Ionic liquids in analytical chemistry

Ionic liquids in analytical chemistry

Analytica Chimica Acta 661 (2010) 1–16 Contents lists available at ScienceDirect Analytica Chimica Acta journal homepage: www.elsevier.com/locate/ac...

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Analytica Chimica Acta 661 (2010) 1–16

Contents lists available at ScienceDirect

Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Review

Ionic liquids in analytical chemistry Ping Sun, Daniel W. Armstrong ∗ Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, TX 76019, USA

a r t i c l e

i n f o

Article history: Received 20 October 2009 Received in revised form 4 December 2009 Accepted 9 December 2009 Available online 16 December 2009 Keywords: Ionic liquids Chemical analysis Chromatography Spectroscopy Electrochemistry

a b s t r a c t Ionic liquids (ILs) are composed entirely of ions and they possess fascinating properties, including low volatility, tunable viscosity and miscibility, and electrolytic conductivity, which make ILs unique and useful for many applications in chemical analysis. The dramatic increase in the number of publications on ILs is indicative of the tremendous interest in this field from analytical chemists. This review summarizes recent efforts in the major subdisciplines of analytical chemistry, including extractions, gas chromatography, liquid chromatography, capillary electrophoresis, mass spectrometry, electrochemistry, sensors, and spectroscopy. © 2009 Elsevier B.V. All rights reserved.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Extractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Liquid–liquid extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Liquid phase microextraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Solid-phase microextraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas chromatography (GC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Initial studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Successful IL-based GC stationary phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Other applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Headspace GC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Liquid chromatography (LC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. TLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. LC mobile phase additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. LC stationary phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Capillary electrophoresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Applications in modifying the capillary wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Non-aqueous CE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Micellar capillary electrophoresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Other CE applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mass spectrometry (MS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. MALDI matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Anion detection by ESI-MS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Unique electrochemical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Electrochemical methods for ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1. Electrochemistry-based sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2. Optical sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +1 817 272 0632. E-mail address: [email protected] (D.W. Armstrong). 0003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2009.12.007

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8.3. Sensors based on quartz crystal microbalance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4. Stochastic sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1. IR and Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. Fluorescence spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3. NMR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The term “ionic liquid” (IL) is currently used to describe a broad class of salts which have appreciable liquid ranges. Generally speaking, ILs melt at or below 100 ◦ C. Room temperature ionic liquids (RTILs) describe a subset of ionic liquids that are liquid at room temperature (∼25 ◦ C). The unique properties of these liquids is the direct result of the fact that they are composed entirely of ions, like classical metallic molten salts such as sodium chloride (NaCl). Consequently, ILs may behave quite differently from common molecular liquids when used as solvents. Most RTILs have organic cations (e.g., imidazolium, pyridinium, pyrrolidinium, phosphonium, ammonium). Anions could be inorganic, including Cl− , PF6 − , BF4 − , and more and more current RTILs consist of organic anions, such as trifluoromethylsulfonate [CF3 SO3 ]− , bis[(trifluoromethyl)sulfonyl]imide [(CF3 SO2 )2 N]− (i. e., NTf2 ), trifluoroethanoate [CF3 CO2 ]− , etc. The structures of common cations and anions of ILs are shown in Fig. 1. The relatively large size of one or both ions in ILs and low symmetry account for the lower melting points of these materials [1]. ILs have many fascinating properties including: wide liquid ranges, low volatilities (negligible vapor pressure), good thermal stabilities, electrolytic conductivity, wide range of viscosities, adjustable miscibility, reusability, nonflammability and so on. One important feature of ILs is that varying the cation or anion may significantly affect physical and chemi-

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cal properties. Consequently, combinations of a variety of cations and anions gives a tremendous number of ILs and makes customsynthesis feasible. It is estimated that there could be up to 1018 ionic liquids available [2]. This provides a large pool, from which ILs can be selected for specific applications. These intrinsic features of ILs, combined with their ease of preparation, have resulted in a remarkable increase in their use in academia and industry. ILs have a surprisingly long history. Gabriel and Weiner found ethanolammonium nitrate (m.p. 52–55 ◦ C) in 1888 [3]. The “first” RTIL ethylammonium nitrate [EtNH3 ][NO3 ] with melting point 12 ◦ C was reported in 1914 [4]. A new class of RTILs that consist of dialkylimidazolium chloroaluminate, were reported by Wilkes et al. in 1982 [5]. However, these chloroaluminate ILs did not receive considerable interest due to their reactivity to moisture and many chemicals. The true emergence of ILs as broadly useful solvents occurred with the first development of air- and moisture-stable imidazolium salts in 1992. Wilkes and Zaworotko synthesized stable RTILs containing weakly complexing anions, such as BF4 − [6]. Compared to their extensive use as solvents in organic synthesis, ILs have been used more recently in analytical chemistry. Since the late 1990s, a plethora of papers have been published, which have demonstrated the enormous potential of ILs for chemical analysis. Applications of ILs in analytical chemistry have been reviewed separately [7–11]. Specific applications in separation science also were examined [12–15]. Koel reviewed ILs in chemical

Fig. 1. Structures of common cations and anions of ionic liquids.

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analysis, including extraction, chromatography, spectroscopy and [16]. Poole reviewed the determination of solvent properties of RTILs with chromatographic and spectroscopic methods [17]. An overview of the applications of ILs in analytical chemistry and representative examples are given in this monograph. It is divided into eight sections including: extractions, gas chromatography (GC), liquid chromatography (LC), capillary electrophoresis (CE), mass spectrometry (MS), electrochemistry, sensors, and spectroscopy. It should be noted that ionic liquids are merely starting materials and do not remain liquids in some applications, such as ionic liquids bonded to silica solid support to make SPME fibers and LC stationary phases. In other cases, ionic liquids are used as additives at low concentrations in CE running buffers, LC mobile phases, and as the complexing agents for MS detection of anions. In these cases, ILs just function as other common salts and some of their special physical properties, such as low volatility are no longer important. However, in order to comprehensively discuss all the related topics, these applications also are included in this review. 2. Extractions Sample preparation plays a vital role in sample analysis, and it includes interferent removal, and analyte preconcentration. Extraction is one of the most popular methods of sample preparation. A recently published review discussed applications of RTILs in extractions and related problems [18]. 2.1. Liquid–liquid extraction Liquid–liquid extraction, as one of the most frequent methods for sample pretreatment, is based on the partitioning of the target compound between two immiscible phases. ILs have been widely applied in liquid–liquid extraction of various compounds, such as metal ions, small organic molecules and biological compounds. Generally, a combination of a complexation reagent (such as crown ethers and calixarenes) and an IL was used to extract metal ions from aqueous solutions [19–24]. Dicyclohexyl-18-crown-6 ether dissolved in imidazolium ILs was used to extract Sr2+ [19]. The ability of pyridinocalix[4]arene to extract Ag+ was improved significantly by dissolving it in 1-alkyl-3-methylimidazolium PF6 , instead of chloroform [23]. Hirayama and co-workers reported the extraction of La3+ and divalent metal ions (Cu2+ , Mn2+ , Co2+ ) using 1-alkyl-3-methylimidazolium NTf2 and thenoyltrifluoroacetone [24,25]. Wei et al. first reported that gold nanoparticles and nanorods were efficiently transferred from an aqueous phase to a [BMIM][PF6 ] RTIL (1-butyl-3-methylimidazolium PF6 ) phase without using thiols or amines [26]. The extraction capabilities of RTILs are ascribed to their alcohol-like polarity and salt-like compositions [26]. ILs have been used to extract small organic compounds, such as aromatic and aliphatic hydrocarbons [27,28], acids [29], phenols and amines [30,31]. Three imidazolium-based IL-aggregates were applied to quantitatively extract polycyclic aromatic hydrocarbons (PAHs) [28]. This extraction procedure only took 7 min and provided good reproducibility. The IL/water (Pil/water ) and IL/heptane (Pil/hep ) distribution coefficients of 40 compounds, including organic acids and bases, amino acids, antioxidants, and neutral compounds, were reported [32]. The Pil/water values of ionizable compounds are greatly affected by pH. Visser et al. reported that the partitioning of thymol blue was pH-dependent and could be reversed by using CO2 and NH3 [33]. Recently, ILs have been widely applied in desulfurization [34–37]. Deep desulfurization of diesel fuel using ILs was first reported in 2001 [34]. ILs provide exceptional extraction capability for dibenzothiophene, which is difficult to remove using normal hydrotreating.

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Large biomolecules, such as proteins [38–40] and DNA [41], could be extracted using ILs as extractants. Bovine serum albumin, trypsin, cytochrome C and ␥-globulins were extracted using imidazolium-based ILs [40]. The authors found that hydrophobic interactions played a vital role in partitioning, and electrostatic interactions and salting-out effects also contributed. The extraction of double-stranded DNA using [BMIM][PF6 ] was reported for the first time by Wang et al. [41]. Lower concentrations of DNA (<5 ng L−1 ) were beneficial for quantitative extractions, and other species (such as proteins and metals) did not interfere. The interactions between the [BMIM]+ cation and P–O bond of the DNA phosphate group played an important role in these extractions. This approach is superior to the existing phenol/chloroform method in that it avoids denaturing DNA and the use of harmful solvents. The structure of ILs affected their extraction selectivity and efficiency for various types of compounds, including metal ions [21,22,42], sulfur-containing compounds [37], and proteins [40]. The distribution coefficients for four tested metal cations increase with decreasing the alkyl chain length in 1-alkyl-3methylimidazolium ILs with the exception of the value for [C4 MIM] [21]. Increasing the alkyl chain length of the IL cations was found to improve the extraction efficiency of proteins [40]. The partition ratios of dibenzothiophene from dodecane to ILs were determined using a series of ILs with different cation classes and anions [37]. The partition ratios follow the order: dimethylpyridinium > methylpyridinium > pyridinium ∼ imidazolium ∼ pyrrolidinium. Because the solubility and miscibility of the ILs could be adjusted by elaborately changing the nature and functionality of cation or anion, the approach of custom-designed ILs for extracting target compounds was proposed [43]. 2.2. Liquid phase microextraction It is well known that liquid–liquid extraction has some disadvantages: it is time- and labor-intensive, and it can require using large amounts of organic solvents, which are harmful (as VOCs) for the environment. To solve these problems, microextraction techniques have been proposed, i.e., liquid phase microextraction (LPME) for example. Jiang and co-workers are among the first who applied ILs as extraction solvents in LPME [44–46]. Polycyclic aromatic hydrocarbons were selected as model analytes and 1octyl-3-methylimidazolium PF6 was used in LPME [44]. The authors reported that the direct-immersion mode provided higher enrichment factors for low volatility analytes, while headspace LPME worked better for most volatile PAHs. Cruz-Vera et al. determined anti-inflammatory drugs and phenothiazine derivatives in urine samples by IL-based dynamic LPME [47,48]. ILs also can function as membrane liquids in hollow fiber supported liquid phase microextraction [49,50]. This technique, combined with HPLC, was utilized to determine sulfonamides and chlorophenols in environmental water samples. Single-drop microextraction (SDME) is an approach that evolved from LPME. In SDME, the extraction solvent is just one drop of liquid, suspended on the syringe needle, which is then immersed in the solution or put in the headspace. Due to their low volatility, wide solubility, and good thermal stability, ILs have been widely used in SDME [51–57]. Vidal et al. analyzed chlorobenzenes in water samples by IL-based SDME [53,54]. Aguilera-Herrador et al. reported analysis of some representative pollutants in water samples by IL-SDME directly connected to GC–MS [52]. They designed a removable interface which can effectively desorb analytes from the IL and transfer them to the GC, while preventing the IL from entering the GC–MS. Analytes of various volatility/polarity could be analyzed in the same run because the solvent delay is avoided. For example, dichloromethane gave an exceptionally short retention time and could not be quantitatively analyzed

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when using methanol as the extractant, while accurate quantitation was achieved by this IL method. The same group also used 1-octyl-3-methyl-imidazolium PF6 in SDME to quantitatively analyze trihalomethanes in water samples [55]. Recently, dispersive liquid–liquid microextraction (DLLME), as a modified LPME, has received attention. In this technique, a mixture of extraction solvent and disperser solvent is injected to the aqueous solution containing target analytes. A cloudy solution forms, consisting of fine droplets of extraction solvent dispersed into the aqueous solution. Compared with SDME, the contact surface between extraction solvent and aqueous solution increases significantly, which is beneficial for extraction. DLLME using an ionic liquid, coupled with HPLC was applied to detect insecticides [58,59] and polycyclic aromatic hydrocarbons in water [60]. Pyrethroid pesticides were extracted by DLLME using 1-hexyl-3methylimidazolium PF6 [59]. This method gave higher extraction efficiencies than conventional SDME. A simple DLLME method using 1-hexylpyridinium PF6 as an extractant solvent was developed to preconcentrate zinc in water and milk samples [61]. A new IL-based DLLME approach using a one-step in-syringe set-up was presented and evaluated by the determination of non-steroidal anti-inflammatory drugs in urine samples [62]. Compared to traditional DLLME, this method was easier to perform, because the centrifugation step was avoided. 2.3. Solid-phase microextraction Solid-phase microextraction (SPME) is a fast, solventless alternative to conventional liquid phase extraction [63–67]. It integrates sampling, extraction, concentration and introduction to chromatography into a single solvent-free step. A SPME method using a disposable IL coating was developed to quantitatively analyze benzene, toluene, ethylbenzene, xylene in paints [63]. This coating material has some obvious advantages: lower cost, minimum carryover, and comparable reproducibility to commercial fibers. The partitioning behavior of different types of compounds in IL-aggregates coated on SPME was studied, and monocationic ILs generally provided higher extraction power than dicationic analogues [68]. Polymeric imidazolium-based IL coatings were synthesized and applied to extract esters [64]. This type of coating showed high thermal stability, long lifetimes, and provided good analyte recoveries, comparable to those using polydimethylsiloxane fibers. Recently, two new ILs, which contained styrene units, were used to prepare silica-bonded polymeric SPME adsorbents [65]. These SPME fibers can work successfully in both the headspace mode and immersion mode.

3.2. Successful IL-based GC stationary phases Competitive imidazolium ILs-based GC stationary phases achieved greater success [73]. 1-Butyl-3-methylimidazolium hexafluorophosphate and chloride salts ([BMIM][Cl] and [BMIM][PF6 ]) were coated onto silica capillaries, working as GC stationary phases. The properties of these stationary phases were investigated by testing the interactions with a variety of probe molecules via inverse GC. It is interesting that ILs display unusual dualnature retention behavior, separating both nonpolor and polar compounds. The IL-based column performed the same as the nonpolar column when retaining relatively nonpolar analytes. However, they behaved significantly different when retaining highly polar analytes and proton-donor analytes. These analytes were retained very strongly on the [BMIM][PF6 ] column. Later, Anderson and Armstrong [74] prepared highly stable GC stationary phases based on 1-benzyl-3-methylimidazolium trifluoromethanesulfonate ([BeMIM][TfO]) and 1-(4-methoxyphenyl)3-methylimidazolium trifluoromethanesulfonate ([MPMIM][TfO]). Their properties were studied by the Abraham solvation model equation. In this model, five interaction parameters (r, s, a, b, l) were used to characterize the stationary phase: r defines the ability to interact with ␲ and n electrons of solute; s is a measure of dipolarity/polarizability; a and b define the hydrogen bonding basicity and acidity, respectively; l is the ability of ILs to separate homologs. It was found that these two ILs had very similar hydrogen bond basicities (a), but their hydrogen bond acidities (b) and abilities to interact via ␲ and n electrons (r) were quite different. [BeMIM][TfO] and [MPMIM][TfO] columns could be operated up to 220 ◦ C, 250 ◦ C, respectively. Fig. 2 indicates their thermal bleeding profiles. These stationary phases provided efficient separation of a wide variety of analyte mixtures including alkanes, alcohols, polycyclic aromatic hydrocarbons, and isomeric sulfoxides. In 2005, a high selectivity/high stability GC stationary phase using a cross-linked ionic liquid was developed [75]. It was prepared by cross-linking the ionic liquid stationary phase using a small amount of free radical initiator and it remained stable up to 280 ◦ C. A more highly cross-linked stationary phase could be used at higher temperature ranges (300–400 ◦ C). The cross-linked IL stationary phases preserved the separation capability of the original coated IL phases. More recently, other new types of highly stable GC stationary phases have been developed [76–80]. The stationary phases based on dicationic RTILs functionalized with poly(ethylene glycol) (PEG) linkages, remained stable up to 350 ◦ C [76]. Phospho-

3. Gas chromatography (GC) 3.1. Initial studies The unique properties of ionic liquids (i.e., low volatility, high viscosity, good thermal stability, and variable polarities) make them ideal for GC stationary phases. Research of using molten salts in GC started in the 1950s. Barber et al. [69] prepared GC stationary phases using molten stearates of Mn, Co, Ni, Cu, and Zn, in order to separate alcohols and amines. Later, ethylammonium nitrate and ethylpyridinium bromide GC stationary phases were prepared and tested by Poole and co-workers [70,71]. They performed as polar stationary phases, and dipolar or H-bonding interactions significantly contributed to retention. Other groups prepared GC columns based on tetraalkylphosphonium salts [72]. However, all these columns suffered from narrow liquid range, low column efficiencies, and/or poor thermal stabilities. Hence they were not adapted as materials for GC stationary phases.

Fig. 2. Thermal bleed diagram illustrating the volatilization temperatures of various traditional RTILs and the two new bulky-type ILs: BMIM, 1,3-dibutylimidazolium, (BBIM), BeMIM, BeHIM, MPMIM, BMIM-Cl (∼145 ◦ C), BMIM-TfO (∼175 ◦ C), BMIMPF6 (∼170 ◦ C), BBIM-Cl (∼180 ◦ C), BMIM-NTf2 (∼185 ◦ C), BeHIM-PF6 (∼200 ◦ C), BeMIM-TfO (∼220 ◦ C), and MPMIM-TfO (∼250 ◦ C). Reprint from Ref. [74].

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nium ILs also were used to prepared GC columns with high thermal stability [79]. Unsymmetrical dicationic liquids and trigonal tricationic ILs were synthesized [77,78]. Among them, 18 ILs are stable up to 350 ◦ C [77]. A new generation of GC stationary phases based on trigonal tricationic ionic liquids were developed [80]. The tested ILs contain four types of core structures: mesitylene, benzene, triethylamine and tri(2-hexanamid)ethylamine. Three imidazolium or phosphonium groups are linked to the core structure. Their solvation parameters are different from those of monocationic and dicationic ILs, so these new IL stationary phases provide unique retention and selectivity for a series of complex mixtures. Both the nature of the IL’s cation and anion affect the separation performance of these stationary phases [73,81,82]. Polyvinyloctylimidazolium IL polymers with different anions were studied as GC stationary phases [81]. Columns using the NTf2 anion were the most efficient. Later, it was found the attached side-chains on the vinylimidazolium cations also affected the separations [82]. It should be noted that based on the work of Armstrong et al. IL-based GC stationary phases are now available commercially for the first time from Supelco (Bellefonte, PA). This is the first class of new stationary phases that has become available in decades [83]. They are expected to have a major impact on the fields of GC and GC × GC. 3.3. Other applications Inverse GC is a useful technique to carry out thermodynamic studies of ILs. Enthalpies and entropies of transfer were determined by dissolving eight n-alkanes in six different ILs [84]. The thermodynamic data can be explained by a simple model, which also provides information about interionic distances and possible dielectric constants for ILs. Activity coefficients of various compounds in trimethyl-butylammonium NTf2 and 1-hexadecyl3-methylimidazolium BF4 were determined [85,86]. Solubilities of the gaseous and liquid solutes (CO2 , hydrocarbons, THF, and etc) and thermodynamics of solubilization in ILs were investigated by inverse GC [87]. ILs could work synergistically with other compounds to improve separations on GC stationary phases. The GC columns, based on a geminal dicationic IL, 1,9-di(3-vinylimidazolium)nonane NTf2 separated essential oils from herbal plants [88]. This paper reported two ways of using ILs as a GC stationary phase: a neat and a mixed phase. The mixed stationary phase was prepare by mixing the dicationic IL, the monocationic IL (1-vinyl-3-nonylimidazolium bis[(trifluoromethyl)sulfonyl]imidate, and traditional polysiloxane. The mixed stationary phase provided a much better selectivity for complex samples than the neat stationary phases. It was found that 74 compounds were identified from the cinnamon essential oil on the mixed column, compared to 65 compounds on two commercial columns. ILs and fullerenes can work synergistically as GC stationary phases [89]. The addition of fullerene to ILs greatly improved the peak efficiency for both nonpolar and polar analytes. It was reported recently that many different types of isotopic compounds were separated on the cavitand-based GC stationary phase, in which ILs were used as solvents to dissolve cavitands and coat them onto the columns [90]. Applications of ILs in 2D-GC have drawn greater attention recently [83,91–93]. The potential impact of ILs used as stationary phases in 2D-GC and GC has been summarized recently [83]. Complex mixtures were separated within a few minutes using the novel dual-column GC [91]. The dual-column set-up is composed of an immobilized cross-linked IL column and a commercial coated Rtx-1 column. A band trajectory model which relates band position with analysis time helped to determine pulse application times. This dual-column connected with DMS achieved separation of 13 components found in the headspace above U.S. currency. Reid et

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al developed 2D-GC with the second column coated with a trifluoromethylsulfonate ionic liquid thin film [92]. The new 2D-GC gave a better separation for a mixture of 32 compounds than using a PEG commercial column in the second dimension. Enantiomeric separations using ionic liquids have been achieved in two ways: a conventional chiral selector was dissolved in an achiral IL to prepare GC chiral stationary phase; or chiral ILs could directly function as chiral selectors of stationary phases. The first way was realized by dissolving cyclodextrins in 1-butyl-3-methylimidazolium chloride [94]. The new stationary phases separated fewer compounds than the respective commercial cyclodextrin columns, since the imidazolium ion pair of ILs could form inclusion complex with the cyclodextrin cavity and block the interaction sites with chiral analytes. This complex also is supported by the fact that efficiency of the IL-CD columns has been significantly improved (up to 10-fold). Ding et al. [95] synthesized enantiopure N,N-dimethylephedrinium-based ionic liquids and prepared the first chiral IL-based GLC stationary phases. Fourteen pairs of enantiomers (including alcohols, sulfoxides, epoxides, and acetylated amines) were separated. The separation mechanism also was studied by evaluating the enantiomeric- and diastereomericIL columns. Ren et al. [96] developed a new chiral stationary phase using the mixture of a chiral IL (hydroxybutyltrimethylammonium salt) and cellulose tris(3,5-dimethylphenyl carbamate). The new column separated 16 pairs of enantiomers out of 22. 3.4. Headspace GC Headspace GC avoids direct liquid or solid probing and greatly decreases matrix interference. One acidic, one basic, and one neutral compound were dissolved in different ILs with appropriate acidity and basicity [97]. They were detected by headspace GC and their detection limits were in the low ppm level. Residual solvents (acetonitrile, dichloromethane, N-methyl-2-pyrrolidone, toluene, DMF, and n-butyl ether) in pharmaceuticals were determined by headspace GC using 1-butyl-3-methylimidazolium BF4 ([BMIM][BF4 ]) as the solvent [98]. These analytes have relative low volatility (B.P.s >150 ◦ C), so higher temperature headspace procedure is necessary. Due to its good thermal stability, [BMIM][BF4 ] worked, exceeding well and gave better sensitivity than DMSO as the solvent. 4. Liquid chromatography (LC) 4.1. TLC Most current HPLC and TLC stationary phases use silica gel as the solid support. It has a common problem: the silica surface has residual acidic silanol groups, which have deleterious effects on separations. For example, unsymmetric (tailing), broad peaks are observed, especially for basic analytes. Therefore, amine additives are often added to block the acidic surface and ameliorate these effects. ILs, with proton acceptor properties, provide the potential for this application. Kaliszan et al. [99] first reported that IL additives in the mobile phase suppressed free silanol effects on the retention of basic drug compounds in TLC. Eight basic compounds were not moved from the application spot either on the bare silica or on the octadecylsilica plates using acetonitrile eluent. Traditional amine additives (triethylamine, dimethyloctylamine, and ammonia) could not completely suppress the effect of free silanols. The tested ILs were imidazolium BF4 types (1-ethyl-3-methylimidazolium [EMIM], 1-methyl-3hexylimidazolium, and 1-hexyl-3-heptyloxymethylimidazolium tetrafluoroborate). ILs decreased the retention of basic analytes more effectively than other alkylamines. It was reported that

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[EMIM][BF4 ] was added to the eluent in order to optimize the separation of peptides on TLC [100]. 4.2. LC mobile phase additives Addition of ILs to the mobile phase in column-based LC separations could be traced to a publication in 1986 [101], which reported ILs used as organic modifiers. Alkylammonium nitrate or thiocyanate salt was mixed with another solvent of low viscosity and used as the mobile phase. Later, the same group conducted detailed studies on the solvent properties of six ILs used in microcolumn RPLC [102]. They found that the solvent selectivities were controlled by proton acceptor–donor and weak dispersive interactions, influenced by the cation size and the nature of the IL anions. More recently, Waichigo et al. published a series of papers that discussed replacing traditional organic modifiers with ILs [103–106]. Replacement of traditional organic solvents by ILs produces some problems: high viscosities (producing high back pressure), UV absorption, corrosiveness to the instrument parts, and higher cost. Compared to these problems, the environmental advantage provided by low volatility of ILs is not important, and currently they cannot replace methanol or acetonitrile. Compared to working as an alternative for organic modifiers in HPLC, ILs as low concentration additives were found to be more useful. Similar to mobile phase additives for TLC, the primary aim of adding ILs to HPLC mobile phases is to shield the acidic silica surface, in order to improve the peak shape and reduce the peak broadening. The retention mechanism using IL additives is very complex since both the cation and anion contribute to the retention of analytes. Jiang and co-workers reported effects of 1-alkyl-3-methylimidazolium ionic liquids as mobile phase additives for separating ephedrines (norephedrine, ephedrine, pseudoephedrine and methylephedrine) on a C18 column [107]. Ephedrines are polar basic compounds, and their peaks show seriously tailing and are broad with aqueous mobile phases. When using imidazolium ILs as additives, the peak shape, efficiency and resolution were dramatically improved. [BMIM][BF4 ] could bind to the stationary phase through two different interactions: electrostatic interaction between imidazolium cations and silanol groups, and hydrophobic interactions between butyl groups and C18 groups. The authors also found the alkyl group on the imidazolium cation and the anion type affected retention and efficiency. Later, they reported effects of 1-alkyl-3-methylimidazolium ILs as additives on separating catecholamines [108] and nucleotides [109] in RPLC. Kaliszan and co-workers also published several papers using IL additives to suppress the silanol effect when separating basic drug molecules [110,111]. The effects of 1-butyl3-methylimidazolium BF4 and triethylamine as mobile phase additives on the separation of seven ␤-blockers were compared [112]. 4.3. LC stationary phases More recently, development of new LC stationary phases based on ILs received greater attention. Liu et al. are among the first to examine this application [113]. They synthesized anion exchange stationary phases with immobilized imidazolium-based ILs [114,115]. The synthesis scheme and separation results are shown in Fig. 3. The new columns successfully separated anions, amines, and nucleotides, and they exhibited both a strong anion exchange character and a reversed phase interaction. Stalcup and co-workers synthesized a new stationary phase by covalently attaching n-butylimidazolium to silica through an n-alkyl tether and characterized it by the linear solvation energy relationship method [116]. The new column retained the probe aromatic compounds in a similar way to the phenyl stationary phase. It should be

Fig. 3. (A) Preparation of N-methylimidazolium functionalized silica; (B) Separation of anions: (1) iodate; (2) chloride; (3) bromide; (4) nitrate; (5) iodide; (6) thiocyanate. Reprint from Ref. [114].

noted that these immobilized imidazolium stationary phases form essentially different types of anion exchange media. Their advantages and/or disadvantages compared to other available anoin exchange columns should be examined. A special application of ILs was found for countercurrent chromatography (CCC). In CCC, both the mobile phase and stationary phase are liquids. Bethod et al applied 40:20:40 w/w water–acetonitrile–[BMIM][PF6 ] biphasic liquid system for CCC [117]. The viscosity of the water-saturated [BMIM][PF6 ] IL phase is about 80 cP, while the ternary phase water–ACN–[BMIM][PF6 ] (40%: 20%: 40%) has a viscosity of 3 cP. Therefore, addition of acetonitrile is necessary to reduce viscosity. This system was used to determine the distribution constants of various probe compounds between the two phases. 5. Capillary electrophoresis CE has become a powerful separation technique in recent years due to low sample and reagent consumption, high efficiency and simplicity. Generally fused-silica capillaries are used for CE, and silanol groups on the inner surface are normally negatively charged. This results in the formation of an electroosmotic flow (EOF). Also, the inner capillary surface could participate in interacting with analytes, further affecting separations. 5.1. Applications in modifying the capillary wall The capillary surface could be chemically modified to reverse EOF when cationic ions absorb or bond to the inner surface. Several groups reported that ILs were covalently bonded to the capillary wall to modify the surface [118–120]. A 1-methylimidazoliumbased IL was covalently bonded to a fused-silica capillary surface to reverse the EOF [118]. While the cationic EOF of the bare silica

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capillary increased with increasing pH, the velocity of the reversed EOF of the IL-bonded capillary decreased at higher pHs. Two drugs (sildenafil and its metabolite UK-103,320) were strongly absorbed onto the bare fused-silica capillary wall, while they passed through the IL-coated capillary with a recovery of 98%, 100%, respectively. On the new IL-bonded capillary, five inorganic ions and seven alkylphosphonic acids/monoesters were separated [120]. In addition, modification of the capillary wall improved its reproducibility. Compared with covalent bonding on the capillary wall, dynamic coating provides the advantage of ease of operation. The dynamic coating of ILs could also reverse the EOF and improve separations. Stalcup et al reported the application of 1-alkyl-3methylimidazolium ILs as the electrolytes for separating phenolic compounds in grape seed extract [121]. Coatings of imidazolium ions produced the anodic EOF. Polyphenols associate with the positively charged imidazolium cations either on the coating wall or in the bulk solution as a free form (shown in Fig. 4(I)). Movement of counterions in the outer most layer of double layer engenders EOF. Fig. 4(II) demonstrates the effects of the alkyl group on the imidazolium cation and the anion nature on the separation of polyphenols. 1-Ethyl-3-methylimidazolium BF4 gave higher EOF values than the butylimidazolium analogue. A new IL (dimethyldinonylammonium bromide) was used for simultaneous determination of eight carboxylates [122]. Other groups also reported applications of ILs as dynamic coating materials to separate acidic and basic proteins [123,124], and bioactive flavone derivatives [125]. ILs have been successfully used on microchips as dynamic coatings to improve separations [126,127]. Most recently, six phosphonium-based ILs (including three monocationic and three dicationic ILs) were tested as additives in the CE buffer for separating inorganic and small organic anions [128]. In this study, the dicationic IL, propane-1,3-bis(tripropylphosphonium) fluoride, appeared to be the most effective additive, in that it significantly suppressed the EOF and allowed the facile manipulation of separation selectivity. Baseline separation of six inorganic and seven organic anions was obtained with 7 min using this additive. A CE-ESI-MS method was developed for the separation, identification, and quantitation of four anions in the positive mode using a dicationic ion paring reagent (N,N’-dibutyl 1,1 pentylenedipyrrolidinium) [129]. This method provided greater sensitivities, compared to the direct MS detection in the negative mode. Limits of detection of monochloroacetate and benzenesulfonate, were found to be 20.9, and 1.31 ng mL−1 , respectively. 5.2. Non-aqueous CE Non-aqueous CE (NACE) has received considerable attention due to its special advantages: Some water-insoluble compounds cannot be separated in traditional aqueous CE. Faster separations can be obtained in NACE because of the lower viscosity solution and the higher EOF, and the use of organic solvents makes direct online MS detection feasible. ILs possess some attractive properties over conventional organic solvent modifiers, such as good electrical conductivity [130–132]. Vaher et al. used dialkylimidazolium ILs as buffer electrolytes to separate water-insoluble dyes in acetonitrilebased NACE [130]. Addition of an IL additive is necessary because all five dyes migrate with the EOF marker in the absence of IL additives. When the 1-butyl-3-methyl imidazolium IL was added as an additive, dyes were well separated. More recently, effects of ILs on the magnitude of EOF in NACE were studied in detail using thermal marks, monitored by a contactless conductivity detection [132].

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analytes. A surfactant is added to the run buffer to form micelles as a pseudo-phase and additional modifiers are added in some cases, in order to improve efficiency and selectivity. Warner and coworkers used polymeric surfactants with ILs as modifiers in MCE to separate both achiral and chiral compounds [133]. Poly(sodium N-undecylelinic sulfate) and poly(sodium oleyl-l-leucylvalinate) were tested as surfactants in order to separate three types of mixtures (alkyl aryl ketones, phenols, and enantiomers of binaphthyl derivatives). Five different ILs were tested as modifiers and [BMIM][BF4 ] worked best. Peak efficiency and resolution were influenced by the IL concentration. The migration orders of ketones indicate that the more hydrophobic analytes partition more to the polymeric pseudo-phase than to the bulk aqueous phase. Besides their use as modifiers in CE, ILs could also be used as surfactants. IL-based cationic surfactants were added to the aqueous buffer to separate neutral methylresorcinol isomers and benzene derivatives, and the aggregation behavior of long chain alkylimidazolium ILs was studied [134]. 1-Butyl-3-methylimidazolium dodecanesulfonate (BAS) was applied in microchip MCE to significantly increase the EOF and improve the separation of proteins [135]. BAS not only provided the appropriate ionic strength, but also alleviated absorption of proteins to the PDMS surface. 5.4. Other CE applications Enantiomeric separations in CE have become useful and it sometimes allows separations, which are not observed by other lower-efficiency techniques. Enantioseparations using chiral ILs as additives were reported [136]. Two chiral ILs (ethyl- and phenylcholine of bis(trifluoromethylsulfonyl)imide) were evaluated. The addition of chiral ILs improved the enantioselectivity for 2-arylpropionic acids using cyclodextrins as chiral selectors. But enantioseparation was not obtained only with chiral ILs. Synergistic effects of the chiral ILs and cyclodextrins were observed. Enantiomers of five “profen” drugs were separated in MCE using permethylated ␤-cyclodextrin and a chiral IL, which formed micells in the running buffer [137]. Furthermore, the apparent binding constant of the cyclodextrin to this chiral IL was estimated by a competitive inhibition method [138]. A chiral IL, S-[3-(chloro2-hydroxypropyl)trimethylammonium] NTf2 was synthesized and used as a co-electrolyte or a chiral selector in CE to separate various enantiomeric pharmaceutical compounds [139]. Shamsi and Rizvi used chiral IL surfactants in MCE to separate enantiomeric acidic analytes [140]. Two chiral ILs, undecenoxycarbonyl-l-pryrrolidinol bromide, and undecenoxycarbonyl-l-leucinol bromide were synthesized in monomeric and polymeric forms. Electrostatic interactions between the acidic analytes and cationic micelle contribute significantly to chiral recognition. CE with IL addtives has been applied in a number of other studies. One interesting area is the separation of single-walled carbon nanotubes (SWCN) bundles [141]. This method is based on the dispersion/solubilization of SWNTs in the ionic liquid 1-butyl-3methylimidazolium BF4 . ILs were applied as run buffer additives in the detection of microbial contamination by CE [142]. The purpose is to stack all different bacteria and fungi in a sample plug into a single peak, and move it away from other contaminants. A dicationic IL was added to decrease the required amount of cationic surfactants used, because high concentrations of surfactants might lyse cells. 6. Mass spectrometry (MS)

5.3. Micellar capillary electrophoresis

6.1. MALDI matrices

As an extension of CE, micellar capillary electrophoresis (MCE) is unique in that it is able to separate both neutral and charged

MALDI (matrix-assisted laser desorption ionization) is a soft ionization technique for mass spectrometry, which allows detection of

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Fig. 4. (I) Mechanism of polyphenols’ separation using 1-alkyl-3-methylimidazolium-based ionic liquids. (II) Separation of polyphenols using (A) 1-ethyl-3methylimidazolium tetrafluoroborate, (B) 1-butyl-3-methylimidazolium tetrafluoroborate, and (C) 1-ethyl-3-methylimidazolium hexafluorophosphate [150 mM]. Reprint from Ref. [121].

intact, large biomolecules (such as proteins and oligonucleotides), and synthetic polymers. The analyte is dissolved in a volatile solvent containing the matrix and then spotted on the MALDI plate. The matrix plays a key role in this technique in preventing analyte molecules from being destroyed by direct laser energy absorption and facilitating their volatilization and ionization. Matrix selection is crucial in MALDI-MS analysis. An ideal matrix should possess the following properties: absorption at the laser wavelength, capabilities of dissolving or cocrystalizing with the sample, low volatility, suppressing analyte decompositions, and promoting the ionization of analytes. Compared with multicomponent matrixes [143–145], single-component matrixes have gained greater popularity. Typical single-component matrixes are small, polar, organic molecules, such as derivatives of cinnamic acid and benzoic acid. A matrix could be a solid or liquid. A problem encountered with the solid matrix is that heterogeneity of the sample makes MS signal greatly affected by the position of the laser shot. Therefore, MALDI with solid matrixes normally may not function as a quantitative method. Although the liquid matrixes provided a homogeneous distribution [146–148], they did not work very successfully, due to peak broadening, adduct formation and volatility problems. The unique properties of ILs (low volatility and wide solubility with various types of compounds), make them suitable for working as MALDI matrixes. Armstrong et al. first developed effective ILs as MALDI matrixes [149]. Typical imidazolium, pyridinium and phosphonium ILs were ineffective matrixes as they did not adequately promote the ionization of the sample. When ILs were formulated using anions of popular solid matrixes and specific prononated cations, then peptides, proteins and poly(ethylene

glycol) could be detected with high sensitivity and good reproducibility. Compared to solid matrixes containing cracks, crystals and incongruities, ILs formed uniform spots. Also, IL matrices existed longer in a high vacuum than conventional solid matrixes such as ␣-cyano-4-hydroxycinnamic acid (CHCA) and sinapinic acid (SA). Recently, a second generation of IL matrixes was developed for MALDI-MS [150,151]. A systematic study was conducted by varying both the cation and anion of the ILs, in hopes of finding the best IL matrix, which is able to provide good sensitivity and a wide mass detection range. The proton affinity and pKa of the cation appeared to significantly affect its ability to function as an effective matrix [150]. As a good IL matrix, the cations must have a high pKa (≥11) and proton affinity (>930 kJ mol−1 ). The capabilities of a new IL matrix (N,N-diisopropylethylammonium ␣-cyano-4-hydroxycinnamate, DEA-CHCA) and five common solid matrices were compared for the characterization of biodegradable polymers [151]. The new IL matrix minimized polymer degradation, and more accurate determinations of average molecular weights and the polydipersity index of polymers were achieved. Other groups also utilized ILs as MALDI matrixes to detect various types of molecules, including peptides and proteins [152–155], DNA [156], saccharides [154,157,158], and synthetic polymers [155], and compounds of low molecular weight [159–161]. A MALDI-MS method using an IL maxtrix was developed for imaging gangliosides in mouse brains [162]. It was reported that the signal reproducibility was significantly improved using IL matrixes [155,158,159]. Fig. 5 shows that spot-to-spot reproducibilities are quite different using traditional solid matrices (2,5-dihydroxybenzoic acid and ␣-cyano-4-hydroxycinnamic acid)

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Fig. 5. (A) Plot of normalized [M + Na]+ ion intensities yielded from 90 different positions on a MALDI-MS preparation of the oligosaccharide maltoheptaose with ionic liquid matrix DHBB (2,5-dihydroxybenzoic acid butylamine) (black squares) in comparison to traditional DHB (2,5-dihydroxybenzoic acid) solid matrix (grey circles). (B) Resulting [M + H]+ ion intensities from 90 positions on a human angiotensin II preparation with ionic liquid matrix CHCAB (␣-cyano-4-hydroxycinnamic acid butylamine) (black triangles) and alternatively with traditional CHCA (␣-cyano-4-hydroxycinnamic acid) matrix (grey squares). The relative standard deviations (RSD) of the data series are given as bar graphs beneath the normalized ion intensity distribution diagrams. Black and grey bars indicate RSD values found using ionic liquid matrixes and traditional MALDI matrixes, respectively. Reprint from Ref. [155].

and their IL analogues [155]. Due to improved reproducibility, MALDI-MS using an IL matrix was applied as a quantitative method without using internal standards [159,163,164]. Quantitation of peptides for monitoring protease-catalyzed reactions by MALDI-MS using IL matrixes was reported [164]. When the matrix-to-analyte ratio is high enough, the relationship between peptide amount and MS signal intensity is linear and the dynamic range of linearity is about one order. Zabet-Moghaddam et al. reported quantitative analysis of low molecular weight compounds (such as amino acids, sugars and vitamins) using IL matrixes [159]. 6.2. Anion detection by ESI-MS Applications of IL additives in ESI-MS focuses on the detection of anions. ESI-MS is one of the most sensitive methods for anion detection. But direct detection of anions in the negative mode in ESI-MS has some problems. Detection of anions often falls into the

MS range that is below the low MS cut-off of some instruments or in a region of high chemical noise, due to the low molecular weight of many anions. Corona discharge occurs more frequently in the negative mode than in the positive mode. Anion detection in the positive mode originated from analyzing perchlorate using ion chromatography-ESI/MS [165]. The sensitivity and selectivity were highly improved by the formation of the positively charged complex ion between a perchlorate anion and the IL dication. Subsequently, a general ESI-MS method for detecting singly charged inorganic and organic anions in the positive ion mode was proposed using dicationic ILs as pairing reagents [166]. Complexation of singly charged anions with dicationic ILs gives a single positively charged ion, and moves the target MS to a higher MS range. Additionally, MS/MS can further improve sensitivity and selectivity. Fig. 6 shows a MS fragmentation pattern for the complex ion formed by the dicationic IL pairing with an anion. Twenty-three dications were evaluated as pairing reagents for anion detection [167]. Recently, trigonal and linear tricationic ILs were used

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Fig. 6. Structure of the dicationic reagent in its synthesized form (A) and proposed fragmentation pathways (B, C) for an anion (A− ) of interest. Reprint from Ref. [166].

as pairing reagents for detecting divalent anions [168–170]. The sensitivity of hexachloroplatinate and o-benzenedisulfonate were significantly improved using cationic pairing agents in the positive mode (Fig. 7) [168]. It was found that the flexibility of the multi-functional cation played an important role in pairing with

anions, affecting the detection sensitivity [167]. More flexible linear cations produced higher sensitivity than rigid trigonal ones. Also, bisphosphonate drugs were detected by ESI-MS in the positive mode using dicationic and tricationic additives as pairing agents [171].

Fig. 7. Comparison of positive (I, II) and negative modes (III, IV) for hexachloroplatinate (I, III) and o-benzenedisulfonate (II, IV). Tricationic reagents in water were introduced into the carrier flow after anion injection in positive ion mode (I, II) while only water was used in negative ion mode (III, IV). Reprint from Ref. [168].

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7. Electrochemistry Applications of ILs in electrochemistry have expanded rapidly, in that ILs have high ionic conductivity and low volatility. There are excellent books and reviews, discussing various aspects of ILs in electrochemistry. Ohno edited the book “Electrochemical aspects of ionic liquids” [172]. It covers studies in different areas of electrochemistry. A book, edited by Endres, focuses on electrodeposition [173]. Hapiot and Lagrost reviewed electrochemical reactivity of ILs [174]. The physical properties of ILs and their applications in electrochemistry have been reviewed by Compton and co-workers, Galinski et al. and Domanska, separately [175–177]. Ivaska and Wei reviewed application of ILs in electrochemical sensors [178]. MacFarlane et al. summarized their work since 1996 related to ILs in electrochemical devices (such as fuel cells, solar cells) and processes [179]. A good review discussed studies about phase interfaces from an electrochemical perspective [180]. 7.1. Unique electrochemical properties The intrinsic conductivity of ILs makes them useful in electrochemical studies. Typically, the conductivity values of ILs are in a broad range 0.1–18 mS cm−1 [176]. The conductivities of ILs are much lower than conventional aqueous solutions, such as H2 SO4 (730 mS cm−1 in 30 wt.% solution) and KOH (540 mS cm−1 in 29.4 wt.% solution). Conductivities of a series of ILs-based on dialkylimidazolium cations were studied [181]. Both the cation and anion structure affected the conductivity significantly. [EMIM][TA] (trifluoroacetate) gave a conductivity of 9.6 mS cm−1 , while [EMIM][HB] (heptafluorobutanoate) gave a conductivity of 2.7 mS cm−1 . ILs with different imidazolium cations and the same anion ([NTf2 ]) have conductivities in the range of 0.98–8.8 mS cm−1 . The conductivity properties of hybrid RTIL-carbon composite materials were investigated using ac impedance technology [182]. Two types of carbon materials are mixed with [BMIM][PF6 ]: multiwall carbon nanotubes (MWNTs) and mesocarbon microbeads (MCMBs). They gave different conductivity characteristics. A unique feature of ILs is their good redox-robustness. Typically, the electrochemical potential window of ILs is 4–5 V, which is similar to that of common organic electrolytes, but significantly larger than that of aqueous electrolytes [176]. Electrolytes based on imidazolium ILs gave a wide electrochemical window of >4 V [183]. A window of over 5.5 V was observed in an IL (trimethylpropylammonium NTf2 ) [184]. This IL provided better cathodic stability than 1-ethyl-3-methylimidazolium based ILs. ILs themselves can be electroactive. Murray and co-workers synthesized a series of electroactive ILs, by directly linking ferrocene to imidazolium [185]. ILs with appropriate viscosities could be tailored by changing the linker. Later, they reported the electron transport properties of these “ferrocenenated” imidazolium ILs [186]. 7.2. Electrochemical methods for ILs Generally, reference potentials in ILs are unavailable and a direct comparison between ILs and conventional solvents is difficult. Wedd and co-workers reported [Co(Cp)2 ]+/0 (Co(Cp)2 = cobaltocene) redox couple could be used as a voltammetric reference standard in [BMIM][PF6 ] and its values were similar to those in acetonitrile [187]. In addition, the voltammetric studies of ferrocene derivatives were carried out when the solids adhered to the electrode surface. All the processes are chemically reversible and diffusion-controlled [187]. Some relatively new approaches have been attempted in the study of electrochemistry in ILs [188,189]. A microchemical method using small amounts of chemicals was developed and it was able to

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provide equivalent voltammetric data to the conventional method [188]. The use of ILs could solve some practical problems during electrochemical studies [190]. Octathio[8]circulene is not soluble in aqueous and organic solvents, and conventional measurements in the solution state are not possible. A thin film was one possible option that could be used to study its electrochemistry, but it easily peeled off during measurements. The problem was solved by using an IL solvent and reproducible results were obtained. ILs could be used to make electrodes with favorable properties [191–194]. Kakiuchi et al. developed a new type of reference electrode [191]. An Ag/AgCl electrode was immersed in or coated with a AgCl-saturated IL, 1-methy-3-octylimidazolium NTf2 . This IL acted not only as a solvent for dissolving the AgCl, but also as a salt bridge. But, long-term use of this reference electrode is still difficult because the IL is slightly soluble in the sample solution. A new composite electrode was developed using multiwall carbon nanotubes (MWCNT) and n-octylpyridinium PF6 [193]. This type of electrode produced better sensitivity and stability than others based on carbon nanotubes. Simultaneous determination of glutathione and glutathione disulfide has been achieved using a nanoscale copper hydroxide composite carbon IL electrode [194]. Electrochemistry is a useful technique to study IL-aqueous interfacial properties, which are useful in investigations of extractions using ILs. Voltammetric experiments were used to measure facilitated transfer of metal ions by crown ethers across RTIL/aqueous solution electrochemically-polarized interfaces [195]. The transfer of metal ions and ammonium ions across a polarized IL memberane facilitated by valinomycin was investigate by Samec and Langmaier [196]. It was reported that a wide polarized potential window at 0.8 V was found at the interface between water and a hydrophobic RTIL, composed of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate anion and ammonium cation [197]. The polarized potential window is beneficial for measurements of ion transfer across the interface and partitioning between these two phases. Recently, electrowetting of ILs received greater attention [189,198–200]. Murray and Wang studied contact angles of an ionic liquid sessile droplet of monolayer-protected Au nanoparticles [189]. A fundamental study about electrowetting properties of nineteen ILs was conducted [199], including mono-, di-, tricationic, and mono-, di-anionic ILs. All of the tested ILs gave electrowetting, and their behaviors depended on the structures of the ILs. When used as electrolytes in dielectric-based microfluidic devices, ILs remained more stable at high voltages than aqueous solutions. Recently, electrowetting properties of linear tricationic ILs were studied and these ILs can act as effective electrowetting materials due to their high structural flexibility [200]. 8. Sensors 8.1. Electrochemistry-based sensors ILs have been widely utilized in sensor construction. Electrochemistry-based sensors have attracted considerable attention for a long time, due to their good sensitivity and selectivity [201–203]. Applications of RTILs in electrochemical gas sensors were reported by Buzzeo et al. [201]. The RTIL functioned as a nonvolatile electrolyte in the gas sensor and eliminated the use of membranes, which were generally necessary to separate the sample from the electrolyte. This sensor was able to work in extreme conditions, in the combustion industry at high temperatures. [EMIM][NTf2 ] was used to develop an ammonia sensor, based on the electro-oxidation of hydroquinone [202]. The use of ILs minimized the absorbed water, therefore the sensor performance was less affected by moisture and humidity. However, the high viscosity of ILs presented some problems, such as slow response times [201] and irreversible redox reactions [202].

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8.2. Optical sensors

9.1. IR and Raman spectroscopy

Oter et al. [204] first reported an optical CO2 sensor using the RTIL ([MBIM][BF4 ] or [MBIM][Br]) as the matrix with 8hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS). The detection of CO2 is based on the fluorescence signal change of HPTS when pairing to CO2 . More recently, the same group reported that ionic liquid modification of an ethyl cellulose matrix extended the detection range to 0–100% pCO2 [205]. A hybrid electrochemical-colorimetric sensing platform for detecting explosives was developed (shown in Fig. 8) [206]. The product of the electrochemical reaction was detected by a colorimetric device. A thin layer of [BMIM][PF6 ] played an important role in this platform: the IL coating selectively preconcentated explosives and quickly transported them to the electrodes; and it facilitated the formation of reduction products.

Binding constants between cyclodextrins and phenols in RTILs were studied using NIR spectrometry [214]. The binding constants in RTILs are much smaller than those in D2 O. The reason for this is that the [BMIM] cation of the IL can form an inclusion complex with cyclodextrin, competing with phenol for the cyclodextrin cavity. Later, a NIR method was developed to determine enantiomeric compositions of pharmaceutical products and amino acids, using enantiopure forms of a chiral RTIL [(3-chloro2-hydroxypropyl)trimethylammonium][NTf2 ] [215]. This chiral IL played dual roles: both as solvent and chiral selector. This method successfully detected enantiomeric excesses (ee) as low as 0.6%. A fluorescence method using the same chiral IL also was developed to determine ee [216]. Kazarian et al. reported an in situ ATR (attenuated total reflectance)-IR study of CO2 dissolved in different ILs at high pressures [217,218]. The test ILs consisted of the same cation and different anions (BF4 and PF6 ) [217]. The results demonstrated that the anions affected the interactions between CO2 and the ILs. Raman spectroscopy was used to study the interactions between ILs and polymer films [219]. The vibrational and rotational relaxation of acetonitrile dissolved in ILs were studied by Aleksa et al. using Raman spectroscopy [220]. The vibrational relaxation of acetonitrile was faster in the IL solution, but the effect of ILs on the rotational relaxation was very small. Fujii et al. [221] studied the anion conformation of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide by Raman spectroscopy. The theoretical calculations show that the bis(fluorosulfonyl)imide anion exists as two conformers (cis and trans) and this correlates well with Raman spectra, where the anion bands are significantly affected by the temperature.

8.3. Sensors based on quartz crystal microbalance Sensors based on the quartz crystal microbalance (QCM) have been reported by several groups. Dai and co-workers first developed a sensor for organic vapors based on QCM using ILs as sensing materials [207]. When analytes were dissolved in an IL, the viscosity of this IL changed rapidly, which gave a frequency shift of the QCM device. The response of the QCM depended on the nature of both the analyte and the IL. Johannsmann and coworkers developed an organic vapor sensor using QCM [208]. The organic vapor is absorbed into small droplets of an IL. A discriminative sensor, using an IL-incorporated plasma polarizedfilm, was used for the detection of methanol, ethanol, n-propanol, and n-butanol in the concentration range of 16–84 ppmv [209]. The performance of seven RTILs (two imidazolium ILs, four phosphonium ILs, and an ammonium IL) as sensor coating materials for organic vapor sensors was investigated [210]. Except for dichloromethane, the sensor gave proportional responses up to 100% for three analytes at room temperature and at elevated temperatures (120 ◦ C). This sensor showed excellent reversibility for the adsorption/desorption processes, and gave the potential for real-time monitoring. Later, the same group developed miniaturized gas sensor arrays using a multichannel monolithic QCM [211]. 8.4. Stochastic sensors ILs were used as supporting electrolytes for nanopore stochastic sensors [212]. This method can detect monovalent cations and liquid explosive components and their sensitizers. The use of ILs makes some difficult measurements possible, such as detection of Na+ and K+ , and the detection of insoluble analytes in the NaCl and KCl solutions. Furthermore, the sensitivity of the stochastic sensor was improved by replacing the NaCl solution with one containing the IL as the supporting electrolyte. It was reported that the translocation velocities of various DNA homo- and copolymers through protein pores in the DNA sensor were significantly decreased by using ILs as the supporting electrolyte [213]. This is beneficial for nanopore DNA sequencing. 9. Spectroscopy The design and optimum use of ILs requires detailed understandings of their properties. Thus it is not surprising that a variety of spectroscopic techniques have been utilized in the study of ILs. In addition, ILs dissolve many types of compounds, and it has proven to be advantageous to use them as solvents in spectrometric studies.

9.2. Fluorescence spectroscopy Given its sensitivity and small sample requirement, fluorescence spectroscopy has been widely used to study ILs. The dynamic solvation of the fluorescence probe, coumarin 153 in different ILs was studied by different groups [222–226]. Compared to conventional solvents, the solvation free energy did not change significantly in ILs [223]. The correlation between the rotation time and the solvent viscosity remained the same for ILs and conventional polar solvents. The interactions between ILs and ␤-cyclodextrin were studied by a competitive method using 2-(p-aminophenyl)-3,3-dimethyl-5ethoxycarbonyl-3H-indole as a fluorescent probe [227]. The results indicated that ILs and cyclodextrin formed a 1:1 complex, which was entropically unfavorable and enthalpy-driven. The chiral ILbased on 1-methylimidazolium and chloromethylmenthyl ether appeared to affect the excited-state properties and the stereoselective fluorescence quenching by photoinduced electron transfer of naproxen analogs [228,229]. 9.3. NMR High performance NMR was developed for neat ILs or ILs mixed with other solutes [230]. The NMR spectra of 1 H and 13 C of neat ILs were similar to those obtained in conventional deuterated solvents. The aggregation of two imidazolium-based ILs in conventional solvents (including water, methanol, acetonitrile, and benzene) was studied by 1 H NMR [231]. Yasaka et al. recently developed an in situ NMR method for the quantitative analysis of acidic impurities in ILs [232]. Water was added and acted as a NMR indicator. The amount of strong acid percentage correlated with the chemical shift of the exchangeable proton peak of water, while the amount of weak acid was related to the spectral width of water (Fig. 9). This sensitive method could be used for many

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Fig. 8. Hybrid electrochemical-colorimetric sensor with a thin layer of ionic liquid (BMIM-PF6) as a selective preconcentration medium. (A) Cyclic voltammograms of blank IL (black line) and 2 ppm TNT in IL (red line) at 100 mV s−1 . Arrows indicate peak currents of TNT. (B) Color (absorbance) changes durig the electrochemical reduction of TNT in BMIM-PF6. The absorbance change for each color is defined as the logarithmic ratio of the intensity in a sensing area (working electrode) to the intensity in a reference area (reference electrode). Insets in part B show two images taken before (0.0 V) and after (−1.5 V) TNT reduction. Reprint from Ref. [206] (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article).

Fig. 9. (A) Exchangeable proton peak in the presence and in the absence of a strong acid in [bmim]+ [NTf2]− . (a) The 1 H NMR spectrum of acid-free [bmim]+ [NTf2]− in the presence of 0.018 mol kg−1 water at 50 ◦ C. The arrow indicates the position of the water signal (very weak). (b) The zoomed spectrum of (a) is shown in the lower part and the same one but in the presence of HNTf2 is shown in the upper part. The arrows indicate the exchangeable proton peak. The chemical shift scale is drawn so that the proton of the terminal methyl group of the butyl chain of [bmim]+ cation (internal chemical shift reference) is at 0.94 ppm. (B) 1 H NMR peaks of water in the absence and in the presence of weak acids. (a) Water is observed at 3.91 ppm as a sharp peak in the absence of acids. The concentration of water is 0.43 mol kg−1 . (b) The water peak broadens as the concentration of acetic acid increases up to 32 mmol kg−1 at a fixed water concentration of 0.43 mol kg−1 . Reprint from Ref. [232].

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ILs, but not ones which have cations and anions that are acids and bases. 10. Conclusions The dramatic increase in publications involving ionic liquids in analytical chemistry are indicative of their vast potential in this field. ILs have been widely applied in most subdisciplines of analytical chemistry, such as sample preparation, separations, and chemical analysis. While this field continues to progress, some problems should be noted. For example, systemic and mechanistic understandings of IL properties are not adequate. Toxicity and long-term stability of ILs can vary widely and must be taken into consideration when choosing ILs for any project. Some ILs are relatively impure, and precautions should be taken since impurities could affect both the properties of the IL and the application in which it is used. Most studies in this area are far from mature, and more challenges continue to be forthcoming. The next generation of ILs, including tunable aryl alkyl ionic liquids will play an important role in analytical chemistry in the future [233]. It is expected that the use of ionic liquids will continue to expand in the area of chemical analysis. This will result both in enhanced techniques and technologies as well as a better understanding of ILs and their properties. Acknowledgement We gratefully acknowledge the Robert A. Welch Foundation (Y0026) for the support of this work. References [1] I. Krossing, J.M. Slattery, C. Daguenet, P.J. Dyson, A. Oleinikova, H. Weingaertner, J. Am. Chem. Soc. 128 (2006) 13427. [2] A.J. Carmichael, K.R. Seddon, J. Phys. Org. Chem. 13 (2000) 591. [3] S. Gabriel, J. Weiner, Ber. 21 (1888) 2669. [4] P. Walden, Bull. Acad. Sci. St. Petersburg (1914) 405. [5] J.S. Wilkes, J.A. Levisky, R.A. Wilson, C.L. Hussey, Inorg. Chem. 21 (1982) 1263. [6] J.S. Wilkes, M.J. Zaworotko, Chem. Commun. (1992) 965. [7] J. Liu, J.A. Jonsson, G. Jiang, TrAC Trends Anal. Chem. 24 (2005) 20. [8] G.A. Baker, S.N. Baker, S. Pandey, F.V. Bright, Analyst 130 (2005) 800. [9] S. Pandey, Anal. Chim. Acta 556 (2006) 38. [10] J.L. Anderson, D.W. Armstrong, G. Wei, Anal. Chem. 78 (2006) 2893. [11] R.J. Soukup-Hein, M.M. Warnke, D.W. Armstrong, Annu. Rev. Anal. Chem. 2 (2009) 145. [12] A. Berthod, M.J. Ruiz-Angel, S. Carda-Broch, J. Chromatogr. A 1184 (2008) 6. [13] S.A. Shamsi, N.D. Danielson, J. Sep. Sci. 30 (2007) 1729. [14] X. Han, D.W. Armstrong, Acc. Chem. Res. 40 (2007) 1079. [15] B. Buszewski, S. Studzin’ska, Chromatographia 68 (2008) 1. [16] M. Koel, Crit. Rev. Anal. Chem. 35 (2005) 177. [17] C.F. Poole, J. Chromatogr. A 1037 (2004) 49. [18] R. Liu, J. Liu, Y. Yin, X. Hu, G. Jiang, Anal. Bioanal. Chem. 393 (2009) 871. [19] S. Dai, Y.H. Ju, C.E. Barnes, J. Chem. Soc., Dalton Trans: Inorg. Chem. (1999) 1201. [20] M.L. Dietz, S. Jakab, K. Yamato, R.A. Bartsch, Green Chem. 10 (2008) 174. [21] H. Luo, S. Dai, P.V. Bonnesen, Anal. Chem. 76 (2004) 2773. [22] H. Luo, S. Dai, P.V. Bonnesen, A.C. Buchanan III, J.D. Holbrey, N.J. Bridges, R.D. Rogers, Anal. Chem. 76 (2004) 3078. [23] K. Shimojo, M. Goto, Anal. Chem. 76 (2004) 5039. [24] N. Hirayama, H. Okamura, K. Kidani, H. Imura, Anal. Sci. 24 (2008) 697. [25] K. Kidani, N. Hirayama, H. Imura, Anal. Sci. 24 (2008) 1251. [26] G. Wei, Z. Yang, C. Lee, H. Yang, C.R.C. Wang, J. Am. Chem. Soc. 126 (2004) 5036. [27] A. Arce, M.J. Earle, S.P. Katdare, H. Rodriguez, K.R. Seddon, Phys. Chem. Chem. Phys. 10 (2008) 2538. [28] L. Guerra-Abreu, V. Pino, J.L. Anderson, A.M. Afonso, J. Chromatogr. A 1214 (2008) 23. [29] G. Absalan, M. Akhond, L. Sheikhian, Talanta 77 (2008) 407. [30] V.M. Egorov, S.V. Smirnova, I.V. Pletnev, Sep. Purif. Technol. 63 (2008) 710. [31] Y. Lu, W. Ma, R. Hu, X. Dai, Y. Pan, J. Chromatogr. A 1208 (2008) 42. [32] S. Carda-Broch, A. Berthod, D.W. Armstrong, Anal. Bioanal. Chem. 375 (2003) 191. [33] A.E. Visser, R.P. Swatloski, R.D. Rogers, Green Chem. 2 (2000) 1. [34] A. Bosmann, L. Datsevich, A. Jess, A. Lauter, C. Schmitz, P. Wasserscheid, Chem. Commun. (2001) 2494.

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