# Returning to Nature for the Design of Sportive Phases in Solid-Phase Microextraction
## 2 Cork  
Cork is defined as the outside bark of the _Quercus super_ L., which is commonly known as cork oak tree. This evergreen oak can be found in southwest Europe and northwest Africa and grows up to 20 m height. Due to its cellular structure and chemical composition, cork exhibits some physical properties such as a high coefficient of friction, resilience, high energy absorption, excellent insulation properties, impermeability, low conductivity, and low density, which makes it an excellent material for a variety of applications. It is mainly used as wine bottle stopper; nevertheless, various other applications have been attributed to cork and its derivatives such as flooring and walls coverings, acoustic and vibrating insulation, ecoceramics, and ecodesign [15; 16]. Cork has unique microscopic features compared to other lignocellulosic materials. Its hollow polyhedral prismatic cells may present honeycomb or brick wall structures when observed from radial or axial directions, respectively. On average, cork cells are 45 mm tall and have 15-20 mm of face and 1-2 mm of thickness. The chemical composition of cork varies according to seasonality and territory but mainly consists of hydrophobic biopolymers such as suberin (\(\pm\)40%) and lignin (\(\pm\)24%), the hydrophilic polysaccharides cellulose and hemicellulose (\(\pm\)20%), and 15% of other extractives e.g., waxes and tannins. The main cork constituent comprises phenolic, long-chain, and hydroxy fatty acids, which are linked by ester groups framing a polyester structure [17]. Moreover, granules and cork powders exhibit surface charge according to the environment pH. The point of zero-charge values between 2.1 and 4.6 can be reached, corresponding to acidic range. Meanwhile, about 40% of the total acid groups correspond to strong acids and 50% correspond to the overall phenolic OH groups. Therefore, its formidable characteristics are promising resources with potential application on adsorption technologies [18]. Herewith cork is a natural, cheap, renewable, fully sustainable, and biodegradable raw material with a very relevant advantage over other carbonaceous materials to be explored as a sorbent in many applications [19]. Since it was first described as an adsorbent in 2005, [20] its use in environmental applications for the removal of a variety of organic and inorganic pollutants has gained considerable attention, and a large number of research articles, as well as reviews, covering this approach have been published [17; 21; 22; 23; 24; 25; 26]. The cork employment in sample preparation for microextraction purposes was firstly reported in 2011 [27] and has been increasing over the years. In the first study, powdered activated carbons (AC) from cork waste were supported for bar adsorptive microextraction (BA\(\upmu\)E) as novel adsorbent phases for the determination of clofibric acid and ibuprofen in environmental and biological matrices [27]. The ACs were prepared by the chemical and physical activation of raw waste cork with K\({}_{2}\)CO\({}_{3}\) at 700 \({}^{\circ}\)C for 1 h and steam at 750 \({}^{\circ}\)C for 1 h, respectively. For the concentrations used in the study, both carbons completely removed the two target compounds. Further, cork was successfully applied as a coating for solid phase microextraction (SPME) devices in the determination of ultra-trace levels of organochlorine pesticides and polycyclic aromatic hydrocarbons (PAHs) in water samples, respectively, by gas chromatography [28; 29]. The preparation of the fibers was performed as follows: the cork powder (200 mesh) was immobilized into wires of NiTi (0.2 mm thickness and 2 cm length) containing epoxy glue. Then, the devices were placed into a heating block for 90 min at 180 \({}^{\circ}\)C and conditioned at 260 \({}^{\circ}\)C for 60 min in the gas chromatograph (GC) inlet port before using. The natural features of the cork were modified due the heating indicating that the biosorbent suffered thermal decomposition at high temperatures.