## 1 Introduction  
Essential oils are obtained from plants usually by water or steam distillation, by mechanical process, or by dry distillation. They are complex mixtures of 20-60 volatile, lipophilic and scented compounds, but some of essential oils may contain more than 300 different compounds. Typically, two or three compounds make up the majority (20-70%), while the remaining substances are present in much smaller amounts. The main components are responsible for the biological and pharmacological properties of the essential oils, but minor components also contribute to their effectiveness [1]. The content of essential oil in plant material usually varies from 1% to 3% of the plant weight, and its quality is influenced by many factors, such as environmental factors, harvesting period, geographic location, growth stage and processing of the plant material [2, 3, 4]. Essential oils have diverse biological properties, including antioxidative, anti-inflammatory, antimicrobial, and antitumor activity, and for this reason, phytochemical characterization as well as biological activity investigation is an important step in the discovery of promising herbal medicines. The analysis of an essential oil's chemical composition is an inevitable step in quality control, with gas chromatography coupled with mass spectrometry (GC-MS) or with flame ionization detector (GC-FID) as the most widely used methods. Though they provide information on the complete essential oil composition, they have several drawbacks, from time consumption to column selection, and reference compounds [5]. New techniques are being investigated to give faster and more cost-effective approaches to essential oil characterization. Fourier-transform infrared spectroscopy (FTIR) is a widely used, non-destructive technique that enables the rapid identification of the functional groups present in the major constituents of essential oils [6]. Nuclear magnetic resonance spectroscopy (NMR) is a more powerful method that can provide additional structural information and the identification of components present in essential oils. The main advantage of FTIR and NMR is obtaining direct information on the molecular structure and functional groups of the constituents without preliminary separation [5;7]. By combining the above spectroscopic methods or by comparing the spectra of the essential oils with the spectra of the individual components, the main components can be identified. Oxidative stress is caused by an imbalance in reactive oxygen species (ROS) and the capacity of living organisms to remove these reactive species. In some cases, endogenous antioxidants are not sufficient to neutralize ROS, which can damage cellular structures and biomolecules leading to many oxidative-stress-related diseases, such as heart disease, Alzheimer's disease, cancer, arthritis and others [3]. There is also the growing need for antimicrobial drugs of natural origin, due to the increasing number of resistant bacteria [8]. Essential oils have been recognized to have antioxidant and antimicrobial properties, and because of their diverse compositions, are ideal candidates for investigation of new applications in food, cosmetics and the pharmaceutical industry [9]. The content of metals in essential oils is associated with plant contamination, plant species, soil composition, harvesting period, geographical origin and climate regime. Fe, Cu, Ag and Zn contribute to the biological activities of essential oils, while Pb, Hg, Cd and As can lead to toxic effects, even at low levels [10;11]. Therefore, it is extremely important to determine the levels of metals in essential oils to predict their effects on human health. Within this research, we evaluated six essential oils: lemon (LEO, from _Citrus limon_ (L). [PERSON]), bay leaf (BLEO from _Laurus nobilis_ L.), clove (CEO, from _Syzygium aromaticum_ (L). [PERSON]. & L.M.Perry), star anise (SAEO, from _Illicium verum_ [PERSON]), oregano (OEO, from _Origanum vulgarte_ L.), and nutmeg (NEO, from _Myristica fragrans_ [PERSON]).