simple and fast method for iron determination in white and red wines using dispersive...

Simple and Fast Method for Iron Determination in White and RedWines Using Dispersive Liquid−Liquid Microextraction andUltraviolet−Visible SpectrophotometryJuliana V. Maciel,† Bruno M. Soares,† Jaime S. Mandlate,† Rochele S. Picoloto,‡ Cezar A. Bizzi,‡

Erico M. M. Flores,‡ and Fabio A. Duarte*,‡

†Escola de Química e Alimentos, Universidade Federal do Rio Grande, Rio Grande, Rio Grande do Sul 96203-900, Brazil‡Departamento de Química, Universidade Federal de Santa Maria, Santa Maria, Rio Grande do Sul 97105-900, Brazil

ABSTRACT: This work reports the development of a method for Fe extraction in white and red wines using dispersive liquid−liquid microextraction (DLLME) and determination by ultraviolet−visible spectrophotometry. For optimization of the DLLMEmethod, the following parameters were evaluated: type and volume of dispersive (1300 μL of acetonitrile) and extraction (80 μLof C2Cl4) solvents, pH (3.0), concentration of ammonium pyrrolidinedithiocarbamate (APDC, 500 μL of 1% m/v APDCsolution), NaCl concentration (not added), and extraction time. The calibration curve was performed using the analyte additionmethod, and the limit of detection and relative standard deviation were 0.2 mg L−1 and below 7%, respectively. The accuracy wasevaluated by comparison of results obtained after Fe determination by graphite furnace atomic absorption spectrometry, withagreement ranging from 94 to 105%. The proposed method was applied for Fe determination in white and red wines withconcentrations ranging from 1.3 to 4.7 mg L−1.

KEYWORDS: dispersive liquid−liquid microextraction, iron, wine, UV−vis spectrophotometry, food analysis, sample preparation,GFAAS

■ INTRODUCTION

Wine is defined as a beverage obtained exclusively by alcoholicfermentation of grapes.1 With regard to its chemicalcomposition, wine has a great concern considering nutritionaland toxicological aspects, because it is a worldwide consumedbeverage.2 Among the main components present in wine, someof them can be cited as volatile organic compounds (ethanol),non-volatile organic compounds (superior alcohols, sugars,organic acids/salts, and others) and major and trace elements.3

The presence of metals in wine can be originated from naturaland/or anthropogenic sources.2 The range and level ofinorganic compounds in wine depends upon several factors,such as the type of grape, soil characteristics, environmentalconditions, and contamination during the manufacturingprocess.4

Iron is an essential element, which plays an important role inbiological systems, participating in the maintenance of cellularhomeostasis and several metabolic and fermentation processes,such as enzymatic activator, stabilizer, or as a functionalcomponent of proteins. Iron deficiency leads manly to anemia,causing changes in muscle metabolism and dysfunctions in theimmunological system.5 Otherwise, excess of Fe causehemochromatosis, characterized by pigmentation in the skin,pancreatic injury, such as diabetes, cirrhosis, and others.6 Thepresence of iron (ferric salts) in wine may cause cloudiness andprecipitation, which modify organoleptic characteristics.7

In general, detection techniques, such as flame atomicabsorption spectrometry (FAAS),7 graphite furnace atomicabsorption spectrometry (GFAAS),8 and inductively coupledplasma−optical emission spectrometry (ICP−OES),9 havebeen applied for Fe determination in wine samples. Although

there are a few works reporting ultraviolet−visible (UV−vis)spectrophotometry for the determination of Fe in wine, it is auseful technique and stands out mainly by its availability,simplicity, and relatively low cost when compared to otherspectrometric techniques already cited.10 However, the directelement determination at low concentrations by UV−vis isoften difficult because of its insufficient sensitivity andselectivity. Besides, some wines (especially the red wines)show an intense color, which causes several interferences forUV−vis detection.11 Therefore, a prior extraction andpreconcentration step is convenient, especially for matrixremoval. Thus, many sample preparation methods, such assolid-phase extraction (SPE),7 cloud point extraction(CPE),12,13 and liquid−liquid extraction (LLE),14,15 havebeen evaluated for subsequent element determination inwine. However, some of these methods demand excessivesolvent consumption, high waste generation, and longextraction times.Recently, dispersive liquid−liquid microextraction (DLLME)

was developed16 as an alternative for conventional LLE. Thismethod shows some advantages, such as simplicity, quickness,low waste generation, and good enrichment factors.17 Initially,DLLME was employed for the extraction of organic analytesfrom aqueous samples.16 However, it has recently been appliedfor element extraction18 and selected chemical species.19 It isimportant to mention that, when DLLME is applied for

Received: May 2, 2014Revised: July 18, 2014Accepted: July 29, 2014Published: July 29, 2014

Article

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© 2014 American Chemical Society 8340 dx.doi.org/10.1021/jf5019774 | J. Agric. Food Chem. 2014, 62, 8340−8345

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extraction of elements (or their species), it is necessary toperform a chemical modification (addition of a complexantagent), which results in a complex with affinity by extractionsolvent, which can be easily extracted.20

In this sense, the aim of this work was to develop a methodfor Fe extraction in white and red wine samples using DLLMEto allow for the determination by UV−vis. Some parameters,such as type and volume of dispersive and extraction solvents,pH, complexant agent [ammonium pyrrolidinedithiocarbamate(APDC)] concentration, NaCl concentration, and extractiontime, were exhaustively investigated. The optimized methodwas applied for Fe determination in commercial white and redwine samples. For accuracy evaluation, Fe was also determinedby GFAAS in all samples after sample digestion.

2. EXPERIMENTAL SECTION2.1. Instrumentation. Iron determination was performed in a

Shimadzu UV−vis spectrophotometer (model UV-2550, Japan) with1.0 cm quartz microcells. Iron was determined [as a complex ofFe(APDC)3] at 587 nm using peak height.18

For results comparison, a graphite furnace atomic absorptionspectrometer (model AAnalyst 800, PerkinElmer, Waltham, MA)equipped with a Zeeman-effect background correction system and anauto sampler (model AS 800, PerkinElmer) were used for the analysisof wine samples previously digested with nitric acid. Transverselyheated pyrolytically coated graphite tubes using standard platformswere used throughout. A Fe hollow cathode lamp (HamamatsuPhotonics, Japan) was (248.3 nm). The spectral bandpass was set at0.2 nm. The main conditions of the heating program were optimized,and the follow temperatures were chosen: drying 1, ramp of 15 °C s−1

up to 90 °C and hold for 15 s; drying 2, ramp of 5 °C s−1 up to 130 °Cand hold for 10 s; pyrolysis, ramp of 15 °C s−1 up to 1300 °C and holdfor 20 s; atomization, ramp of 2000 °C s−1 up to 2400 °C and hold for6 s; and clean out, ramp of 500 °C s−1 up to 2500 °C and hold for 3 s.Results were obtained using integrated absorbance (peak area), with20 μL of reference solutions or sample extract and 5 μL of 1000 mgL−1 Pd solution (Merck, Germany), which were injected into thegraphite tube. Argon (99.999%, White Martins, Brazil) was used as thepurge gas. Wine samples were digested in a heating block with openvessels. About 2 mL of sample were transferred to the vessels andsubmitted to a step for alcohol removal (80 °C for 2 h). After cooling,4 mL of concentrated HNO3 was added, followed by heating at 80 °Cfor 1 h and 120 °C for 1 h. After cooling, the digests were diluted withultrapure water up to 50 mL for further Fe determination by GFAAS.A centrifuge (model 80-2B, Centribio, Brazil) with 15 mL calibrated

centrifuge tubes was used for separation of phases after the DLLMEprocedure. The pH was adjusted using HCl or NaOH solutions with aHanna pH meter (model pH21, Brazil).2.2. Reagents. All reagents used in this work were of analytical

grade. Ultrapure water was purified by a Direct-Q UV3 purificationsystem (resistivity of 18.2 MΩ cm, Millipore, Billerica, MA). Stockreference solutions of 100 mg L−1 Fe were prepared by dissolving anappropriate amount of FeNH4(SO4)2·12H2O (Vetec, Brazil) in 1.0mol L−1 HCl (Merck). Other reference solutions were prepared dailyby serial dilution of stock solution in ultrapure water. The complexantagent (APDC, purity of 99%, Sigma-Aldrich, St. Louis, MO) wasprepared by dissolving an appropriate amount of this reagent inultrapure water. Dichlorobenzene (C6H4Cl2, d = 1.30 g mL−1), carbontetrachloride (CCl4, d = 1.59 g mL−1), monochlorobenzene (C6H5Cl,d = 1.11 g mL−1), and tetrachloroethylene (C2Cl4, d = 1.62 g mL−1)were evaluated as extraction solvents and purchased from Vetec.Acetone, acetonitrile, ethanol, methanol, and tetrahydrofuran (THF)were evaluated as dispersive solvents and obtained from J.T.Baker(Center Valley, PA). Solutions of 0.1 mol L−1 HCl and 0.1 mol L−1

NaOH (both Merck) were used to adjust the pH of wine samples. Allmaterials used in this work were previously cleaned by immersion in20% (v/v) HNO3 (Merck) solution for 24 h and washed withultrapure water.

2.3. Samples. Samples used in this work (two samples of whitewine and five samples of red wine) were produced and purchased inRio Grande do Sul State (Brazil). All samples were stored in glassbottles in the dark at room temperature and were named as WW1 andWW2 (white wine) and RW1, RW2, RW3, RW4, and RW5 (redwine). The alcohol content of samples ranged from 10 to 13.4%.Sample WW1 (1 mL of sample, pH adjustment and dilution up to 5mL with water) was used for method optimization.

2.4. DLLME Optimization. Initially, experiments were performedto evaluate the wavelength with a maximum absorbance signal and lesssusceptible to interferences on Fe determination by UV−vis. Theformation of the sedimented phase using all possible combinationsbetween extraction and dispersive solvents was evaluated. Someparameters of DLLME, such as type of dispersive solvent (acetone,acetonitrile, ethanol, methanol, and THF), volume of dispersivesolvent (100−1500 μL), type of extraction solvent (C6H4Cl2, CCl4,C6H5Cl, and C2Cl4), volume of extraction solvent (20−160 μL), pH(2−4), APDC concentration (0.1−3%, m/v), extraction time (0.5−5min), and NaCl concentration (0−0.5 mol L−1), were evaluated. Forthese experiments, samples were spiked with a Fe(III) solution up to afinal Fe concentration of 2 mg L−1, and results were shown as Ferecovery (%). Even considering the natural Fe concentration, sampleswere spiked to ensure the detectability of analyte and to allow for asuitable comparison of extraction conditions.

2.5. DLLME Procedure. Aliquots of 1 mL of wine and 500 μL ofAPDC solution were placed in a 15 mL glass tube with a conic bottomand diluted up to 5 mL with ultrapure water. The pH was adjustedwith HCl or NaOH solutions. A mixture containing the dispersive andextraction solvent was quickly injected into the sample solution withaid of a 2.5 mL microsyringe. After injection, a cloudy solution wasformed, which was centrifuged at 3000 rpm for 3 min, and smalldroplets of extraction solvent containing the complexed analyte[Fe(APDC)3] were deposited at the bottom of the glass tube. Toreduce some interference during the measurements, the sedimentedphase (extract) was washed with ultrapure water (3 mL andcentrifugation at 3000 rpm for 3 min). The aqueous phase wasremoved, and extract was further diluted up to 3 mL with acetonitrilefor subsequent Fe determination by UV−vis.

All statistical calculations were performed using GraphPad InStatsoftware (GraphPad InStat Software, Inc., version 3.06). A significancelevel of 95% was adopted for all comparisons.

3. RESULTS AND DISCUSSIONInitially, experiments were performed to find the spectral regionwith maximum absorbance for the Fe(APDC)3 complex. In aprevious study of our group,18 APDC was used for complex-ation of Fe(II) and Fe(III) species. It was observed that Fe(III)is instantaneously complexed by APDC, while Fe(II) iscomplexed by APDC and quickly oxidized to Fe(III), makingit difficult to perform Fe speciation analysis.18 It was observed[using an aqueous solution containing Fe(III) and APDC] thatFe(APDC)3 complex shows one absorption band in UV region(wavelength of maximum absorbance at 354.0 nm) and twoabsorption bands in visible region (wavelengths of maximumabsorbance at 499.0 and 587.0 nm). Although the most intenseabsorption band was in the UV region (data not shown), it washighly influenced by the absorption band of APDC, making theuse of this wavelength difficult for analytical purposes. Thus, tominimize possible interferences, the wavelength of 587.0 nmwas selected for Fe determination by UV−vis.

3.1. Effect of the Type of Extraction and DispersiveSolvents. The choice of dispersive and extraction solvents isessential to obtain suitable analyte recoveries using DLLME.The extraction solvent should have the ability to extract theanalytes and present low solubility in water.21 Initially, severalcombinations between extraction and dispersive solvents werequalitatively evaluated (results not showed) for red and white

Journal of Agricultural and Food Chemistry Article

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wines. The main parameters to choose the solvents that wereused for subsequent experiments were taken into account, withthe minimum amount of solids around the drop and theminimum undesirable effects (e.g., changes in baseline) duringmeasurements by UV−vis. In this case, solvents with higherdensity than water, such as dichlorobenzene, carbon tetra-chloride, monochlorobenzene, and tetrachloroethylene, wereevaluated as the extraction solvent. These solvents wereinvestigated using 50 μL of extraction solvent combined with500 μL of acetonitrile as the dispersive solvent. As showed inFigure 1, the higher recovery (48%) was found using

tetrachloroethylene, whereas the recoveries for other solventswere lower than 33%. Additionally, tetrachloroethylenepresented the lowest relative standard deviation (RSD, about3%), which was also considered for choosing this solvent forfurther experiments.For the selection of the dispersive solvent, the key factor is

the miscibility of dispersive solvent into the extraction solventand aqueous sample.22 The effect of dispersive solvent(acetone, acetonitrile, ethanol, methanol, and THF) on Feextraction was evaluated using 500 μL of each solvent and 50μL of tetrachloroethylene (as the extraction solvent). As can beseen in Figure 2, the highest recovery (about 48%) wasobtained using acetonitrile, with RSD below 2%. Thus,acetonitrile was used as the dispersive solvent for subsequentexperiments.3.2. Effect of Volumes of Dispersive and Extraction

Solvents. The volume of the dispersive solvent is an importantparameter in DLLME because, at small volumes, the dispersionof the extraction solvent does not occur and the cloudy solutionis not formed. On the other hand, the use of large volumesincreases the solubility of the extraction solvent (and analyte)in the aqueous phase, decreasing the extraction efficiency.23

After dispersive (acetonitrile) and extraction (tetrachloro-ethylene) solvents have been chosen, experiments wereperformed to evaluate the volume of these solvents. Thevolume of acetonitrile ranged from 200 to 1500 μL incombination with 50 μL of C2Cl4 and 500 μL of 1% (m/v)APDC at pH 3.3. As can be seen in Figure 3, Fe recoveryincreased according to the increase of acetonitrile volume up to1300 μL. A significant difference (Tukey−Kramer test)between 1300 and 1500 μL of acetonitrile was not observed,

and the condition using 1300 μL was selected for furtherexperiments.After optimization of the volume of acetonitrile (1300 μL),

the volume of tetrachloroethylene was evaluated from 20 and160 μL. Results are shown in Figure 4, and a significantdifference (Tukey−Kramer test) was not observed among 40and 160 μL of CCl4. Considering the use of lower volumes of

Figure 1. Effect of the type of extraction solvent on Fe recovery.Conditions: 5 mL of white wine sample (5 times diluted), addition of240 μL of 25 mg L−1 Fe(III) solution, 500 μL of 1% APDC, 50 μL ofextraction solvent, 500 μL of dispersive solvent (acetonitrile), and pH3.3. The error bars represent the standard deviation (n = 5).

Figure 2. Effect of the type of dispersive solvent on Fe recovery.Conditions: 5 mL of white wine sample (5 times diluted), addition of240 μL of 25 mg L−1 Fe(III) solution, 500 μL of 1% APDC, 500 μL ofdispersive solvent, 80 μL of extraction solvent (C2Cl4), and pH 3.3.The error bars represent the standard deviation (n = 5).

Figure 3. Effect of the dispersive solvent volume (acetonitrile) on Ferecovery. Conditions: 5 mL of white wine sample (5 times diluted),addition of 240 μL of 25 mg L−1 Fe(III) solution, 500 μL of 1%APDC, 80 μL of extraction solvent (C2Cl4), and pH 3.3. The errorbars represent the standard deviation (n = 5).

Figure 4. Effect of the extraction solvent volume (C2Cl4) on Ferecovery. Conditions: 5 mL of white wine sample (5 times diluted),addition of 240 μL of 25 mg L−1 Fe(III) solution, 500 μL of 1%APDC, 1300 μL of dispersive solvent (acetonitrile), and pH 3.3. Theerror bars represent the standard deviation (n = 5).



tetrachloroethylene (from 20 to 60 μL), it was difficult tohandle the extract because of the low volume of solvent. Thus,80 μL was chosen for the subsequent experiments, whichpresented the lowest RSD values (about 2%) and also assured asuitable amount of extraction solvent.3.3. Effect of pH. The effect of pH for element extraction

using DLLME has an important role on the formation of thecomplex and its subsequent extraction. The influence of pH inthe Fe recovery was evaluated from 2 to 4. Results are shown inFigure 5. Using pH 3, Fe recovery of about 80% was obtained

(RSD of 3%). For pH values below 3, low recovery values couldbe attributed to the protonation of APDC (complexant agent).Similarly, low recoveries were obtained at pH values higherthan 3, which can be attributed to the formation of Fe−hydroxyl complexes. It can directly affect the transference ofanalyte to the extraction solvent because the formation of theFe(APDC) complex is avoided.24 Therefore, pH 3 was selectedfor further experiments. This pH (3) was in agreement withthose found for Fe extraction using APDC as the complexantagent.25

3.4. Effect of the APDC Concentration. The effect of theAPDC concentration was investigated using 500 μL of solutionranging from 0.1 to 3% (m/v). The maximum recoveries(about 80%) were found using 1% APDC solution, whichremained constant up to 3%. Once significant differences (p >0.05) among the values ranging from 1 to 3% were notobserved, 1% APDC solution was selected for further studies.For the APDC concentration below of 1%, the lower recoveriescan be explained because of the low APDC concentration,which was not enough for complete complexation of theanalyte, especially because of the presence of other ions thatcould compete with the analyte by complexation with APDC.3.5. Effect of the NaCl Addition. Generally, the addition

of salt in the DLLME procedure increases the extractionefficiency because of the salting out effect that promotes thetransfer of analyte to the organic phase by decreasing itssolubility in the aqueous phase.26 To investigate the influenceof ionic strength, experiments were performed by addingvariable amounts of NaCl (from 0 to 25 g L−1). Otherexperimental conditions were kept constant, but at this range ofthe NaCl concentration, no statistical difference (Tukey−Kramer test) on Fe recovery was observed. Thereby, for furtherstudies, NaCl was not added.

3.6. Effect of the Extraction Time. In general, theextraction time for DLLME is defined as the time between theinjection of the mixture of dispersive and extraction solventsbefore the centrifugation.27 The extraction time was evaluatedfrom 0.5 to 5 min, but results (not shown) did not presentedsignificant influence (Tukey−Kramer test) on Fe recovery.Because of the large surface area between the extraction solventand aqueous phase after the formation of a cloudy solution, thecomplex quickly diffuses into the extraction solvent.28 There-fore, the DLLME procedure was considered time-independent,which is an important advantage of the proposed method.

3.7. Analytical Performance. The optimized DLLMEmethod was evaluated considering some parameters, such aslinearity, limit of detection (LOD), limit of quantification(LOQ), accuracy, and precision. A complementary study forevaluating the matrix effect on Fe determination by UV−viswas also performed comparing the slopes of calibration curves,through the DLLME method, in aqueous reference solutionand wine samples. When these slopes were compared,suppression in the analytical curve of about 16 and 26% wasfound for white and red wines, respectively. This suppressionwas also observed during method optimization, once recoverieswere not better than 84%. Thus, to minimize this effect, theinstrument calibration was carried out using a calibration curvewith analyte addition. It is important to point out that thebehavior (slope) for the same type of wine (white or red) wassimilar for all samples. In this sense, for Fe determination inwhite wine, sample WW1 was used for instrument calibration.Similarly, for red wine, sample RW1 was used for instrumentcalibration for further Fe determination. The calibration rangewas linear from 0.75 to 2.5 mg L−1 Fe(III) with coefficients ofdetermination (R2) of 0.9985 and 0.9988, for white and redwines, respectively. It is important to mention that the effect ofcoexisting ions was also investigated. As expected, it wasobserved that transition metals, such as Cd2+, Cr3+, Mn2+, Ni2+,Pb2+, Sn2+, and Zn2+, with concentration below 1 mg L−1, didnot affect Fe extraction/determination by UV−vis spectropho-tometry. For major elements, such as Ca2+, K+, Mg2+, and Na+,the concentration limit is about 5000 mg L−1. It is important tomention that Co2+ and Cu2+ have a strong influence on Feextraction/determination (concentration limit of about 0.4 mgL−1).The LOD and LOQ were estimated using a calibration curve

with analyte addition, defined as 3 and 10 times the standarddeviation of 10 measurements of blanks divided by the slope ofthe calibration curve.29 The LOD and LOQ were 0.2 and 0.8mg L−1, which are significantly lower than the maximum Feconcentration established by Brazilian legislation. Consideringthat the proposed DLLME method is relatively fast and lowcost, it can be an important alternative for Fe monitoring in awine sample easily applied in routine analysis.Because of the lack of a certified reference material

presenting certified values for Fe in wine samples, accuracywas evaluated by comparison to Fe determination by GFAAS. Itis important to mention that Fe determination by GFAAS wasperformed using samples decomposed in a heating block withopen vessels. Results are summarized in Table 1. Bycomparison to results obtained by GFAAS, the proposedmethod showed an agreement ranging from 93 to 105% for allsamples, with RSD lower than 7%. The proposed method wasalso applied for Fe determination in commercial white and redwine samples (Table 1), and the concentration ranged from 1.3to 4.7 mg L−1 and from 2.5 to 4.2 mg L−1, respectively. It is

Figure 5. Effect of pH on Fe recovery. Conditions: 5 mL of white winesample (5 times diluted), addition of 240 μL of 25 mg L−1 Fe(III)solution, 500 μL of 1% APDC, 80 μL of extraction solvent (C2Cl4),and 1300 μL of dispersive solvent (acetonitrile). The error barsrepresent the standard deviation (n = 5).



worth highlighting that results are below the maximum limit ofFe in Brazilian legislation (maximum value of Fe in wine is 15mg L−1), making the proposed extraction procedure appro-priate for Fe control in wines.In summary, the proposed DLLME method combined with

UV−vis spectrophotometry was successfully developed for totalFe determination in wine samples. This method was consideredas environmentally friendly because it uses low volumes oforganic solvents and showed the same suitable features, such assimplicity, quickness, and relatively low cost, when compared toAAS- or ICP-based techniques. In addition, the proposedmethod showed good results in terms of linearity, accuracy, andprecision, providing evidence of the feasibility of UV−visspectrophotometry as an alternative to routine quality controlfor Fe concentration in white and red wines.

■ AUTHOR INFORMATIONCorresponding Author*Fax: +55-55-3220-9445. E-mail: [email protected] authors are grateful to the Conselho Nacional deDesenvolvimento Cientifico e Tecnolo gico (CNPq), theCoordenacao de Aperfeicoamento de Pessoal de Nivel Superior(CAPES), and the Fundacao de Amparo a Pesquisa do Estadodo Rio Grande do Sul (FAPERGS) for supporting this study.NotesThe authors declare no competing financial interest.

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Table 1. Iron Determination in White and Red Wines byUV−vis and GFAAS (Results in mg L−1 ± StandardDeviation; n = 5)

sample UV−vis GFAAS

white wine 1 1.3 ± 0.1 1.4 ± 0.1white wine 2 4.7 ± 0.1 5.0 ± 0.2red wine 1 4.2 ± 0.2 4.4 ± 0.2red wine 2 3.3 ± 0.1 3.2 ± 0.1red wine 3 3.9 ± 0.2 4.1 ± 0.3red wine 4 2.5 ± 0.1 2.5 ± 0.1red wine 5 3.0 ± 0.2 3.0 ± 0.2



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