Ultrasound-assisted emulsification microextraction and preconcentration of trace amounts of silver ions as a cyclam complex
© Khayatian and Pourbahram. 2016
Received: 17 March 2015
Accepted: 12 January 2016
Published: 27 January 2016
Silver has been recognized as a toxic element for biological systems. A low level of exposure to silver compounds is widespread owing to the use of soluble silver compounds to disinfect water for drinking and reaction purposes. The present paper describes an ultrasound-assisted emulsification microextraction method for the preconcentration and determination of silver ions in drinking water.
Determination was carried out using 1,4,8,11-tetraazacyclotetradecane as a complexing reagent and chloroform as extracting solvent, followed by flame atomic absorption determination of silver ions.
The main factors affecting microextraction efficiency, such as extraction solvent type and volume, concentration of chelating reagent, concentration of picrate ions, and pH, were optimized. Under optimal conditions, a limit of detection and enrichment factor of 6.79 ng mL−1 and 9.8 were obtained for silver ions, respectively. The analytical curve was linear in the range of 0.055–1.5 μg mL−1, with a correlation coefficient (R 2) of 0.997.
The use of ultrasound as a powerful energy for the microextraction and determination of silver was proposed. The main advantages of the method are as follows: minimum use of toxic organic solvent and wide linear dynamic range.
KeywordsUltrasound-assisted emulsification microextraction Silver extraction 1,4,8,11-tetraazacyclotetradecane
The increasing use of silver compounds and silver-containing preparations in the industry such as disinfectants and photographic films has resulted in an increased silver content of environmental samples (Kolthoff and Elving 1966; Meian 1991). Thus, accurate determination of trace quantities of silver ions by simple methods is important in chemical and environmental analysis. Direct determination of silver ions of trace level is usually very limited due to the complexity of the sample matrix and the low concentrations of silver ions; therefore, there is a crucial need for the extraction and preconcentration of this element from the sample matrix before its analysis using atomic absorption spectrometry (Xie et al. 2005). The most widely used techniques for separation and preconcentration of trace amounts of silver ions are coprecipitation (Mao et al. 1998), solid-phase extraction (SPE) (Rofouei et al. 2009; Shamsipur et al. 2002; Safavi et al. 2004; Khayatian and Hassanpoor 2012a), liquid–liquid extraction (LLE) (Koh and Sugimoto 1996), and cloud point extraction (CPE) (Manzoori et al. 2007 and Dalali et al. 2008). However, most of these methods are laborious, time-consuming, and require large volumes of toxic organic solvents.
It is well known that ultrasound is a powerful energy for the acceleration of various steps in analytical procedures, such as homogenizing and emulsion forming (Aydin et al. 2006). This type of energy greatly helps in the processes of extraction and preconcentration because it facilitates the acceleration of the mass-transfer process between two immiscible phases (Ozcan et al. 2009). This leads to an increase in the extraction efficiency of the method within a minimum span of time (Luque de Castro and Priego-Capote 2006 and Khayatian and Hassanpoor 2013). As a result, the application of ultrasound radiation to an extraction method such as liquid–liquid extraction provides the advantages of both methods. This new technique is called ultrasound-assisted emulsification microextraction (USAEME). This extraction method offers advantages such as simplicity, rapidity, a high enrichment factor, and safety and low cost due to consumption of very small amounts of toxic organic solvents. In USAEME, an extraction solvent is rapidly injected into an aqueous sample with a syringe. After sonication, a cloudy solution is formed, and the analyte is extracted into the fine droplets of the extraction solvent. After extraction, the phase separation is performed by centrifugation, and the analyte is determined in the sedimented phase by instrumental methods.
In recent years, the USAEME method has been used for the separation and preconcentration of cadmium (Li et al. 2009 and Ma et al. 2009; Khan et al. 2014), palladium (Mohamadi and Mostafavi 2010), gold (De La Calle et al. 2011), silver (Wen et al. 2012), Te(IV) (Fathirada et al. 2012), mercury (Stanisz et al. 2013), and from environmental matrices. However, among many types of ligands used in microextraction of metal ions, less attention has been directed toward using macrocyclic crown ethers as an extracting agent. Therefore, we decided to study application of crown ethers as a new extracting agent for developing a sensitive microextraction method for the determination of silver ions.
In the present work, we report a simple USAEME method for the preconcentration and flame atomic absorption spectrometry (FAAS) determination of silver ions using 1,4,8,11-tetraazacyclotetradecane (cyclam) as a complexing agent and CHCl3 as an extraction solvent.
Chemicals and reagents
Extra pure methanol ≥95.5 %, acetonitrile (AN) ≥99 %, nitric acid 65 %, and hydrochloric acid 32 % were used as received. Stock standard solution (1.000 g L−1) of Ag+ was prepared by dissolving corresponding AgNO3 (Merck) (99.8–100.5 %) in distilled water. The ligand that was used for the extraction procedure was 1,4,8,11-tetraazacyclotetradecane (cyclam, Merck) ≥92.0 % and used without any further purification, except for vacuum drying over P2O5.
Determination of silver and other cations was performed on a Shimadzu AA-670 atomic absorption spectrometer (Kyoto, Japan) under the recommended conditions (wavelength of 328.1 nm and bandwidth 0.6 nm). All pH measurements were made using a Metrohm E-691 digital pH meter with a combined glass electrode. A model Labofuge 400 (Germany) centrifuge was used to accelerate phase separation. A model Parsonic 7500S, 28-kHz, 100-W (220 VAC max) ultrasonic bath with temperature control was used to assist the emulsification process of the microextraction technique.
Preparation of radiological film
A 2-cm2 sheet of a developed film (0.0504 g) was cut into several strips and put into a 100-mL beaker. Then, 0.5 mL of concentrated sulfuric acid was added, followed by six to eight drops of concentrated nitric acid. The beaker was placed on the hot plate and heated until evolution of brown fumes was no longer observed. This procedure was repeated until the radiological film was digested. After complete decomposition, the contents of the beaker should consist of a white to very light tan precipitate and a clear yellowish liquid. The hot plate was set on high heat, and the contents of the beaker were boiled until white fumes appeared (Gansel 1959; Sarafraz-Yazdi and Amiri 2010). Then, the analysis was performed as mentioned previously after diluting the solution 50 times and adjusting the pH with sodium hydroxide to pH 2.
Results and discussion
Optimization of parameters
To obtain high extraction efficiency, the effects of different parameters such as type and volume of the extracting solvent, pH, amount of cyclam, amount of counter ion, extraction time, salt addition, and centrifuging time were optimized. Then, the effect of interfering ions was investigated, and different natural water samples and a radiological film sample were subjected to analysis via the recommended method to evaluate the concentration of silver ions.
The effect of type and volume of the extraction solvent
The extracting solvent in USAEME should be able to form a cloudy solution in the aqueous phase; in addition, it must have high extraction efficiency and low water solubility (Manzoori et al. 2007; Safavi et al. 2004). Thus, to study the type of extraction solvent, different solvents such as chloroform, dichloromethane, carbon tetrachloride, chlorobenzene, and tetrachloroethylene were examined. For this purpose, 200 μL of each solvent was added to 5.0 mL of deionzed water containing 0.5 μg mL−1 of silver ions, and the extraction efficiency of silver ions was measured by the increase in absorbance in the extracted solvent. The obtained results showed that chloroform has the highest extraction efficiency for the silver ions; thus, chloroform was selected as the extraction solvent for further experiments, Table S1 (in Additional file 1).
Effect of pH on the extraction of silver ions
The effect of the pH of the sample solution on the recovery of 0.5 μg mL−1 of silver ions from the 5.0-mL solution was investigated in the pH range of 2.0–7.0. The pH was adjusted by using 0.1 M of either nitric acid or sodium hydroxide solutions. The results of percent of extraction versus pH value are shown in Figure S1 (Additional file 1). As can be seen, the results indicated that the silver ions can be quantitatively extracted at pH 2.0 (≥95 %), and then, at higher pH values, percent extraction slowly decreased. Low extractions that occur at lower pH values (pH 1.0) are probably due to protonation of the ligand. At higher pH values (pH > 3.0), the extraction efficiency gradually decreases, which may be due to the addition of sodium hydroxide for the adjustment of higher pH values and the competition of this ion with silver ion for complex formation with cyclam. Thus, pH 2.0 was chosen as the optimum pH for further studies.
Effect of cyclam concentration on the extraction of silver ions
Effect of counter ions on silver ions absorbance
The silver ion extraction depends not only on the various kinds of counter anions used but also on the concentration of these ions; therefore, different types of counter ions, such as NO3 −, ClO4 −, and picrate ion (from picric acid), were chosen for the extraction of silver ions. Among the different counter ions studied, the picrate ion was the best for the extraction of silver ions from the sample solution. This observation is due to a more lipophilic character of picrate ions that not only neutralize the charged (Ag+-cyclam) complexes but also increase a more lipophilic character to the silver-cyclam complex. These effects cause greater extraction of silver ions than the inorganic anions such as NO3 − and ClO4 − can provide (Shamsipur and Khayatian 2001 and Shamsipur et al. 2003; Rounaghi et al. 2008).
Effect of extraction time
Effect of ionic strength
In the extraction methods, due to salting-out effect, the solubility of metal chelates in aqueous solutions decreases with increasing ionic strength. Thus, for this purpose, extraction efficiency was investigated by the addition of varying concentrations of NaNO3 and NaClO4 salts from 0 to 10 %. Increasing the salt concentration had no significant effect on extraction efficiency, perhaps because of the dual effects of salt addition in USAEME: one involves increasing the volume of the extracted phase, which decreases the enrichment factor, and the other is the salting-out effect that increases the enrichment factor (Zeini Jahromi et al. 2007 and Bidari et al. 2007). These two effects cancel each other out; therefore, the enrichment factor is nearly constant by increasing the amount of the salts, and the extraction experiments were carried out without additional salt.
Effect of time and rotational speed of the centrifugation step on extraction
The effect of centrifugation time on extraction of silver ions was evaluated in the range of 3–9 min, while all of the experimental parameters were kept constant. The obtained results showed that absorbance signals increase slowly by increasing the centrifugation time until 5 min and then it remains constant with further increase in centrifugation time (Additional file 1: Figure S2). The effect of centrifugation speed was also investigated in a rotational speed of 1000–3500 rpm (Additional file 1: Figure S3). The results showed that the best rotational speed for extraction is 2500 rpm. Therefore, a centrifugation time of 5 min at a rotational speed of 2500 rpm was chosen for further experiments.
Effect of interfering ions
Tolerant limits of coexisting ions for the extraction of 0.5 μg mL−1 silver ions
Tolerance limit ion ratio
96 ± 3
95.1 ± 2.2
95.4 ± 2.5
97.1 ± 3.4
96.0 ± 1.5
95.4 ± 2.3
97.32 ± 2.4
96.3 ± 3.3
95.0 ± 2.3
94.0 ± 3.5
98.8 ± 1.4
95.35 ± 2.5
95.76 ± 3.4
95.2 ± 3.5
95.0 ± 4.3
100.4 ± 1.3
95.06 ± 3.2
Analytical performance and method validation
The analytical characteristics of the recommended method, including linear range, limit of detection (LOD), limit of quantification (LOQ), relative standard deviation (RSD), correlation coefficient (R 2), and enrichment factor, were obtained. Under the optimum experimental conditions, the analytical curve was achieved by analyzing 5.0 mL of silver ion standard solution containing a known amount of target ion in the range of 0.01–2.5 μg mL−1. The analytical curve was linear in the range of 0.055–1.5 μg mL−1 with a correlation coefficient (R 2) of 0.997. The regression equation was A = 0.853C − 0.019, where A is the absorbance and C is the concentration of silver in micrograms per milliliter. The limit of detection (n = 10, LOD = 3 σ blank/m) and limit of quantification (n = 10, LOQ = 10 σ blank/m), where m is the slope of the analytical curve in accordance to IUPAC recommendation, were obtained 6.79 and 22.77 ng mL−1, respectively. The RSD for 15 replicate measurements of 0.5 mg L−1 of silver ions was 5.50, and the recovery of extraction calculated according to (C org V org)/(C aq V aq) × 100, where C org and C aq are the concentrations of silver in the organic phase and sample solution, respectively, and V org and V aq are the volumes of the organic and the sample solutions, respectively, was about 98.5 %.
The application of the USAEME method for determination silver ions in water and radiological film samples
Added (μg mL-1)
Found (μg mL-1)
0.0943 ± 0.012b
0.575 ± 0.018
1.103 ± 0.016
Comparison with other methods
Comparison of the published preconcentration methods for silver ion with the recommended method
Linear range (μg L−1)
Detection limit (μg L−1)
(Campagong and Honjo 2002)
(Shemirani et al. 2007)
(Kocúrova et al. 2011)
(Tuzen and Soylak 2009)
On the other hand, the USAEME method has many advantages over the other methods in Table 3, such as it uses little toxic organic material and it is faster than LLE methods. Compared to CPE, this method is faster and does not need heating and cooling of sample solution, which is a tedious process in the CPE. Compared to DLLME, the USAEME method does not need to use a third solvent (disperser solvent) for emulsification, which usually decreases the partition coefficient of the analytes into the extractant solvent (Sarafraz-Yazdi and Amiri 2010). Compared to SPE, the organic phase used for extraction is small (200 μL), cheap, and more available than SPE sorbents that are more expensive than solvents used in the liquid extraction.
The use of ultrasound as a powerful energy for the microextraction and determination of silver was proposed. The main advantages of the method are as follows: minimum use of toxic organic solvent and wide linear dynamic range. In addition to these advantages, USAEME is simple, easy to use, inexpensive, and environmentally friendly.
cloud point extraction
limit of detection
limit of quantification.
ultrasound-assisted emulsification microextraction
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