Ion-association dispersive liquid–liquid microextraction of ultra-trace amount of gold in water samples using Aliquat 336 prior to inductively coupled plasma atomic emission spectrometry determination
© The Author(s). 2016
Received: 5 April 2016
Accepted: 20 October 2016
Published: 5 November 2016
Gold is a rare and noble metal with atomic number of 79. Several techniques including both spectrometric and electrochemical methods are widely used to determine gold level in real samples. As a result of low concentration of gold in environmental samples, sample preparations such as extraction, clean-up, and pre-concentration before instrumental analysis are mandatory. Thus, a pre-treatment method (dispersive liquid–liquid microextraction (DLLME)) for gold pre-concentration and inductively coupled plasma optical emission spectrometry (ICP-OES) determination is proposed.
An extraction method employing DLLME and ICP-OES has been developed for rapid separation, pre-concentration, and determination of ultra-trace amount of Au (III). The extraction of the analyte was performed in the presence of a quaternary ammonium cation, N-methyl-N,N,N-trioctylammonium chloride, (Aliquat 336) as an extractant based on ion-association extraction system.
1-Octanol and acetonitrile were used as extraction and disperser solvents, respectively. The variables affecting the extraction conditions were optimized. A calibration curve in the range of 0.3–100 ng mL−1, the detection limit of 0.09 ng mL−1, an enrichment factor of 150, and an extraction recovery of 74 % were obtained. The precision (RSD %) of the method was 6 % for five replicates and recoveries of 10 ng mL−1 Au(III).
The combined DLLME method with ICP-OES can readily determine Au(III) at trace (μg L−1) level using only 10 mL of sample solution (tap, lake, and mining water) without interference by the matrices. This methodology is simple, fast, and low cost which can be used in routine analytical laboratories.
Gold is a rare and noble metal with atomic number of 79 and 14 radioactive isotopes with half-times from a few seconds to a few days (Aitio et al. 2014). The word noble refers in part to its rarity, beauty, and wide utility in economic and industrial activity due to its chemical stability, high ductility, corrosive resistance, and low impedance (Kagaya et al. 2010). The average concentration of gold in earth’s crust is approximately 4 μg kg−1 (ppb) and the values of 0.05 and 0.2 ng mL−1 (ppb) were found in sea water and river water, respectively (Lee 1996; Rancis et al. 2005). Several techniques including both spectrometric and electrochemical methods are widely used to determine gold in real samples (Shamsipour and Ramezani 2008; Gomez and McLeod 1993; Ye and Beng khoo 1999; Hu et al. 2006; de-Souza Periera et al. 2014; Ye et al. 2014). As a result of a low concentration of gold in environmental samples and matrix interferences, the limiting factor in the analysis is often the sensitivity of the method. Therefore, sample preparations such as extraction, clean-up and pre-concentration before instrumental analysis are mandatory. Solid-phase extraction (Zhang et al. 2011; Sabermahani et al. 2012; Iraji et al. 2013; Afzali et al. 2014; Kazemi et al. 2015), cloud-point extraction (Manzoori et al. 2007; Tavakoli et al. 2008), co-precipitation (Messerschmidt et al. 2000), and liquid–liquid extraction (LLE) (El-Shahawi et al. 2007; Vasilyeva et al. 1975; Ohava et al. 1995; Kolekar and Anuse 2001) are used for sample preparation purposes. LLE has been widely used for separation and pre-concentration of gold ions. This technique is based on transfer of desired analytes from the primary aqueous sample to water-immiscible solvent (Pena-Periera et al. 2009). Gold ion normally exists in an aqueous phase as a hydrated anion complex (AuCl4 −) with little or no tendency to be transferred to organic phase. Suitable cations like quaternary ammonium cations (e.g., tetrabutylammonium), organometallic cations (e.g., Ph3Sn+), and cationic colorants (e.g., Rhodamine B) have been applied to form non-solvated ion-pair with AuCl4 − (Das and Bhatacharyya 1976) to produce extractable species. However, LLE is not without its associated problems such as being time consuming, large amount of toxic organic solvent consumption, and difficulty in automation. Recent trends in analytical chemistry are in the direction of simplification and miniaturization of sample preparation procedures as well as the minimization of solvent and reagent usage. So, dispersive liquid–liquid microextraction (DLLME) has been introduced for organic and inorganic analyte separation from aqueous matrices (Rezaee et al. 2006). DLLME is based on dispersion of extraction solvent in aqueous bulk samples assisted with a disperser solvent. Then, the analytes are allowed to be extracted into fine droplets of extraction solvent during the formation of cloudy solution (Anthemidis and Ionnp 2009). After centrifuging, the separated organic phase will be analyzed with appropriate techniques. The advantages of this technique include simplicity of operation, rapid extraction, and high-enrichment factors (E f ) (Leong et al. 2014). DLLME studies related to ion metals involve the use of complexing reagents, ligandless-dispersive liquid–liquid microextraction (LL–DLLME), and ion-pair forming agents. The two first approaches are widely used for different kinds of metals such as Pd (Mohammadi et al. 2010), Ag (Mohammadi et al. 2009a), and Cu (Mohammadi et al. 2009b) in the cationic form. The ion-pair-based approach has only been applied for gold following the formation of an ion-pair between AuCl4 − complex and different counterions such as Victoria Blue R, dicyclohexylamine, benzyldimethyl tetradecyl ammonium, and between AuCN2 − complex and Astra Phloxin (De La Calle et al. 2011; Kocuroua et al. 2010).
Because of high toxicity and limitation in extracting various analytes of commonly used extraction solvents, two different aspects (I) the use of new solvents, typically ionic liquids and (II) the expansion of the application scope using polar and low-density organic solvents such as long-chained alcohols or hydrocarbons are considered (Yana and Wanga 2013). In this study, a pre-treatment method (DLLME) for Au determination with a quaternary ammonium salt, N-methyl-N,N,N-trioctylammonium chloride, (Aliquat 336) as an extractant reagent based on ion-association solvent extraction system is proposed. The purpose of the present work is to optimize various parameters affecting the efficiency of the combination of microextraction and ICP-OES for the determination of gold in water samples.
Reagents and solutions
Reagent grade 1-octanol, 1-dodecanol, acetonitrile, acetone, ethanol, NaCl, HCl, and HNO3 were purchased from Merck chemical company (Darmstadt, Germany) and N-methyl-N,N,N-trioctylammonium chloride (Aliquat 336) was purchased from Sigma–Aldrich (USA). Doubly distilled deionized water was prepared by MiliQ water (Merck, Millipore) and was used in all experiments. A stock standard solution of gold as AuCl4 − and other trace and major elements (for interfering studies) (1000 μg mL−1-SM 80B-500 VHG Labs-USA) were utilized and diluted to the desired concentration daily prior to use. Tap water (Karaj, Alborz province, Iran), lake water (Orumieh, West Azarbaijan, Iran), and mining water (Anguran mine, Zandjan province, Iran) samples were collected in acid-leached polyethylene bottles and acidified by HNO3.
In this study, a simultaneous radial viewing torch configuration ICP-OES (Varian 735-OES, Australia) equipped with charge-coupled device (CCD) detector has been used for the determination of gold. The control of the spectrometer is provided by PC-based ICP Expert II software. All measurements were carried out in time-scan mode at 242.8 nm ionic emission Au spectrum. A Vision VS 5500N Centrifuge (Korea) was used to speed up the phase separation.
In a 10-mL volumetric flask, 3.5 mL concentrated HCl, 2.0 mL AuCl4 − solution (1.0 mg L−1), and 0.5 g NaCl, salt were added and diluted to the mark with distilled water and transferred to a proper homemade test tube (final concentration of gold was 0.200 mg L−1). Fifty microliters of 1-octanol as extraction solvent containing Aliquat 336 (0.1% w/v) and 1.5 mL acetonitrile as disperser agent was together rapidly injected to test solution by a disposable syringe. A cloudy solution (water/1-octanol-Aliquat 336/acetonitrile) was formed in the test tube where the AuCl4 − is extracted into fine droplet of 1-octanol by aid of Aliquat 336 as an ion-association agent. The mixture was then centrifuged for 10 min at 2900 rpm. After this process, the dispersed fine droplets were collected on the top of the solution in the neck of the homemade test tube. Finally, the collected organic phase (50 ± 3% (μL)) was introduced to ICP-OES instrument through four-channel peristaltic pump, and the gold content was determined in time-scan mode. When running organic samples by ICP-OES, a higher power level (1.6 KW) and lower nebulizer flow rate (0.6 L min−1) are required to ignite and maintain plasma. With correctly chosen conditions, the intense green tongue generated by molecular emission of C2 and CN emission will be visible between the turns of the induction coil but will not extend above the top of the torch (Boorn and Browner 1982). So, the viewing height in this study was chosen 15 mm above the load coil to avoid background emission due to green tongue.
Preparation of water samples
Tap water sample from laboratory of applied geological research center of GSI (Karaj, Alborz province), Lake water as a source for hydro-geochemistry studies from Orumieh lake (Orumieh, West Azarbaijan province), and mining water from Anguran mine (Zandjan province) were collected in acid-leached poly(ethylene) bottles, filtered, and acidified with HNO3 to prevent adsorption of metal ions on the bottle wall.
Preparation of certified reference material (CRM) samples
The proposed method was also applied to the extraction and determination of gold in three geochemical reference samples (GAU 15–17). An accurately measured sample (0.5000 g) of GAUs was dissolved in aqua regia. The solution was evaporated to near dryness to remove the nitrogen oxides. Then, 10 mL distilled water and a few drops of concentrated hydrochloric acid were added. The solution was filtered, and the filtrate was transferred completely into a calibrated 25-mL volumetric flask and diluted to the mark with water. The extraction and determination of gold content was carried out following the general procedure described.
Results and discussion
Nature and amount of extraction solvent
Extraction solvent plays a prominent role in a successful extraction procedure (Rezaee et al. 2010). In general, the extraction solvent must be able to extract the analyte well and has low solubility in aqueous medium. High-density extraction solvents, being mostly halogenated, are generally toxic and have high background emission in ICP-OES, accordingly alcohols with lower densities than aqueous solutions and less volatile were chosen in this test. The solvents that were tested in this work are 1-octanol and 1-dodecanol (due to their compatibility with ICP-OES). The results showed that the organic phase after extraction by 1-octanol had higher overall extraction recovery for the target analyte than 1-dodecanol. Therefore, 1-octanol was selected for further studies.
The results showed that the peak area (proportional to concentration) of the extracted gold decreased with the increasing volume of 1-octanol in the studied range. However, as expected, the normalized peak area (extraction recovery) increased with the increasing volume of 1-octanol. For trace analysis, high-enrichment factor is more critical than extraction efficiency, so 50 μL of solvent was chosen as the optimized volume for analysis.
Type and amount of disperser solvent
The disperser solvent has to be highly miscible with both water and the extraction solvent. Disperser solvent has an important effect on decreasing the interfacial tension between water and extracting solvent and thus makes the droplet size smaller. Thereby, acetone, ethanol, and acetonitrile which have these properties were tested as disperser solvents in this work. Acetonitrile was chosen as disperser solvent due to its highest emission intensity of extracted phase containing target Au(III).
Effect of HCl concentration
Effect of salt concentration
Amount of ion transfer reagent
Effect of foreign ions
The effect of foreign ions on pre-concentration and determination of Au(III) (20 μg L−1)
Extraction recovery (%)
96 ± 6
96 ± 6
96 ± 6
96 ± 6
104 ± 6
95 ± 6
96 ± 6
96 ± 6
96 ± 6
95 ± 6
101 ± 6
96 ± 6
95 ± 6
101 ± 6
98 ± 6
107 ± 6
107 ± 6
105 ± 6
Calibration graph and real sample
Determination of Au(III) in water samples
Added (μg L−1)
Found value(μg L−1)
Anguran mine drainage
9.9 ± 1.4
99 ± 15
10.6 ± 0.7
106 ± 7
61.6 ± 6.4
102 ± 10
Determination of Au(III) in geochemical reference samples
Certified (μg g−1)
Found (μg g−1)
0.30 ± 0.01
1.09 ± 0.04
3.14 ± 0.08
Comparison with other methodologies
Comparison of different method for determination of gold in aqueous samples
LOD (μg L−1)
Linear range (μg L−1)
Shamsipour and Ramezani 2008
Sabermahani et al. 2012
Tavakoli et al. 2008
Zhang et al. 2011
Mohammadi et al. 2010
Tajic and Taher 2011
Au(III) can be selectively extracted from a sample solution in HCl (4 mol L−1) media at the presence of other elements such as Cr, Co, Pt, and Pd. The combined DLLME method with ICP-OES can readily determine Au(III) at μg L−1 level using only 10 mL of sample solution without interference by the matrices. In this method, using toxic-chlorinated solvents that are commonly used as extraction solvents is avoided. As the solvent and circumstances are compatible with ICP-OES, there is no need to evaporate solvent for elemental analysis. This methodology is a simple, fast, green, and low cost which can be used with ICP-OES technique so could be of great interest for Au(III) determination in routine analytical laboratories.
This work has been supported by the Chemistry and Chemical Engineering Research Center of Iran and Applied Geological Research center of Geological Survey of Iran.
All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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