Sample preparation for speciation.
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Editorial↗
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Determination of actinide speciation in solution using high-energy X-ray scattering.
High-energy X-ray scattering (HEXS) has been used to understand the coordination environment of the uranyl ion in a perchlorate solution. Assuming the two coordinating oxo ligands bound to U(VI) are represented in a peak in the pair distribution function (PDF) at 1.766(1) A, integration of the peak intensity is used to quantify the charge located on the oxygens. The dioxo ligands are essentially neutral, as predicted by numerous published calculations, with a charge of -16.4(8) electrons. The peak in the PDF at 2.420(1) A is consistent with equatorial ligating waters. The intensity of this peak is inconsistent with an integral coordination number and is used to propose a solution equilibrium of five and four waters coordinating to the uranyl(VI) ion favoring the five-coordinate species. This equilibrium is then used to experimentally determine that five-coordinate uranyl is 1.19+/-0.42 kcal/mol more stable than its four-coordinate counterpart under the conditions of the experiment. Further peaks in the Fourier transform of the scattering data at 4.50, 7, and 8.7 A are attributed to uranium-solvent correlations.
Actinoid Series Elements↗
Biosynthesis of Cd-bound phytochelatins by Phaeodactylum tricornutum and their speciation by size-exclusion chromatography and ion-pair chromatography coupled to ICP-MS.
Cd-bound phytochelatins (Cd-PCs) have been synthesised by incubation of Phaeodactylum tricornutum cell cultures with Cd and purified by size-exclusion chromatography-UV-Vis. These complexes, which were identified in previous work, have now been used as model substances to develop and optimise ion-pair chromatography (IPC) coupled to inductively coupled plasma-mass spectrometry (ICP-MS) for analysis of Cd-PCs. Subsequent analysis of samples taken from Silene vulgaris plants cultivated under heavy metal stress conditions revealed Cd signals but no Cd-PC signals. By use of isotopically enriched (116)Cd-PCs the sample preparation steps were verified to determine the stability of the analytes. We observed species transformation between Cd-PCs and other unidentified Cd complexes. Consequently, the kinetic and thermodynamic lability of Cd-PCs are decisive factors in their detection.
Cadmium↗
Uptake and speciation of selenium in garlic cultivated in soil amended with symbiotic fungi (mycorrhiza) and selenate.
The scope of the work was to investigate the influence of selenate fertilisation and the addition of symbiotic fungi (mycorrhiza) to soil on selenium and selenium species concentrations in garlic. The selenium species were extracted from garlic cultivated in experimental plots by proteolytic enzymes, which ensured liberation of selenium species contained in peptides or proteins. Separate extractions using an aqueous solution of enzyme-deactivating hydroxylamine hydrochloride counteracted the possible degradation of labile selenium species by enzymes (such as alliinase) that occur naturally in garlic. The selenium content in garlic, which was analysed by ICP-MS, showed that addition of mycorrhiza to the natural soil increased the selenium uptake by garlic tenfold to 15 microg g(-1) (dry mass). Fertilisation with selenate and addition of mycorrhiza strongly increased the selenium content in garlic to around one part per thousand. The parallel analysis of the sample extracts by cation exchange and reversed-phase HPLC with ICP-MS detection showed that gamma-glutamyl-Se-methyl-selenocysteine amounted to 2/3, whereas methylselenocysteine, selenomethionine and selenate each amounted to a few percent of the total chromatographed selenium in all garlic samples. Se-allyl-selenocysteine and Se-propyl-selenocysteine, which are selenium analogues of biologically active sulfur-containing amino acids known to occur in garlic, were searched for but not detected in any of the extracts. The amendment of soil by mycorrhiza and/or by selenate increased the content of selenium but not the distribution of detected selenium species in garlic. Finally, the use of two-dimensional HPLC (size exclusion followed by reversed-phase) allowed the structural characterisation of gamma-glutamyl-Se-methyl-selenocysteine and gamma-glutamyl-Se-methyl-selenomethionine in isolated chromatographic fractions by quadrupole time-of-flight mass spectrometry.
Enzymes↗
Sulfur speciation by capillary zone electrophoresis: conditions for sulfite stabilization and determination in the presence of sulfate, thiosulfate and peroxodisulfate.
Capillary zone electrophoresis (CZE) was used for the separation of the sulfur species SO3(2-), SO4(2-), S2O(3-) and S2O8(2-). Using an electrolyte system with 9.5 mmol L(-1) potassium chromate as UV-absorbing probe and 1 mmol L(-1) diethylenetriamine (DETA) as electroosmotic flow modifier, various possibilities for the stabilization of sulfite and electrophoretic separation of the sulfur anions were investigated. By adding 5% propanol as a stabilizer to both the working electrolyte and the sample solution, a good stabilization for sulfite and a separation of the sulfur anions in a short analysis time (4 min) was achieved. The advantages by using propanol instead of other stabilizers often used in analytical techniques are discussed. The electrophoretic separation of the sulfur anions was optimized with respect to the pH of the working electrolyte and concentration of the electroosmotic flow modifier (DETA). The detection limits achieved for SO3(2-), SO4(2-), S2O3(2-) and S2O8(2-) were 0.35, 0.25, 0.78 and 0.80 mg L(-1), respectively.
Journal Article↗
Speciation of antimony(III) and antimony(V) in cell extracts by anion chromatography/inductively coupled plasma mass spectrometry.
An analytical method for the separation and quantification of Sb(III) and Sb(V) using anion chromatography with ICP-MS is presented. The optimum conditions for the separation of the antimony species were established with 15 mmol/L nitric acid at pH 6 as eluent system on a PRP-X100 column. The retention times for antimony(V) and antimony(III) were 85 s and 300 s with detection limits of 0.06 microg/L and 0.29 microg/L, respectively. The proposed method was applied to cell extracts of Leishmania donovani, which were incubated with antimony(III) and antimony(V). Some metabolism seemed to occur within the cells.
Animals↗
Characterizing speciation of trace elements in the chemistry of life.
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Environment↗
Speciation of butyltin compounds in marine sediments with headspace solid phase microextraction and gas chromatography mass spectrometry.
A method for the determination of organotin compounds (monobutyl = MBT, dibutyl = DBT, and tributyltin = TBT) in marine sediments by headspace Solid Phase Microextraction (SPME) has been developed. The analytical procedure involved 1) extraction of TBT, DBT and MBT from sediments with HCl and methanol mixture, 2) in situ derivatization with sodium tetraethylborate and 3) headspace SPME extraction using a fiber coated with poly(dimethylsiloxane). The derivatized organotin compounds were desorbed into the splitless injector and simultaneously analyzed by gas chromatography - mass spectrometry. The analytical method was optimized with respect to derivatization reaction and extraction conditions. The detection limits obtained for MBT, DBT and TBT ranged from 730 to 969 pg/g as Sn dry weight. Linear calibration curves were obtained for all analytes in the range of 30-1000 ng/L as Sn. Analysis of a standard reference sediment (CRM 462) demonstrates the suitability of this method for the determination of butyltin compounds in marine sediments. The application to the determination of TBT, DBT and MBT in a coastal marine sediment is shown.
Gas Chromatography-Mass Spectrometry↗
Speciation of copper, lead and cadmium in aquatic systems by circulating dialysis combined with flame AAS.
The assessment of free Cu(II), Pb(II) and Cd(II) ions in the presence of complexed species was realised by a circulating dialysis with Cuprophan planar membranes and subsequent quantification by flame atomic absorption spectrometry. The effect of the flow rate, the time of equilibration, pH and the presence of various complexing agents in the donor solutions were studied. The determination of free Cu(II), Pb(II) and Cd(II) ions in the presence of soil humic substances resulted from the above studies.
Journal Article↗
Comparison of sodium tetraethylborate and sodium tetra(n-propyl)borate as derivatization reagent for the speciation of organotin and organolead compounds in water samples.
The influence of pH on the propylation with sodium tetra(n-propyl)borate of butyl- and phenyltins as well as for trimethyl- and triethyllead was investigated. Ethylation and propylation with tetraalkylborates were compared with regard to derivatization yields and figures of merit for organotin compounds in real water samples. Similar results for limit of detection (3-12 ng/L as tin), derivatization yield (40-100%) and relative standard deviation of the method (3-10%) were achieved for derivatization with the two tetraalkylborates. Propylation is thus the preferred method for the simultaneous determination of environmentally relevant organotin and organolead compounds. The handling of the hygroscopic and air sensitive reagents NaBEt4 and NaBPr4 was simplified by dissolving them in tetrahydrofurane. The reagent solutions in tetrahydrofurane can be stored for at least one month at 4 degrees C in the dark without observing any decrease in the derivatization efficiency.
Journal Article↗
Investigating the potential of high-performance liquid chromatography with atmospheric pressure chemical ionization-mass spectrometry as an alternative method for the speciation analysis of organotin compounds.
Liquid chromatography with atmospheric pressure chemical ionization-mass spectrometry (LC-APCI-MS) was applied for the determination of butyl- and phenyltin compounds. Chromatography was performed on a 30 x 2 mm, 3 microm C18 column, enabling the separation of mono-, di- and trisubstituted butyl- and phenyltin compounds in less than 10 min using a water/1% trifluoroacetic acid/methanol gradient. While satisfactory retention and resolution is achieved for the di- and trisubstituted butyl- and phenyltin compounds, monobutyltin and monophenyltin cannot be resolved chromatographically. Depending on the parameter values of the interface, APCI-MS detection allows both specific detection of the molecular ion or cluster ion at low to intermediate fragmentor voltages or quasi-element specific detection of the Sn+ ion released from the organotin compounds at high fragmentor voltages. The sensitivity of MS detection is similar for butyl- and phenyltin compounds, but varies largely from mono- to trisubstituted organotin compounds with tributyl- and triphenyltin being the most sensitively detectable compounds. Detection limits are in the 20-65 pg (abs.) range in SIM mode and in the 750-2000 pg (abs.) range in the scan mode for tributyl- and triphenyltin and for dibutyl- and diphenyltin, respectively. Monobutyl- and monophenyltin can be detected with much lower sensitivity which, together with their unfavorable chromatographic behavior, accounts for the fact that they cannot be analyzed at environmentally relevant concentrations. Although LC-APCI-MS is generally less sensitive than comparable GC methods, it is applicable to the analysis of environmental samples as demonstrated by the analysis of the PACS-2 sediment certified reference material. Although the derivatization of the ionic organotin compounds, which particularly in real samples is a potential source of error, is circumvented when LC-APCI-MS is used, the extraction step is still critical and may lead to underestimation when quantitation is not done by the method of standard addition.
Chromatography, High Pressure Liquid↗
Comparison of chromium speciation by CZE and ion exchange followed by AAS.
The hydrogen chromate anion (HCrO4-), which is the predominant species in acidic solutions and solutions with low chromium concentration, was determined by capillary zone electrophoresis (CZE) using UV detection on-column at 200 nm. A fused-silica capillary (55 cm x 50 microm i.d.) was employed with a high negative voltage of 20 kV. Total chromium was determined after reduction by H2O2 and its complexation by EDTA. The use of H2O2 as reducing agent is advantageous, as it does not increase the conductivity of the solution. Detection limits achieved (for 200 s injection time) were 30 and 8 microg/L for Cr(VI) and Cr(III), respectively. The CZE results obtained for Cr(III) and Cr(VI) were compared with those obtained by ion exchange with subsequent AAS.
Journal Article↗
Speciation of As(III), As(V), MMA and DMA in contaminated soil extracts by HPLC-ICP/MS.
A method to separate and quantify two inorganic arsenic species As(III) and As(V) and two organic arsenic species, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), by HPLC-ICP/MS has been developed. The separation of arsenic species was achieved on the anionic exchange column IonPac AS11 (Dionex) with NaOH as mobile phase. The technique was successfully applied to analyze extracts of two contaminated soils, sampled at a former tannery site (soil 1) and a former paint production site (soil 2). The soils were extracted at pH values similar to the natural environment. Extractions were performed at different pH values with 0.3 M ammonium oxalate (pH = 3), milli-Q water (pH = 5.8), 0.3 M sodium carbonate (pH = 8) and 0.3 M sodium bicarbonate (pH = 11). No organically bound arsenic was found in the extracts. As(V) was the major component. Only up to 0.04% of the total arsenic contained in soil 1 were mobilized. The highest amount of extracted arsenic was found at the highest pH. In the milli-Q water extract of soil 1 As(III) and As(V) were found. High amounts of As(V) were found in the extracts of soil 2. Up to 20% of the total arsenic bound to soil 2 constituents were released. The results show that the mobilization of arsenic depended on the pH value of the extraction solution and the kind of extracted soil. Dramatic consequences have to be expected for pH changes in the environment especially in cases where soils contain high amounts of mobile arsenic.
Arsenic↗
Metals in environmental compartments. Reflections on the 8th Workshop on Progress in Analytical Methodologies for Trace Metal Speciation, Lisbon, Portugal, September 5, 2000.
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Chemistry, Analytic↗
Preconcentration of chromium (III) and speciation of chromium by electrothermal atomic absorption spectrometry using cellulose adsorbent.
A simple and sensitive method has been developed for species selective determination of chromium(III) and chromium(VI) in water by electrothermal atomic absorption spectrometry. The procedure is based on selective absorption of Cr(III) on a cellulose micro column (pH 11, 0.5 mol L(-1) NaCl). Total chromium was subsequently determined after appropriate reduction of Cr(VI) to Cr(III). Recoveries of more than 97% were found. A concentration factor of 100 was achieved. The relative standard deviations (n=10) at the 40 ng L(-1) level for chromium(III) and chromium(VI) were 2.3% and 1.8% and corresponding limits of detection (based on 36) were 1.8 ng L(-1) and 5.1 ng L(-1), respectively. No interference effects have been observed from other investigated species and the method has been successfully applied to natural water samples.
Cellulose↗
Effect of extraction pH on metal speciation in plant root extracts.
The species patterns of nutrient and trace metals (K, Ca, Mg, Mn, Fe, Zn) obtained by extraction of plant roots have been determined as a function of extraction pH in the range 4-9. The extractable metal concentrations were subdivided into low-molecular-weight (<10 kDa) and high-molecular-weight (>10 kDa) metal species by TXRF analysis. Except for pH 9, the low-molecular-weight fraction is predominant. This fraction was further separated by HPLC with AAS detection. It was demonstrated that the nature of the metal species changes dramatically when the extraction pH is changed, especially in the range 9-7. Information about the chemical nature of chromatographically separated metal species is obtained by use of different electrochemical detectors (amperometric detection at a glassy carbon or copper electrode and pulsed amperometric detection) and diode-array UV detection.
Calcium↗
Trace metal and mineral speciation of remediated wastes using electron microscopy.
Electron microscopic techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalyses (EPMA), were used to evaluate metal species and mineralogical phases associated with metal-bearing contaminated soil and industrial wastes that have been solidified and stabilized with Portland cement. Metals present in the wastes included arsenic, barium, cadmium, chromium, copper, lead, nickel, and zinc. In addition, mineral alterations and weathering features that affect the durability and containment of metals in aged remediated wastes were analyzed microscopically. Physical and chemical alteration processes identified included: freeze-thaw cracking; cracking caused by the formation of expansive minerals, such as ettringite and thaumasite; carbonation; and the movement of metals from waste aggregates into the surrounding cement matrix. Preliminary results show that although the extent of degradation after 6 years is considered slight to moderate, evaluations of durability and permanence of metals containment cannot be based on leaching and bulk chemistry analyses alone. The use of electron microscopic analyses is vital in studies that evaluate trace metal and mineral species and that attempt to predict the long-term performance of metal containment in solidified and stabilized wastes.
Industrial Waste↗
Speciation studies by capillary electrophoresis- Simultaneous determination of chromium(III) and chromium(VI).
A method for the simultaneous determination of chromium(III) and chromium(VI) by capillary electrophoresis (CE) has been developed. The chromium(III) has been chelated with 1,2-cyclohexanediaminetetraacetic acid (CDTA) in order to impart a negative charge and similar mobility to both the chromium(III) and the chromium(VI) species. The effects of the amount of the reagent, pH and heating time required to complete the complexation have been studied. Factors affecting the CE behaviour such as the polarity of electrodes and the pH of electrophoretic buffer have been investigated. The separated species have been monitored by direct UV measurements at 214 nm. The detection limits achieved are 10 microg/l for Cr(VI) and 5 microg/l for Cr(III) and linear detector response is observed up to 100 mg/l. The procedure has been applied to the determination of both chromium species in industrial electroplating samples and its accuracy was checked by comparing the results (as total chromium) with those of atomic absorption spectrometry. No interference occurred from transition metal impurities under optimized separation conditions. The method is also shown to be feasible for determining Cr(III) as well as other metal ions capable to form complexes with CDTA (like iron(III), copper(II), zinc(II) and manganese(II)) in pharmaceutical preparations of essential trace elements.
Journal Article↗