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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
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  Talanta 77 (2009) 1290–1294 Contents lists available at ScienceDirect Talanta  journal homepage: A novel fiber-packed column for on-line preconcentration and speciation analysisof chromium in drinking water with flame atomic absorption spectrometry Romina P. Monasterio a , d , Jorgelina C. Altamirano a , b , c , Luis D. Martínez b , e , Rodolfo G. Wuilloud a , b , c , ∗ a Laboratory of Environmental Research and Services of Mendoza (LISAMEN), CCT – CONICET – Mendoza, Av. Ruiz Leal S/N Parque General San Martín, CC 131,M 5502 IRA Mendoza, Argentina b Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina c Instituto de Ciencias Básicas, Universidad Nacional de Cuyo, Mendoza, Argentina d Departamento de Química, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa, Argentina e Departamento de Química Analítica, Facultad de Química, Bioquímica y Farmacia, Universidad Nacional de San Luis, Argentina a r t i c l e i n f o  Article history: Received 29 June 2008Received in revised form 2 September 2008Accepted 3 September 2008Available online 11 September 2008 Keywords: Chromium speciationLlama fiberPreconcentrationFlame atomic absorption spectrometry a b s t r a c t A novel on-line preconcentration and determination system based on a fiber-packed column was devel-oped for speciation analysis of Cr in drinking water samples prior to its determination by flame atomicabsorption spectrometry (FAAS). All variables involved in the development of the preconcentrationmethod including, pH, eluent type, sample and eluent flow rates, interfering effects, etc., were studied inorder 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. TotalCr was determined after oxidation of Cr(III) to Cr(VI) with hydrogen peroxide. The calibration graph waslinearwithacorrelationcoefficientof0.999atlevelsnearthedetectionlimitanduptoatleast50  gL  − 1 .The relative standard deviation (R.S.D.) was 4.3% ( C  =5  gL  − 1 Cr(VI),  n =10, sample volume=25mL). Thelimitofdetection(LOD)forbothCr(III)andCr(VI)specieswas0.3  gL  − 1 .Verificationoftheaccuracywascarried out by the analysis of a standard reference material (NIST SRM 1643e “Trace elements in naturalwater”).ThemethodwassuccessfullyappliedtothedeterminationofCr(III)andCr(VI)speciesindrinkingwater samples.© 2008 Elsevier B.V. All rights reserved. 1. Introduction Chromium is one of the most abundant elements on earth andis found naturally in rocks, soil, plants, animals, volcanic dust andgases[1].Itcanpercolateintothesoilbyleachingandhasthepoten- tial to contaminate groundwater, which can be a major source of drinkingwater[2].Inaqueoussolution,itismainlypresentasCr(III) and Cr(VI) oxidation states [3–6]. The properties of these species are very different from a chemical and toxicological point of view.TrivalentCr,themainchemicalspeciesfoundinfood,isessentialformaintainingnormalglucoseandlipidmetabolism(group3ofIARC)[1,6,7].Ontheotherhand,hexavalentCr-containingcompoundsareconsiderably toxic and known to be carcinogenic and mutagenicfor humans (group 1 of IARC) [6]. Thus, considerable emphasis has ∗ Corresponding author at: Laboratory of Environmental Research and ServicesofMendoza(LISAMEN),CCT–CONICET–Mendoza,Av.RuizLealS/NParqueGeneralSan Martín, CC 131, M 5502 IRA Mendoza, Argentina. Tel.: +54 261 5244064;fax: +54 261 5244001. E-mail address: (R.G. Wuilloud). URL: (R.G. Wuilloud). been given to the development of analytical methodologies for Crspecies separation and determination.Since one of the routes of incorporation of Cr into the humanbody is water, its determination in this type of samples becomesvery important. The Food and Agriculture Organization of theUnited Nations (FAO) and the World Health Organization (WHO)recommend a guideline value of 0.05mgL  − 1 Cr for drinking water[6].Therefore,powerfulanalyticaltechniquesarerequiredandonlyfewofthemshowenoughsensitivity.Amongthem,electrothermalatomic absorption spectrometry (ETAAS) and inductively coupledplasma mass spectrometry (ICP-MS) are the most commonly usedfor trace Cr determination [8]. Despite flame atomic absorption spectrometry (FAAS) continues to be highly employed in routineanalytical laboratories, the low concentration levels of Cr that canbe found in water are not compatible with the detection limitachieved by this technique.Therefore, determination of Cr species at trace levels, usuallyincludes preparation and preconcentration steps prior to elemen-tal detection in order to achieve accurate, reliable and sensitiveresults [9,10]. Some preconcentration methods have used chela- tionwithdiphenylcarbazide[11,12]or4-(2-thiazolilazo)-resorcinol (TAR) [13], as a previous step to metal adsorption on polymeric 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.talanta.2008.09.002  R.P. Monasterio et al. / Talanta 77 (2009) 1290–1294  1291  Table 1 Instrumental and experimental conditionsFlame type Air–C 2 H 2 Wavelength 357.9nmSlit width 0.7nmLamp current 25mAMeasurement mode Peak heightAir flow rate 6Lmin − 1 Acetylene flow rate 2Lmin − 1 Sample introduction flow rate 9mLmin − 1 Column characteristicsEffective bed length 43mmInternal diameter 3mmFiber amount 70mgPreconcentration conditionsLoading flow rate 2mLmin − 1 Loading volume 25mL Eluent NaOH (1molL  − 1 )Stopped-flow time 5min resins, like XAD [14,15]. On the other hand, ionic exchange mate- rials, like alumina [16,17] and commercial polymeric resin, such as Dowex or Amberlite, have been widely used with good results[1,18,19]. Other techniques, such as precipitation and coprecipita-tion [20,21], have also been applied for Cr determination at trace levels. However, many of these methodologies were performed inbatch,requiringconsiderablesamplevolumesinordertoreachlowdetection limits, turning these procedures time-consuming andimpracticalinroutineanalysis.Thissituationhasbeensignificantlyimprovedbyutilizingon-linepreconcentrationsystemscoupledtoelemental detectors, such as FAAS [22].Analternativematerialformetalretention,especiallyforreme-diation purposes, has been wool and/or fiber of animal srcin.Fiber proteins have polar and ionizable groups on the side chainof amino acid residues and bind charged species such as metalions.Infact,featherorsilkproteinshavebeenusedforpurificationof heavy metal-contaminated waste water [23]. Studies on wool keratin for binding of heavy metals as well as coloring with metalcomplexes, such as  o - o  -dihydroxyazo Cr compounds, began in the1950s [23,24]. However, to date fibers have not been implemented in analytical chemistry to develop metal preconcentration.Inthepresentwork,thenovelapplicationofanimalfiberfortheefficientretentionandpreconcentrationofCrisshowed.Anon-lineflowinjectionpreconcentrationsystemconsistingofafiber-packedcolumn for Cr(VI) retention was developed and coupled to FAASdetection.Cr(III)andCr(VI)speciesweredifferentiatedbyoxidationof Cr(III) into Cr(VI) by hydrogen peroxide prior retention of thesecondspeciesinthefiber-packedcolumn.Therefore,totalcontentof Cr was then evaluated based on Cr(VI) species determination.The analytical parameters for optimal speciation along with thepossible mechanisms of Cr retention on the fiber are discussed. 2. Experimental  2.1. Instrumentation The measurements were performed with a PerkinElmer (Uber-lingen, Germany) Model 5100PC atomic absorption spectrometerequipped with a Cr hollow cathode lamp. The FAAS instrumen-tal and operating conditions that provided the best sensitivity forCr(VI) signal are listed in Table 1.The flow injection system is shown in Fig. 1. A Gilson (Villiers Le-Bell, France) Minipuls 3 peristaltic pump equipped with tygon-type pump tubes (Gilson) were employed to propel the sample,reagentandeluent.Thesampleinjectionwasachievedusingasix-way rotary valve from Upchurch Scientific (Oak Harbor, WA, USA). Fig.1.  Schematicdiagramoftheinstrumentalsetup.S,sample;E,eluent;W,waste;P 1  and P 2 , peristaltic pumps; V 1 , six-way valve; V 2 , six-way valve. (a) Load positionand (b) injection position.  2.2. Reagents and chemicals A stock standard solution of 1000mgL  − 1 Cr(III) was preparedfrom7.6930gchromiumnitrate(99.99%)(Cr(NO 3 ) 3 · 9H 2 O)(Merck,Darmstadt, Germany) dissolved in ultrapure water and diluted to1000mL with a final HNO 3  concentration of 0.05molL  − 1 . Workingsolutionswerepreparedbydilutionofthestockstandardsolution.A stock standard solution of 1000mgL  − 1 Cr(VI) was preparedfrom 2.8287g potassium dichromate (99.5%) (K 2 Cr 2 O 7 ) (Aldrich,Milwaukee, WI, USA) dissolved and diluted to 1000mL withultrapure water. Working standard solutions were prepared byappropriate dilution with ultrapure water.We prepared 1molL  − 1 sodium hydroxide solution from NaOH(Aldrich) and used it for Cr(VI) elution from the column. A nitricacidsolutionof1molL  − 1 waspreparedfromproperdilutionof65%(w/w) HNO 3  (Merck). We prepared these solutions as a carrier forconditioning the column and regeneration.Ultrapure water (18M  cm) was obtained from a Milli-Q waterpurification system (Millipore, Paris, France).All reagents were of analytical reagent grade and the presenceof Cr was not detected in the working range.All bottles used for storing samples and standard solutions, aswell as the glassware were washed in 10% (v/v) nitric acid for 24hand finally rinsed with ultrapure water.  2.3. Preparation of the fiber-packed column Llama( lama glama )fiberswithnotreatmentwerepurchasedatlocal stores. The llama fiber was washed before using in an ultra-sonicbathwithadetergentsolutionfollowedby0.5molL  − 1 sodiumhydroxide, 1molL  − 1 nitric acid and finally water. The fiber wasdried at room temperature until constant weight. A glass column(3mm i.d. and 55mm length) was used for preconcentration. Anamountof70mgofllamafiberwasusedtopackthecolumnuptoaneffectivebedlengthof43mm.Thecolumnwasfinallywashedwithultrapure water followed by conditioning with 0.5molL  − 1 sodiumhydroxide and 1molL  − 1 nitric acid.  2.4. Separation and preconcentration procedure Initially, the column was conditioned for preconcentration atthe correct pH with 1molL  − 1 nitric acid, valve V 1  in position B(Fig. 1). The sample solution was then loaded on the fiber-packed column at a flow rate of 2mLmin − 1 , with valve V 1  in S positionandvalveV 2  inloadposition(a).Aftertheloadingtime,theloadinglines and column were washed with 1molL  − 1 nitric acid, with the  1292  R.P. Monasterio et al. / Talanta 77 (2009) 1290–1294 valve V 1  again in position B. Finally, valve V 2  was switched to theinjection position (b) and the column was loaded with 1molL  − 1 sodium hydroxide followed by a 5-min stopped-flow. After thistime the retained Cr species was eluted with 1molL  − 1 sodiumhydroxide solution at a flow rate of 9.5mLmin − 1 , directly into thenebulizer and subsequently the flame. The procedure was basedon the retention of Cr(VI) on the fiber. After oxidation of Cr(III),totalCrwasevaluatedbyapplyingthepreconcentrationproceduredescribedabove.TheconcentrationofCr(III)specieswascalculatedas the difference between the total concentration of Cr and that of Cr(VI).  2.5. Oxidation of Cr(III) species Hydrogen peroxide was used for oxidation of Cr(III) to Cr(VI). Astandard solution (100  gL  − 1 Cr(III)) volume of 70mL was addedwith500  Lof100vol.hydrogenperoxide.Thissolutionwasheatedonathermostaticwaterbathfor40minat93 ◦ Candthenboiledonaheating plate for 10min in order to remove any excess of hydrogenperoxide. After this procedure, the resulting solution was cooledto room temperature and then taken up to 100mL with ultrapurewater.  2.6. Sample collection and conditioning  For the collection of tap water samples, domestic water wasallowed to run for 20min and approximately a volume of 1000mL wascollectedinabeaker.Thewatersampleswerefilteredthrough0.45  m pore size membrane filters (Millipore) immediately aftersampling and acidified to pH 2 with nitric acid. Finally, sampleswere stored in bottles (Nalgene; Nalge, Rochester, NY, USA) at 4 ◦ C. 3. Results and discussion  3.1. Optimization of loading variables Sample pH value plays an important role with respect to theadsorption of Cr(III) and Cr(VI) onto microcolumns. The pH of media is a critical parameter as Cr speciation is dependant on thisfactor. Thus, Cr(VI) can exist primarily as chromic acid (H 2 CrO 4 )and its salts, hydrogen chromate ion (HCrO 4 − ) and chromate ion(CrO 42 − ). The predominant species are H 2 CrO 4  at pH<1, HCrO 4 − at pH 1–6, and CrO 42 − at pH 6 [25]. Loading conditions were opti- mized by monitoring Cr signal with FAAS while changing the pHof the solutions that passed through the column. The effect of pHwas evaluated in the range of 1.1–6.1. As can be seen in Fig. 2a, it is evident that Cr(VI) retention resulted optimum at pH 4 and hence,this value was selected for all experiments.The possibility of using a buffer to keep a constant pH 4 wasinvestigated using a 2-molL  − 1 sodium acetate/acetic acid solu-tion. Buffer concentrations were in the range of 6.7 × 10 − 3 to0.13molL  − 1 .However,whenthisbuffersystemwasused,aconsid-erabledropinsensitivitywasobserved(Fig.2b).Thisphenomenon can be explained considering a possible competition betweenacetateionandCr(VI)anionicspeciesbytheactivesitesofthefiber.Therefore, pH was adjusted by addition of proper amounts of acidor base.A flow rate ranging between 0.5 and 6mLmin − 1 was foundto be suitable for optimal loading on the fiber-packed column.Higher flow rates did not lead to any improvement of Cr retention.This could be probably due to insufficient contact time betweenthe sample solution and the fiber. A flow rate of 2mLmin − 1 wasselected.The influence of the column length was investigated between25 and 115mm. In the analytical range of this work, increasing the Fig. 2.  (a) Dependence of Cr(VI) retention on fiber material with the pH of load-ing solutions. (b) Relationship between buffer concentration and Cr(VI) retention.Preconcentration of 25mL of 100  gL  − 1 Cr(VI) at pH 4.0. Other conditions were asshown in Table 1. fiber amount did not improve the preconcentration and recoveryof Cr(VI). Therefore, a fiber bed length of 43mm was chosen asoptimal.  3.2. Optimization of elution variables InordertoeluteCr(VI)speciesretainedonthefiber,hydroxideswereusedaseluents.Theelutingagentswereammoniumhydrox-ide and sodium hydroxide in concentrations ranging from 0.5 to3molL  − 1 . As it is shown in Fig. 3, the highest efficiency for Cr elu- tion from the column was achieved with 1molL  − 1 NaOH solution.Higherconcentrationsofthiseluentresultedtobelesseffective.Onthe other hand, Cr recoveries obtained with ammonium hydroxidewere lower to that of NaOH for each concentration level. Based ontheseobservations,1molL  − 1 NaOHsolutionwasselectedaseluentfor further experiments.An important variable to be optimized was the contact timeof the eluent solution with the fiber. Thus, it was observed thata minimal contact time was needed in order to achieve totalelution of Cr from the column (Fig. 4). Elution was performed by a stopped-flow procedure filling the column with 1molL  − 1 NaOH and keeping the eluent for 5min before injection intoFAAS.  R.P. Monasterio et al. / Talanta 77 (2009) 1290–1294  1293 Fig. 3.  Effect of types and concentration of eluting agents. (  ) Sodium Hydroxide;(  ) ammonium hydroxide. Other conditions were as shown in Table 1. Fig. 4.  Effect of stopped-flow time on Cr elution efficiency from the fiber-packedcolumn. Other conditions were as mentioned in Table 1.  3.3. Retention efficiency and preconcentration factor  A retention percentage higher than 99.8% was achieved whenthe procedure was carried out under optimal experimental condi-tions(Table1).Therefore,anenrichmentorpreconcentrationfactor of 32 was obtained for a sample volume of 25mL. The amount of Cr(VI) species retained on the fiber was determined in batch. A50-mL portion of 60mgL  − 1 Cr(VI) solution was adjusted to pH 4with nitric acid and shaken with 100mg of fiber in a glass flask for30min. Adsorption capacity of llama fiber material was found tobe25.2mgCr(VI)g − 1 ofdriedfiber.Itcanbestatedthatadsorptioncapacity of llama fiber is higher than those reported for others ionexchange materials such as alumina [10] and nanometer-sized zir- conium oxide immobilized on silica gel [26], but significantly less expensive as compared to these materials.  3.4. Separation of Cr(III) and Cr(VI) species OxidationofCr(III)toCr(VI)wasneededinordertoallowCr(III)retention on the fiber as Cr(VI) species. Hydrogen peroxide wasselected as oxidant due to its high oxidation capacity and the pos-sibility of it being eliminated from the reaction media by simpleheating of the mixture. Therefore, several hydrogen peroxide con-centrations 2.5, 4.9 and 9.8 × 10 − 2 molL  − 1 and at different pH of the media (pH 4, 8.3, and 11) were assayed. Optimal Cr(III) reten-tion and recoveries were achieved for 4.9 × 10 − 2 molL  − 1 hydrogenperoxide at pH 8.3.In order to assess the selectivity of the proposed method forCr(III) and Cr(VI) determination, it was applied to several stan-dardsolutionsatdifferentconcentrationratiosofthetwooxidationstates. Table 2 indicates that Cr(III) and Cr(VI) species were com- pletely separated and quantitatively recovered. The method wasthus shown to have an acceptable performance for selective deter-mination of Cr species under different conditions.  3.5. Interferences Theeffectsofrepresentativepotentialinterferingspecies(attheconcentration levels at which they may occur in the sample stud-ied)weretested.TherecoveryoftheanalytewasnotinfluencedbyCO 32 − , Cl − , SO 42 − , PO 43 − and Fe 3+ ions, probably due to the reten-tioncapacityofthefiberthatavoidedfastsaturationofthecolumn.These ions could be tolerated up to at least 2,000  gL  − 1 .  3.6. Determination of Cr species in water samples After separation/preconcentration by the proposed procedure,thecalibrationgraphsforFAASdeterminationofCr(VI)werelinear,achievingarelativestandarddeviation(R.S.D.)of4.3%( C  =5  gL  − 1 , n =10, sample volume=25mL). Linearity of calibration curve wasobserved at levels near the detection limit and up to at least300  gL  − 1 . The calibration graph showed a correlation coefficientof 0.999. The limit of detection (LOD), calculated based on threetimes the standard deviation of the background signal (3   ), was0.3  gL  − 1 . The preconcentration factor was obtained as the ratioof the slopes of the calibration curves for Cr(VI) with and withoutthe preconcentration step. The accuracy of the proposed methodwas evaluated by analyzing a standard reference material, NISTSRM 1643e “Trace Elements in Water”, with a reported Cr con-tent of 20.40 ± 0.24  gL  − 1 . Using the proposed methodology theCr content determined in this SRM was 20.1 ± 1.9  gL  − 1 .The results of the method applied to Cr(III) and Cr(VI)determination in drinking water samples were in the range of 7.3–14.2  gL  − 1 for Cr(VI) and 1.2–2.4  gL  − 1 for Cr(III). The results  Table 2 Evaluation of the separation of Cr(VI) and Cr(III) speciesCr(VI)/Cr(VI) ratio Cr(VI) Cr(III)Added (  gL  − 1 ) Found (  gL  − 1 ) Recovery (%) Added (  gL  − 1 ) Found (  gL  − 1 ) Recovery (%)21 42 41.5 99.0 2 1.90 96.52.66 32 31.4 98.3 12 11.9 99.81 22 21.5 97.7 22 21.8 99.20.37 12 11.8 98.3 32 32.8 1020.05 2 1.90 95.4 42 79.6 99.8  1294  R.P. Monasterio et al. / Talanta 77 (2009) 1290–1294  Table 3 Analysis of Cr(VI) and Cr(III) in drinking water samples (95% confidence interval;  n =6)Sample Cr(VI) Cr(III)Added (  gL  − 1 ) Found (  gL  − 1 ) Recovery (%) a Added (  gL  − 1 ) Found (  gL  − 1 ) Recovery (%) a 1 0 10.3  ±  1.5 – 0 2.4  ±  0.3 –10 20.1  ±  1.9 99.6 10 12.5  ±  0.9 100.02 0 7.3  ±  0.7 – 0 1.2  ±  0.2 –10 17.6  ±  1.2 100.0 10 11.0  ±  1.6 98.03 0 14.2  ±  1.4 – 0 1.5  ±  0.3 –10 24.1  ±  2.3 99.8 10 11.3  ±  1.7 99.6 a 100 × ((found − base)/added). are shown in Table 3. Concentration levels observed in this work were not significantly different to those reported by Bulut et al.[14], Wuilloud et al. [9], and Saygi et al. [1], for Cr(III) and Cr(VI) species in drinking water samples. 4. Conclusion The methodology developed in this work provides a novel,simple, and inexpensive approach to achieve high retention andseparation of Cr(III) and Cr(VI) species. The results showed thatllama fiber has high and reproducible retention capacity of Cr(VI)speciesandhence,itisproposedasaneffectivealternativetoothermore expensive ionic exchanger materials. The preconcentrationsystem provided an enrichment factor of 32 as a consequence of the high Cr retention (99%) on the fiber.The coupling of an on-line preconcentration system to FAASincreases the speed of the preconcentration and analysis process,while reducing sample consumption and contamination risks. Theproposed on-line preconcentration system associated with FAASdetection allowed the separation and determination of Cr(III) andCr(VI) species in drinking water samples at levels as low as  gL  − 1 with good accuracy and reproducibility.  Acknowledgments ThisworkwassupportedbyConsejoNacionaldeInvestigacionesCientíficas y Técnicas (CONICET), Agencia Nacional de PromociónCientífica y Tecnológica (FONCYT) (PICT-BID) (FONTAR—N ◦ PMT II– CAI/073), and Universidad Nacional de San Luis (Argentina). 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