## 1 Introduction  
Essential oils (EOs) are natural, volatile and complex mixtures of lipophilic compounds, often including terpen, phenol-derived aromatic compounds and aliphatic compounds, that are biosynthesized by aromatic plants as their secondary metabolites and represented by a strong odor [1]. Ecologically, they are important for the plant defense system by providing protection against grazers, taking part in fire and drought tolerance and attracting pollinators and other animals for seed dispersal. The most often reported bioactivities of EOs and their components are antimicrobial, antiviral, antinociceptive, anticancer, anti-inflammatory, digestive, semiochemical and free radical scavenging [2], while new potential applications in human health, nutrition, agriculture and the environment are continuously emerging [1]. The volatile components of EOs are biosynthesized in specialized secretory structures such as glandular trichomes located on the surface, or secretory cells and ducts situated within the tissues of various plant organs of aromatic plants. The conventional techniques for volatile organic compounds (VOCs) extraction include, e.g., hydrodistillation and steam distillation, expression (cold pressing), solvent extraction [3] and _enfeurage_ (a classical method for extracting volatile oil from flowers using a layer of animal fat) [4]. Along with conventional methods used for the extraction of volatile compounds from plants, solid-phase microextraction (SPME) is considered a convenient alternative technique for a variety of sample matrices (liquid/solid samples or headspace (HS) vapors). This technique bypasses the distillation or solvent extraction procedure [2]. It uses a fused silica fiber that is externally coated with an appropriate stationary phase such as polydimethylsiloxane (PDMS), polyacrylamide (PA), PDMS/divinylbenzene (DVB), carbowax (CW)/DVB, and carboxene (CAR)/PDMS on which the headspace VOCs are adsorbed [5]. After the extraction, the fiber is usually directly inserted into the injector for gas chromatography analysis usually coupled with mass spectrometry (GC-MS), which is the most often used technique for the chemical characterization of EOs [2]. SPME has been widely used for chemical analysis of volatile components of different types of food substances, flavors, and medicinal plant materials because it is a simple, rapid, inexpensive and solvent-free technique that demands only a small amount of sample and reduces the degradation of VOCs [6; 7]. Furthermore, headspace solid-phase microextraction (HS-SPME) requires no sample pre-treatment, allows high selectivity for small compounds and reusability of various commercially available SPME fibers [8], which makes this technique generally well accepted for VOCs extraction. The genus _Sideritis_ L. (family Lamiaceae) includes more than 150 species of annual or perennial herbs and shrubs, distributed in temperate and tropical regions of the Northern Hemisphere, with the majority of species found in the Mediterranean area, North Africa, the Iberian Peninsula, the Middle East and in the Macaronessian region [9; 10; 11]. The use of aerial parts of many _Sideritis_ species has been well reported, especially in Albania, Bulgaria, Greece, Macedonia and Turkey, where they are consumed as herbal teas and applied in traditional medicine for the treatment of various disorders of the gastrointestinal and respiratory system, as well as for burns and wounds [10; 12]. In recent decades, many researchers have been trying to justify the traditional uses of _Sideritis_ species by elucidating their phytochemical composition and investigating potentially useful pharmacological activities of these species [10; 13; 14]. Although species from Lamiaceae family are well known for their aromatic properties and are recognized as the most important source of EOs with economical interest among Angiosperms [3], the species of the genus _Sideritis_ have been reported as poor in EOs [10; 15; 16].