Electronic cigarettes (e-cigarettes) are devices designed to vaporize a liquid solution, offering an alternative to traditional tobacco consumption. The identification, detection, and analysis of the compounds present in these devices are crucial for understanding their impacts on health and the environment. Numerous studies have identified a diverse range of compounds emitted by e-cigarettes, including well-known substances such as nicotine, thermal degradation products, and other toxicants that may be harmful or carcinogenic. Although e-cigarettes are often considered an alternative to conventional smoking, they are not without risks. Recent research has increasingly focused on assessing the health impacts of e-cigarettes, integrating findings from various scientific disciplines. Two primary analytical approaches are used for the sample preparation, identification, and quantification of these compounds. The first approach focuses on aerosol analysis, utilizing techniques such as headspace static extraction and gas chromatography coupled with mass spectrometry (GC-MS). The second approach is directed towards liquid analysis, employing liquid–liquid extraction techniques and liquid chromatography (LC) systems. Given the constant publication of new research in this area, a comprehensive review that consolidates information on identified compounds, sample preparation methods, and extraction and analysis techniques is necessary to integrate current knowledge and address emerging findings.

Quercetin, one of the most abundant flavonoids in nature, has attracted the attention of many researchers due to its chemical and biological properties. A series of metal–quercetin complexes (Cu²⁺, Co²⁺, Zn²⁺, Sn²⁺, Al³⁺, Cd²⁺ and Mg²⁺) were synthesized and systematically characterized by Fourier transform infrared spectroscopy (FTIR), UV-visible spectroscopy (UV–Vis) and nuclear magnetic resonance (NMR). These analyses confirmed that the complexes predominantly form through coordination with the 4-carbonyl group and adjacent phenolic hydroxyls. This induces measurable shifts in the ν(C=O), ν(O–H), and π→π* transition bands relative to free quercetin. The antioxidant capacity of the complexes was evaluated using 2,2-Diphenyl-1-Picrylhydrazyl (DPPH^•) radical scavenging method, 2,2′-Azinobis(3-Ethylbenzothiazoline-6-Sulfonic Acid) (ABTS^•)⁺ radical activity, and Ferric Reducing Antioxidant Power (FRAP) assay. Several complexes exhibited higher radical scavenging efficiency than quercetin, with inhibition percentages exceeding 80% in the DPPH^• and ABTS^•+ assays. Others showed reduced activity due to the masking of redox-active hydroxyl groups during metal coordination. FRAP results corroborated these trends, indicating metal-dependent modulation of reducing power. Antimicrobial evaluation revealed that selected complexes were more active than free quercetin, particularly against Staphylococcus aureus and Candida spp., with minimum inhibitory concentrations (MICs) ranging from 75–250 μg mL⁻¹. Overall, metal complexation significantly alters the electronic structure and biological behavior of quercetin, highlighting the potential of metal–flavonoid complexes as multifunctional antioxidants and antimicrobials.