# Returning to Nature for the Design of Sportive Phases in Solid-Phase Microextraction
## 5 Agricultural By-Products
In this context, [PERSON] et al. have proposed the oxidation of the garlic peel with nitric acid to increase the surface acidic functional groups [92]. In this way, the sorption capacity was increased by a factor of 10 for quinolone antibiotics. The authors systematically studied the effect of the oxidation of garlic peel in terms of porosity and functional group distribution. Concerning the superficial morphology, the scanning electron microscopy analysis demonstrated that while the garlic peel exhibits a compact net structure, the oxidized one showed a corroded one and smaller pore irregularly distribution on the surface. The infrared spectra pointed out that after the acidic treatment, some unreactive groups were transformed into carboxylic moieties, while a weaker absorption bandwas observed in the aromatic ring region, which indicates their decomposition during the treatment. Moreover, the evaluation of the cellulose, hemicellulose, and lignin contents also indicates an increase of the former in the oxidized garlic peel, this fact correlatating with the higher retention for quinolones on the modified sorbent. The elimination of dyes, pigments, and other colorants using activated carbon is probably the most effective remediation procedure due to the almost irreversible interaction established between the sorbent and the analyte. Agricultural by-products have been identified as an environmentally friendly source of activated carbon, which is usually generated by carbonization at high temperatures [93]. Following this idea, several carbon-based sorbents have been prepared using banana and pomelo peels as starting materials. Four relevant examples will be briefly presented. [PERSON] et al. prepared a hierarchical porous carbon by the treatment of banana peels at 160 \({}^{\circ}\)C and further carbonization at 900 \({}^{\circ}\)C under an inert atmosphere [94]. The solid showed an intrinsic microporous structure with the presence of meso and macropores, which provides a high surface area (561.8 m\({}^{2}\)/g). The formation of graphitized carbon during the synthesis allows the interaction with aromatic compounds via \(\pi\)-stacking. Therefore, the authors evaluate the potential of the hierarchical porous carbon synthesized as a sorbent for carbamate pesticides potentially present in cucumber and watermelon samples. Cartridge SPE was selected as an extraction mode by packing 10 mg of the solid. The enrichment factors obtained were in the range of 80-114 depending on the carbamate and the sample, which resulted in detection limits between 0.05 and 0.30 ng/g. Pomelo peels have also been used as a precursor material to obtain nanoporous carbon [95]. In this approach, chemical activation with KOH was applied. The grounded and dried pomelo peel was precarbonized at 450 \({}^{\circ}\)C for 2 h in an inert atmosphere and thoroughly mixed with 1 M KOH aqueous solution. After a thermal threaten (up to 800 \({}^{\circ}\)C), the nanoporous carbon was activated being ready to use as a sorbent phase in SPME fiber. For this aim, the solid was immobilized onto a stainless-steel wire via physical coating approach and further protected with a silicone sealant. The SPME fiber was evaluated for the headspace extraction of benzene homologs from waters and soil before their determination by gas chromatography-mass spectrometry. As evaluated by the authors, their approach resulted in better sensitivity and comparable selectivity as regards reported methods for the same analytical problem. Probably, dispersive microextraction can be identified as the simplest and most efficient approach among miniaturized sample treatment techniques [96]. The use of an extractant phase dispersed in the sample matrix facilitates the interaction with the analytes, thus enhancing the efficiency of the whole process. However, the recovery of the extractant enriched with the analytes is challenging.