Elsevier

Talanta

Volume 129, 1 November 2014, Pages 254-262
Talanta

Generation of volatile copper species after in situ ionic liquid formation dispersive liquid–liquid microextraction prior to atomic absorption spectrometric detection

https://doi.org/10.1016/j.talanta.2014.05.048Get rights and content

Highlights

  • A new procedure for copper determination was proposed.

  • The effects of ionic liquids on the generation of volatile Cu species were studied.

  • An extraction with in situ formed ionic liquid was used.

  • The determination by chemical and UV-induced CVG AAS was optimized.

  • An application to copper determination in complex matrices was shown.

Abstract

The new procedure using in situ synthesis of ionic liquid extractant for dispersive liquid–liquid microextraction (in situ IL DLLME) combined with generation of volatile species prior to electrothermal atomic absorption spectrometry (ET AAS) for the determination of copper in soil samples was developed. Analytical signals were obtained without the back-extraction of copper from the IL phase prior to its determination. Under optimal conditions, the extraction in 10 mL of sample solution employing 8 μL of 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide (HmimNTf2) (as the extraction solvent) was conducted. The ionic liquid served as two-task reagent: the efficient extractant and enhancement substance for generation step. The chemical generation of volatile species was performed by reduction of acidified copper solution (HCl 0.8 mol L−1) with NaBH4 (1.5%). Some essential parameters of the chemical generation such as NaBH4 and HCl concentrations, the kind and concentration of ionic liquid, carrier gas (Ar) flow rate, reaction and trapping time as well as pyrolysis and atomization temperatures were studied. For photogeneration the effect of the parameters such as the kind and concentration of low molecular weight organic acids and ionic liquid, carrier gas (Ar) flow rate, UV irradiation and ultrasonication time on the analytical signals were studied. The detection limit was found as 1.8 ng mL−1 and the relative standard deviation (RSD) for seven replicate measurements of 100 µg mL−1 in sample solution was 7%. The accuracy of the proposed method was evaluated by analysis of the certified reference materials. The measured copper contents in the reference materials were in satisfactory agreement with the certified values. The method was successfully applied to analysis of the soil and sediment samples.

Introduction

Copper is known as a necessary micronutrient for plants, animals and humans [1]. It is a vital part of several enzymes (for example, ferroxidases, cytochrome c oxidase, superoxide dismutase, tyrosinase, lysyl oxidase, and dopamine beta hydroxylase) [2], [3].

In human populations absorption of copper is highly variable and dependent upon the age, diet, metal amount ingested, its chemical form and the composition of other dietary components such as zinc, as well as the environmental matrix in which it is contained. Ingestion of a large amount of copper salts causes gastrointestinal disturbances, the first symptom to occur is nausea, which can be observed from approximately 4 mg L−1 of copper in drinking water. In severe cases, harmful effects especially hemolysis, liver and kidney damage, can occur [2], [3].

In different types of uncontaminated soil the global copper contents are reported to range between 2 and 250 mg kg−1 with an average value of 30–40 mg kg−1 [2], [4]. Generally copper is accumulated in the upper few centimeters of soil, but it also has tendency to be adsorbed by organic compounds, carbonates, clay minerals and oxyhydroxidies of manganese and iron in deeper soil layers [4], [5]. Contents of copper are closely associated with soil texture and usually are the lowest in light sandy soils and the highest in loamy soils. The concentration of copper in plant tissues seems to be a function of its level in soil and the nutrient solution. The pattern of this relationship, however, differs among plant species and plant parts [4].

Contamination of soil by copper compounds results from utilization of Cu-containing material such as fertilizers, sprays, and agricultural or municipal wastes as well as industrial emissions [4]. Anthropogenic sources of copper come mainly from mining and smelting operations [1]. Soils that are situated in the neighborhood of non-ferrous metal smelters exhibit considerable amounts of heavy metals [6], [7]. In the case of copper mills, the environment is usually contaminated, primarily by high quantities of copper and lead [4], [8].

The amount of accumulated metals, mainly in top layers of the soil, is dependent upon the distance from shaft furnace chimneys, wind direction and physico-chemical properties of soil. The presence of high copper and lead concentrations in soils poses an increased hazard of their migration into ground and underground water, thus transferring into the biomass of plants growing in these soils [8].

For the routine determination of copper in water, wastewater, acid-digested materials (soil, sediments, plants and animal tissues) and soil–water extracts [1] atomic absorption spectrometry (FAAS, GF AAS) and inductively coupled plasma optical emission spectrometry (ICP-OES) are widely used.

Atomic absorption and emission spectrometry with chemical vapor generation (CVG) of volatile species of copper are also utilized for detection [9], [10], [11], [12], [13], [14], [15], [16]. Recently, many attempts have been made to explore the enhancement reagents in order to improve efficiency of the generation of volatile copper species. Several reagents have been found to enhance the CVG of this metal, such as 1,10-phenanthroline [12], [16], room temperature ionic liquids (RTILs) [15] and organic acids as the reaction medium [10]. For GF AAS in situ furnace trapping procedure is particularly attractive for the determination of very low concentrations of elements that form volatile species [17]. Preconcentration is easily achieved, the generation of volatile analytes and their introduction into an atomizer offers many significant advantages over conventional solution phase pneumatic nebulization of samples [11].

The disadvantages of CVG such as the requirement for preparation of chemical reductants (NaBH4, SnCl2), relatively large waste production, the possibility of contamination from reagents, and large amounts of gaseous byproducts result in the attempt to find new procedures for volatile species generation. For this purpose research into the use of UV radiation [18], [19], [20], [21] and ultrasound energy [22], [23] to generate volatile analytes has been undertaken. The use of UV radiation in the presence of low molecular weight organic acids maintains the advantages of vapor generation while eliminating the need for the utilization of chemical reductants, further simplifying the system [24]. The method (named photochemical or UV-induced generation) is, therefore, a “greener” method than “traditional” CVG for the determination of many metals.

For copper extraction from analytical samples several varieties of sample preparation techniques such as solid-phase extraction [25], [26] and dispersive liquid–liquid microextraction (DLLME) [27], [28], [29], [30] have been developed. The DLLME technique shortens sample preparation and can be used for metal and organic analytes [31]. In this technique mostly volatile organic compounds (VOCs) are used as extraction solvents. Most DLLME procedures require also the use of dispersion solvents (e.g. methanol, acetonitrile) to enable the dissipation of extraction solvents in water samples and the formation of a cloudy solution. Some researchers proposed using ultrasound for better dispersing the extraction solvent [32], [33].

Nowadays, there is a worldwide tendency to use ionic liquids instead of VOCs used in many analytical procedures because ionic liquids are regarded as green solvents. Many extraction methods are now based on the use of ionic liquids. These include single step in-syringe system for liquid microextraction (SSLME) [34], cold induced aggregation microextraction (CIAME) [35], ionic liquid-based headspace microextraction [36], ionic liquid-based single-drop microextraction [37], [38] and dispersive liquid–liquid microextraction [39].

The newest version of DLLME – in situ IL DLLME is an alternative to the former techniques. The extraction solvent (ionic liquid) is formed in situ in a relatively fast chemical reaction within a sample solution and no dispersion solvent is used [40], [41], [42].

This work was undertaken to study the effects of ionic liquids on the generation of volatile copper species. The ionic liquid (1-hexyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl]imide (HmimNTf2)) [43] was used as the extraction solvent and reagent affecting the efficiency of the generation. Optimum conditions for chemical and UV-induced generation procedures in the presence of ionic liquid were investigated.

Section snippets

Instrumentation

All measurements were performed with an AAS 5EA spectrometer (Analytik, Jena, Germany) equipped with deuterium source background correction, a transversely heated graphite atomizer and an MPE5 autosampler. Pyrolytically coated graphite tubes were employed exclusively. Copper hollow cathode lamp (Photron, Victoria, Australia) was used as the radiation source. The software did not include procedure for copper volatile species generation, the conditions were optimized using arsenic procedure for

Results and discussion

The parameters of chemical and UV-induced generation of volatile copper species were optimized to achieve the best analytical performance by investigating each variable in turn with all other variables kept constant. This procedure allowed studying the individual effect of each variable on the analytical signals. In order to investigate the chemical generation of Cu after in situ DLLME the following parameters were studied: HCl concentration in sample solution, NaBH4 concentration, the kind and

Conclusion

in situ synthesis of ionic liquid extractant (HmimNTf2) for dispersive liquid–liquid microextraction combined with generation of volatile species prior to electrothermal atomic absorption spectrometry for the determination of copper in environmental samples was demonstrated. The extraction technique can be employed for isolation and preconcentration of copper from soil and sediment samples. The extraction solvent is formed in situ in relatively fast chemical reaction in a sample solution and no

Acknowledgments

The study was financially supported by the National Science Centre (NCN), Poland (Grant no. UMO-2012/06/A/ST4/00382).

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