## 2 Basis of Solid-Phase Extraction  
For analytical purposes, SPE is usually performed using a small column or cartridge containing an appropriate packing. As an alternative, membranes loaded with distinct resins can be applied for SPE micro-extraction [7]. In a common practice the adsorbed materials are eluted from the resin with a small amount of solvent. The most commonly used material for SPE is chemically bonded silica, usually with a C\({}_{8}\) or C\({}_{18}\) organic group. On the other hand the most common polymeric resin in SPE is porous polystyrene [8]. Both silica- and polystyrene-based phases have distinct disadvantages: (1) silica is unstable to alkali and is known to offer only poor surface contact in case of aqueous samples due to the hydrophilic/hydrophobic balance between the carrier and the carbon chain; (2) polystyrene carriers also possess hydrophilic surfaces; (3) in most cases an activating solvent must be used, which can cause reduced capacity and/or reproducibility, respectively; (4) many types of organic compounds are incompletely extracted from aqueous solutions. Therefore, there is ongoing work with the aim of creating new types of chemically bonded resins to overcome these drawbacks. As one of the first improvements, in 1992 [PERSON] and [PERSON] modified PS-DVB with alcohol and acetyl functional groups [9]. According to the chemical character of the functional group chemically bonded to the copolymer, the resulting phases are classified as non-polar, polar, or ion exchangers. Since then a number of different stationary phases based on different types of materials--poly(carboxylic acid) coated silica [10; 11], divinylbenzene-\(n\)-vinylpyrrolidone copolymer [12], poly(glycolylethanolamine methacrylate-divinylbenzene) (GMA-DVB) [13; 14], fullerene-silica [15], titanium oxide coated steel targets [16], aluminium silicate embedded in poly(styrene-co-divinylbenzene) (PS-DVB) [17; 18], zirconium silicate [19], poly(N-vinylimidazole/ethylene glycol dimethacrylate) [20]--have been designed for the selective extraction of, e.g., pharmaceutically active compounds from medicinal plants. An evaluation of commercial available SPE materials for the selective analysis of monoterpenoids was reported by [PERSON] _et al._[21]. For SPE of biomolecules the number of tailored stationary phases is even higher, e.g., for phosphopeptide and biomarker analysis, cellulose [22], poly(glycolylethanolamine-methacrylate/divinylbenzene) [23; 24; 25; 26], carbon nanotubes [27], diamond layer [28; 29], fullerenes [30; 31; 32; 33], diamond-like-carbon [34; 35], titanium and zirconium oxide [36], graphitic nanofibers [37], chitosan [38] and silica lanthanum oxide [39]. Nowadays the analysis after the enrichment step is mainly carried out by mass spectrometric (MS) techniques, including matrix assisted laser desorption ionization time of flight (MALDI-TOF) [40; 41; 42; 43], electrospray ionization (ESI) and still high-performance liquid chromatography (HPLC) employing UV-absorbance, diode array detection and others [8]. The past, present and future of solid-phase extraction was reviewed by [PERSON] _et al._ in 2012 [44] and [PERSON] in 2014 [45]. Altogether, in 2015 using the SciFinder engine of the American Chemical Society (ACS) 67.010 hits can be found entering the term \"solid phase extraction\". From those only 890 hits are correlated to infrared (IR) spectroscopic analysis, most of them describing a physicochemical characterization of the material itself [46; 47; 48] but only very few the analysis of the interesting analytes bound and/or enriched onto the material, respectively [49]. Compared to mass spectrometry (MS)-based techniques infrared spectroscopy in the wavenumber range between 400 and 13,000 cm\({}^{-1}\) offers especially the advantages of fast, cheap, non-invasive analyses. Furthermore, several parameters can be analysed simultaneously, even in high-throughput mode [50]. The main disadvantage is the low sensitivity compared to MS in the percentage or upper ppm range.