Supramolecular solvent microextraction of gold prior to its determination by microsample injection system coupled with flame atomic absorption spectrometry

Abstract

A supramolecular solvent based liquid–liquid microextraction (SsLLME) system for gold was developed prior to its determination by a microsample injection system coupled with flame atomic absorption spectrometric (MS-FAAS). 1,3,4-Thiadiazole-2,5-dithiol was used as a complexing agent to obtain a hydrophobic complex. The analytical factors affecting the microextraction efficiency, such as the pH, type and volume of supramolecular solvent, amount of complexing agent, ultrasonication and centrifuge time, and sample volume, were investigated. The limit of detection (LOD), enhancement factor (EF) and relative standard deviation (RSD) of the method were 1.5 μg l⁻¹, 51 and 4.2%, respectively. The accuracy of the method was checked by the analysis of CDN-GS-3D Gold Ore certified reference material and addition-recovery tests. The method was successfully applied for the determination of gold in environmental samples.

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1. Introduction

Gold is the most important noble metal because its uses in jewelry, pharmaceutical applications, and industry.^1–4 The concentrations of gold in real samples like environmental and pharmaceutical materials are usually below the detection limit of flame atomic absorption spectrometry, and it cannot be directly determined because of insufficient sensitivity and matrix interference effects.^1,5,6 Therefore, under these circumstances, a simple, sensitive and selective separation and preconcentration procedure for the determination of trace gold in real samples is required prior to flame atomic absorption spectrometric analysis.^1,2,5–7 Various separation and enrichment methods, including solid phase extraction (SPE),^1,8 cloud point extraction,^9,10 liquid–liquid extraction,¹¹ and coprecipitation¹² have been developed by the scientists. However, these methods have certain disadvantages like the high consumption of toxic organic solvents, generation of secondary toxic wastes, difficult and tedious operation and longer time consumptions.^13–17 To eliminate these disadvantages, small-scale preconcentration-separation methods called microextractions, which include dispersive liquid–liquid microextractions,^13,14 hollow fiber liquid phase microextractions,¹⁸ solid phase microextractions¹⁷ and ultrasound-assisted emulsification of solidified floating organic drop microextractions,¹⁹ have been developed for the preconcentration and separation of gold from various media.

Supramolecular solvent based liquid–liquid microextraction (SsLLME) is a new microextraction technique that has been developed for the preconcentration and separation of organic and inorganic species.^20–24 Supramolecular solvents (Ss) are obtained from amphiphile solutions by two well-defined self-assembly global processes. The processes occur on both nano and molecular scales, and are influenced by external effects like the pH, temperature and electrolyte concentration of the sample, and the type and volume of solvent. First, the amphiphiles aggregate to obtain supramolecular assemblies, like reverse micelles or vesicles, in a homogenous solution, and then form coacervates in the second step to give a water-immiscible phase, which is separated from the bulk solution.^20–24 In microextraction studies, the interactions between the analytes in the water phase and the extraction phase are important to increase the extraction efficiency.^21,25 Therefore, the selection of a suitable extraction medium is an important step. Supramolecular solvents have a large interaction area and can interact with organic analytes and metal–ligand complexes through hydrophobic interactions, dispersion forces and hydrogen bonding.^20–24 The interaction areas in supramolecular assemblies provide a high extraction capability and a short extraction time. The preparation of supramolecular solvents at room temperature using the conventional, cheap and harmless chemicals is the most important advantage of their application.^20–24 The supramolecular solvent led to the formation of reverse micelles of 1-decanol in the nano- and microscale regimes that were dispersed in a continuous phase of tetrahydrofuran (THF)–water. The fundamentals of the supramolecular solvent based microextraction method are the hydrophobic and π-cation interactions, and the formation of hydrogen bonds between analyte and supramolecular solvent phase. The hydrophobic character and water immiscibility of certain supramolecular solvents allow their usage in the solvent extraction of hydrophobic compounds.

In the present study, the supramolecular solvent based liquid–liquid microextraction method (SsLLME) for the preconcentration and separation of gold in environmental samples was developed prior to its determination by microsample injection-flame atomic absorption spectrometry.

2. Experimental

2.1. Chemicals and solutions

All the solutions were prepared using reverse osmosis purified water (18.2 MΩ cm, Millipore). Unless otherwise stated, all the chemicals used were of analytical reagent grade, and were used without further purification. The extraction solvents (1-decanol and undecanol) and THF were purchased from Merck (Darmstadt, Germany). Decanoic acid was purchased from Sigma-Aldrich (St. Louis, MO, USA). Concentrated (36% (v/v)) HCl (Merck, Darmstadt, Germany) and 65% HNO₃ (E. Merck, Darmstadt, Germany) were used.

The model solutions of gold(III) were obtained using standard solutions prepared by dilution from 1000 mg l⁻¹ gold(III) stock solution (E. Merck, Darmstadt, Germany). Gold Ore CDN-GS-3D certified reference material (CDN Resource Laboratories Ltd. Canada) was also used. A solution of 0.1% (m/v) of 1,3,4-thiadiazole-2,5-dithiol (Aldrich, USA) was daily prepared in ethanol. To adjust the pH of the sample solutions, buffer solutions were prepared using the combinations of salts and solutions as follows: phosphate buffer solution (pH 2.0–4.0, sodium dihydrogen phosphate/phosphoric acid), acetate buffer solution (pH 5.0–6.0 ammonium acetate/acetic acid), phosphate buffer solution (pH 7.0–7.5 sodium dihydrogen phosphate/disodium hydrogen phosphate).

2.2. Instrumental

A Perkin-Elmer Model 3110 flame atomic absorption spectrometer (Norwalk, CT, USA) including a gold hollow cathode lamp (operating conditions as follows: wavelength 242.8 nm, spectral band width: 0.7 nm and lamp current: 15.0 mA) was used. The air–acetylene flame was used for the absorbance measurements. The continuous aspiration mode was used for the determination of the concentration of the extracted gold in the final volume. The samples were introduced into the nebulizer of the FAAS by using a home made micro-injection unit, including a Teflon funnel connected to the nebulizer by capillary tubing.^25,26 100 μl of samples was injected into the micro-injection unit using an Eppendorf pipette and the peak heights were measured as the signals. In order to produce a supramolecular solvent, an ultrasonic water bath (Norwalk, CT, USA) was used. A Sartorius PT-10 model pH meter with a glass electrode was used for the pH adjustments of the sample solutions (Sartorius Co., Goettingen, Germany). Centrifugations were performed using an ALC PK 120 model centrifuge (Buckinghamshire, England).

2.3. Supramolecular solvent based liquid–liquid microextraction procedure

A graphical diagram of the SLSDE-ILDLLME protocol is shown in Fig. 1. 10 ml of a sample solution containing 0.5 μg Au(III), 0.05 mg 1,3,4-thiadiazole-2,5-dithiol and 2 ml of pH 6 buffer solution was placed into a 50 ml polypropylene centrifuge tube. Then, the extraction solution, which was prepared by mixing 50 μL 1-decanol and 200 μL THF, was injected in to the sample solution and the tube was capped. The mixture was kept in an ultrasonic bath for 3 min, and the supramolecular solvent made up of 1-decanol dispersed in THF–water was spontaneously formed. The obtained mixture was centrifuged at 4000 rpm for 3 min to accelerate the complete separation of the water and extraction phase. The supramolecular solvent phase was situated on the top of the water phase because of its lower density than water; it was taken using micropipette in a container and its volume was completed to 250 μl with ethanol.

Fig. 1 Graphical representation of the supramolecular solvent based liquid–liquid microextraction method (SsLLME).

2.4. Analysis of real samples

The supramolecular solvent based liquid–liquid microextraction procedure was applied to gold ore samples and rock samples obtained from the different cities of Turkey (Ordu, Samsun, Nigde and Erzincan). The ore and rock samples were homogenized with an agate homogenizer and dried at 80 °C for 24 hours.

Ore (0.1 g), rock samples and 0.15 g of certified reference material CDN-GS-3D Gold Ore were weighed, placed into a 100 ml beaker, and digested with 30 ml of aqua regia at 95 °C, until their semidried masses were obtained. The resulting mixture was then mixed with 30 ml of aqua regia, which was evaporated almost to dryness. 5 ml of purified water was added to the residue. The suspension was filtered through a blue band filter paper (Macherey-Nagel, Düren, Germany), and the insoluble part was washed with purified water. Then, the SsLLME procedure given in the section 2.3 was applied to the samples.

The suggested method was also applied to tap water from Canakkale and well water from Sivas, Turkey. The water samples were filtered through a Millipore® cellulose membrane filter (0.45 μm pore size) and the suggested SsLLME procedure (Section 2.3) was applied to the 7.0 ml of water samples.

3. Results and discussion

In order to obtain quantitative recoveries of gold(III), the microextraction procedure was optimized for various analytical parameters. The conditions for the preconcentration of gold ions were selected using the model solutions.

3.1. Influence of pH

The pH of the working media is the most critical parameter in the supramolecular solvent based liquid phase microextraction studies to obtain the supramolecular solvent and for the subsequent extraction of the metal–ligand complex.²⁷ The effect of the pH of the sample solution on the extraction efficiency of gold(III) was investigated in the pH range 2.0–7.5. The results given in Fig. 2 reveal that the best recovery can be achieved when the pH of sample solution was adjusted to 6.0 because of the neutral character of Au(III)-1,3,4-thiadiazole-2,5-dithiol complex in this interval and the type of interactions governing its solubilisation in the supramolecular-solvent phase. Therefore, a pH of 6.0 was chosen as the working pH for the subsequent experiments.

Fig. 2 Effect of the pH on the extraction efficiency of Au(III) (N = 3, 1-decanol volume = 100 μL, THF volume = 600 μL, amount of complexing agent = 0.1 mg).

3.2. Type of supramolecular solvent

The high extraction capability of the supramolecular solvent was a consequence of both the power of the analyte–extractant interactions and the special structure of their aggregates.^21,28 To obtain the best supramolecular solvent phase, three supramolecular solvent including decanoic acid–THF, 1-decanol–THF and undecanol–THF were used in this study. The % recovery values of gold(III) using 1-decanol–THF, undecanol–THF and decanoic acid–THF were 100 ± 2, 53 ± 0 and 63 ± 6, respectively. The 1-decanol–THF supramolecular solvent was used as the extraction phase in further studies.

3.3. Influence of the volume ratio of 1-decanol and THF

The effect of the 1-decanol–THF volume ratio on the recovery of gold was also studied. To study the effect of varying the supramolecular solvent volume on the gold extraction efficiency from a sample solution, the volume of 1-decanol was varied between 50 and 300 μL and the volume of THF was kept constant at 600 μL. A volume of 50 μL of 1-decanol was adequate for the quantitative recovery of gold. The effect of the volume of THF on the extraction recovery was also studied. Different volumes of THF in the range of 100–600 μL with the addition of 50 μL of 1-decanol were examined. The results illustrated in Fig. 3 reveal that the recoveries stayed quantitative between 200–400 μL, and thus 200 μL of THF was selected as the optimum volume.

Fig. 3 Effect of the volume of THF on the extraction efficiency of Au(III) (N = 3, pH = 6.0, 1-decanol volume = 50 μL, amount of complexing agent = 0.1 mg).

3.4. Effect of the amount of the complexing agent

The extraction efficiency depends on the hydrophobicity of the complexing agent, which influences the solubility of the metal complex in the supramolecular solvent phase.²¹ For this purpose, 1,3,4-thiadiazole-2,5-dithiol was used as the complexing agent because of the highly hydrophobic nature of its gold complex. The amount of the complexing agent has a critical effect on the quantitative recovery of the analytes in the liquid phase microextraction.²⁹ Thus, it is highly important to establish the minimal amount of the complexing agent required for the total complex formation with the highest recovery. The effect of the amount of 3,4-thiadiazole-2,5-dithiol was also investigated in the range of 0.0–0.5 mg. The results are depicted in Fig. 4. An amount of 0.05 mg of 3,4-thiadiazole-2,5-dithiol was selected for further experiments.

Fig. 4 Effect of the amount of 1,3,4-thiadiazole-2,5-dithiol on the extraction efficiency of Au(III) (N = 3, pH = 6.0, 1-decanol volume = 50 μL, THF volume = 200 μL).

3.5. Effect of ultrasonication and centrifuge time

It is known that ultrasonic radiation is an important way to improve the kinetics and the extraction efficiency. The ultrasonic radiation increases the interactions between the analyte in the sample solution and extraction phase for an increased mass transfer.^30,31 In this study, an ultrasonic water bath was used for the formation of a supramolecular solvent, which was composed of reverse micelles of 1-decanol that are dispersed in THF–water, and the extraction of Au(III)-1,3,4-thiadiazole-2,5-dithiol complex. The effect of the ultrasonication time on the extraction efficiency was examined in the range of 2–4 minutes. 3 minutes of ultrasonication time was found to be sufficient for the quantitative recovery. To spend the minimum amount of time, 3 minutes of ultrasonication time was used for further work.

The influence of the centrifugation time on the extraction efficiency of Au(III) in the developed procedure was examined at 4000 rpm between 1 and 9 min. Quantitative recovery was obtained within 3 minutes of centrifuging time.

3.6. Study of interferences

The effects of matrix ions on the extraction efficiency and on the selectivity of the developed method were also investigated by using the optimum conditions. A 10 ml model solution, which included 0.5 μg of Au(III) and different amounts of other ions, was prepared and the suggested extraction method was applied to the solution. The obtained results are shown in Table 1. The tolerance limits of the matrix ions were determined to be the values that caused a deviation of more than ±5% in the recovery. Under the optimum conditions, no interferences were observed from most of the ions tested except for Fe³⁺, Cu²⁺, Ni²⁺ and Mn²⁺ ions. In order to eliminate the interference effects of the ions, EDTA, NH₄F, KSCN, ascorbic acid and citric acid were used. In the presence of KSCN, ascorbic acid and citric acid, the extraction of gold(III) was not possible. However, the interference effects of the ions can be eliminated by using a mixture of 25 mg of EDTA and 75 mg of NH₄F as the masking agent.³

Table 1 Effect of some matrix ions on the extraction efficiency of Au(III) (N = 3)

Ion	Added as	Concentration, mg l^−1	Recovery, %
a Masked with 75 mg NH4F.b Masked with 25 mg EDTA.
Na^+	NaNO3	50 000	97 ± 5
K^+	KCl	50 000	99 ± 5
Mg^2+	Mg(NO3)2·6H2O	500	103 ± 4
Ca^2+	Ca(NO3)2·4H2O	500	95 ± 5
Zn^2+	Zn(NO3)2·6H2O	5	100 ± 3
Cr^3+	Cr(NO3)3·9H2O	10	96 ± 3
Pd^2+	Pd(NO3)2	2.5	97 ± 1
Co^2+	Co(NO3)2·6H2O	2.5	99 ± 4
Cl^−	NaCl	77 000	97 ± 5
Fe^3+	Fe(NO3)3·9H2O	100^a	95 ± 0
Cu^2+	Cu(NO3)2·3H2O	50^b	99 ± 2
Mn^2+	Mn(NO3)2·4H2O	50^b	98 ± 0
Ni^2+	Ni(NO3)2·6H2O	10^b	100 ± 1

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3.7. Effect of the sample volume

The effect of the sample volume on the extraction of Au(III) was investigated using 10–40 ml of model solution keeping the other conditions same. It was found that the recovery of gold was quantitative up to 15 ml of the sample solution. Thus, a preconcentration factor of 60 was obtained using a final volume of 0.25 ml.

3.8. Analytical figures of merit

The analytical features of the developed method were evaluated under the optimum conditions. The whole preconcentration procedure took about 10 min, and it could simultaneously treat as many samples as the centrifuge capacity. A calibration curve was constructed using the equation A = 4 × 10⁻³ + 0.834C with a correlation coefficient of R² = 0.996, where A is the absorbance and C is the gold concentration in μg l⁻¹. The limit of detection (LOD), which is defined as C_LOD = 3S_d/m (where C_LOD, S_d and m are the limit of detection, the standard deviation of the eleven blank solutions, and the slope of the calibration graph, respectively), was 1.5 μg l⁻¹. The limit of quantification (LOQ) was found to be 4.95 μg l⁻¹, as calculated from the ratio of ten times the standard deviation of the eleven blank solutions to the slope of the calibration curve.

The preconcentration factor (PF), defined as the ratio of the model solution to the final volume, was 60. The enhancement factor (EF), defined as the ratio of the slope of the calibration curve after and before preconcentration, was 51. The relative standard deviation (RSD), obtained from 10 replicates of the microextraction procedure of 10 ml solution containing 50 μg l⁻¹ Au(III), was 4.2%.

The consumptive index is the volume of samples necessary to attain one unit of the preconcentration factor. The consumptive index (CI) can be found for practical purposes as CI = V_s/EF where V_s is the volume of the sample solution (ml) consumed to achieve the EF value.³² The CI was found to be 0.29.

3.9. Applications of the method

To prove the accuracy and applicability of the developed microextraction method, it was applied for the determination of the amount of gold in the CDN-GS-3D Gold Ore certified reference material. The obtained results for the reference material shown in Table 2 agreed with the certified values.

Table 2 The analysis results of CDN-GS-3D Gold ore certified reference material (N = 5)

Certified value (μg g^−1)	Found (μg g^−1)	Recovery, %
a Mean ± standard deviation.
3.41 ± 0.25	3.38 ± 0.2^a	99

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The accuracy of the suggested method was also verified by spiking various amounts of gold(III) in water and ore samples (Table 3). A good agreement was obtained between the added and recovered amounts of gold. All the results obtained show that the suggested microextraction method was confidential and matrix independent for the determination of gold in a wide range of samples. The present SsLLME procedure was applied to the determination of gold in ore and rock samples obtained from Nigde and Erzincan (Table 4).

Table 3 Addition and recovery tests for the microextraction of gold(III) in water and ore samples (N = 5)

	Added, μg	Found, μg	Recovery, %
a LOQ: limit of quantification.b Mean ± standard deviation.
Tap water from Canakkale	0.0	<LOQ^a	—
	2.0	2.08 ± 0.00^b	104
	4.0	4.04 ± 0.18	100
Well water from Sivas	0.0	<LOQ	—
	0.75	0.77 ± 0.05	102
	1.5	1.47 ± 0.08	98
Ore sample from Ordu	0.0	<LOQ	—
	0.75	0.78 ± 0.07	103
	1.5	1.48 ± 0.07	99
Ore sample from Samsun	0.0	<LOQ	—
	1.0	1.04 ± 0.03	104
	2.0	1.90 ± 0.03	95
Ore sample from Ordu-2	0.0	<LOQ	—
	0.75	0.72 ± 0.02	96
	1.5	1.51 ± 0.10	100

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Table 4 The application of the proposed method for the analysis of gold in ore and rock samples (N = 5)

Sample	Concentration, μg g^−1
a Mean ± standard deviation.b LOQ: limit of quantification.
Ore sample from Nigde-1	1.73 ± 0.16^a
Ore sample from Nigde-2	3.35 ± 0.25
Rock sample from Erzincan-1	0.35 ± 0.06
Rock sample from Erzincan-2	<LOQ^b
Rock sample from Erzincan-3	<LOQ

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3.10. Comparison with other preconcentration techniques

The SsLLME-MS-FAAS method was compared with other preconcentration methods that are used for the determination of gold in real samples in Table 5. The developed SsLLME-MS-FAAS method is environmentally friendly involving low usage of toxic organic solvents. The comparative data of the different analytical characteristics showed that the low detection limit and high enhancement factor obtained for the proposed microextraction method are much better than those reported for the other preconcentration methods for gold determination in real samples.

Table 5 Comparison of the SsLLME-FAAS with other methods for the determination of gold

Method	LOD (μg l^−1)	PF	Samples	Ref.
a Solid phase extraction-flame atomic absorption spectrometry.b Cloud point extraction-flame atomic absorption spectrometry.c Ultrasound-assisted emulsification of solidified floating organic drop microextraction-flame atomic absorption spectrometry.d Liquid–liquid extraction-UV-VIS spectrometry.e Ionic liquid microextraction-flame atomic absorption spectrometry.f Supramolecular solvent based liquid–liquid microextraction-flame atomic absorption spectrometry.
SPE-FAAS^a	1.61	31	Water, soil, sediment	1
SPE-FAAS^a	16.6	200	Water, soil, ore	2
CPE–FAAS^b	3.8	16	Ore	33
USAE-SFODME–FAAS^c	0.45	34.8	Pharmaceuticals, water	34
LLE-spectrophotometric^d	0.5	200	Water, ore	35
ILME-FAAS^e	3.4	40	Water, soil	7
SsLLME-FAAS^f	1.5	60	Water, ore	This work

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4. Conclusion

This study describes a novel and simple supramolecular solvent based liquid–liquid microextraction method for the preconcentration and separation of trace amounts of gold in ore, water and rock samples prior to its determination by microsample introduction-flame atomic absorption spectrometry (FAAS). The important properties of the developed method are as follows: (I) it offers a simple and rapid alternative to the conventional sample preconcentration methods, (II) it can decrease the consumption of toxic organic solvents and the amount of secondary toxic waste that are produced, and (III) it is free from matrix interferences that are associated with the naturally-occurring gold. The suggested method was successfully applied for the determination of low concentrations of gold in water, rock and ore samples with good accuracy and precision.

Acknowledgements

The authors are grateful for the financial support of the Unit of the Scientific Research Projects of Erciyes University (FBD-2014-4991) (Kayseri, Turkey).

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Footnote

† This study is a part of PhD thesis of Erkan Yilmaz.

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