Simple Time-Saving Method for Iron Determination Based on

Article pubs.acs.org/JAFC

Simple Time-Saving Method for Iron Determination Based on Fluorescence Quenching of an Azaflavanon-3-ol Compound Aysel Başoğlu,† Gonca Tosun,§ Miraç Ocak,§ Hakan Alp,§ Nurettin Yaylı,# and Ü mmühan Ocak*,§ †

Vocational School, Bayburt University, Bayburt, Turkey Department of Chemistry, Faculty of Sciences and #Faculty of Pharmaceutical Sciences, Pharmaceutical Sciences, Karadeniz Technical University, 61080 Trabzon, Turkey

§

S Supporting Information *

ABSTRACT: A simple and time-saving spectrofluorometric method developed using an azaflavanon-3-ol compound was used for the determination of iron in various food samples. Nitric acid and hydrogen peroxide were used for digestion of samples in a closed microwave system. The method was validated by analyzing two certified reference materials (CRM-SA-C Sandy Soil C and Mixed Polish Herbs INCT-MPH-2). Measurements were carried out using a modified standard addition method. The standard addition graph was linear until 21.6 mg/L in the determination of iron(III). Detection and quantification limits were 0.81 and 2.4 mg/L, respectively. Satisfactory accuracy was obtained for spinach, dill, mint, purslane, rocket, red lentils, dry beans, and two iron medicinal tablets. High recoveries were found for streamwater samples fortified at three different concentrations. The method is simple, time-saving, cost-effective, and suitable for the determination of the iron content of foods. KEYWORDS: food, iron determination, spectrofluorimetric analysis, azaflavanon-3-ol

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INTRODUCTION For almost all living organisms, iron is a vital element. It is wellknown that iron in the heme group of hemoglobin, a protein in red blood cells, interacts with oxygen and allows it to be soluble in blood.1 Therefore, the iron complex of heme provides transport of oxygen in cells. A reduction in the number of heme groups containing iron causes failure in oxygen transport and cell functions. This important element is also effective in the function of enzymes in protein metabolism2 and enhances the functions of biological elements such as calcium and copper. It has critical roles to produce connective tissues in the body and some of the neurotransmitters in the brain.3 It is also effective in maintenance of the immune system.4 The best sources of iron include dried beans, dried fruits, eggs, liver, red meat, salmon, whole grains, iron-fortified cereals, vegetables, and seeds. Heme iron is found in animal-based foods, which allow easier absorption of iron. Intake of iron from plant-based food is harder. These types of foods, such as prunes, raisins, apricots, lima beans, soybeans, dried beans, broccoli, spinach, collards, wheat, and brown rice, contain nonheme iron. It is particularly important to include plant- and animal-based foods with high iron content in a diet. Iron content of foods is generally determined by AAS and ICP methods.5−9 The atomic methods are very expensive and complicated. Moreover, sample treatment steps are necessary in many cases. Separation processes such as extraction and coprecipitation lead to long determination time. Similarly, preconcentration of iron before analysis requires some chemicals such as adsorbents and stripping reagents.10−13 Therefore, it is important to develop a a simple, reliable, time-saving, and costeffective method for determining the iron content of foods. Spectrofluorometric methods are simple and cost-effective analytical methods, and their other advantages include inherent selectivity and sensitivity. Using fluorescent ligands in selective © 2015 American Chemical Society

and sensitive determination of metal ions is an effective approach.14−16 If a fluorescent compound consists of an ionophore and a fluorophore moiety, it is called a fluoroionophore. The ionophore part forms a complex with analyte, whereas the fluorophore moiety is responsible for analytical response. These types of compounds are used as analytical ligands to develop selective and sensitive methods. In this study, an azaflavanon-3-ol compound was used as a fluoroionophore. The azaflavanon-3-ol structure produces fluorescence emission, whereas the carbonyl and hydroxyl groups probably interact with the iron(III) ion. Some fluorescent compounds have been proposed for the determination of iron in the literature.17−22 However, their application to real samples is limited.23−26 Except for Zhang, others applied their method to tap water, streamwater, and wastewater samples.23−25 Zhang used only multivitamin tablets as a real sample to determine iron content.26 Spectrofluorometric methods for iron determination in foods are also limited.27−29 Because of a long complexation process and pretreatment before analysis, some of these methods are time-consuming.27,28 Some require the use of many chemicals and extra processes.29,30 In the present study, a simple, time-saving, and harmless spectrofluorometric method to determine the iron content of foods is reported. An azaflavanon-3-ol compound was proposed as a fluorescent reagent. According to our knowledge, there is no report of a flavonoid compound used in metal determination in real samples in the literature. In this sense, the present study is a first. Moreover, a modified standard additional method was developed to get accurate results. This modified standard Received: Revised: Accepted: Published: 2654

November 11, 2014 February 26, 2015 February 27, 2015 February 27, 2015 DOI: 10.1021/jf505336d J. Agric. Food Chem. 2015, 63, 2654−2659

Journal of Agricultural and Food Chemistry

Article

The fluorescence intensities of the first tube containing the blank solution and the second tube and the slope of the calibration graph were used to calculate iron(III) concentration. The iron(III) complex has a lower fluorescence intensity compared to the free azaflavanon-3-ol ligand. After the iron(III) complex is formed, excess iron(III) ion (quencher) causes dynamic quenching of the complex, which is regular for increasing iron(III) concentration between 2.4 and 21.6 mg/L, and it is used to determine the iron content of the sample. The modified standard addition method was used to determine the iron concentration in all samples. A constant amount of iron(III) (1 mg/L), 2 mL of ligand (1.3 × 10−5 M), and an aliquot sample solution were added to all tubes. Only the sample was not added to the first tube. Therefore, a blank solution without the sample in the first tube was prepared. Increasing amounts of iron were added to the third and next tubes. The pH of all solutions was set to 7 with citric acid buffer before the final volume was completed to 4 mL. Fluorescence intensity was measured at 503 nm for all solutions. The iron(III) concentration was calculated from eq 1

addition method is also not yet present in the literature. It works when the calibration graph was formed after the equivalent point because of dynamic quenching with excess iron(III) ions.

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MATERIALS AND METHODS

Instrumentation. A PerkinElmer UV−vis dpectrophotometer and a PTI spectrofluorometer (QM 2006 model) were used to measure absorbance and fluorescence intensity, respectively. The excitation wavelength was 375 nm, and fluorescence emission spectra were recorded in the range of 400−650 nm with a slit width of 1.0 nm. A Jenway 3040 ion analyzer was used to measure the pH of the solutions. Reagents. In all analytical measurements, pipets and vessels kept in dilute nitric acid at least overnight and subsequently washed three times with distilled water were used. Analytical grade chemicals and solvents were purchased from Merck (Darmstadt, Germany) and Fluka (Buch, Swetzerland). Stock solutions (1000 mg/L) of metal cations were prepared by dissolving an appropriate amount of nitrate salts in deionized water. Working standard solutions were obtained by appropriate dilution of these stock standard solutions. The buffer solutions (AVS Titrinorm, Merck Certipur) were purchased from Merck. Sandy soil standard (CRM-SA-C) was supplied from HighPurity Standards, Inc., and Mixed Polish Herbs standard (INCTMPH-2) from Institute of Nuclear Chemistry and Technology. 2-(4-Fluorophenyl)-3-hydroxy-2,3-dihydroquinoline-4(1H)-one (Figure S1, Supporting Information) was prepared according to the literature. The synthetic procedure is described in the Supporting Information. A 2.6 × 10−3 mol/L solution of this compound was prepared by dissolving the appropriate amount of this ligand in 100 mL of methanol and was kept in a refrigerator at 4 °C for 1 week. Samples. All food samples were purchased from local supermarkets and iron tablets from pharmacies in Trabzon, Turkey. The streamwater sample was collected from Değirmendere in Trabzon and was filtered through a 0.45 mm cellulose nitrate membrane. The vegetable samples were washed thoroughly with tap water and deionized water. Then, the samples were dried at 105 °C for 24 h and ground. Digestion of the dried samples (0.5 g) was carried out using 6.0 mL of HNO3 and 2.0 mL of H2O2, and digestion of standard reference materials (0.5 g) was carried out using 1.5 mL of HNO3, 4.5 mL of HCl, 1.0 mL of HF, and 2.0 mL of H2O2 in a closed microwave digestion system. After the microwave digestion process, the solutions were filtered through a 0.45 mm cellulose nitrate membrane and evaporated. Then, the volume was completed to 25 mL with deionized water. To digest the dried iron tablets, one iron tablet was dissolved in 60 mL of deionized water using an ultrasonic bath. Then, the temperature was set to 200 °C on the hot plate, and 3 mL of HNO3 was added gradually. The solutions were filtered through a 0.45 mm cellulose nitrate membrane, and the final volume was completed to 25 mL with deionized water. Optimum Conditions. Some parameters of the proposed method, such as solvent and concentration of ligand, solution pH, complexation time, and constant iron(III) concentration, were optimized. DMF, acetonitrile, THF, ethanol, and methanol were tested as a ligand solvent. Among these solvents, methanol was preferred because it is cheap and has high fluorescence intensity. A series of buffer solutions (pH from 2 to 10) was used to optimize solution pH. pH 7 was suitable for the study because of the high fluorescence intensity of the solution at this pH. Only 4 min was enough for complexation. Various constant iron(III) concentrations (0.2−3.0 mg/L) were tested to obtain accurate results with modified standard addition method. The optimum constant iron(III) concentration was 1.0 mg/L. Proposed Method. The proposed method is based on fluorescence quenching of an azaflavanon-3-ol compound, which has been recently reported in the literature, with the iron(III) ion. A modified standard addition method was used to determine the iron content of food samples. Just like a standard addition method, it has not been reported in the literature according to our knowledge. One more tube (first tube) was used in this method. A certain amount of analyte and ligand was added to all tubes and the first tube. A certain amount of sample was added to all tubes, except for the first tube.

Cx = (F0 − F1)/m

(1)

where Cx is the iron concentration of the sample in the tubes, F0 and F1 are the fluorescence intensities of the first and second tubes, respectively, and m is the slope of the standard addition graph. According to this type of standard addition procedure, the difference between F0 and F1 is related to the iron concentration of the sample in the tubes. Figure S2 (Supporting Information) shows the standard addition graph for the determination of iron (2.5 mg/L) in spinach.

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RESULTS AND DISCUSSION When the ligand is excited at 375 nm, it gives a maximum emission band at 503 nm. Excitation and emission spectra of the ligand are given in Figure S3 (Supporting Information). Figure S4 (Supporting Information) shows the effects of Li+, Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Al3+, Sn2+, Pb2+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Ag+, Zn2+, Cd2+, and Hg2+ ions (1.3 × 10−4 M) on the fluorescence spectra of the ligand (1.3 × 10−5 M). As can be seen from Figure S4, there are moderate changes in the fluorescence intensity of the ligand for many ions, except for Fe2+ and Fe3+ ions. Al3+ causes a small shift in fluorescence spectra to short wavelength. However, spectrofluorometric titrations showed that there is no regular change for fluorescence intensity with increasing aluminum concentrations. In contrast, increasing Fe2+ and Fe3+ concentrations cause quenching on the fluorescence intensity of the ligand at 503 nm. Because the samples were digested in acidic media, iron(II) was oxidized to iron(III), and the method was based on quenching of the iron(III) ion. Effect of pH. The effect of pH on the fluorescence intensity of the solution containing equivalent ligand and iron(III) (1.3 × 10−5 M) is shown in Figure S5 (Supporting Information). Citric acid−sodium hydroxide−hydrochloric acid, acetic acid− ammonium acetate, citric acid−sodium hydroxide, HEPES, and boric acid−sodium hydroxide−hydrochloric acid buffers were tested to set the pH of the solutions. As seen from Figure S5, the fluorescence intensity was satisfactorily high and nearly the same between pH 6 and 8. Iron determination was carried out with a variety of buffer solutions. The lowest relative error percent for the standard iron(III) solution was obtained for the citric acid buffer solution (pH 7). Therefore, this buffer solution was used for iron determination. Effect of Reagent Concentration. To determine optimum reagent concentration, spectrofluorometric titrations were performed by using solutions of the ligand in the range from 5.0 × 10−5 to 2.6 × 10−6 M. The values of fluorescence intensity at 503 nm were plotted against the iron(III) concentration. 2655

DOI: 10.1021/jf505336d J. Agric. Food Chem. 2015, 63, 2654−2659


Article

When 1.3 × 10−5 M ligand concentration was used, the highest R2 value (0.9949) was obtained. Therefore, further studies were carried out at this concentration. Effect of Constant Iron Concentration. The constant iron(III) concentration used in the modified standard addition method was optimized by changing from 0.2 to 3.0 mg/L. Measurements were carried out in Sandy Soil C and iron tablet 1 for 2.5 mg/L iron(III) concentration, and the results are shown in Table S1 (Supporting Information). As seen from Table S1, the low relative errors were 1.5 and 1.1% for 1 mg/L in the case of Sandy Soil C and iron tablet 1, respectively. Between constant iron concentrations of 1.3 and 2.0 mg/L, the RE% values were below 2.5. However, the lower concentration (1 mg/L) was preferred as the constant iron concentration. Time before Measurement. The emission spectrum of the mixture containing an equivalent amount of iron(III) and the ligand (1.3 × 10−5 M) was obtained in the range of 1−15 min. It was seen that the fluorescence intensity did not change after 1 min (Figure S6, Supporting Information). There was very little change in the fluorescence intensity from 1.0 until 15 min, which means that a few minutes before the measurements was enough. Composition and Stability of the Complex. The Job method was used to investigate the nature of the complex in methanol/water (1:1). The concentration of ligand and iron(III) was 2.6 × 10−6 M, and total solution volume was 4 mL. As seen from Figure S7 (Supporting Information), the plot of fluorescence intensity versus the ligand volume showed a break point at 2 mL, indicating that complex composition was 1:1. The conditional formation constant (log K ± standard deviation), calculated according to a known method31 using the spectrofluorometric titration data (N = 3), was found to be 3.40 ± 0.09. Effect of Foreign Ions. The potential interference due to common coexisting ions encountered in a natural sample was tested by adding these ions to the solution containing the equivalent ligand and iron(III) (1.3 × 10−5 M). The fluorescence intensity of the solution was compared with that of the solution containing only the ligand and iron(III). The tolerance limit was recognized at ±5% difference in the analytical signals. Ions Li+, Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Al3+, Cr3+, Sn2+, Pb2+, Zn2+, Ni2+, Co2+, Cd2+, Hg2+, Ag+, Cu2+, Mn2+,OH−, C2O42−, CO32−, SO42−, S2O72−, Cr2O72−, CrO42, HCO3−, HSO4−, H2PO4−, CN−, SCN−, HSO4−, F−, Cl−, Br−, I−, and CH3COO− were examined. As seen from Table S2 (Supporting Information), the effect of the tested foreign ions is negligible. Quenching Mechanism. The Stern−Volmer relationship32 (eq 2) expresses the efficiency of quenching of a fluorophore by a quencher. F0/F = 1 + KSV[Q]

concentration until 4 × 10−4 M, after which a positive deviation was observed. This result shows that both static and dynamic quenching exist in the system. As known, static quenching requires a ground state complex and is distinguished from dynamic quenching with temperature effect.23 Dynamic quenching is a kind of collisional quenching, and quenching efficiency increases with increasing temperature. The inset of Figure S8 shows the temperature effect on fluorescence efficiency (F0/F). As seen from the Figure S8 inset, the quenching efficiency does not increase with increasing temperature. Conversely, the lower fluorescence efficiency is observed when the temperature increases from 298 to 333 K. Therefore, it may be concluded from the consideration of Figure S8 that static quenching, proving a formation of complex between ligand and iron(III) ion, is effective in the fluorescence quenching. Analytical Performance. An external calibration line based on fluorescence quenching of the 1.3 × 10−5 M ligand showed linearity within the concentration range of 1.4−21.6 mg/L. However, accuracy was not good in iron determination in samples using the external calibration graph. Because of this, a modified standard addition method was used in iron determination. The analytical performance data for the developed method under the optimized conditions are presented in Table S3 (Supporting Information). The data given in Table S3 indicate that a linear relationship was found between the fluorescence intensity at 503 nm and the concentration of the iron(III) in the range of 2.4−21.6 mg/L. The correlation coefficient was 0.9949, indicating good linearity. Table 1. Determination of Iron in Two Reference Materials (N = 3) sample

found value

RSD%

certified value

Sandy Soil C Mixed Polish Herbs

13.9 0.45

1.96 1.38

13.9 0.46

Table 2. Determination of Iron in Food Samples (N = 3) standard method (ICP-OES)

proposed method

a

(2)

sample

value found

RSD%

value found

MUa

spinach mint rocket purslane dill dry beans red lentils

215.6 642.7 615.7 136.3 78.5 102.6 86.8

1.6 0.7 1.4 0.3 1.7 0.6 0.3

219.2 648.4 607.0 135.7 80.1 102.9 86.4

17.8 52.5 49.2 11.0 6.5 8.3 7.0

Measurement uncertainty.

Table 3. Determination of Iron in Iron Tablet Samples (N = 3)

F0 and F are the fluorescence intensities in the absence and presence of the quencher, respectively. KSV is the Stern− Volmer quenching constant, and [Q] is the concentration of the quencher. If a system obeys the Stern−Volmer equation, a plot of F0/F versus [Q] will give a straight line with a slope of KSV and a y-axis intercept of 1. However, a positive deviation may be observed in the Stern−Volmer graph above a certain quencher concentration. In this case, both dynamic and static quenchings are efficient in the system.33 Figure S8 (Supporting Information) shows the Stern−Volmer plot of the iron(III)−ligand system. As seen from Figure S8, the F0/F term linearly increased with increasing iron(III)

standard method (NMKL 161)

proposed method sample

value found

RSD%

value found

SD

tablet 1 tablet 2

155.2 138.6

0.9 0.2

157.8 138.5

2.1 1.1

Table 4. Recovery Studies of Iron in the Streamwater Sample

2656

Fe added (mg/L)

Fe found (mg/L) ± RSD%

recovery (%)

2.5 5.0 7.5

2.5 ± 0.8 5.0 ± 0.2 7.5 ± 0.3

98.8 99.6 99.6


macaroni, wheat, flour, and some water samples

5-hydroxy-4-ethyl-5,6-dipyridin-2-yl4,5-dihydro-2H-[1,2,4]-triazine-3thione, FAAS zirconium(IV) hydroxide, FAAS

2657

baking soda and baking powder samples

ammonium pyrolidine− dithiocarbamate−Chromosorb 102, Ce(OH)4, FAAS or GFAAS 8-oxyquinoline, FAAS

natural and mineral waters

river water, rainwater, drinking water, and powdered milk

dimethyl (E)-2-[(Z)-1-acetyl)-2hydroxy-1-propenyl]-2-butenedioate, FAAS

natural waters, coffee, fish, tobacco, black and green tea

bovine liver

milk powder and jasmine tea

multivitamin tablets

rocket, dill, mint, spinach, purslane, red lentils, dry beans, and iron medicinal tablets rice, spinach, different water samples

sample

4-hydroxyquinoline, SF

2-(2-hydroxyphenyl)-4(3H)quinazolinone, SF 2-pyridinecarbaldehyde− phenylenedihydrazone, SF

5-(8-hydroxy-2-quinolinylmethyl)-2,8dithia-5-aza-2,6-pyridinophane, SF

2-(4-fluorophenyl)-3-hydroxy-2,3dihydroquinoline-4(1H)-one, SF

reagent, method

dissolving in a mixture of concentrated HNO3 and distilled water before neutralization with NH3 complexation with 8-oxyquinoline solution

powdered milk charring at 300 °C for an hour, turned into ash overnight until becoming a grayish white color

for food samples, dissolving concentrated HNO3 and evaporation to dryness, oxidation with H2O2 digestion in microwave system for 26 min

dissolvied in aqua regia, heated to dryness, dilution to 10 mL with NaOAc−HOAc incineration for 270 and 60 min at 500 °C, for milk powder and jasmine tea, respectively acidic digestion

microwave-assisted digestion for food samples for about 20 min, wet digestion for iron medicinal tablets left at 500 °C for 2 h, dissolving in nitric acid, dilution with an acetate buffer

sample preparation

extraction of the complex to chloroform for 2 min

masking with KF, waiting 20 min for fluorescence stability SPE with the impregnated Amberlite XAD-16 resin, waiting for saturation for 2 h with the ligand solution, PF: 25 coprecipitation with zirconium(IV), centrifugation for 15 min, dissolving the precipitate with 1 mL of concentrated HNO3, PF: 25 extraction with modified octadecyl silica membrane disk using a vacuum, stripping iron(III) from the disk with EDTA solution, PF: 166 SPE with APDC-Chromosorb 102 and coprecipitation with Ce(OH)4, PE: NA

modified standard addition method, waiting only 1−2 min before the measurement preparation of a film, waiting >20 min to soak in buffer solution, and 5 min for fluorescence stability preparation of an optode membrane and dried for 24 h before being used waiting 30 min for fluorescence stability

procedures before measurement

5.0

6.00, 10.5

2.5

8.0

5.0

4.6

2.5−4.5

5.5

7

pH

0.1−4.0

N.A

0.0400−1.0

0.25−5.0

0.28−0.56

0.03−0.56

0.04−7.80

33.6−0.006

2.40−2.60

linear range (mg/L)

0.1

0.26, 0.12

0.0028

0.00153

0.00459

0.168

0.02

0.005

0.0028

0.81

LOD (mg/L)

Table 5. Analytical and Operational Parameters of the Proposed Method and Previously Reported Methods Based on Spectrofluorometry and Atomic Spectrometry ref

9

12

10

13

11

28

27

26

29

PM

Journal of Agricultural and Food Chemistry Article



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The LOD (3 × Sd/m) and LOQ (9 × Sd/m) were determined using the standard deviation of 11 measurements of the blank response (Sd) and the slope of the calibration line (m) according to IUPAC recommendations. Accuracy. The accuracy of the proposed method was verified by analyzing two standard reference materials (CRMSA-C Sandy Soil C and Mixed Polish Herbs INCT-MPH-2). A good agreement was found between obtained values and certified values (Table 1). Statistical analysis of the results using Student’s t test shows no significant difference between obtained values and certified values. The calculated Student’s t value (3.48) was less than the theoretical value (4.30) at a confidence level of 95%. The food samples and two iron tablets were analyzed according to a modified standard addition method. Analysis of the same food samples was carried out by using a standard ICPOES method, whereas standard NMKL 161 method was used to analyze iron tablets. The results are given in Tables 2 and 3. Statistical analysis of the results using Student’s t test showed no significant difference between the results. The accuracy was also tested by using a standard addition technique. The modified standard addition method was applied to a streamwater sample (Değirmendere, Trabzon) spiked at three iron(III) concentration levels, and recoveries between 98.9 and 99.6% were obtained (Table 4). The results showed that the proposed method can be applied for the determination of iron in foods, iron tablets, and water samples. Comparison with Other Methods. A comparison of the analytical and operational parameters of the spectrofluorometric and atomic methods that are already present in the literature and the proposed method for the determination of Fe(III) is given in Table 5. Compared to previous methods, the proposed method is relatively time-saving and simpler.10−13,26−29 Lower detection limits could be obtained by some atomic methods.10,11,13 However, as seen from Table 5, timeconsuming solid phase extraction and coprecipitation methods have been applied to separation and preconcentration of iron(III) ion in these studies. The preconcentration factors (PF) are given for these atomic methods in Table 5. Detection limits of some spectrofluorometric methods are also lower than the proposed method. However, sample preparation and treatment of these methods are more time-consuming than those of the proposed method (Table 5). Advantages of the Proposed Method. The proposed method can be applied to various food samples and iron medicinal tablets. Sample digestion carried out in 20 min in a closed microwave system is not time-consuming compared with many methods in the literature (Table 5). There is no need either for the separation of iron(III) ion from sample solution or sample treatment stage before the measurement. A modified standard addition method handles probable matrix effects. A time of 1−2 min was sufficient before measurement to obtain a stable fluorescence response. Thus, this property of the proposed method is one of its advantages when compared to others (Table 5). The recovery studies showed that the proposed method can also be applied to water samples having proper iron(III) concentrations. Moreover, the used reagent is a kind of flavonoid compound. Flavonoids are known as useful compounds for the human body. In this context, it can be thought that the reagent is environmentally friendly. Spectrofluorometric methods are much cheaper compared with atomic methods. Consequently, these advantages make the proposed method time-saving, simple, and economic compared to several other methods in the literature.

Article

ASSOCIATED CONTENT

S Supporting Information *

Synthesis of the ligand, excitation and emission spectra, and complexation and analytical performance data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(U.O.) Phone: +90 462 377 42 67. Fax: +90 462 325 31 96. E-mail: [email protected]. Funding

This research was supported by Karadeniz Technical University, Scientific Research Foundation (Project 10481). Notes

The authors declare no competing financial interest.

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REFERENCES

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Simple Time-Saving Method for Iron Determination Based on

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