Novel and Validated Spectrophotometric Matrix Matching Method for Simple and Rapid Determination of Chromium in Oily Media

Balkesir University, Faculty of Science and Arts, Chemistry Department, 10145, ?, Balkesir, Turkey

Novel and Validated Spectrophotometric Matrix Matching Method for Simple and Rapid Determination of Chromium in Oily Media

N,N’-bis(4-methoxysalicylidene) ethylenediamine is proposed as analytical reagent for the direct determination of chromium in original matrix of liquid oils.The method is based on the complexation of N,N’-bis(4-methoxysalicylidene) ethylenediamine and chromium in n-hexane∶acetone (1∶4) medium and spectrophotometric determination at 355 nm against reagent blank.Complexation of MSE and chromium is completed in 20 seconds at molar ratio 1∶1.The molar absorptivity and the formation constant of the complex are determined as 9 740 L·mol-1·cm-1and 3.6(±0.1)×105, respectively.Beer’s law is obeyed in range 0.02～1.50 mg·L-1chromium concentration.Limits of detection and quantification of the suggested method were 7.5 and 25.1 μg·kg-1, respectively.The accuracy and precision of the method was checked by finding mean recovery and relative standard deviation by oil-based chromium standard analysis as 96.4%±1.4% and 1.5%, respectively.The practical applicability of elaborate method was tested using oil-based chromium standard spiked and unspiked corn, sunflower, soybean, olive and canola oils.

N,N’-bis(4-methoxysalicylidene) ethylenediamine； Chromium； Molecular absorption spectroscopy； Oil

Introduction

Chromium is one of the most abundant elements on earth and considered as an essential nutrient for trace amounts but it is toxic, carcinogenic and allergenic for large amounts.Oxidation states of chromium define the essentiality and the toxicity.This element usually found in trivalent (Ⅲ) or hexavalent (Ⅵ) forms in nature[1].Chromium (Ⅵ) can easily cross the cell membranes and can mediate cytotoxic effects on various organs such as kidney, testis and brain[2].Trivalent chromium takes part in metabolism of carbohydrates, lipids and proteins by increasing the efficiency of insulin[3-4].

Human exposure to heavy metals from food has risen during the last decades because of the industrial activities.Therefore, monitoring the heavy metal content of fruits and vegetables has gained great importance[5-6].Edible oils are extracted from seeds or produced by pressing whole fruits.Shelf life and the sensorial properties of these products are highly depends on trace metals contents.Chromium and the other elements such as iron, copper, manganese, zinc, nickel and lead might come from soil, genotype of the plant, fertilizers/pesticides or fabrication process and cause formation of carcinogenic free radicals[7-8].When the economical and vital effects are considered, determination of trace amounts of metals in edible oils could be crucial.

Up to now, many well-established sample preparation techniques such as wet or dry ashing, acid extraction, dilution with organic solvents and extraction with chelating agents, have been reported to eliminate the high organic oil matrix.These applications are based on mineralization of organic content, emulsification formation or isolation the components of interest from the matrix[9-14].However, mentioned procedures are time consuming, vulnerable to contamination and/or loss of analyte.Additionally, dilution, lack of repeatability and poor precision are the other drawbacks and restrict the applicability in routine analysis.High qualified instruments, such as neutron activation analysis (NAA)[15]may allow direct determination of elements at low concentration but availability of these high priced detection techniques are limited.

Spectrophotometry is a well-established and seems to be the best analytical approach with sensitive, precise and accurate measurements.Due to low cost, simplicity and wide range of applications, the technique is widely used in research laboratories[16-19].The choromogenic chelating reagents such as bis(salicylaldehyde)orthophenylenediamine (BSOPD)[20]and ammonium pyrrolidinedithiocarbamate (APDC)[21]commonly used for spectrophotometric determination of chromium in environmental, biological and pharmaceutical samples.Schiff bases are firstly introduced by Hugo Schiff and considered as one of the most popular chelating agent among the organic ligands[22].Based on color formation resulting from the reaction between Schiff base and metal ion, UV-visible spectrophotometry is commonly used for determination of metals[23-25].Although quite old and a known technique, a literature review reveals that there is only one research cited for metal determination in oils using spectrophotometry which includes acid extraction step prior to complexation with cinnamaldehyde-4-hydroxybenzoylhydrazone (CMHBH)[26].

Recently, reports have focused on the miniaturized analysis of samples to reduce instrumentation costs and shorten the analysis time.Starting from this point of view, the main goal of the present work was to develop a simple and reliable methodology for direct determination of Cr in oily matrix by spectrophotometry without digestion or any other sample preparation step.N,N’-bis(4-methoxysalicylidene) ethylenediamine (MSE) Schiff base which was previously utilized for extraction and determination of Zn, Fe and Cu in edible oils[8, 27]was used as a chelating agent.The method is based on complexation of chromium with MSE (Fig.1) in organic solvent medium and determination by UV-Vis spectrophotometer at ng·kg-1levels.Various factors which influence the sensitivity and specificity of the method such as wavelength, stability and composition of complex and ranges of applicability of Beer’s law were investigated.

Fig.1 Molecular structure of N,N’-bis(4- methoxysalicylidene) ethylenediamine

1 Experimental

1.1 Apparatus/Instrumentation

All spectral and absorbance measurements were carried out with a PG instruments Ltd T80+ double beam UV-Vis spectrophotometer (Leicestershire, UK) equipped with matched quartz cells of 10 mm path length.The equipment permits multiple expansions in absorbance and wavelength with ±0.001 accuracy.

1.2 Reagents and Solutions

All solutions were prepared with analytical reagents.n-Hexane and acetone were purchased from Merck (Darmstadt, Germany) and used without further purification.Organometallic Conostan chromium (5 000 mg·kg-1, catalog number: 507118) standard was used for the investigation of the complexation behaviors and testing the improved method.N,N’-bis(4-methoxysalicylidene) ethylenediamine (MSE) was synthesized as mentioned before[8].1×10-4mol·L-1MSE solution was prepared by dissolving it in a 1∶4 (φ) n-hexane∶acetone mixture.Sunflower, olive, corn, canola and soybean oil samples used in this research were collected from Turkish markets.2.0 g of oil sample was dissolved in 25 mL of 1∶4 (φ) n-hexane∶acetone mixture and used for analysis.The reagent blank solution was prepared by same amount of oil without MSE.Glass vessels were cleaned by washing with soap water, followed by soaking with 10% HNO3and rinsing with pure water.

1.3 Recommended Procedure

The method of standard additions was utilized for determination of chromium in oil samples.Into a series of 5.0 mL calibrated flasks, 2.0 mL of oil solution, appropriate volumes of chromium standard solution, and 1.0 mL of MSE solution were added and volume was made up to the mark with n-hexane∶acetone mixture.The content of each flask was mixed well at room temperature.A portion of the solutions were transferred into the quartz cell and the absorbance was measured at 355 nm against the reagent blank.The reagent blank solution was prepared similar to oil sample solution without MSE.The concentration of the analyte in the samples was obtained by finding the abscissa of the crossing point of the plotted line and the horizontal axis (at ordinate equal to zero).For all samples, standard addition procedures were replicated 3 times and the average and standard deviation of the results were reported.

2 Results and Discussion

2.1 Selection of Appropriate Solvent

Solubilization of MSE and oil samples was examined with miscible solvents.As it is known, the oil samples were successfully dissolved in n-hexane, however MSE was not soluble in apolar medium.To increase the dissolution of MSE, acetone was added to the Schiff base, n-hexane and oil mixture.Up to rate of 1∶4 (φ) of n-hexane∶acetone mixture, the medium was highly hydrophobic and the solution was turbid.The solubility of oil and MSE in n-hexane-acetone mixture at rate of 1∶4 (φ) was appropriate.Therefore, this mixture was used in all further experiments for dissolution of MSE and preparation of oil solutions.

2.2 Absorption Spectra

The UV-Vis spectra of 5×10-5mol·L-1MSE and Cr-MSE were monitored in the range of 200～900 nm with 0.5 nm interval and given in Fig.2.In case of the absorbance of the MSE was negligible, the Cr-MSE complex gave maximum absorbance at 355 nm.Therefore, all the spectral experiments of the complexes achieved at 355 against to reagent blank solution.Unlike similar reports[28-29], the order of mixing reagents does not affect the absorbance value.The molar absorptivity (ε) of the Cr-MSE was calculated from the absorbance value and found to be 9 740 L·mol-1·cm-1.

Fig.2 The UV-Vis spectra of N,N’-bis(4-methoxysalicylidene) ethylenediamine MSE and Cr-MSE at 5×10-5mol·L-1

2.3 Effect of N, N’-bis(4-methoxysalicylidene) ethylenediamine (MSE) Concentration

The presented spectrophotometric method is based on the formation of a complex between chromium and MSE.Therefore, addition of suitable concentration of MSE was important to obtain the highest complexation efficiency.In this experiment, the influence of MSE concentration was studied in the range of 1×10-5～8×10-5mol·L-1at fixed 1.0 mg·L-1chromium concentration.As seen in Fig.3, 5×10-5mol·L-1MSE concentration is sufficient and used in further experiments.

2.4 Nature and Stability of Complex

The stoichiometry of complexes was established using the continuous variation method (Job’s method).The corrected absorbance values which were obtained by subtracting the absorbance of free MSE from the absorbance of Cr-MSE solution were plotted against volume fraction of chromium.As shown in Fig.4, the ratio of MSE to chromium was found to be as 1∶1.There is not any study about structure of the complex conducted by us.On the other hand, it was estimated that MSE has been bonding oxygen atoms in hydroxyl group and nitrogen atoms in imine group to the chromium.Complex formation constants were determined using standard multi component analysis procedure[30]and found as 3.6(±0.1)×105for Cr-MSE indicating the high complexing ability of MSE with chromium.

Fig.3 The effect of N,N’-bis(4-methoxysalicylidene) ethylenediamine (MSE) concentration on complexation with chromium

Fig.4 Job’s plot: Cr=0.0～1.0 mL of 1×10-4mol·L-1, MSE=1.0～0.0 mL of 1×10-4mol·L-1;λmax=355 nm

Fig.5 Effect of time on complexation of chromium and N,N’-bis(4-methoxysalicylidene) ethylenediamine (MSE)

The time required to complete complexation and to obtain maximum absorbance, kinetic studies were carried out.In order to investigation, standard chromium and MSE solutions were mixed and fixed to 5×10-5mol·L-1.The flask was shaken vigorously and the mixture rapidly transferred to quartz cell to monitor the absorbance changing versus time.Because of the volatility of the solvent, the absorbance of Cr-MSE was screened up to 25 minutes with 1 second intervals.As can be seen in Fig.5, it is observed that complexation was completed within 20 seconds.

2.5 Analytical features of the method

Characteristics of the presented method are summarized in Table 1.The molar absorptivity of Cr-MSE was calculated atλmaxas 9 740.The absorbance of the complex obeys Beer’s law in concentration range of 0.02～1.50 mg·L-1for chromium.The limits of detection (LOD) (3σ) and quantification (LOQ) (10σ) of the suggested work were calculated at appropriate experimental conditions according to IUPAC definitions.Accuracy and precision of the method was determined by analyzing the oil-based chromium standard solution containing 1.0 mg·kg-1Cr (N=5).As given in Table 1, the accuracy is 96.4% as recovery percentage and the precision is 1.5% as relative standard deviation (RSD).

Table 1 Analytical characteristics for the spectrophotometric determination of Cr with MSE

ParameterSolventmixture(V/V)n-hexane∶acetone(1∶4)ConcentrationofMSE(mol·L-1)5×10-5λmax/nm355Molarabsorptivity,ε/(L·mol-1·cm-1)9740Complexformationconstant3.6(±0.1)×105Compositionofcomplex(M∶L)1∶1Complexationtime(sec.)20LOD/(μg·kg-1)7.5LOQ/(μg·kg-1)25.1Beer’slawvalidityrange/(mg·L-1)0.02～1.50Meanrecovery/%96.4±1.4Precision,RSD/%1.5

Table 2 Real sample analysis (N=3)

2.6 Analysis of Oil Samples

The suitability of the above proposed method was examined by applying it to various oils, including sunflower, olive, soybean, canola and corn oils.In all instance, standard addition method was employed.The results for metal spiked and unspiked oil samples are given in Table 2.The recoveries were in the range of 91%～106%.The results are also statistically evaluated in terms of Student’s t-test and the calculated t values are found to be in the range of 0.58～2.89 which is less than the criticaltvalue (4.30) at 95% confidence level.Evidencing thetvalues, there is no systematic errors associated to the determination of chromium by the spectrophotometric procedure.

Table 3 Comparison of reported methods for the determination of chromium with proposed method

2.7 Comparison the utility of the proposed procedure

The proposed procedure was compared with other existing methods used for metal determination in edible oils.As given in Table 3, the suggested procedure seems to be quite fast and cheap for determination of chromium in edible oils with eliminated tedious sample preparation step.Due to the possibility of detection in real oil matrix, this technique offers an application for routine laboratory analysis that is safer and away from potential risks such as explosion and contamination.

3 Conclusions

In this work, MSE was utilized for spectrophotometric determination of chromium in real oil matrix.Salient features of the work:

(1) Low reagent concentration is required for quantitative determination of chromium.

(2) The Schiff base basically forms complex with chromium in n-hexane-acetone medium.

(3) The suggested procedure do not include further decomposition step.

(4) Determination of chromium can be achieved within 1 minute without any complicated equipment.

(5) The given procedure is comparable with reported methods which suffer from interference of high organic matrix of oil samples.

[1] Mount D R, Hockett J R.Water Research, 2000, 34: 1379.

[2] Mishra A K, Mohanty B.Environmental Toxicology and Pharmacology, 2008, 26: 136.

[3] Lewicki S, Zdanowski R, Krzyzowska M, et al.Annals of Agricultural and Environmental Medicine, 2014, 21: 331.

[4] Pechova A, Pavlata L.Veterinarni Medicina, 2007, 52: 1.

[5] Francisco B B A, Caldas L F S, Brum D M, et al.Journal of Hazardous Materials, 2010, 181: 485.

[6] Kassem M A, Amin A S.Food Chemistry, 2013，141: 1941.

[9] Anthemidis A N, Arvanitidis V, Stratis J A.Analytica Chimica Acta, 2005, 537: 271.

[10] Asemave K, Ubwa S T, Anhwange B A, Gbaamende A G.International Journal of Modern Chemistry, 2012, 1: 28.

[12] Fischer J L, Rademeyer C J.Journal of Analytical Atomic Spectrometry, 1994, 9: 623.

[13] Llorent-Martinez E J, Ortega-Barrales P, Cordova M L F, et al.Food Chemistry, 2011, 127: 1257.

[15] Iskander F Y.Journal of the American Oil Chemists’ Society, 1993, 70: 803.

[16] Afkhami A, Abbasi-Tarighat M, Khanmohammadi H.Talanta, 2009, 77: 995.

[17] Al-Kady A S.Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2012, 97: 284.

[18] Hashem E Y, Abu-Bakr M S, Hussain S M.Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2003, 59: 761.

[19] Kamble G S, Ghare A A, Kolekar S S, et al.Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2011, 84: 117.

[20] Soomro R, Ahmed M J, Memon N.Turkish Journal of Chemistry, 2011, 35: 155.

[21] Elbagermi M A, Alajtal A I, Edwards H G M.APCBEE Procedia, 2013, 5: 378.

[22] Saberyan K, Zolfonoun E, Shamsipur M, et al.Acta Chimica Slovenica, 2010, 57: 222.

[23] Carbonaro L, Isola M, La Pegna P, et al.Inorganic Chemistry, 1999, 38: 5519.

[24] Karab?cek N, Karab?cek S, KormalF.Turkish Journal of Chemistry, 2007, 31: 271.

[25] Kianfar A H, Abroshan I.Chemical Science Transactions, 2013, 2: 17.

[26] Krishna D G, Chandrasekhar N D B.International Journal of Analytical and Bioanalytical Chemistry, 2011, 1: 19.

[28] Ni Y, Chen S, Kokot S.Analytica Chimica Acta, 2002, 463: 303.

[29] Sombra L, Luconi M, Silva M F, et al.Analyst, 2001, 126: 1172.

[30] Perkampus HH.UV-Vis Spectroscopy and Its Applications.Berlin: Springer, 1992.

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Foundation item：the Scientific and Technological Research Council of Turkey (TUBITAK-TBAG project number 105T153)

10.3964/j.issn.1000-0593(2016)05-1634-05

Received：2015-09-20; accepted：2015-12-30

e-mail：feyzullahtokay@balikesir.edu.tr； sbagdat@balikesir.edu.tr