## 1 Introduction  
The idea of increasing oxygenates of full to raise and improve octane number and reduce environmental pollution has been commercialized since the 1970s [1]. The octane-booster additives, the compounds that can generate oxygen during the combustion of engine fuel, comprise organometallics, including tetraethyl lead, ferrocene and methylcyclopentadienyl manganese tricarbonyl, and organics such as methyl _tert_-butyl ether (MTBE), ethyl _tert_-butyl ether (ETBE), _tert_-amyl-methyl ether (TAME), and ethanol. The organic additives are generally less effective than organometallic octane boosters, while they are more environmentally friendly chemicals. Nowadays, MTBE is the most commonly used octane booster. It has been used in many countries since the 1990s, due to its facile manufacturing, easy mixing with hydrocarbons, increasing octane number, easy combustion, reducing emissions from engine combustion, and less corrosive than the alcoholic octane boosters. It is also used in research and industry as an alternative solvent to diethyl ether. MTBE is a volatile organic colorless liquid with a strong distinctive odor like turpentine. It has very high solubility in water (~50 g-L-1) at room temperature, which is 20 times higher than that of solvent gasoline. Consequently, it can easily contaminate water and soil resources with a rapid expansion [2]. Despite all benefits, there are many environmental health and safety concerns about MTBE and its possible carcinogenic properties [3]. MTBE has been banned in a few countries, but it is still used as a fuel octane booster in many developing countries. This widespread usage has resulted in a wide range of environmental pollution by MTBE. This issue will be more evident when soil contamination is taken into consideration. Natural samples like soil contain different active sites, created by organic and inorganic constituents. Organic contaminants like MTBE are usually tightly attached to these active sites, and their release from the native matrices are very difficult [4]. Therefore, development of easy, low-cost effective methods for tracing of MTBE especially in solid environmental samples is of particular importance. Several traditional and solvent-free methods for sampling and analysis of MTBE in gasoline, soil, water, and biological fluids are presented in the scientific literature, and each has its own pros and cons. These procedures usually perform in coupling with gas chromatographic (GC) instruments equipped by photo ionization detector (PID), flame ionization detector (FID), and mass spectrometry detector (MSD) [5]. On the other hand, reducing the maximum allowed amount of MTBE in water (20-40 ug-L-1) by the United States Environmental Protection Agency (EPA) [6] made MTBE quantification more difficult, to achieve appropriate limit of detection. In addition to classical extraction methods, in recent years, different newly emerging techniques have been proposed for sampling, separation, and preconcentration of MTBE like luminescent bioassay [7], GC coupled with direct aqueous injection and mass spectrometry (DAI-GC/MS) [8], purge-and-trap coupled with gas chromatography (P&T/GC) [9], headspace sampling followed by GC/PID quantification [10], solid-phase microextraction (SPME), and single-drop headspace sampling coupled with GC-FID (HS-SDME-GC-FID) [11], and headspace SPME coupled with GC/MS (HS-SPME-GC/MS) [12]. DAI-GC is simpler and more operator-friendly than P&T/GC for analysis of MTBE and its degradation products [13]. However, the best alternative is HS-SPME, as its detection limit is lower than the permissible limit of MTBE in drinking water and soil, because it prevents the contact between the sample matrix and the extraction phase. Accordingly, HS-SPME can be easily applied for analysis of samples contaminated by MTBE by using available commercial fibers. However, commercial SPME fibers are expensive, fragile, have a low sorption capacity, and have limited diversity of sorbent type for sampling different analytes in various sample matrices.