and relative standard deviations can therefore be estimated by means of non-linear regression. Analytical bias was monitored plotting results and their estimated standard deviations from individual laboratories versus certified confidence intervals. For concentrations of Se <0.3 mg/kg fluorimetry was selected as a reference method; above this level INAA is recommended; both are capable of producing unbiased results.     

A micromethod for the determination of selenium in tissues and biological fluids by single-test-tube fluorimetry

A sensitive single-test-tube procedure for the fluorimetric determination of ng quantities of selenium with diaminonaphthalene, from small samples of animal origin is described. Several parameters related to the nitric/perchloric/sulphuric acid digestion, subsequent reduction and piazselenol formation are studied using blood as the matrix. The detection limit is 0.45 ng Se. The within-series precision for blood and heart tissue is 4.2% and 2.3% and between series is 5.0% and 3.6%, respectively. Recovery of added selenite and selenomethionine to blood, heart tissue and urine ranges from 98–101%. The correlation coefficient between the proposed and an electrothermal atomic absorption spectrometric method for serum samples is 0.997. This procedure is especially suitable for serial operation with a daily (8-h) throughput of 25 samples in duplicate.     

Rapid one-step derivatization of Se(VI) to a piazselenol for the spectrofluorimetric determination of selenium in biological material

A spectrofluorimetric method for the determination of total selenium in biological samples has been developed. After oxidative destruction, the sample is reacted with a mixture of bromide and 2,3-diamino-1,4-dibromonaphthalene (Br₂-DAN) without pH adjustment. Selenium(VI) is rapidly reduced by bromide to Se(IV) which then forms 4,7-dibromo-5,6-benzopiazselenol (Br₂-DAN-Se). The conversion of Se(VI) to Br₂-DAN-Se is completed in 6 min at 100°C. The piazselenol is extracted to cyclohexane and the fluorescence measured at 577 nm with excitation at 518 nm. The procedure has been validated by determinations on reference materials with selenium concentrations ranging from 4 to 1460 ng g⁻¹. The advantages of the method are the fast reduction of Se(VI) by hydrobromic acid and the properties of Br₂-DAN, which permit the piazselenol formation to be carried out in a strongly acidic medium and at high temperature. The limit of detection is 0.6 ng g⁻¹ as estimated by three times the standard deviation of the blank determination (n = 7).

The rate of reduction of Se(VI) to Se(IV) in hydrobromic acid was investigated as a function of concentration and temperature. The new kinetic data together with previous kinetic and equilibrium data for the piazselenol formation were used to model and optimize the simultaneous reduction-derivatization procedure.     

The determination of selenium in biological material by thermal neutron activation analysis and atomic absorption spectrometry

Neutron activation analysis and atomic absorption spectrometry (graphite furnace) methods for the analysis of selenium in human tissue are described. The sensitivity (10–30 ng/sample), accuracy and precision are of the same order for both techniques and the choice can only be made on grounds of urgency or convenience. AAS should be chosen for the analysis of wet tissue or the urgent analysis of small numbers of dry tissue. NAA should be chosen for the analysis of large numbers of dry tissue samples where time is not important. The selenium concentration of human liver is shown to be in the region of 1 to 2 ppm (dry weight). Selenium may be lost from tissue during freeze drying if the samples are not maintained at −35°C.