development of a dispersive liquid–liquid microextraction method for iron extraction and...
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AnalyticalMethods
PAPER
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Escola de Quımica e Alimentos, Universida
96203-900, Rio Grande do Sul, Brazil. E-m
32336967
Cite this: Anal. Methods, 2013, 5, 2273
Received 30th October 2012Accepted 15th March 2013
DOI: 10.1039/c3ay26294d
www.rsc.org/methods
This journal is ª The Royal Society of
Development of a dispersive liquid–liquidmicroextraction method for iron extraction andpreconcentration in water samples with differentsalinities
Ederson R. Pereira, Bruno M. Soares, Juliana V. Maciel, Sergiane S. Caldas,Carlos F. F. Andrade, Ednei G. Primel and Fabio A. Duarte*
A new, efficient, fast and simple method for iron determination in environmental water samples with
different salinities by a dispersive liquid–liquid microextraction method, followed by UV-Vis
spectrophotometric determination, was developed. In this study, Fe was complexed with ammonium
pyrrolidinedithiocarbamate and extracted into 1,2-dichlorobenzene droplets after the injection of a
mixture composed of 1,2-dichlorobenzene (extraction solvent) and ethanol (dispersive solvent) into the
sample solution. Factors which affected the efficiency of Fe extraction and its subsequent
spectrophotometric determination were studied and optimized. Under optimized conditions, the
calibration curve was linear and ranged from 0.02 to 2.5 mg L�1 (r ¼ 0.999). The limits of method
detection and quantification were 6.1 and 18.3 mg L�1, respectively, with a preconcentration factor of
3.3. The accuracy evaluated in terms of recovery was between 90 and 100% with RSD lower than 12%.
In addition, a certified reference material (SRM 1643e, Trace Elements in Water) was analyzed with
agreement of 104% and RSD about 5%. The method was applied to total Fe determination in
environmental water samples with different salinities and satisfactory results were obtained.
A Introduction
Iron is one of the most important elements in environmentaland biological systems. It is an essential element for humansand animals since it participates in metabolic and fermentationprocesses as an enzyme activator, stabilizer and functionalcomponent of proteins. In humans, one of the most severe andimportant deciencies in the world today, is mainly related toanemia.1–4 Likewise, high Fe concentration can also causeseveral health problems, especially those associated with cancerand other illnesses.5,6 Anthropogenic activities may addconsiderable amounts of iron to the aquatic system and exert anegative effect on this ecosystem and human health.4 Due to itsimportance to human health and, mainly, to the environment,sensitive and accurate determination of iron in the aquaticenvironment has been continuously carried out.
Several techniques, such as spectrophotometry (UV-Vis),7–9
ame atomic absorption spectrometry (FAAS),10,11 electro-thermal atomic absorption spectrometry (ETAAS),12 inductivelycoupled plasma optical emission spectrometry (ICPOES)13 andinductively coupled plasma mass spectrometry (ICP-MS)14 have
de Federal do Rio Grande, Rio Grande,
ail: [email protected]; Tel: +55 53
Chemistry 2013
been reported in studies of Fe determination in water samples.The spectrophotometric technique is useful for iron determi-nation because of its availability, simplicity, versatility, speed,accuracy, precision and cost-effectiveness.15 UV-Vis is evencheaper than spectrometric (e.g., AAS) and plasma-based tech-niques. However, direct determination of low concentrations byspectrophotometric techniques is oen hard to be carried outbecause of insufficient sensitivity. As a result, prior extractionand analyte preconcentration of matrices are required.
Different procedures, such as liquid–liquid extraction(LLE),8,16 solid phase extraction (SPE),10,13 co-precipitation,17
cloud point extraction (CPE)18,19 and Chelex-100 resin20 (espe-cially in seawater) have been developed for Fe extraction andpreconcentration. However, these methods not only needcomplex equipment and high extraction time but also consumesignicant solvent amounts and generate much secondarywaste.
Dispersive liquid–liquid microextraction (DLLME) is a pre-concentration method based on a ternary system of solvents. Itwas rst reported for the extraction and preconcentration oforganic compounds.21 However, several studies have reported itsuse for the extraction and preconcentration of inorganiccompounds,22–26 and have shown its advantages, such as easyoperation, small amounts of organic solvents, low timeconsumption, low cost and high recovery and enrichment factors.
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Fig. 1 Structural formula of APDC.
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In extractions by DLLME, a mixture containing appropriateamounts of extraction and disperser solvents is rapidly injectedinto an aqueous sample with a syringe. A cloudy solution isformed and the analyte is instantaneously extracted into thedroplets of the extraction solvent. Aer the extraction, the phaseseparation is accomplished by centrifugation; the analyte isdetermined in the sedimented phase.27–29 For element determi-nation, a complexing agent is required if the analyte is not boundto organic groups. Therefore, ammonium pyrrolidinedithio-carbamate (APDC) has attracted much interest as a chelatingagent because of its ability to form colored and extractable chelatecomplexes with divalent and trivalent elements.30 As a result,APDC has been successfully used for complexation and extractionof different elements, such as arsenic,31 lead and cadmium,24 inwater samples. Studies of different water samples have shown thatthe most difficult ones to be analyzed are the ones with highsalinity. Several studies have reported low extraction efficiency ofelements in seawater samples in comparison with natural, river ortap water.24,32,33
This study aimed at developing and optimizing a method foriron extraction and preconcentration in water samples withdifferent salinities by DLLME and determination by UV-Vis.APDC was selected to be the chelating reagent and the factorsthat affect the extraction efficiency by DLLME and subsequentUV-Vis determination were evaluated. Moreover, this methodwas applied to iron determination in mineral, tap, well, estua-rine and sea waters. Results were compared with those gotten bya spectrophotometry method with 1,10-phenanthroline. Accu-racy was evaluated by analyte spiking and CRM (SRM 1643e,Trace Elements in Water) analysis.
B ExperimentalInstrumentation
UV-Vis spectrophotometric measurements were performed by aShimadzu UV-Vis spectrophotometer (model UV-2550, Japan)with 1.0 cm quartz micro-cells. A centrifuge (Centribio, model80-2B, Brazil) with 15 mL calibrated centrifuge tubes was usedfor the phase separation. Before analysis, in order to removesuspended particulate matter, water samples were ltered on acellulose acetate membrane (0.45 mm) by a vacuum pump(Tecnal, model TE-058, Brazil). The pH was adjusted by a HannapH-meter (model pH21, Brazil). The salinity was measured by aYellow Springs conductivity instrument (model 33 SCT, USA).
Reagents
All reagents were of analytical grade. Ultrapure water waspuried by a Direct-Q UV3� (resistivity 18.2 MU cm, Millipore,USA) water purication system. Reference stock solutions ofFe(III) and Fe(II) of 1000 mg L�1 were prepared by dissolvingFeNH4(SO4)2$12H2O (Vetec, Brazil) and Fe(NH4)2(SO4)2$6H2O(Sigma Aldrich, Brazil) in 1 mol L�1 HCl (Merck, Germany),respectively. Working standard solutions were prepared daily byserial dilution of stock solutions with ultrapure water.
An ammonium pyrrolidinedithiocarbamate (Sigma Aldrich,Brazil) solution was prepared by dissolving appropriate
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amounts of this reagent in ethanol (J.T Baker, USA), kept at 4 �C.The APDC structural formula is shown in Fig. 1. A 1% (m/v)hydroxylamine (NH2OH$HCl) solution (Vetec, Brazil) was usedas the reducing reagent to prepare the Fe(II) solution; appro-priate amounts of NH2OH$HCl were dissolved in ultrapurewater. 1,2-Dichlorobenzene (C6H4Cl2), tetrachloroethylene(C2Cl4), monochlorobenzene (C6H5Cl) and carbon tetrachloride(CCl4), the selected extraction solvents, were bought from Vetec(Brazil). Methanol, ethanol, acetonitrile, tetrahydrofuran (THF)and acetone were used as dispersive solvents and purchasedfrom J.T. Baker (USA). Solutions of 0.1 mol L�1 HCl and NaOH(Merck, Germany) were used for the pH adjustment of the watersamples. All glass vessels were kept in a 20% HNO3 solution(Merck, Germany) for at least 24 h and, subsequently, washed inultrapure water twice, before use.
Samples
For the method optimization, a sample of tap water was used torepresent the sample with low salinity (ranging from 0 to 0.5).Tap water was collected from the authors' laboratory aerallowing the water to ow for 5min. Water samples with averagesalinity (ranging from 0.5 to 30) and high salinity (ranging from30 to 35) were supplied by the Institute of Oceanography (at theUniversidade Federal do Rio Grande). In order to evaluate theefficiency of the proposed DLLME method, ve water samples,i.e., mineral, tap, well, estuarine and seawater were selected todetermine their total iron content. The mineral water sampleswere purchased at local supermarkets (Rio Grande, Brazil) andnamed A, B, C, D and E. The well water was collected in RioGrande (Brazil). The estuarine and seawater samples werecollected from the coast and 200 miles offshore (South AtlanticOcean), respectively, in Rio Grande (Brazil). Mineral, tap andwell waters represented the samples with low salinity. Theestuarine water sample represented the samples with averagesalinity whereas the samples collected 200 miles offshore rep-resented the samples with high salinity. The salinity of thesamples was measured by insertion of an electrode in thesolution; then, samples were acidied to 0.1 mol L�1 HNO3
(pH 2), ltered and stored in the dark at 4 �C until analysis.
DLLME procedure
All experiments for DLLME optimization were performed withthe water sample with high salinity. Firstly, aliquots of 10 mL of awater sample containing a known concentration of Fe(III) weretransferred to 15mL centrifuge glass tubes with a conical bottom,which facilitates the collection of the extraction solvent. The pH
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Fig. 3 Effects of pH on the complex between Fe(III) and APDC. Conditions:ultrapure water: 10 mL; 1.0 mg L�1 Fe(III); APDC amount: 300 mL of a 1% (m/v)solution. The error bars represent the standard deviation (n ¼ 3).
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was adjusted with 0.1 mol L�1 HCl or 0.1 mol L�1 NaOH. Thecomplexing agent (300 mL APDC solution) was added to thesample and mixed. Aerwards, the tubes were le to stand for 1min. In this step, Fe ions reacted with APDC to form the complex.A dispersive solvent containing the extraction solvent was rapidlyinjected into the sample solution with a 2.5 mL syringe (Hamil-ton, USA). A cloudy solution was rapidly produced, resulting inne droplets; the complex was then extracted into these nedroplets. The mixture was centrifuged at 3000 rpm for 5 min andthe dispersed ne droplets of the extraction solvent were settled.The aqueous phase was removed and the organic phase wasdiluted up to 3 mL with the dispersive solvent for subsequentabsorbance measurement by UV-Vis.
All statistical calculations were performed by the GraphPadInStat (GraphPad InStat Soware Inc, Version 3.00, 1997) so-ware. A 95% signicance level was adopted for all comparisons.The comparison between two averages was performed by thet-Student test, whereas the Tukey–Kramer test was used forcomparing three or more averages.
C Results and discussion
The spectrophotometric technique used for Fe determinationwas based on the formation of a Fe(APDC)3 complex. Ammo-nium pyrrolidinedithiocarbamate reacts with metallic ions;thus, a very stable complex is formed. Numerous applications totrace element separation and preconcentration can befound.18,19,31,34 In this study, APDC was used for complexingFe(II) and Fe(III) species. Fe(III) is complexed instantly by thecomplexing agent while Fe(II) is complexed by APDC and rapidlyoxidized to Fe(III). As can be seen in Fig. 2, the resulting complexshows two bands in the visible region (505.0 and 595.0 nm) andone band in the UV region (357.0 nm). However, the UV regionis highly inuenced by the APDC absorption band. In order tominimize the interferences, 595.0 nm was selected for the irondeterminations.
Effects of pH
The pH of the sample is one of the essential factors for theformation of a metal-chelate. In this step, the formation of a
Fig. 2 UV-Vis spectrum of ultrapure water (10 mL, pH 4.0) containing: (a) 300 mLof a 1% (m/v) APDC solution; and (b) 300 mL of a 1% (m/v) APDC solution + 1.0mg L�1 Fe(III).
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complex with enough hydrophobicity occurs: it can be extractedinto the small volume of the extraction solvent in the DLLMEprocedure. In this study, the inuence of the pH on theextraction efficiency was studied with pH values ranging from2.0 to 8.0 (Fig. 3). For these experiments, 10 mL of ultrapurewater, containing 1.0 mg L�1 Fe(III) and 300 mL of 1% (m/v)APDC solution, was used. In the range under evaluation, whenpH values are lower than 3.0, the analytical signal decreases.This fact is probably the result of the complex dissociation viahydrolysis of APDC. Similarly, when pH values are higher than3.5, the analytical signal decreases, probably due to the forma-tion of Fe–hydroxy complexes, which could make the analytetransference to the extraction solvent difficult.35 Thus,maximum absorbance was obtained at pH 3.3, which wasselected for further optimization.
The pH range found in this study was below the values foundby other authors who used the same complex. They had usedcloud point extraction followed by FAAS determination, with pHvalues about 5.0.18,19
Effects of types of extraction and dispersive solvents
The types of extraction and dispersive solvents used in DLLMEplay an important role in the extraction efficiency. The extrac-tion solvent should be able to extract compounds of interesthaving higher density than water and low solubility in water.20,27
Thus, 1,2-dichlorobenzene, tetrachloroethylene, mono-chlorobenzene and carbon tetrachloride were selected to bepotential extraction solvents.
Initially, the type of extraction solvent was evaluated bycombination of 50 mL of each solvent (1,2-dichlorobenzene, tet-rachloroethylene, monochlorobenzene and carbon tetrachloride)with 500 mL ethanol as the dispersive solvent. Results are shownin Fig. 4. Among the four extraction solvents, 1,2-dichloroben-zene and monochlorobenzene had the highest extraction recov-eries (50% and 82%, respectively), whereas recoveries of carbontetrachloride and tetrachloroethylene were lower than 41%. It isimportant to mention that the solubility of the complexFe(APDC)3 in 1,2-dichlorobenzene is greater than that in othersolvents due to the interactions between the hydrophobic groupof the complex and non-polar properties of 1,2-dichlorobenzene.
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Fig. 4 Effects of various extraction solvents (50 mL) on DLLME recoveries.Conditions: sample volume: 10 mL; 1.0 mg L�1 Fe(III); sample pH: 3.3; APDCamount: 300 mL of a 1% (m/v) solution; volume of the dispersive solvent(ethanol): 500 mL. The error bars represent the standard deviation (n ¼ 3).
Fig. 6 Effects of volume of 1,2-dichlorobenzene (extraction solvent) on DLLMErecoveries. Conditions: sample volume: 10 mL; 1.0 mg L�1 Fe(III); sample pH: 3.3;APDC amount: 300 mL of a 1% (m/v) solution; dispersive solvent volume (ethanol):500 mL. The bars represent the standard deviation (n ¼ 3).
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Therefore, 1,2-dichlorobenzene was selected to be the extractionsolvent for the optimization of subsequent experiments due to itshigh extraction recoveries and low relative standard deviation(RSD, 2%).
The 1,2-dichlorobenzene, which has high extraction capabilityand low solubility in water, has also been used for DLLMEcombined with ethanol (dispersive solvent) for the preconcen-tration of trace amounts of other elements in water samples (tap,river and well waters), with recoveries ranging from 99 to 102%.26
In order to evaluate the type of the dispersive solvent,methanol, ethanol, acetonitrile, THF, and acetone were studied.In this case, the combination of 50 mL 1,2-dichlorobenzene and500 mL of each dispersive solvent was evaluated. As shown inFig. 5, ethanol and acetone had recoveries above 78%. Thus,ethanol was selected to be the dispersive solvent for furtherexperiments due to its high extraction efficiency, low toxicityand low cost. In another study, ethanol was also chosen to bethe dispersive solvent for extraction and spectrophotometricdetermination of Fe in water samples by DLLME with solidi-cation of oating organic drops (DLLME-SFO).36 However,ethanol was combined with 1-undecanol (extraction solvent),and 2-thenoyltriuoroacetone (TTA) was used as the complex-ing agent.
Fig. 5 Effects of different dispersive solvents (500 mL) on DLLME recoveries.Conditions: sample volume: 10 mL; 1.0 mg L�1 Fe(III); sample pH: 3.3; APDCamount: 300 mL of a 1% (m/v) solution; volume of the extraction solvent (1,2-dichlorobenzene): 50 mL. The bars represent the standard deviation (n ¼ 3).
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Effects of volumes of extraction and dispersive solvents
The effect of the volume of the extraction solvent on theextraction efficiency was investigated. Experiments were per-formed by combination of different volumes of 1,2-dichloro-benzene (from 10 to 100 mL) keeping the volume of ethanol at500 mL. Results in Fig. 6 show that the extraction recoveries werenot signicantly different (Tukey–Kramer test) since volumes of1,2-dichlorobenzene were between 30 and 80 mL. A smalldecrease in the recovery (reached 64%) was observed when thevolume of the extraction solvent was 10 mL, probably due to thedecrease of the ratio of dispersive solvent : extraction solvent,decreasing the dispersion of the extraction solvent in anaqueous solution. With a 30 mL extraction solvent, the recov-eries were higher than 90%. However, drop removal was diffi-cult due to the small volume of the sedimented phase. Thus,50 mL of the extraction solvent was used in further experiments.
In order to examine the effect of the volume of the dispersivesolvent, solutions containing different volumes of ethanol(ranging from 200 to 1000 mL) and 50 mL of 1,2-dichlorobenzenewere subjected to the same DLLME procedure. As can be seen inFig. 7, the recoveries reached their maximum value (93%) at700 mL ethanol. At low volumes of the dispersive solvent,ethanol could not disperse 1,2-dichlorobenzene properly and a
Fig. 7 Effects of volumes of ethanol (dispersive solvents) on DLLME recoveries.Conditions: sample volume: 10 mL; 1.0 mg L�1 Fe(III); sample pH: 3.3; APDCamount: 300 mL of a 1% (m/v) solution; extraction solvent volume (1,2-dichlo-robenzene): 50 mL. The bars represent the standard deviation (n ¼ 3).
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Table 1 Tolerance limits of ions for the determination of 0.1 mg L�1 Fe(III)
Interference species Tolerance limits (mg L�1)
Na+, K+ 10 000Mg2+ 5000Ca2+ 1000Mn2+, Hg2+ 1.0Al3+, Zn2+, Ni2+, As3+ 0.5Cd2+, Pb2+, Cr3+ 0.3Co2+, Cu2+ 0.1Cl� 20 000PO4
3�, SO42� 1000
F� 10.0
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cloudy solution was not formed completely. It is clear that byincreasing the volume of ethanol, the solubility of the complexin water increases.
Effects of APDC concentration
The effect of APDC concentration on the Fe recoveries wasexamined with concentrations of APDC ranging from 0.2 to 5.5mmol L�1. Results are shown in Fig. 8. As can be seen, even onimproving the APDC concentration from 0.9 to 5.5 mmol L�1,recovery improvement was insignicant (Tukey–Kramer test).
However, Fig. 8 shows RSD increase in APDC concentrationsabove 1.8 mmol L�1, with considerable increase in the instru-mental noise, probably due to the incomplete dissolution ofAPDC. With 0.2 mmol L�1 APDC, recoveries are signicantlyreduced due to the incomplete complexation of iron ions. It isimportant to mention that APDC concentration about 1.8 mmolL�1 can provide suitable recovery, considering the stoichiometry(reaction (1)).
3APDC + Fe3+ / Fe(APDC)3 (1)
Therefore, 1.8 mmol L�1 APDC was selected for furtherexperiments.
Effects of extraction time
In DLLME, extraction time is dened as the interval between theinjection of the mixture of dispersive and extraction solventsand the beginning of centrifugation.37 The extraction timeranged from 0.5 to 30 min under constant experimentalconditions. Results showed that the extraction time exerted nosignicant inuence (Tukey–Kramer test) on the extraction effi-ciency. Because of the large surface area between the extractionsolvent and the aqueous phase aer the formation of the cloudysolution, the complex is rapidly spread into the extractionsolvent and the equilibrium state is also quickly achieved, a factthat makes the DLLME method time-independent.20,27,38
Therefore, this parameter is the most meaningful advantage ofthis method. Similar results have been found by other authorswho used the DLLME method for element extraction and pre-concentration in different water samples.22,31,32,37,39
Fig. 8 Effects of different APDC concentrations on DLLME recoveries. Conditions:sample volume: 10 mL; 1.0 mg L�1 Fe(III); sample pH: 3.3; extraction solventvolume (1,2-dichlorobenzene): 50 mL; dispersive solvent volume (ethanol): 500 mL.The bars represent the standard deviation (n ¼ 3).
This journal is ª The Royal Society of Chemistry 2013
Interferences
Effects of several ions which could reduce the extraction effi-ciency or react with APDC and lead to spectral interferences. Inthis study, 10 mL ultrapure water containing 0.1 mg L�1 Fe(III)and different amounts of interfering ions was treated incompliance with the optimized procedure. The tolerance limitsof the coexisting ions were dened as the highest amounts thatcause a decrease about 10% in the recoveries. Results are shownin Table 1. Especially for transition metals and arsenic, anadditional study was performed to investigate the possibleabsorption bands in the same region of the complex Fe(APDC)3,since APDC reacts with metals and forms colored complexes.35
Arsenic and some transition metals were observed to reactstrongly with the complexing agent even at low concentrations(<0.5 mg L�1). However, absorption bands for these elementswere not observed in the same region of the complex Fe(APDC)3.On the other hand, halogens (Cl� and F�) and major elements(e.g., Ca2+, K+, Mg2+ and Na+) showed interference at higherconcentrations. However, element concentration found inaquatic environments is signicantly lower in comparison withthe concentration limits evaluated in this study.
A complementary study to evaluate the species conversionsbetween Fe(II) and Fe(III) caused by the complexing was alsoperformed in order to prove that the total Fe content in thesample was completely measured by the proposed method.Table 2 shows the results of this study for different spikinglevels (150 and 300 mg L�1) of Fe(II) and Fe(III) in high salinitywater. Both species showed excellent recoveries, ranging from97 to 100%, with total conversion from Fe(II) to Fe(III).
Table 2 Accuracy data for the determination of total Fe from different species,Fe(III) and Fe(II), in high salinity water. The results are shown in mg L�1 (mean �standard deviation, n ¼ 5)
Sample
AddedFound(total Fe)
Recovery(%)Fe(III) Fe(II)
High salinity 0 0 <18.1 —300 0 293 � 27 98
0 300 290 � 3 97150 150 300 � 6 100
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Table 3 Precision and accuracy for the determination of Fe(III) in waters withdifferent salinities. Results are shown in mg L�1 (mean� standard deviation, n¼ 5)
Water Added Found Recovery (%)
Low salinity — 48 � 2 —60 113 � 2 100
300 345 � 17 100
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Validation
In order to ensure that the results produced in this study havereliable analytical data, the optimized DLLME procedure wasevaluated by checking the following factors: linearity, limit ofdetection of the method (LODm), limit of quantication of themethod (LOQm), accuracy and precision.
600 596 � 12 92Average salinity — 23 � 3 —
60 81 � 2 100300 302 � 3 97600 566 � 5 92
High salinity — <18.1 —60 61 � 1 102
300 272 � 2 90600 537 � 3 90
NIST SRM 1643e — 102 � 5 104a
a Agreement with a certied value (98.1 � 1 mg L�1 Fe).
Linearity, LODm and LOQm
A calibration curve was obtained by 10 mL of reference solu-tions of Fe(III) with nine concentration levels (ranging from 0.01to 3.00 mg L�1) in ethanol under optimized experimentalconditions. The calibration range was linear from 0.02 to 2.5mg L�1 of Fe(III) with a determination coefficient (r) of 0.999; itindicates good linearity in the previously mentioned concen-tration range. A study of the matrix inuence on the analyterecovery was also performed. To this end, a calibration curvewith analyte addition with a subsequent DLLME procedure wasplotted against the standard curve with the solvent. Theseresults showed that the slopes were similar for both calibrationcurves (difference below 6%), allowing the use of referencesolutions prepared in the solvent for instrument calibration.
The LODm and LOQm, dened as three and ten times thestandard deviation of ten measurements of blanks (n ¼ 10)divided by the slope of the calibration curve, were 6.1 and 18.3mg L�1, respectively. It is important to mention that the LODfound in this study is too low in comparison with that of thestandardmethod, based on the reaction between Fe(II) and 1,10-phenanthroline,40 and with the method recommended by theEnvironmental Protection Agency (EPA), whose determinationis performed by FAAS.41 For these methods, the LOD was about30 mg L�1.
Table 4 Analytical characteristics of different extraction methodsa
MethodDetectiontechnique
LDR(mg L�1) R PF
LOD(mg L�1) Reference
CPE FAAS 0.06–0.35 0.997 20 19 19CPE UV-Vis 0.02–0.16 0.997 50 7 18SPE ICPOES NI — 156 0.053 13LLE UV-Vis 0.07–3.02 0.999 20 29 8LLE FAAS 0.10–4.00 — — 100 16DLLME UV-Vis 0.02–2.50 0.999 3.3 6.1 This
study
a LDR: linear dynamic range; PF: preconcentration factor; NI: notinformed.
Precision and accuracy
Precision and accuracy evaluation was based on determinationsat three different Fe(III) concentrations (60, 300, and 600 mg L�1
of Fe(III), n ¼ 5). The accuracy was also evaluated by the analysisof a CRM. Results are summarized in Table 3. Precision wasestimated as the RSD; it ranged from 2 to 5% for water sampleswith low salinity, from 1 to 12% for samples with averagesalinity and from 1 to 9% for samples with high salinity.Accuracy was estimated as the recovery ranging from 90 to 100%for samples with different water salinities. In otherstudies,24,32,33 low efficiency was also found for seawater (highsalinity) in comparison with samples with low salinities and alow matrix effect. Thus, these results indicated that no signi-cant matrix effect was observed in the procedure proposed inthis study. Results for the CRM were in agreement with thecertied value (about 104%), with 5% RSD.
A comparison among methods for iron determination andextraction is shown in Table 4. In comparison with spectro-metric techniques and other extraction methods, DLLMEprovides low LODm and good reproducibility with UV-Vis.Furthermore, this study has proposed an affordable alternativeto expensive instruments for Fe determination.
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Method applicability
The method proposed by this study was applied to total irondetermination inmineral, well, estuarine and sea waters. In thiscase, spiking was performed at two levels (60 and 300 mg L�1) ina sample with low salinity (mineral water A), a sample withaverage salinity (estuarine) and a sample with high salinity(seawater). Results are shown in Table 5. Recoveries rangedfrom 86 to 102% with RSD below 9% for three different salin-ities in two levels of fortication whereas iron concentrationwas below the LOQm for all samples. Fe concentration inmineral water samples was below that of LOQm, except formineral water E and well water whose iron concentrations were25 � 1 and 35 � 2 mg L�1, respectively. For seawater, theconcentration was below that of LOQm. Results found in thisstudy were compared with those achieved by a well-establishedspectrophotometric method (Table 5). When results obtainedby both methods for well water were compared, good agreement(95%) with no statistical difference (t-Student test, p > 0.05) wasfound.
It is worth mentioning that the preconcentration step withthe DLLME method enables iron to be determined with UV-Visin the sample E (25 � 2 mg L�1) whereas the official method(1,10-phenanthroline) would not detect iron in the same sampledue to the LOQ, which is about 30 mg L�1 Fe(II). This resulthighlights the importance of the preconcentration step by
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Table 5 Comparison of analytical results for the determination of total Fe insamples with different salinities by proposed method and 1,10-phenanthrolinemethod. Results are shown in mg L�1 (mean � standard deviation, n ¼ 5)
Salinity Samples Proposed method 1,10-phenanthroline
Low Mineral water A <18.1 <30Mineral water B <18.1 <30Mineral water C <18.1 <30Mineral water D <18.1 <30Mineral water E 25 � 1 <30Well water 35 � 2 37 � 2
Average Estuarine water <18.1 <30High Seawater <18.1 <30
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DLLME for iron determination in water samples when it isfound in very low concentration. Besides, it eliminates the stepsof species reduction, making it simpler for the DLLME methodto determine total Fe.
D Conclusions
A new DLLMEmethod combined with UV-Vis was developed forFe determination in water samples with different salinities. Themethod, based on the formation of the complex Fe(APDC)3, issimple, rapid and inexpensive. It also has low LOD in compar-ison with spectrometric techniques because of its preconcen-tration step. Besides, the DLLME method is environmentallyfriendly since it uses low volumes of organic solvents. Finally,this method can contribute to monitoring studies of waters withdifferent salinities and low concentrations of Fe, without theuse of additional reagents or previous reduction/oxidation stepswhich can affect the analysis if the species are not completelyconverted into those of interest.
Acknowledgements
The authors are grateful to CNPq, CAPES and FAPERGS forsupporting this study.
Notes and references
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