Stability of total selenium and selenium species in lyophilised oysters and in their enzymatic extracts

To obtain reliable information on speciation analysis it is necessary to previously evaluate the stability of the species in the sample of interest. Furthermore, in those cases in which sample treatment to extract the species is time-consuming, an evaluation of how to maintain species integrity in the extracts is paramount. Thus, the present paper reports the stability of total Se, SeMet and TMSe+ in freeze-dried oyster and in the enzymatic extracts stored in Pyrex and polyethylene containers at different temperatures (-18, 4 and 20 degrees C). Total selenium determinations and Se speciation were carried out by HG-AAS after acid digestion in a microwave oven and by on-line coupling of cation exchange HPLC-ICP-MS after enzymatic hydrolysis, respectively.The results obtained for the freeze-dried sample showed that total Se and the selenium species evaluated are stable for at least 12 months, under all the conditions tested. However, Se species in the enzymatic extracts are only stable for 10 days if stored at 4 degrees C in Pyrex containers. These results show that the extracts do not necessarily have to be analysed just after sample treatment.     

The preparation of a certified reference material of polar pesticides in freeze-dried water (CRM 606)

The preparation of a certified reference material of polar pesticides in freeze-dried water is described. The pesticides selected were atrazine, simazine, carbaryl, propanil, linuron, fenamiphos and permethrin which were added to 6000 litres of tap water at 50–80 μg · L^–1 (200–320 μg · L^–1 for permethrin) level in presence of NaCl (2.5 g · L^–1) prior lyophilization. After the freeze-drying process the residue was rehomogenized, filled into amber glass bottles and stored at –20 °C, +4 °C and +20 °C. All pesticides were determined by HPLC/diode array detector, except permethrin which was determined by GC/ECD. The results obtained for atrazine, simazine, carbaryl, propanil, linuron and fenamiphos showed no within- or between-bottle inhomogeneity, however the material was non-homogeneous for permethrin and therefore this was withdrawn from further studies. With respect to the stability for over one year, all pesticides were stable at –20 °C. At +4 °C all pesticides were stable for at least 9 months and at +20 °C the stability was demonstrated only during the first month of storage. The content (mass fractions) of atrazine, simazine, carbaryl, propanil and linuron in freeze-dried water (CRM 606) was certified by an interlaboratory testing and a certification campaign. Received: 7 September 1998 / Revised: 13 November 1998 / Accepted: 21 November 1998     

Certification of total arsenic, dimethylarsinic acid and arsenobetaine contents in a tuna fish powder (BCR-CRM 627)

Tuna fishes were collected in the Straits of Messina (Italy), were dissected and dorsal muscles minced, freeze-dried, ground and sieved. The obtained powder was stabilised by γ-irradiation and filled into brown glass bottles. The homogeneity and stability at +20 °C and +40 °C were verified with regards to the total arsenic, dimethylarsinic acid and arsenobetaine contents. Ten laboratories participated in the certification study. All participants had demonstrated beforehand their ability to produce accurate results for the As speciation in fish tissue. The certified values are: total arsenic (4.8 ± 0.3) mg/kg, arsenobetaine (52 ± 3) μmol/kg, dimethylarsinic acid (2.0 ± 0.3) μmol/kg. The material is available from the BCR since early 1998. Received: 31 March 1998 / Revised: 20 July 1998 / Accepted: 25 July 1998     

The determination of total Se in urine and serum by graphite furnace atomic absorption spectrometry using Ir as permanent modifier and in situ oxidation for complete trimethylselenonium recovery

The present work evaluated the use of iridium (Ir) as permanent modifier for the determination of total selenium in urine and serum by graphite furnace atomic absorption spectrometry. Concerning urine, the presence of trimethylselenonium (TMSe⁺) was especially considered. Pyrolysis and atomization temperatures of 1,000 and 2,100°C, respectively, were used. For nondigested urine and serum samples, 0.2% v/v HNO₃ and Triton X-100 were used as diluents, respectively, and the same initial platform Ir treatment was effective for up to 1,100 atomization cycles. Good precision [less than 5% relative standard deviation (RSD)] can be achieved with the proposed method. Low TMSe⁺ recovery was observed for nondigested urine samples. Thus, if this species is to be considered in urine analysis, a previous external mineralization step was found to be necessary. Alternatively, an in situ oxidation treatment was developed. Detection limits of 8, 10, and 7 μg l⁻¹ were obtained after dilution, microwave-assisted digestion, and in situ oxidation procedures, respectively. The accuracy of the method was validated by the analysis of certified reference or commercial quality control materials and spiked samples.     

Evaluation of on-line coupling size exclusion chromatography electrothermal atomic absorption spectrometry for selenium speciation

 Chromatographic effluents were on-line analyzed by Zeeman-ETAAS, using a flow-through cell placed in a graphite furnace autosampler as interface. To obtain high sampling rates, the use of fast graphite furnace programmes was studied. Conventional programmes of 96 s were reduced to 18 s by using hot injection (120 °C) and reducing the charring step to 2 s. The increase of the injection volume from 20 to 60 μl lengthened the programme to 46 s. Nickel had to be added to get a comparable response for both inorganic and organic selenium species (selenite and selenomethionine) and to reduce the interferent effect produced in presence of the chromatographic eluent (TRIS 0.01 mol l^-1, NH₄NO₃ 0.1 mol l^-1, pH 7). The optimized conditions were applied to the speciation of selenium in human erythrocyte lysates by size exclusion LC-ETAAS. Using a high performance size exclusion column selenium could be assigned to proteins of 100 and 35 kDa. Detection limits in the range of 1 ng (2 μg l^-1 for 500 μl injection volume) were obtained for the combined technique. Received: 9 October 1996/Revised: 8 July 1996/Accepted: 14 July 1996     

An anion exchange method for the determination of241Am and plutonium in environmental and biological samples

Irradiation and post-irradiation losses in 7 crude oils encapsulated in polyethylene and irradiated in a flux of 10¹²n·cm⁻² sec⁻¹ have been thoroughly studied. The fraction of mercury released during irradiations ranging from 10 minutes to 2 hours is proportional to the integrated flux received by the samples and does not differ substantially from one oil to another. Post-irradiation loss rates at 20°C range between 0.017 and 0.027%·hr⁻¹. Storage at−20°C reduces them by at least a factor of 2. The influence on the activity of¹⁹⁷Hg due to losses in sample weight occuring during the post-irradiation period has been examined. The fraction of radiomercury retained inside the walls of the irradiation vials versus the irradiation time has been assessed. Contamination of an inner irradiation site following prolonged irradiations of oils containing 150 μg/g of this element has been evaluated and found to have a negligible impact for trace measurements above the ng/g level.     

Determination of the hydride concentration in alkyldiboranes(6) by 11B NMR spectroscopy – Reduction of inorganic selenium and tellurium compounds

As an alternative to volumetric analysis, ¹¹B NMR spectroscopy can be used to determine the hydride concentration and the relative amount of different active hydride species in mixtures of alkyldiboranes(6) (alkyl = Et, Pr). Inorganic selenium(IV) compounds react with propyldiboranes(6) at first to give elemental selenium (hydride number HN = 4) and after prolonged heating at 130 °C to selenides (HN = 6). In contrast, tellurium(IV) and tellurium(VI) compounds are reduced only to elemental tellurium. Received: 31 March 1998 / Accepted: 26 April 1998     

Industrial production and purification of <Superscript>32</Superscript>P by sulfur irradiation with partially moderated neutron fluxes and target melting

Target purification of S_α is carried out by distillation at 444±2 °C under N atmosphere and diluting the vapors in CS₂. The solution is filtered through fiberglass, Teflon and cellulose to obtain S_α by CS₂ evaporation. Once 30 g of this target are irradiated with fast neutron fluxes from 4.5 to 7.4·10¹² n·cm⁻²s⁻¹ from 6 to 12 hours, the nuclear reaction ³²S(n,p)³²P takes place. So, the irradiated S_α sample is placed in a Pyrex container situated inside a furnace as the most important piece of equipment in one aluminum and Lucite glove box. The distillation of irradiated sulfur takes place at 444±2 °C under N atmosphere during 1–2 hours. The vapors are connected to a sulfur diluter containing 20% CS₂ aqueous solution, followed by an activated carbon filter and the two similar additional sulfur diluters. Once cooled, the distillation chamber keeps the radioactive, carrier-free ³²P stuck to the wall. Then 25–50 ml of 0.1N HCl acid was injected by suction and heated again at 110±2 °C during 1 hour. The corresponding chemical reaction takes place and the labeled H₃ ³²PO₄ solution is produced. In such a way, industrial production of ³²P labeled molecules has started in Mexico, with an initial production of 3700–5550 MBq per week.     

Preservation and stability of inorganic selenium compounds at ppb levels in water samples

The stability of inorganic selenium(IV) and selenium(VI) species at levels of 1 and 10μg l^-1 has been studied under various conditions of pH, type of water and type of container. Polyethylene containers and adjustment to pH 1.5 provide optimum conditions of preservation for both distilled and natural water samples up to 125 days. Algal growth is detrimental to solution stability at natural pH values of 5.4–7.2, but adjustment to pH 1.5 with sulfuric acid successfully avoids this effect. Container size and temperature are also discussed. Storage of samples at 4°C gives satisfactory stability but is less practicable. The essential nature of preservation techniques applicable to interlaboratory quality control studies is emphasized.     

Quantitative selenium speciation in human urine by using liquid chromatography–electrospray tandem mass spectrometry

A liquid chromatography–electrospray-tandem mass spectrometry (ES-MS/MS) method was developed for the speciation analysis of four organic selenium species of relevance to human urinary metabolism, namely trimethylselenomium ion (TMSe⁺), selenomethionine (SeMet) and the two selenosugars, methyl 2-acetamido-2-deoxy-1-seleno-β-d-galactos/-glucos-amine (SeGalNAc and SeGluNAc, respectively). Their chromatographic separation was achieved by using a cation exchange pre-column coupled in-series with a reversed-phase high-performance liquid chromatography column, along with an isocratic mobile phase. Online detection was performed using ES-MS/MS in selective reaction monitoring mode. SeGalNAc was detected as the major human urinary metabolite of selenium in the samples analysed, whereas TMSe⁺ was detected in the urine of one volunteer before and after receiving a selenium supplement. SeMet was not detected as a urine excretory metabolite in this study. Spiking experiments performed with the urine samples revealed significant signal suppression caused by coeluting matrix constituents. To overcome such interferences, isotopically labelled ¹³CD₃⁸²SeGalNAc was used as an internal standard, whereas in the absence of an isotopically labelled internal standard for TMSe⁺, the standard addition method was applied. Quality control for the accurate quantitation of TMSe⁺ and SeGalNAc was carried out by analysing spiked human urine samples with appropriate selenium standards over a concentration range of 10–50 μg Se L⁻¹. The method has achieved a limit of detection in the presence of urine matrix comparable to that of HPLC-inductively coupled plasma-mass spectrometry for the four selenium species: 1.0 μg Se L⁻¹ for TMSe⁺, 5.6 μg Se L⁻¹ for SeMet, and 0.1 μg Se L⁻¹ for both SeGalNAc and SeGluNAc.     

Morphology and electric conductivity of cross-linked polyethylene–carbon black blends prepared by gelation/ crystallization from solutions

 Ultra-high-molecular-weight polyethylene (UHMWPE) – carbon black (CB) blends were prepared by gelation/ crystallization from PE dilute solutions containing CB particles. The UHMWPE/CB composition chosen were 1/0.15, 1/0.25, 1/0.5, 1/0.75, 1/1, 1/3, 1/5, and 1/9, etc. The cross-linking of PE chains was performed by chemical reaction of dicumyl-peroxide at 160 ^°C. X-ray diffraction patterns indicate that the crystallinity of PE within the blends decreased drastically through the chemical reaction at high temperature. The sample preparation method by gelation/crystallization provided the UHMWPE–CB system with various CB contents up to 90% and the conductivities for the resultant specimens were in the range from 10^-9 to 1 Ω^-1 cm^-1 corresponding to the electric conductivity range of semiconductors. The blends assured thermal stability of electric conductivity by cross-linking of PE chains, although the mechanical property such as the storage and loss moduli were very sensitive to temperature. The conductivity of the blends with CB content ≥20% were almost independent of temperature up to 220 ^°C and the values in the heating and cooling processes were almost the same. On the other hand, for the UHMWPE–CB blends with 13% CB content corresponding to the critical one, temperature dependence of electric resistivity showed positive temperature coefficient (PTC) effect. The PTC intensities for non-cross-linked and cross-linked materials were lower than that of the corresponding low-molecular-weight-polyethylene (LMWPE)–CB blend but the maximum peak appeared at 160 ^°C which is higher than the peak temperature of LMWPE–CB blend. Received: 10 December 1997 Accepted: 9 April 1998     

Separation of organic and inorganic arsenic species by HPLC-ICP-MS

The HPLC separation of eight anionic, cationic or neutral arsenic species (arsenite, arsenate, monomethylarsonic acid, dimethylarsinic acid, arsenobetaine, arsenocholine, trimethylarsine oxide and tetramethylarsonium ion) on a high-capacity, anion-exchange column (Ion Pac AS 7, Dionex) was studied. The separation was performed during one run with a nitric acid gradient ranging from pH 4–1.3. The influence of sodium dodecyl sulfate (SDS), sodium octyl sulfate (SOS) and 1,2-benzenedisulfonic acid (BDSA) as ion pairing eluent modifiers was investigated. In addition the effect of elevated temperatures (30 to 40 °C) was studied. The best results were obtained at room temperature of 20 °C with 0.05 mM benzenedisulfonic acid as the eluent modifier. The chromatograph was connected to an ICP-MS via a cross-flow nebulizer. Detection limits obtained with the optimized chromatographic separation were 0.16–0.60 μg As L^–1 for different species. The proposed speciation method was applied to the determination of arsenic species in the DORM-2 reference material (Dogfish Muscle) and in aqueous extracts of mushrooms collected on arsenic contaminated ground. Received: 3 August 1998 / Revised: 17 September 1998 / Accepted: 21 September 1998     

Retention study of inorganic and organic selenium compounds on a silica-based reversed phase column with mixed ion-pairing reagents

Summary Separation of seven inorganic and organic selenium compounds, namely selenic acid [Se(VI)], selenous acid [Se(IV)], trimethylselenonium iodide (TMSe⁺), selenocystine (SeCys), selenomethionine (SeMet), selenoethionine (Seet), and selenocystamine (SeCM), has been performed on a LiChrosorb C 18 column by using mixed ion-pair reagents; 1-butanesulfonic acid and tetramethylammonium hydroxide. Flame atomic absorption spectrometry (FAAS) was used as an element-specific detector. The retention behaviors of selenium compounds in terms of several chromatographic parameters, such as pH of the mobile phase, the concentrations of ion-pair reagents, and the content of organic modifier (methanol) were investigated. It was found that the separation of both inorganic and organic selenium compounds can be achieved within 12 min with a mobile phase of 10 mM 1-butanesulfonic acid −4 mM tetramethylammonium hydroxide −4 mM malonic acid −0.05% methanol adjusted to pH 4.5 at a flow rate of 1.0 mL min⁻¹. The results obtained in this study showed that the use of mixed ion-pair reagents is very useful to improve the separation of selenium compounds. The applicability of this technique for the speciation of selenium compounds in real samples was demonstrated by the determination of selenium compounds in a selenium nutritional supplement. The results were found to be in good agreement with those obtained by ion-exchange HPLC-ICP-MS.     

Determination of selenium in seawater by Zeeman GFAAS using nickel plus NH4NO3 modifier

The effectiveness of nickel plus NH₄NO₃ as a chemical modifier for the determination of selenium in seawater by Zeeman electrothermal atomic absorption spectrometry has been evaluated. The effect of changing the modifier mass and pyrolysis hold time on the integrated absorbance of selenium and the background absorbance has been investigated. Nickel and NH₄NO₃ allow the quantitative stabilization of selenium in the seawater matrix up to 1300° C as compared with 600° C without modifier. The modifier further reduces the background absorbance caused by seawater. The tolerable amounts of various inteferences such as chloride, sulphate and phosphorus in the presence of nickel plus NH₄NO₃ are evaluated for the determination of selenium in seawater. Received: 31 August 1995 / Revised: 21 December 1995 / Accepted: 6 January 1996     

Field determination of molybdenum in soils

A new procedure for the field characterization of the available amount of molybdenum in soil extract is described. It is based on the preconcentration of the Mo(V)-SCN^– complex into toluene solution of N-octylacetamide (OAA) and subsequent heating of the extract for ≈ 10 min at 60° C over a water bath. The molar absorptivity of the complex is (3.50) × 10⁴ 1 mole^–1 cm^–1 at λ_max 470 nm. The detection limit of the method is 4 ppb Mo. It provides a significantly increased tolerance limit for iron (up to 1000 ppm) and is applicable to the field characterization of Mo in soil extracts. Received: 29 May 1996 / Revised: 17 July 1996 / Accepted: 19 July 1996     

Selenium determination in tissue samples of Nile tilapia using ultrasound-assisted extraction

This paper proposes a method to determine selenium in samples of fish muscle and liver tissue using ultrasound assisted extraction process, and analysed by graphite furnace atomic absorption spectrometry (GFAAS). The selenium content was extracted by 0.10 M HCl at the optimal extraction conditions which were established as follows: sample mass of 100 mg; granulometry of the sample <60 μm; sonication time of five 40 s cycles; and sonication power of 136 W. The selenium determinations were performed by GFAAS, at a drying temperature of 120°C/250°C, pyrolysis temperature of 1300°C, atomization temperature of 2300°C, and cleaning temperature of 2800°C. Palladium nitrate was used as a chemical modifier coinjected with the samples, and tungsten as a permanent modifier. The concentration of selenium determined in the pool of fish muscle and liver tissue were 280.4±4.2 e 592.3±6.7 μg kg⁻¹, respectively. The accuracy and precision of the proposed extraction method were evaluated using certified standard Bovine Muscle — NIST 8414. The results obtained by the ultrasonic extraction method were equivalent to those obtained by the method of acid mineralization of samples in a microwave oven     

Influence of water on the dielectric behaviour of chitosan films

 The dielectric properties of chitosan films with a degree of deacetylation of 86% have been investigated in the frequency range of 10^3–10⁶ Hz covering a broad range of temperatures from −150 to 150 °C. The variation of the dielectric pro-perties with temperature has been associated with two dielectric processes: (a) a local relaxation attributed to the presence of hydrogen-bonded water appearing at low temperatures (b) a conduction process related to water molecules which becomes desorbed upon heating at T>80 °C. Isothermal dielectric experiments have been performed in order to follow, in real time, the occurrence of both, the water sorption and desorption processes. Received: 20 June 1996 Accepted: 19 November 1996     

Investigation of water diffusion into quartz using ion beam analysis techniques

Diffusion of water into quartz was studied by measuring H and ¹⁸O concentration profiles in surface layers of quartz samples treated hydrothermally in the range of 125° C to 200° C. Sample surfaces were orientated normal to the c-axis. The measurements were performed using the nuclear reactions ¹H(¹⁵N,αγ)¹²C and ¹⁸O(p,α)¹⁵N. The diffusion profiles have widths of up to 500 nm. Diffusion rate constants derived from the profiles are in the range of 10^–15 cm²/s to 10^–18 cm²/s and show a distinct temperature dependence, yielding a rough estimate of about 60 kJ/mole for the activation energy. Received: 24 June 1996 / Revised: 20 January 1997 / Accepted: 22 January 1997     

Observation of nanoparticles and film formation by scanning tunneling microscopy: methyl methacrylate/butyl methacrylate nanoparticles prepared by microemulsion polymerization

 Latexes as dispersions of poly(methyl methacrylate-co-butyl methacrylate) copolymeric nanoparticles within water were produced by microemulsion polymerization of the respective comonomers. Polymer yield, number-average and weight-average molecular weights, polydispersity index, and the glass-transition temperature of the copolymer produced were 50%, 8.8 × 10⁴, 2.54 × 10⁵, 2.87, and 45 °C. Scanning tunneling microscopy (STM) images of the latex nanoparticles and film formation on highly oriented pyrolitic graphite (HOPG) were obtained with a 2 V sample bias and a tunneling current of 20 pA. The STM pictures revealed that the particle size was 18 ± 3 nm. There was no film formation in the case of dehydration at room temperature. There was some coalesence of particles when the HOPG surface was preheated at 55 °C, while complete film formation was achieved when the latexes were annealed at 55 °C in an oven for about 10 min. Received: 23 August 1999 Accepted: 17 January 2000     

TEM and SNMS studies on the oxidation behaviour of NiCrAlY-based coatings

The effect of minor Ti additions content (0% Ti, 0.4% Ti, 1% Ti, 2% Ti) on the oxidation behaviour of Ni-20Cr-10Al-0.4Y (in weight-%) model alloys was investigated in the temperature range 950° C to 1100° C up to 200 h in Ar – 20% O₂. Alloy microstructure, oxide scale morphology and microstructure of the scale were characterized by SEM/EDX and TEM. The growth mechanisms of the alumina scales formed on the model alloys were studied by two-stage oxidation experiments with ¹⁸O₂-tracer and subsequent SNMS-analyses. The microstructural observations were correlated with the oxide scale properties in respect to growth rates and spalling resistance, which was tested during cyclic oxidation. Received: 24 June 1996 / Accepted: 18 November 1996