# Recent Advances in Applications of Ionic Liquids in Miniaturized Microextraction Techniques
## 1 Introduction  
Nowadays, sample preparation procedures are more than ever linked with the protection of the environment following the philosophy of Green Analytical Chemistry (GAC). A typical analytical procedure consists of three main parts: sampling, sample preparation and final analysis. It is generally known that almost the 75% of its time is spent in the stage of preparation, so it is a critical part of the analytical procedure. According to GAC, new environmentally friendly instrumentation and methodologies with the minimum emissions of pollutants and environmental sustainability in terms of cost and energy of chemical laboratories are the major aims in the field of sample preparation [1; 2; 3]. Among the sample preparation methods used to clean up and concentrate analytes, liquid-liquid extraction (LLE) and solid-phase extraction (SPE) are the most famous and widely used. Despite their universal application, these methods are accompanied with some drawbacks, such as time-consumption, high cost, inability to extract polar compounds (mainly for LLE), use and disposal of great amount of toxic solvents, complication to automate and potential evaporation and dissolution in a proper solvent prior the analysis, which adds an extra step in the whole procedure. Miniaturization in sample preparation techniques is the key to overcome these above drawbacks. Furthermore, the combination of ionic liquids with the miniaturized sample preparation techniques could be the panacea for the pre-stated limitations [4; 5]. Ionic liquids (ILs) are organic molten salts with melting point lower than 100 \({}^{\circ}\)C in contrast with the common molten salts (fused salts), such as sodium chloride which becomes liquid at 801 \({}^{\circ}\)C. \"Room temperature ionic liquids\" (RTILs) is a term which describes a group of ionic liquids with melting points at or below room temperature (25 \({}^{\circ}\)C). ILs are composed of positive and negative ions, usually a bulky organic nitrogen-containing cation, such as imidazolium, pyridinium, pyrrolinium, phosphonium or ammonium, and a halogen-based organic or inorganic anion, including trifluoromethylsulfonate [CF\({}_{3}\)SO\({}_{3}\)]\({}^{-}\), bis[(trifluoromethylsulfonyl)sulfonyl]imide [(CF\({}_{3}\)SO\({}_{2}\))\({}_{2}\)N]\({}^{-}\) (i.e., NTF\({}_{2}\)), trifluoroethane [CF\({}_{3}\)CO\({}_{2}\)]\({}^{-}\) or Cl\({}^{-}\), PF\({}_{6}\)\({}^{-}\), BF\({}_{4}\)\({}^{-}\), respectively, whereas water and other organic solvents are composed of molecules [6; 7]. The first discovered RTIL is the ethylammonium nitrate [EtNH\({}_{3}\)][NO\({}_{3}\)] with melting point at 12 \({}^{\circ}\)C by [PERSON] in 1914 [8]. However, remarkable progress has been achieved in the field of ionic liquids since 1992, when [PERSON] and [PERSON] introduced a promiscuous generation of ionic liquids, the air and water stable 1-ethyl-3-methylimidazolium-based ILs, [EtMeim]BF\({}_{4}\) and [EtMeim]MeCO\({}_{2}\)[9]. The future of ILs belongs to the deep eutectic solvents (DESs), an interesting subclass of ILs which are less toxic, they have a stronger ecofriendly profile and they can be produced easier and cheaper than the ILs. One of their main drawbacks is their solid state in room temperature and their high viscosity [10]. The structures of the most commonly used ILs are represented in Figure 1. ILs owe the fact that they remain liquid in temperatures below 100 \({}^{\circ}\)C to the large size of the ions that they consist of, avoiding the packing of ionic lattice, which happens in inorganic salts. Compared with conventional solvents, ILs have some special physicochemical properties, such as low volatility, high electrical conductivity, long-term thermal and chemical stability, low flammability, and low vapor pressure. The negligible vapor pressure is one of the properties that makes ILs \"green solvents\", as they contribute to the reduction of atmospheric pollution and the health risks that are entailed.