Microchemical Journal 93 (2009) 220–224

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Microchemical Journal

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A procedure for determination of cobalt in water samples after dispersiveliquid–liquid microextraction

Patrícia Xavier Baliza a,b, Leonardo Sena Gomes Teixeira a, Valfredo Azevedo Lemos b,⁎a Universidade Federal da Bahia, Instituto de Química, Campus Universitário de Ondina, 40.170-280 Salvador, Bahia, Brazilb Universidade Estadual do Sudoeste da Bahia, Núcleo de Química Analítica da Bahia (NQA), Laboratório de Química Analítica (LQA), Campus de Jequié, 45.506-191 Jequié, Bahia, Brazil

⁎ Corresponding author. Fax: +55 73 35289630.E-mail address: valfredoazevedo@yahoo.com.br (V.A

0026-265X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.microc.2009.07.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 July 2009Accepted 27 July 2009Available online 3 August 2009

Keywords:PreconcentrationDispersive liquid–liquid microextractionCobaltFAASWater samples

In this work, a procedure for preconcentration of cobalt using dispersive liquid–liquid microextraction(DLLME) with the reagent Br-TAO as complexing reagent was developed. The procedure is based on aternary system of solvents, where appropriate amounts of the extraction solvent, disperser solvent and thechelating agent Br-TAO are directly injected into an aqueous solution containing Co(II). A cloudy mixture isformed and the ions are extracted in the fine droplets of the extraction solvent. After extraction, the phaseseparation is performed with a rapid centrifugation, and cobalt is determined in the enriched phase by FAAS.Under the optimized conditions, the detection limit obtained was 0.9 µg L−1. The enrichment factor and theconsumptive index were 16 and 0.31 mL, respectively. The accuracy of the method was tested by thedetermination of cobalt in certified reference material of spinach leaves, NIST 1570a. The proposedprocedure was successfully applied to the determination of cobalt in water samples.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Some trace elements, such as cobalt, are essential to man, whosedaily requirement is only a few milligrams. However, if ingested inhigh levels, this can be harmful to human health. Thus, the elementalcomposition is essential to ensure food quality. Furthermore, thecontent of the element inwater, sediment, plants and animals can alsoprovide important information on the levels of contamination in theenvironment [1,2].

The quantification of metal species in various matrices has beenperformed by different techniques, including spectrophotometry,atomic absorption spectrometry (AAS), and inductively coupledplasma optical emission spectrometry (ICP OES), among others.However, these techniques do not have adequate sensitivity andselectivity for some analyses. Thus, procedures of separation orpreconcentration may be required before the spectrometric determi-nation of trace elements [3,4].

Techniques such as co-precipitation [5], solid phase extraction (SPE)[6–8], cloud point extraction (CPE) [9,10] and liquid–liquid extraction(LLE) [11,12] are widely used in the separation and preconcentration oftrace elements. A new trend in analytical chemistry isminiaturization ofpreconcentration systems with the aim of minimizing reagent con-sumption and waste generation [13]. For liquid–liquid extraction,alternatives of miniaturization can be employed with strategies suchas liquid–liquid microextraction (LLME) [14], single-drop microextrac-tion (SDME)anddispersive liquid–liquidmicroextraction (DLLME) [15].

. Lemos).

ll rights reserved.

Dispersive liquid–liquid microextraction is a preconcentrationtechnique that employs a ternary system of solvents. This techniquewas reported for the first time in a procedure for the determination oforganophosphorus pesticides in water [16]. However DLLME has alsobeen used for the extraction and preconcentration of inorganiccompounds, offering advantages such as ease of operation, use ofsmall quantities of sample and organic solvents, speed of analysis, lowcost and high recoveries and enrichment factors [17–21].

In DLLME, a mixture containing appropriate amounts of extractionsolvent and disperser solvent is injected rapidly into an aqueoussample with the aid of a syringe. Then, a cloudy solution is formed andthe analyte is extracted into the interior of the droplets of theextraction solvent. After extraction, the phase separation is accom-plished by centrifugation, and the analyte is determined in thesedimented phase. For the determination of metal trace elements, acomplexing reagent should be dissolved in the mixture [22].

In this work, a procedure for preconcentration of cobalt usingdispersive liquid–liquid microextraction with the reagent Br-TAO ascomplexing reagent was developed. After optimization of experimentalvariables and determination of analytical features, the procedure wasapplied to the determination of cobalt in water samples.

2. Experimental

2.1. Instrumentation

A Perkin Elmer model AAnalyst 200 flame atomic absorptionspectrometer equipped with deuterium lamp for background correc-tionwas used formeasures of absorbance. The hollow cathode lamp of

221P.X. Baliza et al. / Microchemical Journal 93 (2009) 220–224

Co was used as a source of radiation and operated at a wavelength of279.4 nm. Acetylene and air flow rates were 2.5 and 10.0 Lmin−1,respectively. Nebulizer flow rate was 4.0 mL min−1.

A centrifuge (Bio Eng, model BE 5000) was used to accelerate thesedimentation of the rich phase in the process of microextraction. Thesystem for the introduction of the rich phase in the spectrometerconsisted of a six-port manual valve (Rheodyne, model 5041) and afour-channel peristaltic pump (Milan, Model 204). Silicone and Teflontubing and a plastic syringe were also used. For pH measurements, aDigimed, model DM 20 pH meter was used. The digestion of certifiedreference material was performed using a Parr model 4749 aciddigestion bomb.

Fig. 1. System for injection of rich phase in flame atomic absorption spectrometer(FAAS). V, six-port rotary valve; L, loop; Sy, syringe; P, peristaltic pump; S, rich phase. A,filling of the loop; B, transport to the spectrometer.

2.2. Reagents

Deionized water was used in the preparation of all solutions. Theglassware was kept overnight in a 5% (v/v) nitric acid solution at leastovernight and subsequently washed with deionized water. Allreagents used were of analytical grade. Working solutions of cobaltat µg L−1 level were prepared daily by diluting a 1000 µg mL−1 stocksolution (Merck).

Solutions of Br-TAO were prepared by dissolving appropriateamounts of the laboratory-prepared solid [23] in absolute ethanol(Merck). The pH of cobalt solutions was adjusted with the aid ofacetate (pH 4.7–5.5), phosphate (pH 6.0–6.5), borate (pH 7.0–8.0) andammonia (pH 9.0–9.3) buffer solutions. Chloroform (Synth), carbontetrachloride (Merck), methanol (Synth), acetone, isopropanol andethanol (Synth) were used in the procedure for microextraction asextractor or dispersive solvents. For accuracy studies, certifiedreference material of spinach leaves, NIST 1570a, was analyzed.

2.3. General procedure for DLLME

A solution containing cobalt was adjusted to pH with anappropriate buffer solution. Five milliliters of the cobalt solutionwere added to a test tube. Then, a solution containing 50.0 µL ofcarbon tetrachloride, 2.0 mL of methanol, and 50.0 µL of a Br-TAOsolution was prepared. The methanolic solution was immediatelyinjected in the solution of the metal using a 5.0 mL glass syringe. Theresulting cloudy solution was centrifuged for 2.0 min at 5000 rpm.After centrifugation, a reddish residue was sedimented at the bottomof the tube. This residuewas injected into the flame atomic absorptionspectrometer (FAAS) using the system described in the followingitem.

2.4. Introduction of the rich phase in detection equipment

Many procedures using DLLME for preconcentration of metalsemploy electrothermal atomic absorption spectrometry (ETAAS) as atechnique of detection [24]. In this case, the introduction of rich phaseis simple, because the detection equipment requires a small anddefined sample volume. When FAAS is used, special attention shouldbe taken in the introduction of the rich phase, because the samplevolume required for the direct reading in equipment is usually greaterthan that obtained when microextraction procedure is employed. Inthis work, a simple injection device was used in FAAS. The schematicrepresentation of the strategy used for sample introduction is shownin Fig. 1. The system consists of a six-port valve, a peristaltic pump, aplastic syringe and capillary tubes of Teflon. The rich phase isaspirated using a plastic syringe until the liquid fill the entire volumeof the sample loop. Then, the position of the valve is changed and astream of water carries the enriched phase into the FAAS and atransient signal is obtained. The volume of sample loop was set at20 µL.

2.5. Sample preparation

Water samples were collected in tap, well and river in Jequié,Bahia, Brazil. These water samples were filtered; the pH was adjustedwith buffer solution and analyzed immediately after collection. Thefollowing certified reference material was also analyzed: NIST 1570a,spinach leaves purchased from the National Institute of Standards andTechnology. For the digestion of this material, approximately 0.10 g ofsample was weighed into a PTFE cup and four milliliters of a 1:1 (v/v)nitric acid solution were added [14]. The cup containing the mixturewas sealed in an acid digestion bomb. The system was then kept inoven for 4.0 h at 150 °C. After cooling, the pH of the mixture wasadjusted with sodium hydroxide and hydrochloric acid solutions.Then a buffer solution was then added and each sample was subjectedto the preconcentration procedure.

3. Results and discussion

3.1. Optimization of experimental conditions

A univariate optimization strategy was used in order to evaluatethe experimental parameters that affect the dispersive liquid–liquidmicroextraction of cobalt. A 100.0 µg L−1 Co (II) solution was used inthese experiments.

In procedures using DLLME, the dispersive solvent must be misciblein both aqueous phase and extraction solvent. Thus, acetone, ethanol,isopropanol andmethanol were tested as dispersive solvents. The effectof these solvents in preconcentration of cobalt by DLLME wasinvestigated, using 2000 µL of each solvent with 50 µL of the extractionsolvent, carbon tetrachloride, and 50 µL of 1.3×10−3mol L−1 Br-TAOsolution. According to the results presented in Fig. 2, higher analyticalsignals were obtained using methanol as dispersive solvent. Thus, thissolvent was used as dispersive solvent.

The effect of the amount of methanol, the dispersive solvent, wasalso evaluated. Experiments were performed using different volumesof methanol, ranging from 0.50 to 2.50 mL. The volume of rich phaseshould be kept constant, for a better comparison of results. Thus, thevolume of the extraction solvent, carbon tetrachloride was varied

Fig. 2. Influence of dispersive solvent on DLLME of cobalt using Br-TAO.

Fig. 3. Influence of amount of extraction solvent on dispersive solvent in DLLME ofcobalt using Br-TAO.

Fig. 4. Influence of pH in dispersive solvent on dispersive li DLLME of cobalt using Br-TAO.

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simultaneously with the amount of dispersive solvent. The volumes ofdispersive solvent of 0.50, 1.00, 1.50, 2.00 and 2.50 mL, correspondingto the volumes of extraction solvent of 48.5, 50.0, 51.0, 52.0 and53.5 µL, respectively, were tested. The volume of rich phase wasobtained experimentally by approximately 25 µL. Best results wereobtained when a volume of 2.0 mL of methanol was used. When asmall amount of methanol was used, it failed to properly disperse theextraction solvent, and the turbid solution was not formed completely.In contrast, when an excessive amount of dispersive solvent was used,the solubility of the complex of cobalt with the Br-TAO in the aqueousphase increases, damaging the extraction. Thus, a volume of 2.0 mL ofmethanol was used in subsequent experiments.

The extraction solvent used in DLLME should present the samedesirable characteristics for solvents used in conventional liquid–liquid extraction. It is desirable that this solvent has low solubility inwater, high capacity for extraction of components of interest, in thiscase the system Co(II) — Br-TAO, and density greater than that of theaqueous phase. Two solvents that have these properties were tested:chloroform and carbon tetrachloride. The solution remained clearwhen chloroformwas used as solvent for extraction. In this case, therewas no separation of phases after centrifugation. The use of carbontetrachloride allowed the formation of a cloudy solution. Then, carbontetrachloride was adopted as extraction solvent.

The influence of the amount of carbon tetrachloride used indispersive liquid–liquid microextraction of cobalt using Br-TAO wasinvestigated. Experiments were performed using volumes of carbontetrachloride ranging from 50, 60, 70 and 80 µL. The results are shownin Fig. 3 and, as can be seen, the signal obtained decreases for anincrease in the volume of the extraction solvent. This is due to increasein volume for the same mass of the component analyzed. Volumessmaller than 50 µL were not tested because of the difficulty inintroducing these quantities in FAAS. In subsequent experiments,50 µL of extraction solvent was used.

The pH of the aqueous solution is an important factor in dispersiveliquid–liquid microextraction of cobalt using Br-TAO, because thisparameter is directly related to the formation of metal-ligand species.The pH of the cobalt solution was varied by using acetate (pH 4.8 and5.5), phosphate (pH 6.0 and 6.5), borate (pH 7.0, 7.5, and 8.0) andammonia (pH 9.0 and 9.25) buffer solutions. Solutions of cobalt100.0 µg L−1 with each buffer solutionwere prepared. These solutionswere submitted to preconcentration procedure. According to Fig. 4,the pH range with maximum extraction of cobalt is between 7.0 and8.0. Borate buffer pH 7.5 was used in subsequent experiments toadjust the pH of the cobalt solution.

The influence of the quantity of reagent Br-TAO in preconcentra-tion of cobalt was evaluated, varying the concentration of the solutionof this reagent. Br-TAO solutions at different concentrations in ethanolwere prepared. Each of these solutions was mixed with the extractionsolvent and dispersive solvent, and injected in aqueous solution of

cobalt. The result obtained for each solution is shown in Fig. 5. Bestresultswere obtainedwhen concentrations in the range of 9.6×10−4 to1.6×10−3mol L−1 were used. Two solutions of Br-TAO (1.3×10−3molL−1) were prepared with different solvents, ethanol and methanol.These solutions were submitted to preconcentration procedure. Theresultswere similar for both solvents. Then, ethanolwasused topreparea 1.3×10−3mol L−1 Br-TAO solution, due to its lower toxicity.

The effect of time in dispersive liquid–liquid microextraction ofcobalt using Br-TAO was studied. Aliquots of 10.0 mL of 100 mg L−1

cobalt solution were added to different test tubes. The solutioncontaining Br-TAO, methanol and carbon tetrachloride was theninjected into each tube. The time interval between injection and thestart of centrifugation was then varied, using the values of 0.0, 1.0, 2.0,3.0, 4.0 and 5.0 min. Results revealed that the time, in the rangestudied, does not affect the efficiency of microextraction. Thisindicates that the extraction process is very fast, probably due to thelarge area of contact between the extraction solvent and aqueousphase. In subsequent experiments, the phase separation by centrifu-gation was performed immediately after mixing the reagents.

The influence of centrifugation time in dispersive liquid–liquidmicroextraction of cobalt using Br-TAO was also studied. The resultsshowed that the maximum extraction is obtained when the time ofcentrifugation was between 1.0 and 3.0 min. If the period ofcentrifugation is more than 3.0 min, the analytical signal decreases.Presumably this is due to the dispersion of droplets of the extraction

Fig. 5. Influence of concentration of complexing reagent on dispersive solvent in DLLMEof cobalt using Br-TAO.

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solvent in the aqueous phase. To enable a rapid procedure, animportant feature in many analytical applications, a time ofcentrifugation of 2.0 min was used in all subsequent experiments.

3.2. Effect of other ions

The effect of various ions in dispersive liquid–liquid microextrac-tion of cobalt using Br-TAO was studied under the optimizedconditions. Solutions containing cobalt (100.0 µg L−1) and otherions at various amounts were prepared and were subjected topreconcentration procedure. The criterion for interference of eachspecies was set at ±5.0% in the analytical signal obtained for asolution containing cobalt, without any interfering. Table 1 shows thesubstances studied and their maximum amounts tolerable.

Table 2Analytical features of proposed procedure.

Limit of detection 0.9 µg L−1

3.3. Analytical features

The analytical characteristics of the proposed procedure werecalculated under the optimized conditions. These characteristics werecalculated using the values of the signals for analytical curve. Thedetection limit, calculated as 3 sb/b, where sb is the standarddeviation of the blank and b is slope of the linear section of calibrationgraph, was 0.9 µg L−1. The limit of quantification (10 sb/b) was3.0 µg L−1. The system shows linearity between 3.0 and 100.0 µg L−1.The calibration curve over this interval was determined to beA=3.90×10−3+1.39×10−4 C, where A is the analytical signal,

Table 1Effect of foreign ions on dispersive solvent in dispersive liquid–liquid microextractionof cobalt using Br-TAO.

Substance Maximum amount tolerable

Al (III) 10.0 mg L−1

Br (-I) 5.0 g L−1

Ca (II) 10.0 g L−1

Cd (II) 5.0 mg L−1

Cl (-I) 50.0 g L−1

Cu (II) 5.0 mg L−1

Fe (III) 2.0 mg L−1

K (I) 10.0 g L−1

Mg (II) 1.0 g L−1

Na (I) 50.0 g L−1

Ni (II) 1.0 mg L−1

NO3 (-I) 10.0 g L−1

Pb (II) 1.0 mg L−1

SO4 (-II) 5.0 g L−1

Zn (II) 10.0 mg L−1

measured as absorbance, and C is the concentration of cobalt in thesolution (µg L−1).

The precision of the measurements was assessed as the relativestandard deviation for each value of concentration, calculated as (s/X)×100, where s is the standard deviation for seven measurementsand X the mean value of these measures. The enrichment factor(EF) was calculated by the ratio of angular coefficients of thecalibration curves with and without preconcentration [25]. Thecalibration curve for the direct determination of cobalt using flameatomic absorption spectrometry is represented by equationA=1.00×10−3 +8.76×10−5 C. The consumptive index wascalculated by the ratio of the volume of solution of the metalused in the preconcentration and enrichment factor. This parameteris defined as the sample volume, in milliliters, consumed to achievea unit of EF. It is an interesting parameter to compare proceduresusing different volumes of sample. A summary of the analyticalcharacteristics of dispersive liquid–liquid microextraction for cobaltusing Br-TAO is presented in Table 2.

3.4. Application of the method

The proposed method was applied to the determination of cobaltin real samples of water and certified reference material NIST 1570a,spinach leaves. In order to determine the accuracy of the method, thereference material was digested and the liquid sample was submittedto preconcentration procedure. The content of cobalt obtained by theproposed procedure was 0.43±0.07 µg g−1, which agrees with thecertified value 0.39±0.05 µg g−1. Cobalt was determined afterapplication of the proposed procedure to samples of drinking water,river water and well water, collected in the city of Jequié, Bahia. Theresults for three individual determinations are shown in Table 3.Known amounts of metal were also added for the calculation ofrecovery for each sample. The values of recoveries ranged from 94 to104%, demonstrating the applicability of the method.

4. Conclusion

In the proposed procedure, the reagent Br-TAO was successfullyused as complexing for preconcentration of cobalt using dispersiveliquid–liquid a microextraction. The method is simple, easy to use andeconomic. The low cost is related mainly to small amounts of solventsrequired. The small amounts of carbon tetrachloride and methanol

Limit of quantification 3.0 µg L−1

Enrichment factor 16Sample volume 5.0 mLConsumptive índex 0.31 mLPrecision 2.3 a 5.8%Linear range 3.00 a 100 µg L−1

Table 3Results of determination of cobalt in water samples after dispersive liquid–liquidmicroextraction using Br-TAO.

Sample Cobalt amount (µg L−1) Recovery(%)

Added Found

Drinking water 0.0 bLOD 965.0 4.8±0.3

River water 0.0 3.2±0.3 1045.0 8.4±0.4

Well water 0.0 bLOD 945.0 4.7±0.1

LOD, limit of detection. Confidence interval 95%, n=3.

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also minimizes the toxicity of the method. Another interesting featureof the method is speed. After injection of the mixture Br-TAO/methanol/ carbon tetrachloride, the solution was immediately cloudy.The rich phase is injected into the FAAS after rapid centrifugation.There is a need for low volume of sample, particularly desirableproperty when there is limitation in the sample amount. Theadvantages cited and the analytical characteristics obtained makethe method a good alternative to the determination of cobalt inroutine analysis.

Acknowledgments

The authors acknowledge the financial support of ConselhoNacional de Desenvolvimento Científico e Tecnológico (CNPq) andFundação de Amparo à Pesquisa do Estado da Bahia (FAPESB).

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