Evaluation of polychlorotrifluoroethylene as sorbent material for on-line solid phase extraction systems: determination of copper and lead by flame atomic absorption spectrometry in water samples

Polychlorotrifluoroethylene (PCTFE) in the form of beads was applied, as packing material for flow injection on-line column preconcentration and separation systems coupled with flame atomic absorption spectrometry (FAAS). Its performance characteristics were evaluated for trace copper determination in environmental samples. The on-line formed complex of metal with diethyldithiophosphate (DDPA) was sorbed on the PCTFE surface. Isobutyl methyl ketone (IBMK) at a flow rate of 2.8 mL min⁻¹ was used to elute the analyte complex directly into the nebulizer-burner system of spectrophotometer. The proposed sorbent material reveal, excellent chemical and mechanical resistance, fast adsorption kinetics permitting the use of high sample flow rates up to 15 mL min⁻¹ without loss of retention efficiency. For copper determination, with 90 s preconcentration time the sample frequency was 30 h⁻¹, the enhancement factor was 250, which could be further improved by increasing the loading (preconcentration) time. The detection limit (3s) was c_L = 0.07 μg L⁻¹, and the precision (R.S.D.) was 1.8%, at the 2.0 μg L⁻¹ Cu(II) level. For lead determination, the detection limit was c_L = 2.7 μg L⁻¹, and the precision (R.S.D.) 2.2%, at the 40.0 μg L⁻¹ Pb(II) level. The accuracy of the developed method was evaluated by analyzing certified reference materials and by recovery measurements on spiked natural water samples.     

A novel fiber-packed column for on-line preconcentration and speciation analysis of chromium in drinking water with flame atomic absorption spectrometry

A novel on-line preconcentration and determination system based on a fiber-packed column was developed for speciation analysis of Cr in drinking water samples prior to its determination by flame atomic absorption spectrometry (FAAS). All variables involved in the development of the preconcentration method including, pH, eluent type, sample and eluent flow rates, interfering effects, etc., were studied in order to achieve the best analytical performance. A preconcentration factor of 32 was obtained for Cr(VI) and Cr(III). The levels of Cr(III) species were calculated by difference of total Cr and Cr(VI) levels. Total Cr was determined after oxidation of Cr(III) to Cr(VI) with hydrogen peroxide. The calibration graph was linear with a correlation coefficient of 0.999 at levels near the detection limit and up to at least 50 μg L⁻¹. The relative standard deviation (R.S.D.) was 4.3% (C = 5 μg L⁻¹ Cr(VI), n = 10, sample volume = 25 mL). The limit of detection (LOD) for both Cr(III) and Cr(VI) species was 0.3 μg L⁻¹. Verification of the accuracy was carried out by the analysis of a standard reference material (NIST SRM 1643e “Trace elements in natural water”). The method was successfully applied to the determination of Cr(III) and Cr(VI) species in drinking water samples.     

Preconcentration and speciation of chromium in drinking water samples by coupling of on-line sorption on activated carbon to ETAAS determination

An on-line flow injection (FI) preconcentration-electrothermal atomic absorption spectrometry (ETAAS) method is developed for trace determination of chromium in drinking water samples by sorption on a conical minicolumn packed with activated carbon (AC) at pH 5.0. The chromium was removed from the minicolumn with 1.0% (v/v) nitric acid. An enrichment factor (EF) of 35-fold for a sample volume of 10 ml was obtained. The detection limit (DL) value for the preconcentration method proposed was 3.0 ng l⁻¹. The precision for 10 replicate determinations at the 0.5 μg l⁻¹ Cr level was 4.0% relative standard deviation (R.S.D.), calculate with the peak heights obtained. The calibration graph using the preconcentration system for chromium was linear with a correlation coefficient of 0.9992 at levels near the detection limits up to at least 50 μg l⁻¹. The method was successfully applied to the determination of Cr(III) and Cr(VI) in drinking water samples.     

Indirect determination of hexavalent chromium ion in complex matrices by adsorptive stripping voltammetry at a mercury electrode

A sensitive method for the determination of chromium ion(VI) in complex matrices such as crude oil and sludge is presented based on the decreasing effect of Cr(VI) on cathodic adsorptive stripping peak height of Cu-adenine complex. Under the optimum experimental conditions (pH 7.5 Britton-Robinson buffer, 5 × 10⁻⁵ M copper, 8 × 10⁻⁶ M adenine and accumulation potential −250 mV versus Ag/AgCl), a linear decrease of the peak current of Cu-adenine was observed, when the chromium(VI) concentration was increased from 5 μg L⁻¹ to 120 μg L⁻¹. Detection limit of 2 μg L⁻¹ was achieved for 120 s accumulation time. The relative standard deviations (R.S.D., %) were 1.8% and 4% for chromium(VI) concentrations of 18 μg L⁻¹ and 100 μg L⁻¹, respectively. The method was applied to the determination of chromium(VI) in the presence of high levels of chromium(III), in various real samples such as crude oil, crude oil tank button sludge, waste water and tap water samples. Effects of foreign ions and surfactants on the voltammetric peak and the influences of instrumental and analytical parameters were investigated in detail. The accuracy of the results was checked by ICP and/or AA.     

Facile preparation of glutathione-stabilized gold nanoclusters for selective determination of chromium (III) and chromium (VI) in environmental water samples

A novel method for selective determination of Cr(III) and Cr(VI) in environmental water samples was developed based on target-induced fluorescence quenching of glutathione-stabilized gold nanoclusters (GSH-Au NCs). Fluorescent GSH-Au NCs were synthesized by a one-step approach employing GSH as reducing/protecting reagent. It was found that Cr(III) and Cr(VI) showed pH-dependent fluorescence quenching capabilities for GSH-Au NCs, and thus selective determination of Cr(III) and Cr(VI) could be achieved at different pHs. Addition of EDTA was able to effectively eliminate the interferences from other metal ions, leading to a good selectivity for this method. Under optimized conditions, Cr(III) showed a linear range of 25–3800 μg L⁻¹ and a limit of detection (LOD) of 2.5 μg L⁻¹. The Cr(VI) ion demonstrated a linear range of 5–500 μg L⁻¹ and LOD of 0.5 μg L⁻¹. The run-to-run relative standard deviations (n = 5) for Cr(III) and Cr(VI) were 3.9% and 2.8%, respectively. The recoveries of Cr(III) and Cr(VI) in environmental water samples were also satisfactory (76.3–116%). This method, with its simplicity, low cost, high selectivity and sensitivity, could be used as a promising tool for chromium analysis in environmental water samples.     

Speciated isotope dilution analysis of Cr(III) and Cr(VI) in water by ICP-DRC-MS

An isotope dilution method has been developed for the speciation analysis of chromium in natural waters which accounts for species interconversions without the requirement of a separation instrument connected to the mass spectrometer. The method involves (i) in-situ spiking of the sample with isotopically enriched chromium species; (ii) separation of chromium species by precipitation with iron hydroxide; (iii) careful measurement of isotope ratios using an inductively coupled plasma mass spectrometer (ICP-MS) with a dynamic reaction cell (DRC) to remove isobaric polyatomic interferences. The method detection limits are 0.4 μg L⁻¹ for Cr(III) and 0.04 μg L⁻¹ for Cr(VI). The method is demonstrated for the speciation of Cr(III) and Cr(VI) in local nullah and synthetically spiked water samples. The percentage of conversion from Cr(III) to Cr(VI) increased from 5.9% to 9.3% with increase of the concentration of Cr(VI) and Cr(III) from 1 to 100 μg L⁻¹, while the reverse conversion from Cr(VI) to Cr(III) was observed within a range between 0.9% and 1.9%. The equilibrium constant for the conversion was found to be independent of the initial concentrations of Cr(III) and Cr(VI) and in the range of 1.0 (at pH 3) to 1.8 (at pH 10). The precision of the method is better than that of the DPC method for Cr(VI) analysis, with the added bonuses of freedom from interferences and simultaneous Cr(III) determination.     

Modified mesoporous silica materials for on-line separation and preconcentration of hexavalent chromium using a microcolumn coupled with flame atomic absorption spectrometry

A modified SBA-15 mesoporous silica material NH₂-SBA-15 was synthesized successfully by grafting γ-aminopropyl-triethoxysilane. The material was characterized using transmission electron microscopy (TEM) and Fourier transform infrared/Raman (FT-IR/Raman) spectroscopy, and used for the first time in a flow injection on-line solid phase extraction (SPE) coupled with flame atomic absorption spectrometry (FAAS) to detect trace Cr (VI). Effective sorption of Cr (VI) was achieved at pH 2.0 with no interference from Cr (III) and other ions and 0.5 mol L⁻¹ NH₃·H₂O solution was found optimal for the complete elution of Cr (VI). An enrichment factor of 44 and was achieved under optimized experimental conditions at a sample loading of 2.0 mL min⁻¹ sample loading (300 s) and an elution flow rate of 2.0 mL min⁻¹ (24 s). The precision of the 11 replicate Cr (VI) measurements was 2.1% at the 100 μg L⁻¹ level with a detection limit of 0.2 μg L⁻¹ (3 s, n = 10) using the FAAS. The developed method was successfully applied to trace chromium determination in waste water. The accuracy was validated using a certified reference material of riverine water (GBW08607).     

Speciation of chromium in environmental samples by dual electromembrane extraction system followed by high performance liquid chromatography

This study proposes the dual electromembrane extraction followed by high performance liquid chromatography for selective separation-preconcentration of Cr(VI) and Cr(III) in different environmental samples. The method was based on the electrokinetic migration of chromium species toward the electrodes with opposite charge into the two different hollow fibers. The extractant was then complexed with ammonium pyrrolidinedithiocarbamate for HPLC analysis. The effects of analytical parameters including pH, type of organic solvent, sample volume, stirring rate, time of extraction and applied voltage were investigated. The results showed that Cr(III) and Cr(VI) could be simultaneously extracted into the two different hollow fibers. Under optimized conditions, the analytes were quantified by HPLC instrument, with acceptable linearity ranging from 20 to 500 μg L⁻¹ (R² values ≥ 0.9979), and repeatability (RSD) ranging between 9.8% and 13.7% (n = 5). Also, preconcentration factors of 21.8–33 that corresponded to recoveries ranging from 31.1% to 47.2% were achieved for Cr(III) and Cr(VI), respectively. The estimated detection limits (S/N ratio of 3:1) were less than 5.4 μg L⁻¹. Finally, the proposed method was successfully applied to determine Cr(III) and Cr(VI) species in some real water samples.     

Flow injection direct spectrophotometric assay for the speciation of trace chromium(III) and chromium(VI) using chromotropic acid as chromogenic reagent

A new rapid and sensitive FI assay is reported for the simultaneous direct spectrophotometric determination of trace Cr(VI) and Cr(III) in real samples. The method is based upon the reaction of Cr(VI) with chromotropic acid (CA) in highly acidic medium to form a water-soluble complex (λ_max = 370 nm). Cr(III) reacts with CA only after its on-line oxidation to Cr(VI) by alkaline KIO₄. The determination of each chromium species in the sample was achieved by absorbance differences. The calibration curves were linear over the range 3-4000 μg l⁻¹ and 30-1200 μg l⁻¹ for Cr(VI) and Cr(III), respectively, while the precision close to the quantitation limit was satisfactory in both cases (s_r = 3.0% for Cr(VI) and 4.0% for Cr(III) (n = 10) at 10 and 50 μg l⁻¹ level, respectively). The method developed proved to be adequately selective and sensitive (c_L = 1 and 10 μg l⁻¹ for Cr(VI) and Cr(III), respectively). The application of the method to the analysis of water samples (tap and mineral water) gave accurate results based on recovery studies (93-106%). Analytical results of real sample analysis were in good agreement with certified values.     

Further study on a flow injection on-line multiplexed sorption preconcentration coupled with flame atomic absorption spectrometry for trace element determination

A further study on a newly developed flow injection (FI) on-line multiplexed sorption preconcentration (MSP) using a knotted reactor coupled with flame atomic absorption spectrometry (FAAS) was carried out to demonstrate its applicability and limitation for trace element determination. For this purpose, Cr(VI), Cu(II), Ni(II) and Co(II) were selected as the analytes, and detailed comparison was made between the MSP-FAAS and conventional FI on-line sorption preconcentration FAAS in respect to retention efficiency and linear ranges of absorbance versus sample loading flow rate and total preconcentration time. Introduction of an air-flow for removal of the residual solution in the KR after each sub-injection in the MSP procedure played a decisive role in the improvement of retention efficiency. The linearity of absorbance versus sample loading flow rate or total preconcentration time was extended to a more degree for the metal ions with less stability of their PDC (pyrrolidine dithiocarbamate) complexes than those with more stable PDC complexes. It seems that the MSP procedure behaves advantages beyond the inflection points in the diagrams of absorbance versus total preconcentration time and sample loading flow rate obtained by conventional (a single continuous) preconcentration procedure. With a sample loading flow rate of 6.0 ml min⁻¹ and a total preconcentration time of 180 s, the retention efficiencies were increased from 25, 46, 41 and 63% with a single continuous sorption preconcentration to 44, 78, 65 and 75% with a six sub-injection preconcentration procedure for Cr(VI), Co(II), Ni(II), and Cu(II), respectively. The detection limits were 0.40, 0.33, 0.31 and 0.26 μg l⁻¹ for Cr(VI), Co(II), Ni(II), and Cu(II), respectively. The precision (R.S.D.) for eleven replicate determination of 2 μg l⁻¹ Cr(VI), Co(II) and Ni(II), and 1 μg l⁻¹ Cu(II), was 2.1, 4.1, 2.6 and 1.7%, respectively.     

Determination of trace heavy metals in herbs by sequential injection analysis-anodic stripping voltammetry using screen-printed carbon nanotubes electrodes

A method for the simultaneous determination of Pb(II), Cd(II), and Zn(II) at low μg L⁻¹ concentration levels by sequential injection analysis-anodic stripping voltammetry (SIA-ASV) using screen-printed carbon nanotubes electrodes (SPCNTE) was developed. A bismuth film was prepared by in situ plating of bismuth on the screen-printed carbon nanotubes electrode. Operational parameters such as ratio of carbon nanotubes to carbon ink, bismuth concentration, deposition time and flow rate during preconcentration step were optimized. Under the optimal conditions, the linear ranges were found to be 2-100 μg L⁻¹ for Pb(II) and Cd(II), and 12-100 μg L⁻¹ for Zn(II). The limits of detection (S_bl/S = 3) were 0.2 μg L⁻¹ for Pb(II), 0.8 μg L⁻¹ for Cd(II) and 11 μg L⁻¹ for Zn(II). The measurement frequency was found to be 10-15 stripping cycle h⁻¹. The present method offers high sensitivity and high throughput for on-line monitoring of trace heavy metals. The practical utility of our method was also demonstrated with the determination of Pb(II), Cd(II), and Zn(II) by spiking procedure in herb samples. Our methodology produced results that were correlated with ICP-AES data. Therefore, we propose a method that can be used for the automatic and sensitive evaluation of heavy metals contaminated in herb items.     

Speciation of chromium in natural waters by micropumping multicommutated light emitting diode photometry

A simple and sensitive multicommutated flow procedure, implemented by employing a homemade light emitting diode (LED) based photometer, has been developed for the determination of chromium (VI) and total chromium in water. The flow system comprised a set of four solenoid micro-pumps, which were assembled to work as fluid propelling and as commutating devices. The core of the detection unit comprised a green LED source, a photodiode and a homemade flow cell of 100 mm length and 2 mm inner diameter. The photometric procedure for the speciation of chromium in natural waters was based on the reaction of Cr (VI) with 1,5-diphenylcarbazide. Cr (III) was previously oxidized to Cr (VI) and determined as the difference between total Cr and Cr (VI). After carrying out the assays to select the best operational conditions the features of the method included: a linear response ranging from 10 to 200 μg l⁻¹ Cr (III) and Cr (VI) (r = 0.999, n = 7); limits of detection of 2.05 and 1.0 μg l⁻¹ for Cr (III) and Cr (VI), respectively; a relative standard deviation lower than 2.0% (n = 20) for a typical solution containing 50 μg l⁻¹ Cr; a sampling throughput of 67 and 105 determinations per hour for total Cr and Cr (VI), respectively, and recovery values within the range of 93-108% for spiked concentrations of the order of 50 μg l⁻¹.     

A new chelating resin containing azophenolcarboxylate functionality: synthesis, characterization and application to chromium speciation in wastewater

The synthesis of a new stable chelating resin from the polystyrene divinylbenzene copolymer is reported. The polystyrene is first functionalized with a phenolic group and then allowed to couple with diazotized anthranilic acid through the NN bond. The resulting polymer containing azophenolcarboxylate with an ONO chelating environment has been characterized by elemental analysis, hydrogen ion capacity, and water regain value. Its stability towards thermal and different chemical environments has been evaluated. The sorption capacity of the chelating resin for Cr(III) and Cr(VI) as a function of pH has been studied. The interesting point is that chromium(III) is selectively retained at ca. pH 5.0 and chromium(VI) at ca. pH 2.0. When packed in a column, the new material is able to separate Cr(III) from Cr(VI). Five replicate determinations of 10 μg Cr(III) and 10 μg Cr(VI) present in 100 mL solution gave recoveries of 96.9 ± 2.9% (for Cr(III)) and 96.2 ± 2.1% (for Cr(VI)) at the 95% confidence level. Calibration graph was linear over the concentration range of 0-250 μg L⁻¹ of chromium species with correlation coefficient (R) of 0.99994. The detection limits based on 3σ criterion were determined to be 0.6 μg L⁻¹ for Cr(III) and 0.9 μg L⁻¹ for Cr(VI). The developed method was successfully used for the speciation of chromium in wastewater.     

Selective, peroxidase substrate based “signal-on” colorimetric assay for the detection of chromium (VI)

Due to the potentially adverse effects of the chromium (VI) on the human health and also on the environment, the quantitative determination of Cr(VI) is of particular interest. This work herein reports a facile, selective and rapid colorimetric determination of Cr(VI) based on the peroxidase substrate-2,2′-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid) diammonium salt (ABTS) as the color developing agent. ABTS, which was usually acted as peroxidase substrate for the enzyme linked immunosorbent assay, is used here for the first time to fabricate the “signal-on” colorimetric Assay for Cr(VI). The ABTS was chosen instead of the commonly used 1,5-diphenylcarbazide (DPC) due to its good solubility, stability, sensitivity and low background. This method provided a convenient colorimetric detection of Cr(VI) with a wider linear range from 8.33 μg L⁻¹ to 1.25 mg L⁻¹ by recording the absorption spectra at the wavelength of 419 nm and a low detection limit of 7.87 μg L⁻¹. In addition, the entire detection takes less than 10 min.     

A novel flow injection chemiluminescence determination of Cr(VI) with Dichlorotris(1,10-phenanthroline)ruthenium(II)

A selective novel reverse flow injection system with chemiluminescence detection (rFI-CL) for the determination of Cr(VI) in presence of Cr(III) with Dichlorotris (1,10-phenanthroline)ruthenium(II), (Ru(phen)₃Cl₂), is described in this work. This new method is based on the oxidation capacity of Cr(VI) in H₂SO₄ media. First, the Ruthenium(II) complex is oxidized to Ruthenium(III) complex by Cr(VI) and afterwards it is reduced to the excited state of the Ruthenium(II) complex by a sodium oxalate solution, emitting light inside the detector. The intensity of chemiluminescence (CL) is proportional to the concentration of Cr(VI) and, under optimum conditions, it can be determined over the range of 3-300 μg L⁻¹ with a detection limit of 0.9 μg L⁻¹. The RSD was 8.4% and 1.5% at 5 and 50 μg L⁻¹, respectively. For the rFI-CL method various analytical parameters were optimized: flow rate (1 mL min⁻¹), H₂SO₄ carrier concentration (20% w/V), Ru(phen)₃Cl₂ concentration (5 mM) and sodium oxalate concentration (0.1 M). The effect of Cr(III), Fe(III), Al(III), Cd(II), Zn(II), Hg(II), Pb(II), Ca(II) and Mg(II), was studied. The method is highly sensitive and selective, allowing a fast, on-line determination of Cr(VI) in the presence of Cr(III). Finally, the method was tested in four different water samples (tap, reservoir, well and mineral), with good recovery percentage.     

Chromium speciation and preconcentration using zirconium(IV) and zirconium(IV) phosphate chemically immobilized onto silica gel surface using a flow system and F AAS

A procedure for chromium speciation by F AAS using a flow system has been proposed. In this system, Cr(III) and Cr(VI) ions were adsorbed sequentially onto a mini-column packed with silica gel modified with zirconium phosphate and a mini-column packed with silica gel modified with zirconium oxide, respectively. The elution of Cr(III) and Cr(VI) was made with, respectively, nitric acid solution and tris(hydroxymethyl)methylamine (THAM) solution in reverse mode and determination by flame atomic absorption spectrometry without interference of the matrix. Chemical and flow variables as well as concomitant ions were studied in the developed procedure. The enrichment factor for Cr(III) and Cr(VI) was 20.8 and 24.9, respectively, using a preconcentration time of 3.75 min. The limit of detection for Cr(III) and Cr(VI) was 1.9, and 2.3 μg l⁻¹, respectively. The precision of the method, evaluated as the relative standard deviation in solutions containing 100 μg l⁻¹ of chromium species, by analyzing a series of seven replicates, was lower than 3.0%. The accuracy was assessed through recovery experiments of water samples and using another methodology.     

Poly(etheretherketone)-turnings a novel sorbent material for lead determination by flow injection flame atomic absorption spectrometry and factorial design optimization

A novel hydrophobic sorbent material for on-line column preconcentration and separation systems coupled with atomic spectrometry was developed. Poly(etheretherketone) (PEEK) in the form of turnings was used as packing material and evaluated for trace lead determination in environmental samples. Sample and ammonium diethyl-dithiophosphate (DDPA) reagent were mixed on-line and the Pb(II)-DDPA complex was retained effectively on PEEK-turnings. Isobutyl methyl ketone (IBMK) was adopted for efficient analyte complex elution and subsequently transportation into the nebulizer-burner system for atomization. The developed sorbent material has shown, excellent chemical and mechanical resistance, fast adsorption kinetics permitting the use of high sample flow rates without significant loss of retention efficiency. For 120 s sample preconcentration time the sampling frequency was 20 h⁻¹, the enhancement factor was 110, the detection limit (3 s) was c_L = 0.32 μg L⁻¹, and the relative standard deviation (RSD) was s_R = 2.2%, at the 50.0 μg L⁻¹ Pb(II) level. The accuracy of the developed method was evaluated by analyzing certified reference materials.     

Preconcentrative separation of chromium(VI) species from chromium(III) by coprecipitation of its ethyl xanthate complex onto naphthalene

A rapid, sensitive and selective method is described for the determination of chromium(VI) in presence of 100-fold amounts of chromium(III) by flame atomic absorption spectrometry (FAAS) in conjunction with coprecipitative preconcentration of its ethyl xanthate complex onto naphthalene. The solid mixture consisting of the chromium(VI) complex together with naphthalene is dissolved in 8.0 ml of dimethyl formamide (DMF) and chromium(VI) content was established by FAAS. Calibration graphs were rectilinear over the chromium(VI) concentration in the range 0-200 μg l⁻¹. Five replicate determinations of 20 μg of chromium(VI) present in 1.0 l of sample solution gave a relative standard deviation of 3.1%. The detection limit corresponding to three times the standard deviation of the blank was found to be 0.5 μg l⁻¹. The developed procedure has been successfully utilized for the estimation of chromium(VI), chromium(total) (after oxidation with bromate) and chromium(III) (by subtracting chromium(VI) content from chromium(total) value contents of several tannery industries.     

Flow injection on-line preconcentration of low levels of Cr(VI) with detection by ETAAS: Comparison of using an open tubular PTFE knotted reactor and a column reactor packed with PTFE beads

A flow injection (FI) on-line sorption preconcentration procedure utilizing a packed column reactor and combined with electrothermal atomic absorption spectrometry (ETAAS) is proposed for the determination of low levels of Cr(VI) in water samples. Polytetrafluoroethylene (PTFE) beads packed in a mini-column is used as sorbent material. The complex formed between Cr(VI) and ammonium pyrrolidine dithiocarbamate (APDC) is sorbed on the PTFE beads, and is subsequently eluted by an air-monosegmented discrete zone of absolute ethanol (35 μl), the analyte being quantified by ETAAS.The preconcentration procedure using the proposed column significantly enhances the preconcentration efficiency as compared with the preconcentration approach incorporating an open tubular PTFE knotted reactor (KR). Comparing the two procedure for equal surface sorption area, the advantages of using a packed column are observed in terms of limit of detection, enrichment factor and retention efficiency. With a preconcentration time of 60 s, and a sample flow rate of 5.0 ml l⁻¹, the enrichment factor (30.1) and the retention efficiency (24.1%) were doubled, yielding a detection limit (3σ) as low as 8.8 ng l⁻¹. The sample frequency was 16.7 h⁻¹. The concentration efficiency was 8.38 and the precision was 1.05% at 0.5 μg l⁻¹ of Cr(VI). The proposed column has been applied successfully to the analysis of natural water and synthetic seawater. Its performance was verified by the analysis of two certified Cr(VI)-reference materials and by recovery measurements on spiked samples.     

On-line preconcentration and speciation analysis of Cr(III) and Cr(VI) using baker's yeast cells immobilised on controlled pore glass

A study was undertaken to evaluate Saccharomyces cerevisiae as a substrate for the biosorption of Cr(III) and Cr(VI) aiming to the selective determination of these species in aqueous solutions. The yeast cells were covalently immobilised on controlled pore glass (CPG), packed in a minicolumn and incorporated in an on-line flow injection system. The effect of chemical and physical variables affecting the biosorption process was tested in order to select the optimal analytical conditions for the Cr retention by S. cerevisiae. Cr(III) was retained by the immobilised cells and Cr(VI) were retained by CPG. The speciation was possible by selective and sequential elution of Cr(III) with 0.05 mol L⁻¹ HCl and 2.0 mol L⁻¹ HNO₃ for Cr(VI). The influence of some concomitant ions up to 20 mg L⁻¹ was also tested. Quantitative determinations of Cr were carried out by means of inductively coupled plasma optical emission spectrometry (ICP OES). Preconcentration factors of 12 were achieved for Cr(III) and 5 for Cr(VI) when 1.7 mL of sample were processed reaching detection limits of 0.45 for Cr(III) and 1.5 μg L⁻¹ for Cr(VI). The speciation of inorganic Cr in different kinds of natural waters was performed following the proposed method. Spiked water samples were also analysed and the recoveries were in all cases between 81 and 103%.