Ultrasound-Assisted Emulsification Dispersive Liquid-Liquid Microextraction Based on Solidification of Floating Organic Droplet for Separation of Trace Gold Prior to Flame Atomic Absorption Spectroscopy Determination

doi:10.4236/ajac.2011.22029

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American Journal of Anal yt ical Chemistry, 2011, 2, 243-249

doi:10.4236/ajac.2011.22029 Published Online May 2011 (http://www.SciRP.org/journal/ajac)

Ultrasound-Assisted Emulsification Dispersive

Liquid-Liquid Microextraction Based on Solidification of

Floating Organic Droplet for Separation of Trace Gold

Prior to Flame Atomic Absorption Spectroscopy

Determination

Sayed Zia Mohammadi1,2, Mohammad Ali Karimi1,3, Ali Shiebani4, Laleh Karimzadeh2

1Payame Noor University, Te hran, Iran

2Department of Chemistry, Payame Noor University, Kerman, Iran

3Department of Chemistry & Nanoscience and Nanotechnology Research Laboratory (NNRL),

Faculty of Sciences, Payame Noor University, Sirjan, Iran

4Department of Chemistry, Islamic Azad University, Yazd Branch, Yazd, Iran

E-mail: [email protected]

Received January 3, 2011; revised January 16, 2011; accepted January 17, 2011

Abstract

In the present work, a ultrasound-assisted emulsification dispersive liquid-liquid microextraction based on

solidification of floating organic droplet method has been developed as a sample preparation method prior to

flame atomic absorption spectrometry determination of trace amounts of gold in the standard, wastewater

and river water samples. In the proposed method, 1-dodecanol and 5-(4-dimethylamino-benzylidene) were

used as extraction solvent and chelating agent, respectively. Several factors that may be affected on the ex-

traction process, such as type and volume of the extraction solvent, ionic strength, pH of the aqueous solu-

tion, extraction temperature and extraction time were studied and optimized. Under the best experimental

conditions, the calibration curve exhibited linearity over the range of 8.0 ng·mL−1- 3.0 µg·mL−1 with a corre-

lation coefficient of 0.9978 and detection limit based on three times the standard deviation of the blank sig-

nal was 1.5 ng·mL−1. Eight replicate determinations of 0.2 and 1.0 g·mL−1 of gold gave a mean absorbance

of 0.051 and 0.253 with relative standard deviations of ±2.3% and ±1.5%, respectively. Finally, the devel-

oped method was successfully applied to the extraction and determination of gold ions in a silica ore, waste-

water, river water and standard samples and satisfactory results were obtained.

Keywords: Ultrasound-Assisted Emulsification, Microextraction, Solidification, Floating Organic Droplet,

Preconcentration, Gold Determination, Silica Ore

1. Introduction

Gold belongs to the group of elements which occur on

the Earth in very low natural contents. The concentration

of gold in the natural waters is extremely low and is in

the range of 0.05 - 0.2 ng·mL−1 in seawater and river

water, respectively [1]. It is well known that gold is one

of the most interesting micro amount elements due to its

significant role on biology, environment and industry. It

could be used as a drug in the supervised therapy of ar-

thritis and cancer [2] in the forms of different Au(I) and

Au(III) compounds, or in radiotherapy of cancer in the

form of radioactive isotope 198Au. Also, it could be very

toxic for human, animal and plants and account as a pol-

lutant, because of its inhibiting effect upon the activity of

many enzymes and its preventing effect upon the DNA

separation [2]. Numerous methods such as, spectrometric

methods [1], UV-Vis spectrophotometry [2,3], electro-

thermal atomic absorption spectrometry [4], inductively

coupled plasma combined with optical emission spec-

trometry [5,6] or mass spectrometry [7] and flame atomic

absorption spectrometry (FAAS) have been proposed for

S. Z. MOHAMMADI ET AL.

244

the determination of gold in different environmental

samples. However, most of aforementioned methods

except to FAAS involve a greater cost and increased in-

strumentation complexity, limiting its widespread appli-

cation to routine analytical works. FAAS is still being

used because it combines a fast analysis time, a relative

simplicity and a cheaper cost. All of these features have

been responsible for its broad utilization in the determi-

nation of trace elements in different samples [8-10].

However, trace level determination of gold by FAAS is

difficult due to lower levels of gold than detection limit

of FAAS and effects of the matrix components [9]. To

overcome these limitations on the determination of gold

by FAAS, separation-enrichment techniques including

solid phase extraction (SPE), cloud point extraction, liq-

uid–liquid extraction (LLE), coprecipitation, etc. [11-17]

have been used by the researchers around the world.

For many decades, the most common methods for the

performing of the analytical separation were LLE. LLE

is time-consuming and needs to large amounts of toxic

organic solvents. These shortcomings have led to the

development of the new cost-effective methods with

special emphasis on resolving the mentioned difficulties

and detecting analytes at very low concentrations. Re-

cently, efforts have been directed towards miniaturiza-

tion of the LLE procedure by reducing of the amount of

organic solvent and leading to the development of liq-

uid-phase microextraction methods. Liquid-phase mi-

croextraction (LPME) was introduced by Jeannot and

Cantwell in 1996 [18], however, the major disadvantage

of LPME is time-consuming. Efforts to overcome of this

particular limitation led to the development of dispersive

liquid–liquid microextraction (DLLME) method [19]. In

this method, an appropriate mixture of extraction and

disperser solvent are used. The surface area between the

extraction solvent and sample solution are infinitely large

because a cloudy solution can be formed. Therefore, the

extraction equilibrium can reach quickly. The method

has attracted much attention due to their advantages such

as fast analysis, low consumption of organic solvent and

simplicity [20,21]. However, the extraction solvent is

limited in solvents, which should have higher density

than water, such as chlorobenzene, chloroform, tetrachlo-

romethane and carbon disulfide, and all of them are toxic

and environment-unfriendly.

The application of ultrasounic-assisted radiation in the

LLE methods (USALLE) has been reported by Luque de

Castro and Priego-Capote [22,23]. They also success-

fully applied ultrasound-assisted emulsification (USAE)

for the first time to determine some polar and non-polar

compounds in solid plant samples [24]. High extraction

efficiency in a short period of time is the main advantage

of the USALLE. Regueiro et al. applied a miniaturized

approach to USALLE by using a micro volume of or-

ganic phase to provide the advantage of both DLLME

and USALLE [25]. They successfully applied ultra-

sound-assisted emulsification microextraction (USAEME)

to determination of some emergent contaminants and

pesticides in environmental waters. Fontana et al. applied

this method for determination of polybrominated flame

retardants in water samples [26]. They demonstrated that

USAEME is an efficient, simple, and rapid as well as

cheap extraction technique prior to the GC analysis.

Recently, a new mode of liquid-phase microextraction

based on solidification of floating organic droplet

(LPME-SFO) was developed [27,28]. In this method, no

specific holder, such as the needle tip of micro syringe,

the hollow fiber and polychloroprene rubber tube, is re-

quired for supporting the organic micro drop due to the

using of organic solvent with low density and proper

melting point. Furthermore, the extractant droplet can be

collected easily by solidifying it in the lower temperature.

However, the extraction time was somewhat long, thus it

cannot satisfy the demand of fast analysis.

Recently, Xu and co-workers in 2009 [29] combined

the advantages of the two methods (DLLME and LLME-

SFO) and develop a new method named dispersive liq-

uid–liquid microextraction based on solidification of

floating organic droplet (DLLME-SFO). In this method,

the enormous contact area between the organic droplets

and sample solution is beneficial for the fast mass trans-

fer from the aqueous phase to the organic phase. Accord-

ingly the analysis time is shortened greatly. Moreover,

the transfer of the solidified phase from aqueous phase

can be carried out easily.

Pourreza [30] and Afzali [31] reported that 5-4-dime-

thylamino-benzylidene (rhodanine) is suitable as a sensi-

tive and selective reagent for gold. Therefore, rhodanine

was selected as chelating agent in the present work.

The aim of this work is combination of USAEME with

SFO and developing a new method as name USAEME-

SFO for the determination of trace gold in river water

and wastewater samples. All main factors were investi-

gated and optimized. The USAEME-SFO method was

evaluated by analyzing Certified Reference Materials

and spiked samples.

2. Experimental

2.1. Instrumentation

A SensAA GBC (Dandenong, Australia) atomic absorp-

tion spectrometer equipped with deuterium background

correction and gold hollow cathode lamp was used for

absorbance measurements at wavelength of 242.8 nm.

The instrumental parameters were adjusted according to

S. Z. MOHAMMADI ET AL.245

the manufacturer’s recommendations. Acetylene flow rate

and burner height were adjusted in order to obtain the

maximum absorbance signal, while aspirating the analyte

solution. A Metrohm 692 pH meter (Herisau, Switzer-

land) was used for pH measurements. An ultrasonic bath

with temperature control (FALC instruments S.V.l Tre-

viglio, Italy) model LBS2 was used to assist the emulsi-

fication process of the microextraction technique.

2.2. Reagents and Solutions

All chemicals were analytical-reagent grade (Merck,

Darmstadt, Germany) and were used without previous

purification. The laboratory glassware was kept over-

night in a 1.4 mol·L−1 HNO3 solution. Before using, the

glassware was washed with de-ionized water and dried.

Stock solution of gold at a concentration of 1000.0

μg·mL−1 was prepared by dissolving an appropriate

amount of HAuCl4·3H2O (Merck) in double distilled

water. The working reference solutions were obtained

daily by stepwise dilution from stock solution. A 0.05%

(w/v) solution of 5-(4-dimethylamino-benzylidene) (Rho-

danine) (Mer- ck) was prepared by dissolving it in etha-

nol. The solutions of alkali metal salt (1% w/v) and var-

ious metal salts (0.1% w/v) were used to study the inter-

ference of anions and cations, respectively.

2.3. USAMEM-SFO Procedure

All standards and samples were prepared for analysis

according to the following procedure. Eight mL of each

sample was placed in a screw cap glass test tube. To each

test tube, 1 mL of 0.1 mol·L−1 phosphate buffer (pH 3),

100 µL Rhodanine 0.05% in ethanol and 1 mL of 10%

(w/v) NaCl were added. Then, 25.0 µL 1-dodecanol was

injected into each solution and all samples were soni-

cated for 10 minutes at 42˚C in ultrasonic bath. As a re-

sult, oil-in-water emulsions of 1-dodecanol in water were

formed. After centrifuging at 4000 rpm for 4 min, the

organic solvent droplet was floated on the surface of the

aqueous solution due to low density below water. Then,

the sample vial was put into an ice bath for 5 min, at this

time the floated solvent was solidified because of low

melting point (24˚C). Then, the solidified solvent was

transferred to a conical test tube. The extractant in the

test tube was washed by iced water for 2 times, and the

water in the vial was drawn out by a syringe. The solidi-

fied organic solvent melted quickly at room temperature.

Prior to analysis of gold by FAAS, the extractant was

mixed with 0.5 mL of dimethyleformamid.

2.4. Sample Preparation

In order to test the reliability of the proposed USAMEM-

SFO procedure for extraction and determination of gold

in the real samples, two Canadian Certified Reference

Materials Project (CCRMP) and a silica ore sample

(Muteh gold mine, Isfahan, Iran) were analyzed. For this

purpose, 200.0 mg of CCRMPs and 1.00 g of silica ore

sample were taken and dissolved completely in a mixture

of nitric acid, hydrochloric acid and HF (2:6:1) with

heating. The solutions were cooled, diluted and filtered.

Then, the filtrates were made to 50.0 mL with deionized

water in a volumetric flask.

River water and wastewater samples were collected in

acid leached polyethylene bottles. River water sample

was collected from Shahdad (Kerman, Iran) and waste-

water samples were collected from copper factory in

Sarcheshmeh area (Kerman, Iran) and Bahonar copper

factory in Kerman. The only pretreatment was acidifica-

tion to pH 2 with nitric acid, which was performed im-

mediately after collection, in order to prevent adsorption

of the metal ions on the flask walls. The samples were

filtered before analyses through a cellulose membrane

(Millipore, Bedford, MA, USA).

3. Results and Discussion

3.1. Type and Volume of the Extraction Solvent

Selection of the extraction solvent is important in the

optimization of USAEME-SFO conditions. It should

have low solubility in water, high affinity to analytes,

lower melting point than room temperature and lower

density than water. In this work, 1-undecanol and

1-dodecanol were selected as extraction solvent, and

their extraction efficiency were studied. The results were

shown that both of solvent are suitable for extraction of

gold. 1-undecanol is more expensive than 1-dodecanol

(DOD), therefore DOD was selected as the extraction

solvent in the subsequent experiments. The effect of the

DOD volume on the extraction efficiency was also in-

vestigated. Therefore, some experiments were performed

with different volumes of DOD (15.0 - 50.0 µL) as the

extraction solvent and keeping the other variable con-

stant. The results are shown in Figure 1. It was observed

that the maximum extraction efficiency was obtained at

the volume range of 20.0 to 50.0 µL of DOD. Thereby,

25.0 µL DOD was used as extraction solvent in the sub-

sequent experiments.

3.2. Effect of pH on the USAEME-SFO

Procedure

The pH plays a unique role on the metal-chelate formation

and the subsequent extraction. Therefore, the effect of pH

on the USAEME-SFO extraction of gold was studied in

S. Z. MOHAMMADI ET AL.

246

110

10 2030 40 50

Volume o f extraction solvent (L)

Recovery (%)

Figure 1. Effect of the extraction solvent volume (DOD) on

the USAEME-SFO extraction of gold. Conditions: Au(III),

8.0 g; Buffer with pH 3, 1 mL; NaCl 1% (w/v); Extraction

time, 10 min; Extraction temperature, 42˚C.

the pH range of 1 - 9 and keeping the other variable con-

stant. The results are shown in Figure 2. As can be seen

in Figure 2, the highest extraction efficiency of gold was

obtained at the pH range of 2.5 - 3.5. Therefore, pH 3

was selected for the further experiments.

3.3. Effect of the Extraction Temperature

Temperature affects organic solvent solubility in water as

well as the emulsification phenomenon. Thus, this affects

the mass-transfer process and the extraction efficiency.

To determine the influence of the extraction temperature,

8.0 mL aqueous solution containing 8.0 μg of gold was

extracted at different temperatures ranging from 30˚C to

60˚C. The results are shown in Figure 3. It was observed

that the highest extraction efficiency was obtained at the

range of 40˚C - 45˚C. Hence, 42˚C was used for further

experiments.

3.4. Effect of the Extraction Time

In USAEME, the extraction time is defined as interval

time between the injection of the extraction solvent and

the starting of centrifuge. Effect of the extraction time

was examined in the range of 5 to 20 min and keeping

the other variable constant. The results are shown in

Figure 4. It was observed that the highest extraction

110

0123456789

Recovery (%)

Figure 2. Effect the pH of sample solution on the USAEME-

SFO extraction of gold. Conditions were the same as Figure

1 except to pH.

110

25 30 35 40 45 50 55 60 65

Extraction temperature (



Recovery (% )

Figure 3. Effect of the extraction temperature on the

USAEME-SFO extraction of gold. Conditions were the same

as Figure 1 except to extraction temperature.

100

110

051015 20 25

Ex tr actio n time (min )

Reco v er y (%)

Figure 4. Effect of the extraction time on the USAEME-

SFO extraction of gold. Conditions were the same as Figure

1 except to extraction time.

efficiency was obtained at 7.5 - 12.5 min. Based on these

observations; an extraction time of 10 min was used for

further experiments.

3.5. Effect of Ionic Strength

In the extraction methods, the solubility of many analytes

in aqueous solutions decreases with increasing ionic

strength due to salting out effect. For investigating the

influence of the effect ionic strength on the USAEME-

SFO extraction of Au(III) ion, sodium chloride solution

was used in the concentration range of 0.25% to 2.5%

(w/v). The results were shown that the highest recovery

percent of gold ion was obtained at 0.75% - 1.25% NaCl

concentration. Below or above this concentration range,

a decrease on the extraction efficiency was observed.

Therefore, 1% NaCl concentration was used in the sub-

sequent experiments.

3.6. Interference

The efficiency of the USAEME-SFO procedure in the

extraction and preconcentration of gold ions were also

studied in the presence of various cations and anions. In

order to, interference ions in different interference-to-

analyte ratios were added to a solution containing 8.0 µg

S. Z. MOHAMMADI ET AL.247

of Au(III) and were subjected to the USAEME-SFO

procedure. The tolerance limit was set as the amount of

ion required to cause ±5% error in the determination of

gold. The results are given in Table 1. The results were

shown that the presence of large amounts of species

commonly present in water samples have no significant

effect on the extraction of gold.

3.7. Analytical Figures of Merit

Figures of merit of the USAEME-SFO procedure were

obtained by processing of the standard solution of gold.

For a sample volume of 8 mL, the calibration graph ex-

hibited linearity over the range of 8.0 ng·mL−1 - 3.0

µg·mL−1 with a correlation coefficient of 0.9978 (A=

0.2549C + 0.0026, where A is the absorbance value and

C is the concentration of gold (µg·mL−1). The relative

standard deviations (n = 8) at 0.2 and 1.0 g·L−1 of gold

were ±2.3% and ±1.5%, respectively. The limit of detec-

tion, based on three times the standard deviation of the

blank signal was 1.5 ng·mL−1 of gold. The enrichment

factor was calculated as the ratio of the analytical signal

of Au obtained after and before extraction. The enrich-

ment factor was 14.2 for 8.0 mL sample solution.

Table 1. Tolerance limit of interference ions.

Interference ions Interference/gold ratio Recovery (%)

PO43–, H2PO4–, HPO42– >5000 95

K+ 3000 104

Na+ 3500 105

Ca2+ 3000 95

Mg2+ 2500 105

Co2+ 700 95

Mn2+ 500 105

Ni2+ 600 98

Cu2+ 400 105

Pb2+ 500 95

Cd2+ 600 96

Pd2+ 600 104

Rh3+ 800 105

Zn2+ 300 95

Ag+ 200 95

Fe3+ 100 96

Al3+ 50 95

Conditions were the same as Figure 1.

3.8. Accuracy of the Method

The accuracy of the USAEME-SFO procedure was

checked to the determination of gold in CCRMP (MA-1b

and CCU-1b). An aliquot of the sample solution was

taken and gold was determined after the USAEME-SFO

procedure. The results are given in Table 2 and are in

good agreement with the certified value.

3.9. Application

The USAEME-SFO procedure was applied to the deter-

mination of gold in a silica ore, wastewater and river

water samples. The recovery of gold from the silica ore,

wastewater and river water samples spiked with the

known amounts of gold ions was also studied. The ob-

tained results are shown in Table 3. According to these

results the added gold ions spiked to the wastewater and

river water samples can be quantitatively recovered, and

no significant interference was observed.

Furthermore, the ore sample was analysed by induc-

tively coupled mass spectrometry (ICP-MS) technique

for verifying the result obtained by the USAEME-SFO

procedure developed in this paper. As it is obvious from

Table 2. Determination of gold in canadian certified refer-

ence materials project.

Sample Certified value

(g g−1)

Founda

(g g−1)

MA-1b reference

gold ore 17.0 17.1  0.5

CCU-1b copper

flottation concentrate 5.89 ± 0.10 5.86  0.17

aAveragestandard deviation (n = 4).

Table 3. Determination of gold in real samples.

Gold amount (ng·mL–1)

Sample

Added Founda

Recovery

(%)

Silica ore (Muteh

gold mine, Isfahan,

Iran)

0.0

10.0

41.40 ± 1.80b

51.23 ± 0.5

98.3

Shoor river

(Shahdad, Kerman)

0.0

10.0

N.D.c

9.8 ± 0.3

98.0

Wastewater (copper

factory, Sarchashmeh,

Rafsanjan)

0.0

10.0

B.L.R.d

10.6 ± 0.4

106.0

Wastewater (copper

factory, Shahid

Bahonar, Kerman)

0.0

10.0

B.L.R.

10.4 ± 0.4

104.0

aMean ± standard deviation (n = 4).bThe quantitative analysis of gold con-

tent in the silica ore sample by the proposed method and ICP-MS was found

to be 2.07 ± 0.09 µg g−1 and 2.11 ± 0.12 µg·g−1, respectively. cNot detect.

dBelow of linear range.

S. Z. MOHAMMADI ET AL.

248

Table 3, there is a satisfactory agreement between the

results obtained by ICP-MS (2.11 ± 0.12 µg·g−1) and the

proposed method (2.07 ± 0.09 µg·g−1).

4. Conclusions

In this paper, we introduced a USAEME-SFO method

for the analysis of trace amounts of gold in wastewater

and river water samples. The USAEME-SFO procedure

has numerous advantages such as: low cost, low toxic,

simplicity of operation, rapidity and high selectivity. In

addition, it is important to point out that USAEME-SFO

is a low organic solvent consuming extraction technique,

which turns it into a low cost and also an environmen-

tally friendly technique. In this method, the consumption

of the toxic organic solvent (at microlitre level) was

minimized without affecting sensitivity of the method.

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