Short communicationIsotachophoretic separation of selected imidazolium ionic liquids
Introduction
Room-temperature ionic liquids (RTIL), salts that are liquid at ambient temperature, are normally composed of relatively large organic cations and inorganic or organic anions. Unlike molecular liquids, ionic liquids, being polar solvents, are environmentally benign, nonvolatile, nonflammable; most of them have good solubility in water and are stable in air [1]. In addition, by varying the lengths and branching of alkyl chains of the anionic core and the cationic precursor, it is possible to design solvents for specific applications. Because of these characteristic properties, ionic liquids are widely used as new solvent media in heterogeneous catalysis, organic synthesis, electrochemistry, sensors, battery applications, analysis and separation [1], [2]. Typical ionic liquids consist of an organic cation with delocalized charges and a small anion, most often weakly coordinating fluoroanions such as BF4 or PF6 [3]. Table 1 presents primary structures of the room-temperature ionic liquids. Millions of possible cation and anion combinations offer their widely tunable properties with regard to polarity, hydrophobicity and solvent miscibility behavior [3], [4].
Currently, scientists are interested in the applications of ionic liquids in separation and analysis. Reports on ionic liquids used as a modifier of background electrolytes (BGEs) in capillary electrophoresis (CE) appeared frequently [5], [6], [7]. Other scientists systematically studied the stationary phase of gas chromatography based on ionic liquids. Ionic liquids were used as additives in eluents to separate in high performance liquid chromatography (HPLC) [1], [4], [8] and countercurrent chromatography (CCC) [9].
Ionic liquids are more toxic xenobiotics and probably can be rating to persistent organic pollutants (POPs) [10]. From this point of view, analyses of ionic liquids in different parts of ecosystem (water, soils, sediments) are important and necessary. Therefore, the development of new analytical methods for the rapid and reproducible separation and identification of ionic liquids is a prerequisite for future biological and environmental research into these substances. Additionally, ionic liquids can yield photodegradation or biodegradation and analytical control of secondary products of these processes is also necessary.
Ionic liquids are already associated with the term “green” and “green chemistry”, but only because they have got no measurable vapour pressure, it means that they emit no volatile organic compounds (VOCs). Knowledge about their toxicological and ecotoxicological properties is deficient in spite of great importance from ecological point of view.
Isotachophoresis (ITP) with direct conductivity detection is useful and powerful method for high-sensitive determination of both anionic and cationic species in environmental analysis of waters, wastewaters, sewage sludges and composts [11], [12], [13], [14], [15], [16], [17], [18], [19], [20] and can be successfully used in ionic species (eg. ionic liquids) analysis.
The main aim of this study is to demonstrate the possibility of using a new isotachophoretic method to determination of selected room-temperature ionic liquids. The effects of the molar mass on different ionic liquids in the separation was observed. The methods quantitative analytical performance was satisfying and basic validation parameters were assigned.
Section snippets
Instrumentation
An isotachophoretic analyzer EA 102 (Villa Labeco, Spisska Nova Ves, Slovakia) was used. This analyzer is provided with two FEP (fluoroethylenepropylene polymer) columns (a pre-separation one: 0.8 mm × 90 mm and an analytical one: 0.3 mm × 160 mm), two contact conductometric detectors, a sample loop 30 μL. Driving current in detection step was 50 μA in analytical column on both modes (anionic and cationic).
The isotachophoretic data were acquired and processed by an ITPPro 32 software (KasComp,
Results and discussion
Table 2 presents basic physicochemical properties of selected ionic liquids used in isotachophoretic experiments [21]. Five from them have got the same anion (BF4) and different homologous imidazolium cations:
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PMIM (1-n-propyl-3-methyl-imidazolium cation),
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BMIM (1-butyl-3-methyl-imidazolium cation),
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AMIM (1-n-amyl-3-methyl-imidazolium cation),
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HMIM (1-n-hexyl-3-methyl-imidazolium cation),
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OMIM (1-n-octyl-3-methyl-imidazolium cation).
Additionally, in
Concluding remarks
The presented results allow to state that the isotachophoresis offers a very promising alternative to the chromatographic methods for analysis of the ionic liquids. Possibility of determination of cationic and anionic core of room-temperature ionic liquids in reasonable time (up to 20 min in cationic mode) is the main advantage of ITP in this type of analysis.
In water or wastewater samples no sample pre-treatment or only filtration is required before analysis. In analysis of solid samples
Acknowledgements
This work was supported by a grant No. 3 T09D 035 29 and 2 P04G 083 29 from the Ministry of Education and Science (Warsaw, Poland).
The support of Dr. Piotr Stepnowski (University of Gdańsk, Poland) for this study is gratefully acknowledged, especially for samples of ionic liquids.
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