Speciation of volatile selenium species in plants using gas chromatography/inductively coupled plasma mass spectrometry

Gas chromatography/inductively coupled plasma mass spectrometry (GC/ICP-MS) coupled with solid phase micro-extraction can provide a simple, extremely selective and sensitive technique for the analysis of volatile sulfur and selenium compounds in the headspace of growing plants. In this work, the technique was used to evaluate the volatilization of selenium in wild-type and genetically-modified Brassica juncea seedlings. By converting toxic inorganic selenium in the soil to less toxic, volatile organic selenium, B. juncea might be useful in bioremediation of selenium contaminated soil.

Retention of Cr(III) by high-performance chelation ion chromatography interfaced to inductively-coupled plasma mass spectrometric detection with collision cell

High-performance chelation ion chromatography (HPCIC) was employed to retain cationic Cr(III) on an anion-exchange column and hence allow the separation of the two most prevalent forms of chromium, Cr(II) and Cr(VI). A mobile phase of nitric acid was utilized at pH = 1.5; additionally, 2,6-pyridinedicarboxylic acid was used at a concentration of 6 mM. Additives with different structural characteristics were used in an effort to elucidate retention mechanisms. Inductively-coupled plasma mass spectrometry was used for chromium detection. A collision cell was utilized to reduce chloride-based polyatomic ions that may interfere with the detection of Cr(III), and a detection limit study yielded levels in the low part-per-billion range. The newly developed method was applied to the chromatographic analysis of samples of an incubation medium containing Cr(VI) incubated with cell nuclei.

Plasma spectrometry for elemental speciation and characterization in beverages

A review is presented on recent applications of plasma spectrometric techniques for speciation analysis in beverages. The 3 most common ways of characterizing beverages in addition to elemental speciation are also discussed: total element analysis, isotope ratio, and statistical characterization. To date, most of the elemental characterization in beverages with plasma techniques has been performed in the statistical evaluation of wines and in teas. However, despite the increasing interest in elemental speciation, most of the plasma-based techniques in beverage studies are used for total metal detection and the full capabilities of inductively coupled plasma techniques have yet to be realized.

Identification of selenium species in urine by ion-pairing HPLC-ICP-MS using laboratory-synthesized standards

This study focused on the detection/identification of possible selenium metabolites in human urine. Organoselenium compounds not commercially unavailable were synthesized and characterized by electrospray mass spectrometry. Separation of selenomethionine, methylselenomethionine, trimethylselonium, selenoethionine, and selenoadenosylmethionine was achieved by ion-pairing HPLC with a mobile phase of 2 mmol L(-1) hexanesulfonic acid, 0.4% acetic acid, 0.2% triethanolamine (pH 2.5), and 5% methanol. The column effluent was introduced on-line to inductively coupled plasma-mass spectrometry for selenium-specific detection ((77)Se and (78)Se). For selenium speciation in urine, solid-phase extraction was carried out using C(18) cartridges modified with hexanesulfonic acid. Selective retention of cationic species was observed from acidified urine (perchloric acid, pH 2.0). After elution with methanol, evaporation, and dissolution in the mobile phase, the sample was introduced to the HPLC-ICP-MS system and the chromatographic peaks were assigned by adding standards. The species identified in urine were selenomethionine, trimethylselonium ion, and selenoadenosylmethionine. The last species was detected for the first time and our results suggest that selenomethionine might enter the metabolic pathway of its sulfur analog in the activated methylation cycle.

Elemental speciation studies--new directions for trace metal analysis

Elemental speciation is reviewed as a new approach to trace metals analysis. The importance of metal speciation relative to total metal analysis is considered. Various types of chromatographic separation methods using inductively coupled plasma-mass spectrometry (ICP-MS) detection are discussed. A brief introduction to the instrumental techniques is given, as well a discussion of coupling the instrumentation. Various chromatographies are included with an emphasis on liquid chromatography (LC). Gas chromatography (GC) and capillary electrophoresis (CE) are also discussed. Application examples contrast and compare the advantages and disadvantages of ICP- MS with other detection methods. Summary tables are included for two of the LC methods.

Studies of selenium-containing volatiles in roasted coffee

Coffee has been an important and heavily used beverage in many cultures over a long period of time. Although sulfur species have been found to be abundant constituents, no work to date has explored the presence of selenium analogues. Investigation of volatile selenium species from green coffee beans, roasted beans, and brewed coffee drink was performed using solid phase microextraction (SPME) sample preconcentration in conjunction with GC/ICP-MS. Several volatile selenium species at trace levels were detected from roasted coffee beans as well as in the steam from brewed coffee drinks. No detectable selenium (and sulfur) species, however, were found in the headspace of green beans, indicating that selenium-containing volatiles are formed during roasting, as is the case for the sulfur volatiles. Matching standards were prepared and used to identify the compounds found in coffee. Artificial supplementation of the green coffee beans with selenium before roasting was performed to further characterize the selenium-containing volatiles formed during the coffee-roasting process.

Liquid chromatography-inductively coupled plasma mass spectrometry

It is known that while many elements are considered essential to human health, many others can be toxic. However, because the intake, accumulation, transport, storage and interaction of these different metals and metalloids in nature is strongly influenced by their specific elemental form, complete characterization of the element is essential when assessing its benefits and/or risk. Consequently, interest has grown rapidly in determining oxidation state, chemical ligand association, and complex forms of a many different elements. Elemental speciation, or the analyses that lead to determining the distribution of an element's particular chemical species in a sample, typically involves the coupling of a separation technique and an element specific detector. A large number of methods have been developed which utilize a multitude of different separation mechanisms and detection instruments. Yet, because of its versatility, robustness, sensitivity and multi-elemental capabilities, the coupling of liquid chromatography to inductively coupled plasma mass spectrometry (LC-ICP-MS) has become one of the most popular techniques for elemental speciation studies. This review focuses on the basic principles of LC-ICP-MS, its historical development and the many ways in which this technique can be applied. Different liquid chromatography separations are discussed as well as the factors that must be considered when coupling each to ICP-MS. Recent applications of LC-ICP-MS to the speciation of environmental, biological and clinical samples are also presented.

Sequential extractions of selenium soils from Stewart Lake: total selenium and speciation measurements with ICP-MS detection

Different techniques have been employed in order to evaluate the most efficient procedure for the extraction of selenium from soil as required for speciation. Selenium contaminated sediments from Stewart Lake Wetland, California were used. A strong acid mineralization of the samples gives quantitative total selenium, which is then used to estimate recoveries for the milder extraction methods. The different extraction methodologies involve the sequential use of water, buffer (phosphate, pH 7) and either acid solution (e.g. HNO3 or HCl) or basic solutions (e.g. ammonium acetate, NaOH or TMAH). Pyrophosphate extraction was also evaluated and showed that selenium was not associated with humic acids. The extractants were subsequently analyzed by size exclusion chromatography (SEC) with UV (254 and 400 nm) and on-line ICP-MS detection; anion exchange chromatography, and ion-pair reversed phase chromatography with ICP-MS detection. For sequential extractions the extraction efficiencies showed that the basic extractions were more efficient than the acidic. The difference between the acidic and the basic extraction efficiency is carried to the sulfite extraction, suggesting that whatever is not extracted by the acid is subsequently extracted by the sulfite. The species identified with the different chromatographies were selenate, selenite, elemental selenium and some organic selenium.

Trace humic and fulvic acid determination in natural water by cloud point extraction/preconcentration using non-ionic and cationic surfactants with FI-UV detection

A preconcentration and determination method for humic and fulvic acids at trace levels in natural water samples was developed. Cloud point extraction was successfully employed for the preconcentration of humic acid (HA) and fulvic acid (FA) prior to the determination by using a flow injection (FI) system coupled to a spectrophotometric UV-Vis detector. The quantitative extraction of HA and FA within the pH range 1-12 was obtained by neutralization of the anionic charge on the humic substances with a cationic surfactant, hexadecyltrimethylammonium bromide (CTAB). This generated a hydrophobic species that was subsequently incorporated (solubilized) into the micelles of a non-ionic surfactant polyethylene glycol, tert-octylphenyl ether (Triton X-114). The FI method for HA and FA determination was developed by injection of 100 microl of the extracted surfactant-rich phase using an HPLC pump with spectrophotometric detection at 350 nm. A 50 ml sample solution preconcentration allowed an enrichment factor of 167. The limit of detection (LOD) obtained under the optimal conditions was 5 microg l(-1). The precision for ten replicate determinations at 0.2 mg l(-1) HA was 3.1% relative standard deviation (RSD), calculated from the peak heights. The calibration using the preconcentration system for HA and FA was linear with a correlation coefficient (r2) of 0.9997 at levels near the detection limits up to at least 1 mg l(-1). The method was successfully applied to the determination of HA and FA in natural water samples (river water).

Effect of metal ions on the molecular weight distribution of humic substances derived from municipal compost: ultrafiltration and size exclusion chromatography with spectrophotometric and inductively coupled plasma-MS detection

The effect of metal ions (Co, Cu, Ni, Pb, Zn) on the molecular weight distribution of humic substances (HSs) obtained from compost is studied. We believe this is the first of this type of study applied in this way to humic substances. Size exclusion chromatography is coupled with two on-line detection systems (spectrophotometric and ICPMS) to study the binding of metal ions by humic substances leached from compost. ICPMS provided highly specific, sensitive, and multielement analytical information that enabled obtaining direct experimental evidence for the participation of metal ions in molecular size distributions of humic compounds. The compost extract or its high molecular weight fraction (>5,000) was put in contact with EDTA or citrate ions, thereby competing with HSs for binding metals. The experiments were carried out by varying the pH maintained by Tris-HCl or CAPS buffer (pH 8.0 and 10.3) and keeping the ionic strength constant. The elution profile of humic substances using UV/ visible detection was compared with those from ICPMS detection of Co, Cu, Ni, Pb, and Zn in the same chromatographic runs. The results obtained suggested that both bridging between small molecules and complexation/ chelation by individual molecules are involved in metal ion binding to humic substances. The use of ICPMS to study the role of metal ions in aggregation/disassociation of humic substances proposed in this work is promising. Coupling element-specific detection with SEC or other separation systems allows better understanding of the mobility and bioaccessibility of elemental species in the environment and further elucidation of the dissolved humic structure.

Hydrolysis of proteins with methanesulfonic acid for improved HPLC-ICP-MS determination of seleno-methionine in yeast and nuts

In this work, the use of methanesulfonic acid for protein hydrolysis is proposed for evaluation of Se-methionine in yeast, Brazil nuts, and possibly other selenium-rich biological samples. The hydrolysis was carried out by heating the sample with 4 mol L(-1) acid at reflux for 8 h. Two chromatographic techniques (size-exclusion and ion-pairing) coupled with ICP-MS detection were used to compare the release of Se-methionine from proteins by enzymatic (proteinase K, protease XIV) and acid hydrolyses. A more efficient liberation of Se-methionine was observed by acid hydrolysis. For quantification, the sample extracts were introduced onto a C8 Alltima column, and the separation was achieved with a mobile phase containing 5 mmol L(-1) hexanesulfonic acid in citrate buffer (pH 4.5)/methanol (95:5). The results obtained by standard addition showed 816+/-17 micro g g(-1) and 36.2+/-1.5 micro g g(-1) of selenium in the form of Se-methionine in yeast and nuts, respectively (65% and 75% of total selenium).

SEC-ICP-MS studies for elements binding to different molecular weight fractions of humic substances in compost extract obtained from urban solid waste

In this work, the speciation of elements in compost was studied with emphasis on their binding to humic substances. In order to assess the distribution of As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, U, Th and Zn among molecular weight fractions of humic substances, the compost extract (extracted by 0.1 mol l(-1) sodium pyrophosphate) was analyzed by size exclusion chromatography coupled on-line with UV-Vis spectrophotometric and ICP-MS detection. Similar chromatograms were obtained for standard humic acid (Fluka) and for compost extract (254 nm, 400 nm) and three size fractions were operationally defined that corresponded to the apparent molecular weight ranges > 15 kDa, 1-15 kDa and < 1 kDa. The percentage of total element content in compost that was leached to the extract ranged from 30% up to 100% for different elements. The elution profiles of Co, Cr, Cu, Ni and Pb (ICP-MS) followed that of humic substances, while for other elements the bulk elution peak matched the retention time observed for the element in the absence of compost extract. Spiking experiments were carried out to confirm elements' binding and to estimate the affinity of individual elements for humic substances derived from compost. The results obtained indicated the following order of decreasing affinity: Cu > Ni > Co > Pb > Cd > (Cr, U, Th) >> (As, Mn, Mo, Zn). After standard addition, further binding of Cu, Ni and Co with the two molecular weight fractions of humic substances was observed, indicating that humic substances derived from compost were not saturated with these elements.

Simultaneous monitoring of volatile selenium and sulfur species from se accumulating plants (wild type and genetically modified) by GC/MS and GC/ICPMS using solid-phase microextraction for sample introduction

A sensitive method for determining ultratrace volatile Se species produced from Brassica juncea seedlings is described. The use of a new commercially available GC/ ICPMS interface in conjunction with solid-phase micro-extraction is a promising way to perform these studies. The addition of optional gases (O2 and N2) to the argon discharge proved to increase the sensitivity for Se and S as well as for Xe, which as a trace contaminant gas, was used for ICPMS optimization studies. However, the optimization parameters differ when an optional gas is added. In the best conditions, limits of detection ranging from 1 to 10 ppt can be obtained depending on the Se compound and 30 to 300 ppt for the volatile S species. The use of GC/MS with similar sample introduction permits the characterization of several unknown species produced as artifacts from the standards. The method allows the virtually simultaneous monitoring of S and Se species from the headspace of several plants (e.g., onions, garlic, etc.) although the present work is focused on the B. juncea seedlings grown in closed vials and treated with Se. Dimethyl selenide and dimethyl diselenide were detected as the primary volatile Se components in the headspace. Sulfur species also were present as allyl (2-propenyl) isothiocyanate and 3-butenyl isothiocyanate as characterized by GC/MS.

Capillary electrophoresis-inductively coupled plasma-mass spectrometry: an attractive complementary technique for elemental speciation analysis

Some basic and practical aspects of interfacing capillary electrophoresis to inductively coupled plasma-mass spectrometry (CE-ICP-MS) are reviewed in this article with emphasis on the use of this hyphenated technique for elemental speciation analysis. The principles behind the techniques of both CE and ICP-MS are introduced. The interfacing of CE to ICP-MS is discussed including several devices and nebulizers reported in literature. A brief account of their advantages and limitations is given. The various CE-ICP-MS applications for elemental speciation analysis are also reviewed. Some issues concerning the future of CE-ICP-MS for the elemental speciation analyses are discussed.

Elemental speciation by chromatographic separation with inductively coupled plasma mass spectrometry detection

Separation techniques coupled to inductively coupled plasma mass spectrometry (ICP-MS) is reviewed. ICP-MS technique is described briefly. Coupling of the different separation techniques are described, together with the most common applications used for each technique that has been described in the literature. An overview for the future of separation techniques coupled to ICP-MS with regard to elemental speciation is discussed.

Characterization of selenium species in Brazil nuts by HPLC-ICP-MS and ES-MS

Brazil nuts have been classified as the foodstuffs that contain the highest level of unadulterated selenium, an essential trace element that appears to prevent cancer. To date, characterization of the selenium species in brazil nuts has not yet been investigated. In this work, various sample preparation approaches, including microwave extractions and enzymatic treatments, are examined with the goal of species preservation and subsequent selenium speciation; of these approaches, an enzymatic treatment with Proteinase K proved most effective. High-performance liquid chromatography (HPLC) separation strategies and inductively coupled plasma mass spectrometry (ICP-MS) detection schemes will also be presented. Extracts are evaluated against available standards for the commercially obtainable seleno-amino acids, selenomethionine (SeMet), selenoethionine (SeEt), and selenocystine (SeCys); selenomethionine was demonstrated to be the most abundant of these seleno-amino acids. Further characterization of unidentified selenium-containing peaks is attempted by the employment of several procedures, including electrospray-mass spectrometry (ES-MS). A peptide structure was identified; however, this was considered a tentative proposal due to the large background produced by the extremely complicated brazil nut matrix.

Release of cytochrome c and activation of pro-caspase-9 following lysosomal photodamage involves Bid cleavage

Photodynamic therapy (PDT) protocols employing lysosomal sensitizers induce apoptosis via a mechanism that causes cytochrome c release prior to loss of mitochondrial membrane potential (DeltaPsi(m)). The current study was designed to determine how lysosomal photodamage initiates mitochondrial-mediated apoptosis in murine hepatoma 1c1c7 cells. Fluorescence microscopy demonstrated that the photosensitizer N-aspartyl chlorin e6 (NPe6) localized to the lysosomes. Irradiation of cultures preloaded with NPe6 induced the rapid destruction of lysosomes, and subsequent cleavage/activation of Bid, pro-caspases-9 and -3. Pro-caspase-8 was not activated. Release of cytochrome c occurred at about the time of Bid cleavage and preceded the loss of DeltaPsi(m). Extracts of purified lysosomes catalyzed the in vitro cleavage of cytosolic Bid, but not pro-caspase-3 activation. Pharmacological inhibition of cathepsin B, L and D activities did not suppress Bid cleavage or pro-caspases-9 and -3 activation. These studies demonstrate that photodamaged lysosomes trigger the mitochondrial apoptotic pathway by releasing proteases that activate Bid.

Trace element determination in vitamin E using ICP-MS

Vitamin E supplements are either isolated from plants sources or prepared synthetically. Isolation from plants includes eight different tocopherol structures. Vitamin E synthesis includes seven different stereoisomers, which involves the use of several catalysts that may lead to trace element contamination in the vitamin. The use of ICP-MS is an ideal technique for detecting these trace elements. However, the oily nature of the samples requires the development of a sample preparation methodology. This study was done upon the request of synthetic vitamin E manufacturers to test the trace metal purity of their samples. In this work, the comparison of an acid microwave digestion and emulsion preparation is discussed. Cromium, nickel, tin and lead were found in the synthetic vitamin E analyzed and 200, 60, 9 and 45 ppb were the concentrations found respectively for these elements. Digesting the samples gives slightly lower detection limits compared to the emulsion preparation.

A study of method robustness for arsenic speciation in drinking water samples by anion exchange HPLC-ICP-MS

Regulating arsenic species in drinking waters is a reasonable objective, since the various species have different toxicological impacts. However, developing robust and sensitive speciation methods is mandatory prior to any such regulations. Numerous arsenic speciation publications exist, but the question of robustness or ruggedness for a regulatory method has not been fully explored. The present work illustrates the use of anion exchange chromatography coupled to ICP-MS with a commercially available "speciation kit" option. The mobile phase containing 2 mM NaH(2)PO(4) and 0.2 mM EDTA at pH 6 allowed adequate separation of four As species (As(III), As(V), MMAA, DMAA) in less than 10 min. The analytical performance characteristics studied, including method detection limits (lower than 100 ng L(-1) for all the species evaluated), proved the suitability of the method to fulfill the current regulation. Other parameters evaluated such as laboratory fortified blanks, spiked recoveries, and reproducibility over a certain period of time produced adequate results. The samples analyzed were taken from water utilities in different areas of the United States and were provided by the U.S. EPA. The data suggests the speciation setup performs to U.S. EPA specifications but sample treatment and chemistry are also important factors for achieving good recoveries for samples spiked with As(III) as arsenite and As(V) as arsenate.

Comparison of extraction procedures for the determination of arsenic and other elements in lobster tissue by inductively coupled plasma mass spectrometry

A variety of extraction procedures were evaluated for the extraction of arsenic and other analytes from lobster tissue samples using inductively coupled plasma mass spectrometry (ICP-MS) detection. Soxhlet, room temperature mixing, sonication, microwave assisted, supercritical carbon dioxide and subcritical water extractions were evaluated for a variety of solvent systems and optimum conditions determined using a partially defatted Lobster Hepatopancreas marine certified reference material, TORT-2 (National Research Council of Canada). The solubility trends and solvents into which the analytes extracted gave an indication as to the polar/non-polar nature of the compounds present. Analytes that prefer water are probably more polar or inorganic, while those preferring methanol solutions are less polar or organic in nature. Arsenic, cadmium, cobalt, molybdenum and selenium were probably all present in TORT-2 in both polar inorganic and non-polar organic forms. While TORT-2 may have contained similar amounts of selenium in both forms, the results suggested that more of the arsenic was present as less polar or more organic compounds, and cobalt existed mainly as more polar or inorganic species. Most of the extraction techniques suggested that, although there may be some less polar organic forms present, more of the cadmium was probably present as polar inorganic compounds. Additionally, most techniques indicated that molybdenum was possibly all less polar or more organic in nature. In general, microwave assisted extraction (MAE) yielded comparable or improved recoveries for all of the analytes monitored and usually required less solvent. Additionally, MAE proved to be the mildest, fastest, least complicated and most reproducible extraction technique evaluated. MAE at 75 degrees C for 2 min exposure time yielded quantitative recovery of arsenic from TORT-2. These conditions were evaluated for lobster tissue samples purchased from a local restaurant. Separate evaluation of the lobster meat and organs resulted in quantitative recoveries of arsenic from both tissue samples. The results indicated that the extraction efficiencies might have some dependence upon the extraction technique, extraction conditions, analyte, solvent, and sample matrix.

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).

Selenium incorporation into Saccharomyces cerevisiae cells: a study of different incorporation methods

AIMS

To study the effects of the selenium enrichment protocols in yeast at various points in the cell cycle, total selenium accumulation and the forms of selenium incorporated.

METHODS AND RESULTS

The use of selenized yeast as enriched selenium supplements in human nutrition has become a topic of increasing interest over the last decade. Four enrichment procedures have been evaluated using sodium selenite as the selenium source: enrichment during the growth phase; enrichment at the non-growth phase, both of these at different selenium levels; enrichment by seeding in a fermentable carbon source (glucose); Se-enrichment with a non-fermentable carbon source (glycerol). A nitric acid digestion of the yeast samples prepared under different conditions has been performed in order to evaluate the total selenium incorporated into the yeast cells. Also, an enzymatic digestion of the yeast samples with pepsin has been carried out as an initial step to begin the process of determining which of the different possible selenium species are formed. The cell count evaluations of the selenium-enriched yeast showed that the growth phase, seeding and the use of YEPG media is influenced by the addition of Se, while the non-growth phase is not. Total selenium incorporation studies showed that seeding the yeast permits more accumulation of selenium. Speciation studies of the enriched yeast showed that the growth phase increases the formation of L-Se-methionine.

CONCLUSIONS

When the aim of enriching yeast with selenium is the formation of L-Se-methionine, the best enrichment procedure is using the growth phase with small concentrations of sodium selenite.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of selenium supplements is widespread and most of the supplements use selenium-enriched yeast in their formulation. Studies made on supplements do not have the appropriate Se-species for optimal absorption in the human body. This study presents and compares methods for the best selenium yeast enrichment that could ultimately be used in selenium supplement formulations.