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Journal of Trace Analysis in Food and Drugs

(2013) 1: 1-13

doi:10.7726/jtafd.2013.1001

Research Article

______________________________________________________________________________________________________________________________

*Corresponding e-mail: [email protected]

Faculty of Chemistry and Chemical Engineering, Malek Ashtar University of Technology, Tehran,

15875-1774, Iran1

A New Separation and Preconcentration SystemBased on Dispersive Liquid-Liquid Microextraction for

Spectrophotometric Determination of

Trace Amounts of Boron in Water Samples

Ali Reza Zarei*, Sonia Nobakht, and Mohammad Ali Zaree

Received 15 July 2012; Published online 3 November 2012

© The author(s) 2012. Published with open access at uscip.org

Abstract

A simple and sensitive method based on the dispersive liquid-liquid microextraction (DLLME) technique

combined with UV-Vis spectrophotometry analysis has been developed for the determination of boron

concentration in water samples. The method is based on ion-association formation of boron as boron

tetraflouride (BF4-) with methylene blue (MB+) and extraction of the hydrophobic complex (BF4

-:MB+) using

the DLLME technique. Some important parameters, such as reaction conditions and the kind and volume of

extraction solvent and disperser solvent were studied and optimized. Under the optimized experimental

conditions, Beer’s law was obeyed over the concentration ranges of 0.50-50 ng mL -1. The apparent molar

absorptivity and Sandell’s sensitivity of ion-pairing product were 3.39×105 L mol-1 cm-1 and 0.032 ng cm-2,

respectively. Also, the enrichment factor and extraction recovery obtained were 24.68% and 98.74%,

respectively. The proposed method was successfully applied for determination of boron concentration in

water samples. In addition, excellent agreement was observed between this proposed method and the

reference method.

Keywords: Boron; Dispersive liquid–liquid microextraction; Spectrophotometry; Preconcentration

1. Introduction

Boron is an important micronutrient for plants, animals and humans and its presence has been

demonstrated to affect the function or composition of several body compartments (such as the

brain, the skeleton and the immune system) in humans and animals (Power and Woods, 1997;

Nielsen, 1997). Too high an amount of boron can limit plant growth and even lead to plant death,

and for humans an excessive amount may result in nausea, vomiting, diarrhea, dermatitis and



Ali Reza Zarei, Sonia Nobakht, and Mohammad Ali Zaree / Journal of Trace Analysis in Food and Drugs

(2013) 1: 1-13

2

lethargy (Nielsen, 1993). Therefore, the consumption of food and water with high boron content

can be potentially hazardous to health. As a consequence, the World Health Organization (WHO)

has suggested that a safe range of boron intake for adult human beings is 1-13 mg per day (WHO,

1998). Boron is usually present in nutrients (water and milk samples) at the ug L-1 level (Hunt et al.,2004; Nielsen, 1992). Thus, the control of boron levels in samples related to animal and human

nutrition is needed. However, the reliable determination of boron at these levels is not a simple

analytical task, so the development of new microextraction techniques and low cost instrumental

methods could be very interesting for the determination of trace amounts of boron.

Different methods have been reported for the determination of boron, including electrothermalatomic absorption spectrometry (ETAAS) (Nowka et al., 2000; Burguera et al., 2001; Resano et al.,

2007; Lopez-Garcia et al, 2009), inductively coupled plasma-atomic emission spectrometry (ICP-AES) (Tamat and Moore, 1987; Krejcova and Cernohorsky, 2003; Simsek et al., 2003; Ozcan and

Yilmaz, 2005; Kataoka et al., 2008), voltammetry (Thunus, 1996; Domenech-Carbo et al., 2004;Sahin and Nakiboglu, 2006), and liquid chromatography (Katagiri et al., 2006; Raol and Aggarwal,

2008) and spectrophotometry (Sanchez-Ramos et al., 1998; Thangavel et al., 2004; Carrero, et al.,2005; Zaijun et al., 2006; Li and Zhang, 2007; Rusnakova et al., 2011). Spectrophotometricdetermination of boron with curcumin as complexing agent is widely used for routine analysis

because of simplicity and high sensitivity. But the interference of water in the reaction betweenboron and curcumin is a serious problem and it is necessary to remove water from samples. To

overcome this problem, boron has to be selectively separated from the matrix either by solvent extraction with diols (such as 2-ethyl hexane 1,3-diol (EHD) (Dyrssen and Uppstrom, 1969) or by

isothermal distillation of borate ester (Thangavel et al., 2004). Therefore, sample preparation for

the determination of ultra-trace amounts of boron (ng mL-1) using these methods will be time

consuming and suffer from lack of good sensitivity.

Sample preparation has a direct impact on accuracy, precision, and quantitation limits and is often

the rate-determining step of an analytical process, especially when trace determination is thepurpose (Nerin, 2007). Liquid-liquid extraction (LLE) is a versatile classical sample preparation

technique prescribed in many standard analytical methods. However, conventional LLE uses large

amounts of potentially toxic organic solvents, which are often hazardous and explosive (EPA, 1986).

To overcome these drawbacks, some techniques such as liquid-phase microextraction (LPME) and

solid-phase microextraction (SPME) have been developed. Recent research trends involve

miniaturization of the traditional liquid-liquid extraction principle.

Dispersive liquid-liquid microextraction (DLLME) is a new liquid phase microextraction (LPME)

technique based on a ternary component solvent system that is similar to the homogeneous liquid–

liquid extraction method (HLLE) (Rezaee et al., 2006; Anthemidis and Loannou, 2009; Hassan et al.,

2011) and cloud point extraction (CPE) (Zarei, 2007). This method consists of two steps: (1) the

injection of an appropriate mixture of extraction and disperser solvent into an aqueous samplecontaining analytes. In this step, extraction solvent is dispersed into the aqueous sample as very

fine droplets and analytes are enriched into it. Because of the infinitely large surface area betweenextraction solvent and aqueous sample, the equilibrium state is achieved quickly and extraction is

independent of time. This is the most important advantage of this method. (2) The centrifugation of the cloudy solution. After centrifugation, the determination of analytes in the sedimented phase can

be performed by instrumental analysis. Rapidity, high enrichment factor, simplicity of operation




(2013) 1: 1-13

3

and low cost are some of the advantages of this method. Recently, Rusnakova et al. (2011) have

reported a method for determination of boron based on dispersive liquid-liquid microextraction

that was applicable for determination of boron in the range of 0.22-18.7 mg L-1. This method was

applied for spiked waters (Rusnakova et al., 2011).

This work is mainly focused on the suitability of DLLME combined with UV-Vis spectrophotometry

for the determination of ultra-trace amounts of boron in the ranges of 0.50-50 ng mL-1. The

influence of the different experimental parameters on the yield of the sample preparation step is

described and discussed. To evaluate the applicability of the proposed method, it was then applied

for the determination of boron in water samples and compared with results obtained by ICP-AES asa reference method.

2. Experimental

2.1 ApparatusA Hitachi model 3310 UV-Vis spectrophotometer with 1-cm quartz micro cells was used for

recording absorbance spectra. All spectral measurements were performed using the blank solution

as a reference. A Hettich centrifuge (EBA 20) with 10 mL calibrated centrifuge tubes was used to

accelerate the phase separation process.

2.2 Reagents

All reagents used were of analytical grade and all solutions were prepared with distilled water. All

solutions were prepared in polypropylene volumetric flasks. Stock boron solution (1000 µg mL-1)was prepared by dissolving 0.5636 g boric acid (Merck) in water and diluting to 100 mL in a

volumetric flask. A 5.0 (w/v %) NaF was prepared by dissolving 5.0 g sodium fluoride in water anddiluting to 100 mL in a volumetric flask. A 1.50 mM methylene blue solution (Merck) was prepared

by dissolving 0.0480 g methylene blue in water and diluting to 100 mL in a volumetric flask. A 2mol L-1 sulfuric acid solution was prepared by dissolving an appropriate amount of concentratedacid (Merck) in distilled water.

2.3 Dispersive Liquid-Liquid Microextraction Procedure

An aliquot of the solution containing 5.0-500 ng of boron, 1.0 mL of 5.0 (w/v %) NaF, 1.0 mL of 1.50mM methylene blue solution and 1.0 mL of 2 mol L-1 sulfuric acid solution was transferred into a 10

mL centrifuge tube and the contents were mixed well. The contents were diluted with water and

then 400 μL of acetonitrile (as disperser solvent) containing 100 μL of 1,2-dichloroethane (as

extraction solvent) was injected rapidly into a sample solution using a 2 mL syringe. A cloudy

solution was formed in the test tube. In this step the ion-pair product was extracted into the fine

droplets of 1,2-dichloroethane. The mixture was centrifuged at 3800 rpm for 2 min and the

dispersed fine droplets of 1,2-dichloroethane were settled. The supernatant aqueous phase wasreadily decanted with a Pasteur pipette. The remaining organic phase was diluted to 400 µL with

acetonitrile and the absorbance measured at 657 nm against blank. The blank solution was

prepared as for the sample solution except that distilled water was used instead of boron solution.

2.4 Reference Method (ICP-AES Method)

The measurements were carried out with an ICP atomic emission spectrometer (ICP-AES) Perkin

Elmer model 5300 DV. The operation conditions for ICP-OES were as follows: emission line: B (I)




(2013) 1: 1-13

4

249.773 nm (other lines, 208.893, 208.959, 249.678 nm were also used to confirm the results in

analyses), plasma power supply: 1.0 kW, observation height: 6 mm, plasma gas flow: 10.0 L min-1,

auxiliary gas flow: 0.5 L min-1, nebuliser gas flow: 0.6 L min-1, photomultiplier voltage: 600 V,

sample uptake rate: 1.7 mL min-1, integration time: 1 s and replicates: 3.

3. Results and Discussion

In acidic media, after conversion of boron to tetrafluoroborate anion, it reacts with methylene blueto form an ion-pair complex that can be extracted into 1,2-dichloroethane. Therefore, it can be a

suitable method for separation and preconcentration of boron by DLLME. Fig. 1 shows the visible

absorption spectra of the ion pair complex of tetrafluoroborate with methylene blue after DLLME,which exhibits a maximum absorbance at 657 nm. Therefore, all absorbance measurements were

performed at this wavelength. To obtain high sensitivity, it is necessary to investigate the effect of all parameters that could influence the ion-association formation reaction and the performance of

DLLME.

0

0.5

1

1.5

2

550 600 650 700 750

Wavelength /nm

A b s o r b a n c e

(1)

(7)

657 nm

Fig 1. Absorption spectra of the ion pairing product of boron with methylene blue after DLLME,Conditions: Boron, (1) 1.0 (2) 5.0 (3)10 (4) 20 (5) 30 (6) 40 (7) 50 ng mL-1; Methylene blue, 150

µM; NaF, 0.5 (w/v %); Sulfuric acid, 0.20 M.

3.1 Parameters Affecting the Ion-Association Formation Reaction

3.1.1 Effect of the Sodium Fluoride Concentration

For the extraction of boron via ion pair, boric acid converts to boron tetrafluorate anion. The effect

of sodium fluoride concentration on the absorbance of the system was investigated within the

range 0.05-2.0 (w/v %) NaF. The results revealed that the absorbance increased by increasingreagent concentration up to 0.5 (w/v%) NaF and remained nearly constant at higher

concentrations (Fig. 2). Therefore, a concentration of 0.5 (w/v %) NaF was applied in the proposed

method.




(2013) 1: 1-13

5

3.1.2 Effect of the Methylene Blue Concentration

The effect of methylene blue concentration on the absorbance of the system was investigated

within the range 50-300 µM. As can be seen in Fig. 3, the maximum absorbance was achieved at 150

µM, and remained nearly constant at higher concentrations. Therefore, a concentration of 150 µMmethylene blue was applied in the proposed method.

0

0.1

0.2

0.3

0.4

0 0.3 0.6 0.9 1.2

Concentration of sodium fluoride/ (w/v%)

A b s o r b a n c e

Fig 2. Effect of sodium fluoride concentration on the analytical signals, Conditions: Boron,

10 ng mL-1; Methylene blue, 150 µM; Sulfuric acid, 0.20 M.

0

0.1

0.2

0.3

0.4

0 50 100 150 200 250

Concentration of Methylene blue/ µM

A b s o r b a

n c e

Fig 3. Effect of methylene blue concentration on the absorbance of the system, Conditions: Boron,

10 ng mL-1; NaF, 0.5 (w/v %); Sulfuric acid, 0.20 M.

3.1.3 Effect of Sulfuric Acid ConcentrationThe boron tetrafluorate anion that forms in acidic medium is extractable by DLLME. It is best to usesulfuric acid, because sulfate anions, unlike the anions of other strong acids, do not form extractable

ion pairs with the methylene blue. The effect of sulfuric acid concentration on the absorbance of the

system was investigated within the range 0.05-1.0 M. The results revealed that the absorbance

increased by increasing reagent concentration up to 0.20 M, and decreased at higher concentrations

(Fig. 4). Therefore, a concentration of 0.20 M sulfuric acid was applied in the proposed method.




(2013) 1: 1-13

6

0

0.1

0.2

0.3

0.4

0 0.2 0.4 0.6 0.8 1

Concentration of sulfuric acid/ M

A b s o r b a n c e

Fig 4. Effect of sulfuric acid concentration on the analytical signals, Conditions: Boron, 10 ng mL-1;

Methylene blue, 150 µM; NaF, 0.5 (w/v %).

3.2 Parameters Affecting the Extraction Efficiency of DLLME

3.2.1 Selection of the Extraction and Disperser Solvent Type

In DLLME, the extraction solvent has to meet three requirements. It should demonstrate (i) higher

density than water, which makes it possible to separate the extraction solvent from the aqueous

phase by centrifugation; (ii) extraction capability for the relevant compounds and (iii) low

solubility in water (Hashemi, 2010). Halogenated hydrocarbons are usually selected as an

extraction solvent because of their high density. In this work, chloroform (CHCl 3), carbon

tetrachloride (CCl4), 1,2-dichloroethane (C2H4Cl2), dichloromethane (CH2Cl2) and 1,2-

dichloroethylene (C2H2Cl2) were examined in order to find the most suitable solvent for DLLME. For

this purpose, a series of sample solutions were treated with 400 µL of disperser solvents (methanol,ethanol, acetonitrile and acetone) containing 100 µL of the extraction solvents. The results obtained

are shown in Fig. 5. As seen, 1,2-dichloroethane has high extraction efficiency among the tested

solvents, so it was selected as the extraction solvent in further experiments.

Fig 5. Effect of the type of extraction and dispersant solvent on the analytical signals, Conditions:

Boron, 10 ng mL-1; NaF, 0.5 (w/v %); Sulfuric acid, 0.20 M.

C h l o r o f

o r m

C a r b o n t e t r a c h l o r i d e

1 , 2 - D i c h l o r o e t h a n e

1 , 2 - D i c h l o r o e t h y l e n e

D i c h l r o r o m e t h a n e

A c e t o n e

E t h a n o l

M e t h a n o

l

A c e t o n i t r i l e

0

0.1

0.2

0.3

0.4

A b s o r b a n c e




(2013) 1: 1-13

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The main criteria for choosing the disperser solvent in DLLME is its miscibility in the organic phase

(extraction solvent) and also in the aqueous phase (sample solution), which enables the extraction

solvent to be dispersed as fine particles in the aqueous phase to form a cloudy solution

(water/disperser solvent/extraction solvent). The commonly used disperser solvents that illustratethis ability include methanol, ethanol, acetonitrile and acetone. In this study, all combinations of

1,2-dichloroethane (100 µL) as extraction solvents and methanol, ethanol, acetonitrile and acetone

(400 µL) as disperser solvents were examined. In the case of 1,2-dichloroethane with acetonitrile, a

stable two-phase system and higher signal was observed (Fig. 5). We selected 1,2-dichloroethane-

acetonitrile as a suitable set for microextraction of boron.

3.2.2 Effect of the Extraction Solvent Volume

The extraction solvent volume has great effects on the enrichment factor. In order to evaluate onthe extraction efficiency, additional experiments were performed using 400 µL acetonitrile

containing different volumes of 1,2-dichloroethane (Fig. 6).

0

0.1

0.2

0.3

0.4

0 50 100 150 200 250

Extraction solvent volume/ µL

A b s o r b a n c e

Fig 6. Effect of the extraction solvent (1,2-dichloroethane) volume on the analytical responses after

DLLME, Conditions: Boron, 10 ng mL-1; NaF, 0.5 (w/v %); Sulfuric acid, 0.20 M.

As can be seen, by increasing the extraction solvent volumes from 20 to 200 µL, the absorbanceincreased by increasing the volume of the 1,2-dichloroethane to 100 µL and then remained

approximately constant with further increasing of its volume. Therefore, 100 µL of extractionsolvent was selected as volume optimum.

3.2.3 Effect of the Disperser Solvent VolumeThe disperser solvent volume directly affects the formation of the cloudy solution

(water/disperser/extraction solvent), the degree of the dispersion of the extraction solvent inaqueous phase, and subsequently, the extraction efficiency. To study the effect of disperser solvent

volume on the extraction efficiency, all experimental conditions were fixed except the volume of acetonitrile, which varied from 100-800 µL. The results are shown in Fig. 7. According to the

obtained results, the absorbance reached to its maximum value at 400 µL acetonitrile and then




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gradually decreased by further increasing of its volume. It is noted that at low volumes of

acetonitrile, the cloudy state is not formed well; in the high volumes of acetonitrile, solubility of the

extraction solvent in water increases, therefore, the extraction efficiency decreases because of

distribution coefficient decreasing. A 400 µL volume was chosen as an optimum volume fordisperser solvent.

0

0.1

0.2

0.3

0.4

0 200 400 600 800

Dispersive solvent volume / µL

A b s o r b a n c e

Fig 7. Effect of the dispersant solvent (acetonitrile) volume on the analytical responses after

DLLME, Conditions: Boron, 10 ng mL-1; NaF, 0.5 (w/v %); Sulfuric acid, 0.20 M. 3.3 Method Validation

Under the optimized experimental conditions for boron determination, the standard calibration

curves were constructed by plotting absorbance versus concentration. The linear regression, the

Beer’s law range, molar absorptivity, Sandell’s sensitivity, correlation coefficient, limits of detection,

enrichment factor and extraction recovery are summarized in Table 1. Beer’s law is obeyed over the

concentration range of 0.50-50 ng mL-1 of boron at 657 nm. The limit of detection (LOD) defined asC L=3S B /m where C L , S B, and m are the limit of detection, standard deviation of the blank, and slope of the calibration graph (Miller and Miller, 2000).

The enrichment factor (EF) was defined as the ratio between the analyte concentration in the

sedimented phase (Csed) and the initial concentration of the analyte (C0) in the aqueous sample

(Yazdi et al., 2008; Caldas et al., 2010)

EF= Csed/ C0 (1)

The extraction recovery (R%) was defined as the percentage of the total analyte which wasextracted in the sedimented phase.

R%= EF×(Vsed/Vaq) ×100 (2)

Where R%, Vsed, and Vaq are the extraction recovery, the volume of the sedimented phase, and the

volume of the aqueous sample, respectively. In order to examine the enrichment factor, threereplicate extractions were performed at optimal conditions from aqueous solutions containing 10

ng mL-1 of boron. The enrichment factor was calculated as the ratio of final concentration of analyte




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in sedimented phase (Csed) and its concentration in the original solution (C0). Csed was calculated

from the calibration graph obtained by the conventional LLE with 5 mL 1,2-dichloroethane. Based

on Eqs. (1) and (2), the enrichment factor and extraction recovery (R%) obtained were 24.52% and

98.08%, respectively.

3.4 Selectivity

To study the common ion effect or selectivity of the proposed method, the effect of various species

on the determination of 10 ng mL-1 of boron was tested under the optimum conditions. The

tolerance limit was defined as the concentration of added species caused less than ±5% relative

error. It was found that most of the investigated species did not interfere (Table 2). Therefore, theresults presented in this table show the good selectivity of the procedure.

Table 1 Regression and analytical parameters.

λmax (nm) 657

Regression equation using DLLME (n=10) A= 0.0308C + 0.026, R2 = 0.998

Regression equation (n=10) using conventional LLE A= 0.00125C + 0.0097, R2 = 0.999

Molar absorptivity (Lit mol-1 cm-1) 3.39×105 (3.97×103)a

Sandell’s sensitivity (ngcm-2) 0.032 (2.8)

Linear range (ng mL-1) 0.50-50 (50-1000)a Limit of detection (ng mL-1) 0.30 (30)a

Reproducibility (R.S.D., %) 1.86Enrichment factor (EF) 24.52

Extraction recovery (R%) 98.08

a For conventional LLE.

Table 2 Tolerance limit of diverse ions on the determination of 10 ng mL-1 of boron.

Species Tolerance ratio (wion/wboron)

Na+, K+, Mg2+, NH4+, Ba2+, Zn2+, Cd2+, Ni2+, Ca2+, SO4

2-,NO3

-, Cl-

5000:1

Fe3+, NO2-, Br-, F-, CO3

2-, PO43- 2000:1

Fe2+, SCN-, CN- 500:1

3.5 Applications

In order to the test reliability of the proposed method, it was applied to the determination of boronin several water samples. The results are presented in Table 3. The results are in good agreement

with those obtained by ICP-AES. The results indicated that the proposed method is helpful for thedetermination of boron in the natural water samples.




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Table 3 Determination of boron in water samples by the proposed method

ICP-AES

(ng mL-1) Recovery

(%) Boron (ng mL-1) Sample

Founda Added 40.0± 1.2 - 41.1 ± 2.1 - Potable water

96.045.9 ± 1.95.0

105 51.6 ± 2.1 10.0 267 ± 2.5 - 280.3 ± 2.1 - Well water

104285.5 ± 1.45.0

95.0 289.8 ± 1.8 10.0 31.5 ± 1.7 - 33.0 ± 1.4 - Mineral water

97.437.87 ± 1.35.0

98.0 42.8 ± 2.6 10.0 aAverage of three determination ± standard deviation

4. Conclusion

This study proposes the use of DLLME as a method for extraction and preconcentration of boron as

a prior step to its determination by spectrophotometry. The DLLME is versatile and simple andprovides good enrichment factors and efficient separation. In comparison to solvent extraction

methods, it is much safer, since only a small amount of the solvent is used. The results of this studyclearly show the potential and versatility of this method, which could be applied to monitoring of

boron in water samples. Finally, the coupling of DLLME with UV-Vis spectrophotometry gave a fast

and low-cost procedure for determination of boron without requiring sophisticated instrumentssuch as electrophoresis, AAS and ICP-AES.

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http://dx.doi.org/10.1016/j.ab.2007.06.039

PMid:17706585

a new separation and preconcentration system based on dispersive liquid-liquid microextraction for...

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