An investigation of concentration variations of trace elements during storage of natural water samples

Contamination and adsorption of trace elements during storage of natural water samples in polyethylene containers have been studied. Variations of concentrations of 29 elements in pH=1.5 natural water samples stored in polyethylene bottles for 15 and 30 days respectively were determined by the method of freeze-drying preconcentration and NAA. The percentage adsorption loss of radioactive indicators⁶⁰Co,¹²⁴Sb,⁶⁵Zn,⁷⁵Se,¹³⁴Cs and⁴⁶Sc in the pH=1.5–8.0 reservoir water, tap water and distilled water stored for 1–2 months were also determined. The optimized storage condition for natural water is obtained.     

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/KBrO₃, 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.     

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.     

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.     

Selenium speciation by high-performance liquid chromatography with on-line detection by atomic absorption spectrometry

Several approaches to the determination of selenomethionine, selenocystine, selenite and selenate by high-performance liquid chromatography with online detection by atomic absorption spectrometry are described. The N?2,4-dinitrophenyl derivatives of selenomethionine, selenoethionine, selenocystine and phenylmercury(II) cystineselenoate were recovered from aqueous solution, separated on a Nucleosil 5-NO₂ reversed-phase HPLC column with a methanolic mobile phase containing acetic acid and triethylamine, and detected with a quartz thermochemical hydride-generating interface–atomic absorption spectrometry (AA) system. The restriction of having to perform chromatography with an organic mobile phase (to support the combusion process) was overcome with a new interface design capable of operation with either organic or aqueous HPLC mobile phases. Using aqueous acetic acid (0.015% v/v) containing 0.1% (w/v) ammonium acetate delivered at 0.5cm³ min^?1, selenate, selenite, selenomethionine, selenocystine and selenoethionine were separated virtually to baseline on a cyanopropyl-bonded phase HPLC column. Other selenium compounds which were investigated included methane seleninic and methane selenonic acids as well as the crude oxidation product mixtures resulting from the treatment of selenomethionine and selenocystine with hydrogen peroxide. A procedure for extracting selenate, selenite, selenomethionine, selenocystine and selenoethionine from spiked water or ground feed supplement into liquefied phenol resulted in acceptable recoveries for the latter four analytes but was unacceptably low for selenate.     

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.     

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.     

Long term stability of organic selenium species in aqueous solutions

Long term stability of organic selenium compounds (selenocystine, selenomethionine, trimethylselenonium ion) has been studied over a one year period for 2 analyte concentrations: 25 and 150 μg/L Se, at pH 4.5 in the dark, under different storage conditions: temperature of –20°C, 4°C, 20°C, 40°C; in Pyrex, Teflon, or polyethylene containers; in an aqueous matrix or in the presence of a chromatographic counter ion (pentyl sulfonate at 10^–4 mol/L concentration). Light effects have also been tested. The stability of the selenium species was monitored by HPLC-ICP/MS. Storage conditions can drastically alter the stability of organic selenium species. Organoselenium compounds were shown to be stable in the dark over a one year period in an aqueous matrix at pH 4.5 in Pyrex containers at both 4°C and 20°C. Pyrex vials exposed to natural sunlight at room temperature resulted in a steady decrease of the selenoamino acid concentration. Teflon containers caused losses of less than 25% at both 4° C and 20° C in the dark. However, polyethylene vials presented, at all temperatures tested, a rapid decrease of the TMSe⁺ concentration. The stability of the Se species studied did not show significant differences between 4° C and 20° C in any container material used. Storage of solutions at 40° C led to slight differences between the Pyrex and Teflon containers. However, polyethylene presented a drastic decrease of the three species over time at this higher temperature. Solutions frozen at –20° C in polyethylene vials did not stabilize the TMSe⁺ signal. Finally, concentrations and matrices of the samples did not significantly affect the stability of the species. Received: 15 July 1996 / Revised: 14 July 1997 / Accepted: 18 July 1997     

Comparison of ultrasound-assisted emulsification and dispersive liquid–liquid microextraction methods for the speciation of inorganic selenium in environmental water samples using low density extraction solvents

Herein, ultrasound-assisted emulsification microextraction (USAEME) and dispersive liquid–liquid microextraction (DLLME) methods based on applying low-density organic solvents have been critically compared for the speciation of inorganic selenium, Se(IV) (selenite) and Se(VI) (selenate) in environmental water samples by gas chromatography-flame ionization detection (GC-FID). At pH 2 and T = 75 °C for 7 min, only Se(IV) was able to form the piazselenol complex with 4-nitro-o-phenylenediamine. Piazselenol was extracted using an extraction solvent and was injected into a GC-FID instrument for the determination of Se(IV). Conveniently, Se(VI) remained in the aqueous phase. Total inorganic selenium was determined after the reduction of Se(VI) to Se(IV) and prior to the above procedures. The Se(VI) concentration was calculated as the difference between the measured total inorganic selenium and Se(IV) content. The effect of various experimental parameters on the efficiencies of the two methods and their optimum values were studied with the aid of response surface methodology and experimental design. Under the optimal conditions, the limit of detections (LODs) for Se(IV) obtained by USAEME-GC-FID and DLLME-GC-FID were 0.05 and 0.11 ng mL⁻¹, respectively. The relative standard deviations (RSDs, n = 6) for the measurement 10 ng mL⁻¹ of Se(IV) were 5.32% and 4.57% with the enrichment factors of 2491 and 1129 for USAEME-GC-FID and DLLME-GC-FID, respectively. Both methods were successfully applied to the analysis of inorganic selenium in different environmental water samples and certified reference material (NIST SRM 1643e).     

Development of an on-line UV decomposition system for direct coupling of liquid chromatography to atomic-fluorescence spectrometry for selenium speciation analysis

Speciation analysis of four selenium species (selenite, selenate, selenocystine, and selenomethionine) has been performed by on-line coupling of liquid chromatography (LC), UV decomposition, hydride generation (HG), and atomic-fluorescence spectrometry (AFS). Because only selenite (Se^IV) can generate hydrides, on-line conversion of organic and inorganic selenium species is discussed. Preliminary study showed that the use of only UV light was not sufficient to reduce selenate, because no absorption is observed for this compound at the main wavelength of the low-pressure mercury lamp (253.7 nm). Thus, new conditions based on addition of a reducing reagent (I⁻) were developed. Mechanisms of action are proposed to explain selenium species conversions. Because of their compatibility with on-line treatment, phosphate buffers were used for chromatographic separation on an anion exchange column (Hamilton PRP-X100). Detection limits (19–60 pg Se) and repeatability of the technique were close to those obtained by conventional quadrupole ICPMS. Applications to real samples such as water and oysters are presented and emphasize the robustness of the system.     

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.     

Determination of Selenium Species in Cordyceps militaris by High-performance Liquid Chromatography Coupled to Hydride Generation Atomic Fluorescence Spectrometry

The purpose of this work is to develop a high-efficiency extraction method for determining the selenium species in Cordyceps militaris. Six extraction solutions, including hot water, HCl, methanol–water, ammonium acetate, protease XIV, and protease K, combined with ultrasound-assisted extraction, were utilized in the measurements. The selenium species in the extracts were separated and characterized by high-performance liquid chromatography. Their concentrations were subsequently determined by hydride generation atomic fluorescence spectrometry. The 25?mM ammonium acetate was selected as the extraction solution due to its advantages in cost and efficiency. Validation was performed, and the selenium species recoveries were 69–97% for selenocystine, selenite, selenomethionine, and selenate with good linearity and precision. The major selenium species in C. militaris were selenocystine and selenomethionine that accounted for almost 73.1?±?1.6% of the total selenium.     

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.     

The stability of selected herbicides preconcentrated from estuarine river waters on solid-phase extraction disks

Summary The stability of atrazine, simazine, alachlor, metolachlor, and deethylatrazine on C₁₈ Empore disks has been determined. Estuarine water (100 mL) spiked at 3 g L^–1 with the target pesticide mixture was preconcentrated on the disks; the disks were then stored at –20°C, 4°C, and at room temperature for periods up to three months and were analyzed by gas chromatography with nitrogen-phosphorus detection. Complete recovery was observed after storage at –20°C throughout the period of the study. Losses up to maximum of 10% were observed after storage at 4°C. Higher losses (up to 24% for alachlor) occurred only at room temperature; the coefficient of variation for these determinations (8–11%) was also higher than that for the others (3–5%). The stability of the pesticides was dependent on the water matrix, on storage temperature, and on properties such as vapor pressure and water solubility.     

On-line pre-reduction of Se(VI) by thiourea for selenium speciation by hydride generation

In this study, thiourea (TU) was novelly developed as a reduction reagent for on-line pre-reduction of selenium(VI) before conventional hydride generation (HG) by KBH₄/NaOH–HCl. After TU on-line pre-reduction, the HG efficiency of Se(VI) has been greatly improved and because even higher than that of the same amount of Se(IV) obtained in the conventional HG system. The possible pre-reduction mechanism is discussed. The detection limit (DL) of selenate reaches 10 pg mL^− 1 when using on-line TU pre-reduction followed by HG atomic fluorescence detection. When TU pre-reduction followed by HG is used as an interface between ion-pair high performance liquid chromatography and atomic fluorescence spectrometry, selenocystine, selenomethionine, selenite and selenate can be measured simultaneously and quantitatively. The DLs of these are 0.06, 0.08, 0.05 and 0.04 ng mL^− 1, respectively, and the relative standard deviations of 9 duplicate runs for all the 4 species are less than 5%. Furthermore, it was successfully applied to Se speciation analysis of cultured garlic samples, and validated by determination of total selenium and selenium species in certified reference material NIST 1946.     

Influence of the storage conditions on the concentration of clenbuterol in cattle urine samples

A study of the stability of clenbuterol in cattle urine samples under different time and temperature storing conditions is reported. At the farm, urine samples were spiked with several amounts of this compound. Then, they were immediately transported to the laboratory and stored at room temperature, between 4 and 8 °C and frozen. Clenbuterol was quantified by using high performance liquid chromatography with diode-array detection (HPLC-PDA) on days 1, 7 and 42 after sample collection. The drug concentration in frozen urine samples was apparently stable during the whole studied period. However, there was a notable decrease in the concentration of this analyte when samples were stored at room temperature and between 4 and 8 °C, specially during the first week. A complementary study showed that the decrease in clenbuterol concentration was particularly significant from 1st to 4th day after sample collection and when urine specimens were kept at room temperature. Based on the results of this study, it was concluded that sample freezing as soon as possible after collection is the most adequate way to guarantee that the reliable concentration of clenbuterol in urine specimens remains unchanging until sample analysis.     

Saponin Stabilization via Progressive Freeze Concentration and Sterilization Treatment

Saponin is a biopesticide used to suppress the growth of the golden apple snail population. This study aims to determine the stabilized conditions for saponin storage. The maceration process was used for saponin extraction, and for saponin concentration, progressive freeze concentration (PFC) was used. Afterwards, stability analysis was performed by storing the sample for 21 days in two conditions: Room temperature (26 °C) and cold room (10 °C). The samples kept in a cold room were sterilized samples that undergo thermal treatment by placing the sample in the water bath. The non-sterilized samples were kept in room temperature condition for 21 days. The results showed that saponin stored in the cold room (sterilized sample) has low degradation with higher concentration than those stored at room temperature in stability analysis with the highest saponin concentration (0.730 mg/mL) at a concentration temperature of −6 °C and concentration time of 15 min. The lowest saponin concentration obtained by saponin stored at room temperature (non-sterilized sample) is 0.025 mg/mL at a concentration temperature of −6 °C and concentration time of 10 min. Thus, the finding concluded that saponin is sensitive to temperature. Hence, the best storage condition to store saponin after thermal treatment is to keep it in a cold room at 10 °C.     

Capabilities of mixed-mode liquid chromatography coupled to inductively coupled plasma mass spectrometry for the simultaneous speciation analysis of inorganic and organically-bound selenium

This work investigates for the first time the potential of mixed-mode (anion-exchange with reversed-phase) high performance liquid chromatography coupled to inductively coupled plasma mass spectrometry (ICP-MS) for the simultaneous retention and selective separation of a range of inorganic and organically-bound selenium (Se) species. Baseline separation and detection of selenocystine (SeCys₂), Se-methyl-selenocysteine (SeMC), selenomethionine (SeMet), methylseleninic acid (MSA), selenite, γ-glutamyl-methyl-selenocysteine (γ-glutamyl-SeMC), and selenate in a Se standard mixture by mixed-mode HPLC-ICP-MS was achieved by switching between two citrate mobile phases of different pH and ionic strength within a single chromatographic run of 20 min. Limits of detection obtained for these Se species ranged from 80 ng kg^?1 (for SeMC) to 123 ng kg^?1 (for selenate). Using this approach as developed for selenium speciation, an adequate separation of inorganic and organic As compounds was also achieved. These include arsenite, arsenate, arsenobetaine (AsB) and dimethylarsenic acid (DMA), which may coexist with Se species in biological samples. Application of the newly proposed methodology to the investigation of the elemental species distribution in watercress (used as the model sample) after enzymatic hydrolysis or leaching in water by accelerated solvent extraction (ASE) was addressed. Only SeMet, SeMC and selenate could be tentatively identified in watercress extracts by mixed-mode HPLC-ICP-MS and retention time matching with standards. Recoveries (n = 3) of these Se species from samples spiked with standards averaged 102% (for SeMC), 94.9% (for SeMet) and 98.3% (for selenate). Verification of the presence of SeMet and SeMC in an enzymatic watercress extract was achieved by on-line HPLC-ESI MS/MS in selected reaction monitoring (SRM) mode.     

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.     

Determination of selenite and selenate in drinking water: a fully automatic on-line separation/pre-concentration system coupled to electrothermal atomic spectrometry with permanent chemical modifiers

A time-based flow injection (FI) separation pre-concentration system coupled to an electrothermal atomic absorption spectrometer (graphite furnace) has been developed for the direct ultra-trace determination of selenite and selenate in drinking water. The pre-concentration of both forms of selenium is carried out onto a micro-column packed with an anionic resin (Dowex 1X8) that is placed in the robotic arm of the autosampling device. Selenite and selenate are sequentially eluted with HCl 0.1 M and HCl 4 M, respectively. The interference of large quantities of chloride during selenium atomisation is prevented by using iridium as a “permanent” chemical modifier. The features of the pre-concentration separation system for both species are: 53% efficiency of retention and an enhancement factor of 82 for a pre-concentration time of 180 s (sample flow rate=3 ml min⁻¹) with HCl elution volumes of 100 μl. The detection limit (3 s) is 10 ng l⁻¹ for the two species and the relative standard deviation (n=10) at the 200 ng l⁻¹ level is 3.5% for selenite and 5.6% for selenate. The addition of selenite and selenate stock standard solutions to tap water samples yields a 97-103% recovery of both species.