OPEN ACCESS

Eurasian Journal of Analytical Chemistry ISSN: 1306-3057

2017 12(3):197-209 DOI 10.12973/ejac.2017.00163a

© Authors. Terms and conditions of Creative Commons Attribution 4.0 International (CC BY 4.0) apply.

Correspondence: Mashaallah Rahmani, Department of Chemistry, Faculty of Sciences, University of Sistan and

Baluchestan, Zahedan 98135-674, Iran.

rahmani341@gmail.com

Application of Dispersive Liquid-Liquid Microextraction in Narrow-Bore Tube for Preconcentration and

Spectrophotometric Determination of Cadmium in Aqueous Samples

Mashaallah Rahmani University of Sistan and Baluchestan, IRAN

Massoud Kaykhaii University of Sistan and Baluchestan, IRAN

Malihe Harati No University of Sistan and Baluchestan, IRAN

Received 25 June 2016 ▪ Revised 20 September 2016 ▪ Accepted 26 September 2016

ABSTRACT

A novel liquid phase microextraction based on narrow bore-dispersive liquid-liquid

microextraction (NB-DLLME) is introduced and developed for preconcentration and

extraction of cadmium in aquoeus samples using spectrophotometry. Unlike previous NB-

DLLME methods, in this research work a solvent with a density higher than water (carbon

tetrachloride) is used as extractant solvent. The effects of different parameters such as kind

and volume of extractant and dispersive solvent, internal diameter and lengnth of narrow

bore, and concentration of salt are studied and optimized. Under optimum conditions,

dynamic range of calibration curve was linear in the range of 50-900 μg.L-1. The detection

limit and relative standard deveiation were calculated to be 6.3 μg.L-1 and 6.4%, respectively.

Keywords: Narrow bore-dispersive liquid-liquid microextraction, spectrophotometry,

water analysis, cadmium

INTRODUCTION

The environmental pollution by heavy metals is of great concern nowadays [1, 2]. Heavy metal

contamination can cause severe toxicological effects on living organisms [3]. Water pollution

by heavy metals is causing serious ecological problems in many parts of the world [4, 5].

Industrial development and urbanization are two important reasons of contamination of

waters and other environmental sources with heavy metals [6]. Cadmium is one of the most

hazardous elements to human health, which is listed as the sixth most poisonous substance

jeopardizing human health [7,8]. Cadmium can be poisonous even at low concentrations.

When entered to human body, it can bind to the sulfhydryl groups of critical mitochondria

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198

proteins, which lead to mitochondria dysfunction and cell poisoning [9]. Widespread presence

of cadmium in the environment comes from anthropogenic activities. Therefore, many

analytical techniques have been developed for preconcentration and determination of trace

amounts of Cd [10, 11].

UV-visible spectrophotometry is an available and easy-to-operate analytical instrument,

and is one of the most extensively used detection techniques for various analytes including

metals [12, 13]. However, due to the insufficient sensitivity and matrix interference of samples,

it cannot be used for direct determination of trace heavy metals in samples. Therefore, for

determination of traces of metals and achievement of low detection limits, a preconcentration

and/or isolation step is necessary before introduction of samples to UV-visible

spectrophotometry. Many methods have been proposed for preconcentration and separation

of metals such as liquid–liquid extraction [14-16], cloud point extraction [12, 17], co-

precipitation [18-21], ion exchange [22, 23], and membrane techniques [24-26]. The traditional

liquid–liquid extraction method and other conventional separation methods are generally

time-consuming and labor-intensive, and usually consume large amount of toxic organic

solvents (with high purity), which after extraction procedures, have to be disposed of properly.

In the recent decades, liquid phase microextraction methods have been introduced and

developed as adequate replacements for traditional liquid-liquid extraction [27, 28]. The most

important feature of microextraction techniques is minimization of extraction methods, which

leads to reducing the consumption of organic solvents. Some microextraction methods that

have been used in determination of Cd are dispersive liquid-liquid microextraction [29-31],

solidified floating organic drop microextraction (SFODME) [32, 33], and single drop

microextraction (SDME) [34, 35].

Dispersive liquid-liquid microextraction is one of the most popular liquid phase

extraction methods [36]. This technique is based on extension of contact surface between

aqueous and organic phases. This purpose is achieved by utilizing a third solvent, usually

known as dispersive solvent. Dispersive solvents must be soluble in both aqueous and organic

solvent, and therefore, methanol, ethanol, acetone, and acetonitrile are usually applied as

dispersive solvents. Due to high performance of DLLME, many DLLME methods have been

developed [37]. Some DLLME methods try to eliminate dispersive solvent like vortex-assisted

dispersive liquid-liquid microextraction (VA-DLLME) [38] and ultrasound-assisted dispersive

liquid-liquid microextraction (UA-DLLME) [39], and some DLLME methods try to eliminate

centrifugation step like in syringe dispersive liquid-liquid microextraction (IS-DLLME) [40]

and narrow bore dispersive liquid-liquid microextraction (NB-DLLME) [41]. NB-DLLME is

usually applied with solvents with densities lower than water. In our previous work, we

applied IS-DLLME and NB-DLLME for determination of BTEX, with nonanol used as

extractant solvent [42]. In this paper, we applied NB-DLLME for determination of traces of Cd.

Instead of using solvents with densities lower than water, which is common in NB-DLLME,

we used solvents with densities higher than water like chloroform and carbon tetrachloride.

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For ease in operation and collection of organic phase at the bottom of tube, a conventional

burette was used as the extraction unit. The details of this procedure are illustrated in Figure

1. 1,5-Diphenylthiocarbazone, mostly known as dithizone (DTZ), was used as a well-known

and most applied chelating agent for Cd. Dithizone forms a red or pink complex with Cd [43].

EXPERIMENTAL

Reagents and chemicals

Cd(NO3)2 and 1,5-diphenylthiocarbazone (dithizone), were purchased from Merck

(Germany). Other reagents and solvents were purchased from the same company, and were

of analytical grade. A standard stock solution of Cd containing 1000 mg.L-1 of Cd was prepared

by dissolving 0.2771 g Cd(NO3) in a doubly distilled water in 100 mL measuring flask, and

was stored in refrigerator. Working standard solutions were prepared daily by diluting the

stock solution.

To remove the oxidation products of dithizone, purification step was necessary. For this

purpose, 0.5006 g dithizone was dissolved in 50 mL chloroform, transferred into a separating

funnel, and extracted with four portions of 1% ammonia (20 mL each time). To the ammoniac

Figure 1. Narrow bore-DLLME procedure in burette: injection of extractant and dispersive solvent into

sample solution and formation of cloudy solution (a), extractant phase sedimented at the bottom of

burette tube (b), collecting extractant phase by opening burette valve (c), and collecting 10 μL of

sedimented phase for spectrophotometric determination (d)

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200

solution containing dithizone as the ammonium salt, HCl (0.5 mol.L-1) was added to

precipitate dithizone. Then dithizone was extracted with 60 mL of chloroform in three steps.

The chloroform phase was washed with two portions of distilled water and then was

transferred to a beaker. After evaporation of chloroform under fume hood in room

temperature, 0.05 g of purified dithizone powder was dissolved in 10 mL carbon tetrachloride

(extractant solvent) in a colored measuring flask. This stock solution, which contained 0.019

mol.L-1 dithizone was stored in refrigerator, and working solutions were prepared weekly by

diluting this solution. Doubly distilled water was used throughout all the procedures.

Instrument

A UV/visible spectrophotometer, UV-160 (Shimadzu, Japan) equipped with two 10 µL

microcells (Starna, UK), was used for measuring the absorbance and recording the spectra.

Narrow bore-dispersive liquid-liquid microextraction procedure

The proposed NB-DLLME method is quite simple and effective, and only needs a

narrow bore as the extractant unit; for simplicity, a simple 20-mL laboratory burette (100 cm ×

0.5 cm) was used. First, it was necessary to prepare the sample solution. Therefore, 18 mL of

the sample solution containing an appropriate amount of Cd (in the range of 50-900 μg.L-1),

alongside with 6 mL sodium potassium tartrate (20% w/v) as reducing agent and 1.5 mL

dimethylglyoxime (1% w/v) and 3 mL hydroxylamine hydrochloride (10% w/v) as masking

agents was prepared. To adjust pH of sample solution to pH≥10, 0.5 mL ammonia (1% w/v)

and 0.5 mL sodium hydroxide (1% w/v) was added to it. This solution was transferred to a

20-mL burette. In a vial, 200 μL carbon tetrachloride (extractant solvent) containing 0.019

mol.L-1 dithizone was mixed with 300 μL methanol (dispersive solvent). This binary solution

was sucked into a 2-mL glass syringe and was injected into the aqueous solution on top of the

burette. Since the rapid injection will lead to agglomeration of the extractant at the up of the

sample solution in burette, therefore the injection was carried out during 30 seconds.

Immediately after injection of binary solution, a cloudy solution was formed on top of the

sample solution containing tiny droplets of the extractant dispersed throughout the aqueous

solution. Due to higher density of carbon tetrachloride, this cloudy solution moved down the

tube, and the analyte was extracted into fine droplets of extractant through the tube. In less

than one minute almost all the fine extractant droplets were settle at the bottom of burette.

Then the extractant phase was easily collected in a vial by opening the burette valve. 10 μL of

extractant was transferred to the microcell, and its absorbance was recorded against blank

reagent at 520 nm.

RESULTS AND DISCUSSION

Selection of extractant solvent

There are two important issues involving in choosing extractant solvent in dispersive

liquid-liquid microextraction, being insoluble in water and its ability in extraction of analytes.

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201

In NB-DLLME usually solvents with densities lower than water are applied as extractant.

Here, we applied NB-DLLME with a solvent of higher density than water as extractant. Due

to strong dipole interaction, solubility of dithizone is very good in chlorinated solvents [44].

Therefore the ability of chloroform, dichloromethane, carbon tetrachloride, and

tetrachloroethylene as the extractant solvents were investigated. For this purpose, 100 μL of

each solvent containing 0.019 mol.L-1 of dithizone was mixed using 500 μL methanol. This

binary solution was applied for extraction of 800 μg.L-1 Cd in 10 mL sample solution.

According to the data obtained, showed in Figure 2, carbon tetrachloride had the best

absorbance. Therefore, this solvent was chosen as the extractant solvent.

Effect of volume of extractant solvent

Volume of extractant solvent can affect the enrichment factor (EF), repeatability, and the

volume of extractant phase collected at the end of extraction at the bottom of burette. Thus

different volumes of carbon tetrachloride were investigated. For this purpose the experiment

condition were the same as previous step, only the volume of extractant (carbon tetrachloride)

was changed from 100-700 μL (containing dithizone in the range of 0.0019 to 0.0133 mmole).

Absorbance of Cd-DTZ was increased slightly with addition of extractant from 100 to 200 μL

but after 200 μL, the absorbance showed a decrease, which is probably due to dilution of

complex in more volume of extractant. Collection of extractant was a little difficult below 200

μL, accordingly, 200 μL carbon tetrachloride containing 0.038 mmole dithizone was chosen as

the best volume of extractant solvent.

Figure 2. Effect of extractant solvent on extraction of Cd-DTZ. NB-DLLME conditions: Cd, 800 mg.L-1;

dithizone concentration, 0.019 mol.L-1; sample volume, 15 mL; extractant volume, 100 μL; pH≥10;

dispersive solvent, methanol; dispersive solvent volume, 500 μL

0,0

0,3

0,6

0,9

1,2

1,5

carbon tetrachloride chloroform dichloromethane tetrachloroethylene

Ab

sorb

an

ce

Extractant solvent

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Selection of dispersive solvent

DLLME procedures are based on extension of surface contact between aqueous and

organic phases with the help of tiny droplets of extractant dispersed throughout the aqueous

phase. Usually, this is achieved by utilization of dispersive solvent, which must be soluble in

both aqueous and organic phases. Different solvents (methanol, ethanol, acetone, and

acetonitrile) were investigated for their ability in dispersing carbon tetrachloride in sample

solution. 500 μL of each dispersive solvent was mixed with 200 μL of carbon tetrachloride as

extractant, and they were subjected to the same NB-DLLME procedure. The data obtained

showed that methanol had the best absorbance for analyte, and therefore, methanol was the

chosen dispersive solvent (Figure 3).

Effect of volume of dispersive solvent

Volume of dispersive solvent is very important in DLLME because it can affects the size

of extractant droplets, polarity of aqueous phase, and volume of collected extractant at the end

of DLLME procedure. All these parameters are vital for the microextraction yield. Therefore,

it was necessary to study and optimize the best volume for dispersive solvent. For this

purpose, different volumes of methanol as dispersive solvent (100-600 μL) were investigated

(Figure 4). In volumes less than 300 μL, large droplets of extractant were formed.

Consequently, efficiency of microextraction and absorbance of complex tended to be low. In

volumes more than 300 μL, a decrease was observed in absorbance of Cd-DTZ complex, that

is probably because in high concentrations of methanol, the polarity of aqueous phase

decreases which leads to more solubility of organic phase in aqueous phase. Therefore, 300 μL

methanol was chosen as the best volume for dispersive solvent.

Figure 3. Effect of dispersive solvent on extraction of Cd-DTZ. NB-DLLME conditions: Cd, 800 mg.L-1;

dithizone, 0.019 mol.L-1; sample volume, 15 mL; extractant, carbon tetrachloride; carbon tetrachloride

volume, 200 μL; pH≥10; dispersive solvent volume, 500 μL

0

0,3

0,6

0,9

1,2

1,5

methanol acetone ethanol acetonitrile

Ab

sorb

an

ce

Dispersive solvent

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Effect of volume of sample solution

To investigate the effect of volume of sample solution on extraction procedure, NB-

DLLME was carried out in different burettes with various lengths and internal diameters, as

the extractant unit. Lengths of burettes were in range of 47-100 cm with internal diameters of

0.2-0.5 cm, which had the capacity of 6-20 mL of sample solution. For this purpose, two set of

experiments were carreid out. First, a set of sample solution were prepared which had the

same concentration of analyte in different volumes; meaning 800 μg.L-1 of Cd in sample

solutions with volumes ranging from 5 to 20 mL. Then a second set of sample solutions with

Figure 4. Effect of volume of dispersive solvent on extraction of Cd-DTZ. NB-DLLME conditions: Cd, 800

mg.L-1; dithizone, 0.019 mol.L-1; sample volume, 15 mL; extractant, carbon tetrachloride; carbon

tetrachloride volume, 200 μL; pH≥10; dispersive solvent, metha

Figure 5. Effect of volume of sample solution on recovery of 800 μg.L-1 (♦) and 5 μg (●) Cd. NB-DLLME

conditions: dithizone concentration, 0.019 mol.L-1; extractant, carbon tetrachloride; carbon tetrachloride

volume, 200 μL; pH≥10; dispersive solvent, methanol; volume of dispersive solvent, 300 μL

0,0

0,3

0,6

0,9

1,2

1,5

1,8

100 200 300 400 500 600

Ab

sorb

an

ce

Volume of dispersive solvent (μL)

40

50

60

70

80

90

100

110

4 7 10 13 16 19 22

Rec

ob

ery

(%

)

Volume of sample solution (mL)

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various volumes were prepared and to all of them 5 μg of Cd was added to study the effect of

mass transfer in the microextraction procedure. Afetr microextraction procedure, the recovery

of analyte was calculated. Based on the obtained results, presented in Figure 5 we decided to

choose 18 mL in a 100 × 0.5 cm burette for the optimized sampl solution.

Effect of salt concentration

The effect of ionic strength on efficiency of extraction can be explained by the salting-out

effect. According to the salting-out phenamena, water molecules form hydration spheres

around the salt ions that reduce the concentration of available water to dissolve analyte

molecules and thus analyte molecules will be forced to enter extractant solvent. On the other

hand, addition of a salt may lead to changes in the physical properties of the disperse film of

extractant in the solution through increasing the density of sample solution. To investigate the

effect of ionic strength of sample solution on extraction of Cd-DTZ, different sample solutions

containing various concentrations of KCl (0-5% w/v) were prepared and the same NB-DLLME

was applied on them. It was observed that with addition of salt the extraction efficiency

decreases. Therefore, the subsequent NB-DLLME procedures were carried out without

addition of a salt.

Effect of interferences ions

The recovery of Cd with the NDLLME, in presence of interference ions were studied

under optimum condition. The results of these experiments are presented in Table 1. To

reduce the effect of interferences, maskind agents were employed (dimethylglyoxime and

hydroxylamine hydrochloride). The studied ions were Fe+3, Zn+2, Pb+2, Na+, and Ca+2. For this

purpose, a fixed amount of foreign ions was added to a sample solution containing 800 μg.L-1

of Cd. After NB-DLLME procedure, the recovery of Cd was calculated. As can be seen from

the results, the recovery of analytes were higher than 91%.

Table 1. Effect of interferences ions on recovery of 800 μg.L-1 Cd

Ion Added as Concentration (mg.L-1) Recovery of Cd (%) Fe+3 NO3

− 10 92.75 Zn+2 NO3

− 10 95.33 Pb+2 NO3

− 10 91.78 Na+ Cl- 50 98.46 Ca+2 Cl- 50 98.29

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Linear range, limit of detection and precision

Analytical figures of merit for the proposed method obtained under optimal condition

are shown in Table 2. To determine the dynamic range of calibration curve, 15 standard

solutions containing Cd in the range of 10 to 1500 μg.L-1 were prepared. The dynamic range of

calibration curve was found to be 50-900 μg.L-1 (with 9 standard solutions). Detection limit

(LOD) and limit of quantification (LOQ) were calculated based on 3Sb/m and 10Sb/m criteria,

respectively, where Sb is standard deviation of 10 blank measurements and m is slope of

calibration graph. The repeatability of the method, expressed as relative standard deviation

(RSD), was calculated for five replicates of standard sample solution containing 500 μg.L-1 of

Cd. The enrichment factor (EF), calculated as the ratio of slope of calibration graphs after NB-

DLLME (mNB) and after classic extraction (mCE) as shown in Eq. 1. For classic extraction, two

portions of 5 mL carbon tetrachloride were applied for extraction of Cd in 18 mL sample in a

funnel. The other condition were as NB-DLLME. EF was found to be 70 fold.

𝐸𝐹 = 𝑚𝑁𝐵/𝑚𝐶𝐸 (1)

A comparison between data obtained with NB-DLLME with those recently obtained

with other liquid-phase microextraction methods for determination of Cd is summarized in

Table 3.

Table 2. Analytical figures of merits for extraction and determination of Cd with NB-DLLME

Parameter Analytical feature

Equation of calibration curve 𝐀 = 𝟎. 𝟎𝟎𝟐 𝐂𝐂𝐝+𝟐 − 𝟎. 𝟎𝟎𝟑𝟓

Dynamic range (μg.L-1) 50-900

R2 (correlation coefficient) 0.9926

Repeatabilityb (RSD %, n=5) 6.4

Limit of detection (μg.L-1) 6.3

Limit of quantification (μg.L-1) 21

Enrichment factor (fold) 70

Total analysis time (min) < 2

Table 3. comparison of NB-DLLME with other LPME methods for determination of Cd

Microextraction

method/Instrument

Linearity

(μg.L-1)

LOD

(μg.L-

1)

EF

(fold) RSD% Recovery%

Analysis

Timea

(min)

Ref.

DLLME/GF-AAS 0.002-0.020 0.0006 125 3.5 87-108 3 31

SFODMEb/GF-AASc 0.08-30 0.0079 635 5.4 97-104 16 33

SDME/UV-Vis 10-600 0.91 13.4 2.56 92.5-101.7 25 45

NB-DLLME/UV-Vis 50-900 6.3 70 6.4 91.6-103.8 2 Present work a Approximated time of analysis b Solidified floating organic drop microextraction c Graphite furnace-atomic absorption spectrometry

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Analysis of real samples

The proposed NB-DLLME procedure was applied on tap water for determination of Cd.

Since no detectable analyte was observed, the samples were spiked with 3 different

concentrations of Cd to investigate the matrix effect. The results obtained are shown in Table

4. As it can be seen, very good recoveries were achieved between 91.6 and 103.8%.

CONCLUSION

In this work, a novel, fast, economical, effective, and one-step narrow bore-dispersive

liquid-liquid microextraction was developed for preconcentration and determination of traces

of Cd in aqueous samples. Unlike previous NB-DLLMEs, which usually use organic solvents

with lower density than water, here we used an organic solvents with a density higher than

water (carbon tetrachloride). For ease in operation and collection of extractant at the end of

NB-DLLME procedure, a simple 20-mL burette was used as extractant unit. Thus,

centrifugation step was totally eliminated and the whole DLLME procedure took place in a

one step and in less than 2 min. Analytical response of analyte was recorded with UV-visible

spectrophotometry equipped with microcells. This technique is simple, fast, and reliable. Also

being available and inexpensive instruments, the simple and fast NB-DLLME-UV-Vis method

can be applied in every laboratory.

ACKNOWLEDGEMENTS

This research work was supported by The University of Sistan and Baluchestan, Iran.

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