## 2 SPME Techniques  
The underlying principle of the SPME methodology is based on the partitioning of analytes between a coated fiber and a sample. The reusable fiber is coated with a stationary phase, which can be a liquid polymer, a solid sorbent, or a combination of both [9]. A mass transfer begins after exposure to the vapour phase above a solution (HS-SPME) or direct immersion in the solution (DI-SPME). This process is driven by the second law of thermodynamics. Thereby the chemical potential of each compound should be equal. Once equilibrium is reached, the extracted compounds on the fiber are desorbed and inserted into the injection port of a chromatograph. The release of the analytes is accomplished in GC by thermal desorption and in HPLC by dissolution and subsequent injection of the elution solvent [32]. Both methods lead to detection of the substances by the analytical instrument detector. The application of SPME-LC has lagged behind SPME-GC, which might be due to the small number of commercially available SPME sorbents for LC applications, the lack of commercially available interfaces, long equilibration times, and the lack of automation [2].  
The aim of SPME is to reach the equilibrium between the sample matrix and the coating of the SPME device as rapidly as possible [33]. Further exposure of the fiber after equilibrium has been reached does not increase the concentration of extracted compounds. For that reason, sample extraction and pre-concentration could be carried out in one single step [34].  
SPME has several advantages over traditional extraction methods. It is not just a rapid, simple and solvent-free method, but it is also sensitive, provides linear results for a wide range of concentrations and analytes [2]. Notwithstanding the low concentrations of analytes, quantitative or semi-quantitative data are provided and losses that can occur during the extraction, concentration and clean-up steps of traditional sample procedures are mostly avoidable [2].  
One of the main drawbacks of SPME techniques is the limited number of commercially available stationary phases (fiber materials) only roughly covering the scale of polarity of target analytes. In particular, the extraction of polar analytes from samples with a polar matrix poses a problem [35, 36, 37, 38, 39]. Other challenges are the relatively low recommended operating temperature (240-280 \({}^{\circ}\)C), the instability and swelling in organic solvents, breakage of the fiber, stripping of coatings, bending of the needle, and the cost as well as the limited lifetime of the fiber [2, 36]. Furthermore, sample carry-over may occur [40] and high molecular weight compounds cannot be analyzed in combination with GC [36]. In certain cases, low extraction efficiencies are reported, in particular in case of very volatile, polar, or thermally unstable analytes [2]. The early theoretical and practical aspects of SPME have undergone continuous technical development over the years.  
SPME techniques cover a wide range of sampling techniques, including field, _in situ_ and air sampling. The two different implementations fiber SPME and in-tube SPME are frequently used [9]. Detailed SPME theory has been explained in a number of reviews [6, 15, 41, 42].