Speciation of chromium by in-capillary reaction and capillary electrophoresis with chemiluminescence detection

A sensitive method for the simultaneous determination of chromium(III) (Cr3+) and chromium(VI) (CrO4(2-)) using in-capillary reaction, capillary electrophoresis (CE) separation and chemiluminescence (CL) detection was developed. The chemiluminescence reaction was based on luminol oxidation by hydrogen peroxide in basic aqueous solution catalyzed by Cr3+ ion followed by capillary electrophoresis separation. Based on in-capillary reduction, chromium(VI) can be reduced by acidic sodium hydrogensulfite to form chromium(III) while the sample is running through the capillary. Before the electrophoresis procedure, the sample (Cr3+ and CrO4(2-)), buffer and acidic sodium hydrogensulfite solution segments were injected in that order into the capillary, followed by application of an appropriate running voltage between both ends. As both chromium species have opposite charges, Cr3+ ions migrate to the cathode, while CrO4(2-) ions, moving in the opposite direction toward the anode, react with acidic sodium hydrogensulfite which results in the formation of Cr3+ ions. Because of the migration time difference of both Cr3+ ions, Cr(III) and Cr(VI) could be separated. The running buffer was composed of 0.02 mol l(-1) acetate buffer (pH 4.7) with 1 x 10(-3) mol l(-1) EDTA. Parameters affecting CE-CL separation and detection, such as reductant (sodium hydrogensulfite) concentration, mixing mode of the analytes with CL reagent, CL reaction reagent pH and concentration, were optimized. The limits of detection (LODs) of Cr(III) and Cr(VI) were 6 x 10(-13) and 8 x 10(-12) mol l(-1) (S/N=3), respectively. The mass LODs for Cr(III) and Cr(VI) were 1.2 x 10(-20) mol (12 zmol) and 3.8 x 10(-19) mol (380 zmol), respectively.     

Simultaneous, sensitive and selective on-line chemiluminescence determination of Cr(III) and Cr(VI) by capillary electrophoresis

A novel in-capillary reduction and capillary electrophoretic (CE)-chemiluminescence (CL) method was developed for the sensitive and selective determination of chromium(III) and chromium(VI). The proposed method was based on the in-capillary reduction of Cr(VI) with acidic H₂O₂ to form Cr(III) using the zone-passing technique and chemiluminescence detection of Cr(III). The sample [Cr³⁺ and CrO₄²⁻], hydrochloric acid, and H₂O₂ (reductant) solution segments were injected for specified periods of time in this order from the anodic end of a capillary, followed by application of an appropriate running voltage between both ends. As both chromium species have opposite charges, Cr³⁺ migrates to the cathode while CrO₄²⁻ ion, moving oppositely to the anode, reacts with acidic H₂O_2, resulted in formation of Cr³⁺. Based on the migration time difference of both Cr³⁺ ions, they were separated by zone electrophoresis. Running buffer was composed of 0.02 mol l⁻¹ HAc-NaAc (pH 4.7) with 1×10⁻³ mol l⁻¹ EDTA. Parameters affecting CE-CL separation and detection, such as reductant concentration, mixing mode of the analytes with CL reagent, CL reaction reagent pH and concentration, stability of luminol-hydrogen peroxide mixed solution were optimized. The limits of detection for chromium(III) and chromium(VI) (3σ) were 6×10⁻¹³ mol l⁻¹ (mass concentration 12 zmol) and 8×10⁻¹² mol l⁻¹ (160 zmol), respectively. This method offered potential advantages of simplicity, sensitivity, selectivity and applicability to the determination of Cr(III) and Cr(VI) in environmental water.     

Sequential injection analysis for Cr(VI) and Cr(III) with renewable surface reflection spectrophotometry.

On the basis of the chromogenic reaction of chromium(VI) with 1,5-diphenylcarbohydrazide (DPC) on the surface of Polysorb C-18 beads and the sequential injection renewable surface technique (SI-RST), a highly sensitive reflect spectrophotometric method for the determination of chromium(III) and chromium(VI) was proposed. Considerations of system and flow cell design, and factors that influence the determination performance were discussed. With 300 microl of sample loaded and 0.6 mg of beads trapped, the linear response range was 0.02 - 0.5 mg l(-1) Cr(VI) with a detection limit (3 sigma) of 2.4 microg l(-1) Cr(VI). The method achieves a precision of 1.3% RSD (n = 11) and a throughput of 53 samples per hour. The determination of Cr(III) was based on the same reaction for the determination of Cr(VI) after being oxidized by (NH4)2S2O8. The precision of the oxidation method was evaluated using a 0.2 mg l(-1) Cr(III) standard, yielding an RSD of 2.5% (n = 11). The average recovery of Cr(III) oxidized was tested to be 99.1%. The proposed method was used in the simultaneous determination of Cr(VI) and Cr(III) in water samples, and the error was less than 3%.     

Amperometric Determination of Cr(III) and Cr(VI) by Flow Injection Analysis

Speciation of Cr(III) and Cr(VI) can be attained by flow injection analysis with amperometric detection. Cr(VI) is reduced in an acidic medium to Cr(III) with a glassy carbon electrode at —0.1 V vs. Ag/AgCl and the current is recorded. Cr(III) is oxidised on-line to Cr(VI) with alkaline hydrogen peroxide solution. From the difference of the total chromium and Cr(VI), the amount of Cr(III) was obtained. A linear calibration curve for Cr(VI) was obtained for the concentration ranges 0.01-5.0ppm of Cr(VI) and we have calculated the limit of determination to be about 0.5ppb. We have studied the degree of reproducibility obtained using the solid electrodes under various conditions. The influence of flow rate, coil length, interfenences and the extent of reaction were studied.     

Speciation of chromium by selective separation and preconcentration of Cr(III) on an immobilized nanometer titanium dioxide microcolumn

Nanometer titanium dioxide immobilized on silica gel (immobilized nanometer-scale TiO2 particles) was prepared by a sol-gel method and characterized by X-ray diffraction and scanning electron microscopy. The adsorptive behavior of Cr(III) and Cr(VI) on immobilized nanometer TiO2 was assessed. Cr(III) was selectively sorbed on immobilized nanometer TiO2 in the pH range of 7-9, while Cr(VI) was found to remain in solution. A sensitive and selective method has been developed for the speciation of chromium in water samples using an immobilized nanometer TiO2 microcolumn and inductively coupled plasma atomic emission spectrometry. Under optimized conditions (pH 7.0, flow rate 2.0 mL/min), Cr(III) was retained on the column, then eluted with 0.5 mol/L HNO3 and determined by ICP-AES. Total chromium was determined after the reduction of Cr(VI) to Cr(III) by ascorbic acid. The adsorption capacity of immobilized nanometer TiO2 for Cr(III) was found to be 7.04 mg/g. The detection limit for Cr(III) was 0.22 ng/mL and the RSD was 3.5% (n = 11, c = 100 ng/ mL) with an enrichment factor of 50. The proposed method has been applied to the speciation of chromium in water samples with satisfactory results.     

Speciation of Cr(III) and Cr(VI) in potable waters by using activated neutral alumina as collector and ET-AAS for determination

The speciation of Cr(III) and Cr(VI) has been performed by using activated neutral alumina as adsorbent. Both species were quantitatively adsorbed on a small column filled with neutral alumina. The adsorbed Cr(III) was eluted with 4 mol L(-1) HNO(3) and Cr(VI) with 1.0 mol L(-1) ammonia solution. Recoveries of Cr(III) and Cr(VI) were 99% and 100%, respectively. Using ET-AAS for Cr determination the limit of detection in the sample was 0.01 microg L(-1). The combined procedure is fast and sensitive. It can be applied for routine analysis of water samples at sub-microg L(-1) levels with a relative standard deviation (RSD) of 2-10% (three determinations).     

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.     

Preconcentration and separation of Cr(III) and Cr(VI) using sawdust as a sorbent

A simple, inexpensive method based on solid-phase extraction (SPE) on sawdust from Cedrus deodera has been developed for speciation of Cr(III) and Cr(VI) in environmental water samples. Because different exchange capacities were observed for the two forms of chromium at different pH—Cr(III) was selectively retained at pH 3 to 4 whereas Cr(VI) was retained at pH 1—complete separation of the two forms of chromium is possible. Retained species were eluted with 2.5 mL 0.1 mol L⁻¹ HCl and 0.1 mol L⁻¹ NaOH. Detection limits of 0.05 and 0.04 μg mL⁻¹ were achieved for Cr(III) and Cr(VI), respectively, with enrichment factors of 100 and 80. Recovery was quantitative using 250 mL sample volume for Cr(III) and 200 mL for Cr(VI). Different kinetic and thermodynamic properties that affect sorption of the chromium species on the sawdust were also determined. Metal ion concentration was measured as the Cr(VI)–diphenylcarbazide complex by UV–visible spectroscopy. The method was successfully applied for speciation of chromium in environmental and industrial water samples.     

Speciation of Cr(III) and Cr(VI) by Means of Melamine-Urea-Formaldehyde Resin and FAAS

 A method is described for the quantitative preconcentration and separation of trace chromium in water by adsorption on melamine-urea-formaldehyde resin. Cr(VI) is enriched from aqueous solutions on the resin. After elution the Cr(VI) is determined by FAAS. The capacity of the resin is maximal at ∼ pH 2. Total chromium can be determined by the method after oxidation of Cr(III) to Cr(VI) by hydrogen peroxide. The relative standard deviations (10 replicate analyses) for 10 mg/L levels of Cr(VI), Cr(III) and total chromium were 1.5, 3.5 and 2.8% respectively. The procedure has been applied to the determination and speciation of chromium in lake water, tap water and chromium-plating baths.     

Separation of Cr(III) and Cr(VI) using melamine-formaldehyde resin and determination of both species in water by FAAS

A method is described for the determination of Cr(VI) and total chromium by FAAS. Cr(VI) is separated from Cr(III) by adsorption on melamine-formaldehyde resin. After elution of Cr(VI) with 0.1 mol/l NaAc solution, it is analysed by FAAS. Total chromium is determined by FAAS after conversion of Cr(III) to Cr(VI) by oxidation with hydrogen peroxide, total Cr(VI) is concentrated as above. If the total concentration of chromium is sufficient, the determination can be directly made by FAAS. Cr(III) can then be calculated by subtracting Cr(VI) from the total Cr. This method was successfully applied to the determination of chromium in lake water.     

Speciation of Cr(III) and Cr(VI) in water after preconcentration of its 1,5-diphenylcarbazone complex on amberlite XAD-16 resin and determination by FAAS

A simple and sensitive method for the speciation, separation and preconcentration of Cr(VI) and Cr(III) in tap water was developed. Cr(VI) has been separated from Cr(III) and preconcentrated as its 1,5-diphenylcarbazone complex by using a column containing Amberlite XAD-16 resin and determined by FAAS. Total chromium has also been determined by FAAS after conversion of Cr(III) to Cr(VI) by oxidation with KMnO₄. Then, Cr(III) has been calculated by subtracting Cr(VI) from the total. The effect of acidity, amount of adsorbent, eluent type and flow rate of the sample solution on to the preconcentration procedure has been investigated. The retained Cr(VI) complex was eluated with 10 ml of 0.05 mol l⁻¹ H₂SO₄ solution in methanol. The recovery of Cr(VI) was 99.7±0.7 at 95% confidence level. The highest preconcentration factor was 25 for a 250 ml sample volume. The detection limit of Cr(VI) was found as 45 μg l⁻¹. The adsorption capacity of the resin was found as 0.4 mg g⁻¹ for Cr (VI). The effect of interfering ions has also been studied. The proposed method was applied to tap water samples and chromium species have been determined with the relative error <3%.     

Cr(III) and Cr(VI) on-line preconcentration and determination with high performance flow flame emission spectrometry in natural samples

Methods for the on-line chromatographic preconcentration of Cr(III) and Cr(VI) have been developed. Cr(VI) has been preconcentrated on an RP C18 silica based column with tetrabutylammonium-bromide (TBABr) as ion-pairing agent. Specially for Cr(III) a new and effective preconcentration technique based on the sorption of Cr(III)-ions in a C18 column in presence of KH-phthalate has been developed. The efficiency of sample introduction into the atomic emission spectrometer could be improved by hydraulic high pressure nebulization. For the detection of chromium the acetylene/N(2)O flame has been used as a powerful emission spectrometric source. Applying these steps the detection limit (3sigma) could be improved to 25 pg/mL for Cr(III) and to 20 pg/mL for Cr(VI). The method has been applied for the chromium speciation in natural water samples.     

Determination of Cr3+, CrO42-, and Cr2O72- in environmental matrixes by high-performance liquid chromatography with diode-array detection (HPLC-DAD)

A high-performance liquid chromatographic method with diode array detection (HPLC-DAD), based on chelation with ammonium pyrrolidinedithiocarbamate (APDC), has been developed for the determination of chromium species. Determination of Cr3+, CrO42-, and Cr2O72- was performed for standards and synthetic environmental matrixes. This method is robust, rugged, and can be used for rapid routine determination of chromium species with high precision and reliability. Sample pretreatment is simple. The method is capable of discriminating not only between Cr(III) and Cr(VI) but also between the chemical forms of Cr(VI) - CrO42- and Cr2O72-. By analysis of numerous samples the method has been shown to be selective, sensitive, and free from matrix interference, which is crucial for the determination of chromium species in difficult-to-analyze environmental matrixes. This method has been validated by means of an interlaboratory study. Although different speciation techniques were used during this study, there was good agreement between results from the two laboratories. The method detection limits were 7 and 4 mg L(-1) for Cr3+ and Cr2O72-, respectively. Recoveries of the analytes from spiked samples were 98% and 100% for Cr3+ and Cr2O72-, respectively. Both were based on a 10-mL sample volume spiked with 0.4 mg L(-1) chromium.     

Rapid speciation analysis of Cr(VI) and Cr(III) by reversed-phase high-performance liquid chromatography with UV detection

A simple and rapid method is developed for the simultaneous determination of Cr(VI) and Cr(III) based on the formation of their different complexes with ammonium pyrrolidine-dithiocarbamate (APDC). Separation is performed using reversed-phase high-performance liquid chromatography coupled with UV detection. The conditions for complex formation and speciation are determined, such as solution pH, amount of APDC, temperature, and type of mobile phase. In order to substantially reduce the analysis time, the separation is carried out without extraction of chromium-APDC complexes from the mother liquor. Under the optimum analysis conditions, the chromatograms obtained show good peak separation, and the absolute detection limits (3s) are 2.2 microg/L for Cr(VI) and 4.5 microg/L for Cr(III). The calibration curves are linear from 3 to 5000 microg/L for Cr(VI) and 5 to 3000 microg/L for Cr(III). The relative standard deviations of peak areas in five measurements using a sample solution of 200 microg/L are less than 2% for Cr(VI) and 4% for Cr(III), indicating good reproducibility for this analytical method. Furthermore, simultaneous determination of Cr(VI) and Cr(III) is successful with the application of the proposed procedure in the synthetic wastewaters containing common heavy metal ions: Fe(III), Pb(II), Cd(II), Cu(II), and Zn(II).     

Separation of Cr(III) and Cr(VI) in river and reservoir water with 8-hydroxyquinoline immobilized polyacrylonitrile fiber for determination by inductively coupled plasma mass spectrometry

A rapid and simple method has been developed for the separation of chromium (III) and Cr(VI) species in river and reservoir water. Chromium (III) can be chelated with 8-hydroxyquinoline immobilized polyacrylonitrile (PAN) fiber, whereas Cr(VI) cannot. Chelated Cr(III) can be eluted with 2 mol l(-1) HCl-0.1 mol l(-1) HNO(3). Cr(VI) in the filtrate and Cr(III) in the eluant were determined by inductively coupled plasma mass spectrometry. The effect of pH, sample flow rate, eluant type and its volume on the concentration effectiveness of Cr(III) was investigated. The recommended method has been applied for the separation and determination of Cr(III) and Cr(VI) in river and reservoir water. The results indicated that the recovery of each individual Cr species ranged from 96 to 107% and the R.S.D. were found to be <10% at the level of ng ml(-1). The effect of HNO(3) added in the sampling procedure was also evaluated.     

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.     

Cr(III) and Cr(VI) on-line preconcentration and determination with high performance flow flame emission spectrometry in natural samples

Methods for the on-line chromatographic preconcentration of Cr(III) and Cr(VI) have been developed. Cr(VI) has been preconcentrated on an RP C18 silica based column with tetrabutylammonium-bromide (TBABr) as ion-pairing agent. Specially for Cr(III) a new and effective preconcentration technique based on the sorption of Cr(III)-ions in a C18 column in presence of KH-phthalate has been developed. The efficiency of sample introduction into the atomic emission spectrometer could be improved by hydraulic high pressure nebulization. For the detection of chromium the acetylene/N₂O flame has been used as a powerful emission spectrometric source. Applying these steps the detection limit (3) could be improved to 25 pg/mL for Cr(III) and to 20 pg/mL for Cr(VI). The method has been applied for the chromium speciation in natural water samples.     

Speciation of chromium (III) and chromium (VI) by capillary electrophoresis with contactless conductometric detection and dual opposite end injection

A sensitive, rapid and inexpensive capillary electrophoretic method for the determination of Cr(III) and Cr(VI) species is presented. The method is based on the dual opposite end injection principle and contactless conductometric detection. The sample containing cationic and anionic species is injected into the opposite ends of the separation capillary and after the high voltage is applied, the analytes migrate towards the capillary center, where the cell of a contactless conductivity detector is placed. The method does not require any sample pretreatment, except dilution with deionized water. The separation of Cr(III), Cr(VI) and other common inorganic anions and cations is achieved in less than 4 min. The parameters of the separation electrolyte solution, such as pH and concentration of L-histidine, were optimized. Best results were achieved with electrolyte solution consisting of 4.5 mM L-histidine, adjusted to pH 3.40 with acetic acid. The detection limits achieved for Cr(III) and Cr(VI) were 10 and 39 microg.L(-1), respectively. The repeatability of migration times and peak areas was better than 0.3% and 2.8%, respectively. The developed method was applied to the analyses of rinse water samples from the galvanic industry. The results for the determination of Cr(III) and Cr(VI) were in good agreement with the results obtained by certified differential spectrophotometric method using diphenylcarbazide (CN 83 0520-40) and with the results for the total chromium concentrations determined by electrothermal atomic absorbance spectrometry (ET-AAS) and inductively coupled plasma-mass spectrometry (ICP-MS).     

Cr(III)/Cr(VI) speciation in aerosol particles by extractive separation and thermal ionization isotope dilution mass spectrometry

 An isotope dilution mass spectrometric (IDMS) method, using the formation of positive thermal ions, was developed for Cr(III) and Cr(VI) speciation in aerosol particles. Cr(III) and Cr(VI) spike species, enriched in ⁵³Cr, were applied for the isotope dilution step. After leaching of filter collected aerosol samples by an alkaline solution at pH 13, species separation was carried out by extraction with a liquid anion exchanger in methyl isobutyl ketone. Cr(VI) in the organic phase was re-extracted into an ammoniacal solution and chromium was then isolated from both fractions of species by electrodeposition. Detection limits of 30 pg/m³ for Cr(III) and of 8 pg/m³ for Cr(VI) were achieved in atmospheric aerosols for volumes of air samples of about 120 m³. These low detection limits allowed the determination of chromium species in continental aerosol particles in dependence on different seasons. The Cr(III) /Cr(VI) ratio was always found to be about 0.3 whereas dust from soil erosion, which is probably the primary source of chromium in the atmosphere, showed higher ratios. This indicates that chromium is oxidized in the atmosphere. The accuracy of the method was demonstrated in two interlaboratory comparisons of Cr(VI) determinations in welding dust samples. The IDMS method also contributed to the certification of a corresponding standard reference material organized by the Standard Reference Bureau of the European Union. Chromium speciation, including the determination of elemental chromium Cr(0), was carried out in aerosols of different welding processes for stainless steel. These analyses showed distinct differences in the distribution of chromium species in the welding process and can be used as an exact calibration method for routine methods in this important field of monitoring corresponding working places. Received: 19 August 1996/Revised: 24 September 1996/Accepted: 28 September 1996     

Speciation of Cr(III) and Cr(VI) in Environmental Samples after Solid Phase Extraction on Amberlite XAD–2000

A method for speciation of Cr(III) and Cr(VI) in real samples has been developed. Cr(VI) has been separated from Cr(III) and preconcentrated as its pyrrolidinedithiocarbamate (APDC) complex by using a column containing Amberlite XAD–2000 resin and determined by FAAS. Total chromium has also been determined by FAAS after conversion of Cr(III) to Cr(VI) by oxidation with KMnO₄. Cr(III) has been calculated by subtracting Cr(VI) from the total. The effect of pH, flow‐rate, adsorption and batch capacity and effect of various metal cations and salt anions on the sorption onto the resin were investigated. The adsorption is quantitative in the pH range of 1.5–2.5, and Cr(VI) ion was desorbed by using H₂SO₄ in acetone. The recovery of Cr(VI) was 97 ± 4 at a 95% confidence level. The highest preconcentration factor was 80 for a 200 mL sample volume. The adsorption and batch capacity of sorbent were 7.4 and 8.0 mg g^?1 Cr(VI), respectively, and loading half time was 5.0 min. The detection limit of Cr(VI) is 0.6 μg/L. The procedure has been applied to the determination and speciation of chromium in stream water, tap water, mineral spring water and spring water. Also, the proposed method was applied to total chromium preconcentration in microwave digested moss and rock samples with satisfactory results. The developed method was validated with CRM‐TMDW‐500 (Certified Reference Material Trace Metals in Drinking Water) and BCR‐CRM 144R s (Certified Reference Material Sewage Sludge, Domestic Origin) and the results obtained were in good agreement with the certified values. The relative standard deviations were below 6%.