Selectivity in mixed-mode HPLC can be governed through mobile phase content, selectivity factor (\(\alpha\)) values of critical peak pairs were set as optimization goals (\(\alpha_{\text{BP/FT}}\), \(\alpha_{\text{TH/BP}}\), \(\alpha_{\text{LOF/DOX}}\) > 1.2). The retention factor of last eluting analyte (k\({}_{\text{DOX}}\)) was desired to not be greater than 10 in order to assure the rational analysis time. The experimental plan and mathematical models were obtained with Design-Expert 7.0.0 software. Obtained models were statistically evaluated (R\({}^{2}\), pred. R\({}^{2}\), and adj. R\({}^{2}\) > 0.99). The most pronounced effect on the responses which followed had the organic solvent content, whose increase lead to \(\alpha_{\text{TH/BP}}\) enhancing, and had the opposite effect on \(\alpha_{\text{BP/FT}}\), \(\alpha_{\text{LOF/DOX}}\), and k\({}_{\text{DOX}}\). Higher ionic strength corresponded to better separation of BP from both FT and TH. This can be assigned to competitive behaviour between analytes and ions present in mobile phase [2]. Time analysis shortening resulted as a consequence of higher values of organic solvent, ionic strength, and temperature. However, mobile phase pH had the opposite effect, which can be related to more expressed cationic interactions and greater retention of basic analytes [2]. Taking into account effects of all factors, and according to generated Derringer's desirability function for multiobjective decision making, the values of the factors that give the optimal responses are selected to be 44% of acetonitrile, ionic strength 36 mM, temperature of 38 \({}^{\circ}\)C, and mobile phase pH of 5.1. ## References  
* Orlovsky and Zelechonok (2011) Orlovsky, V.; Zelechonok, Y. Evolution of Mixed-Mode Chromatography. _Chromatogr. Today_**2011**, 24-28. Available online: [[https://www.chromatographyoday.com/article/gc-mdgc/32/sielc-technologies/evolution-of-mixed-mode-chromatography/1070](https://www.chromatographyoday.com/article/gc-mdgc/32/sielc-technologies/evolution-of-mixed-mode-chromatography/1070)]([https://www.chromatographyoday.com/article/gc-mdgc/32/sielc-technologies/evolution-of-mixed-mode-chromatography/1070](https://www.chromatographyoday.com/article/gc-mdgc/32/sielc-technologies/evolution-of-mixed-mode-chromatography/1070)) (accessed on 8 November 2011). * Lemasson et al. (2018) Lemasson, E.; Richer, Y.; Bertin, S.; Hennig, P.; West, C. Characterization of Retention Mechanisms in Mixed-Mode HPLC with a Bimodal Re-versed-Phase/Cation-Exchange Stationary Phase. _Chromatographia_**2018**, _81_, 387-399. * _16.9. Development and Validation of a LC-DAD-ESI-MS Method for the Determination of Phenolic Compounds of the Solid Residues from the Essential Oil Industry_
* _M. Irakli 1,* P. Chatzopoulou 1 and C. Biliaderis_**2**
* Hellenic Agricultural Organization--Demeter Institute of Plant Breeding and Genetic Resources, Thermi, 57001 Thessaloniki, Greece
* \(2\) Department of Food Science and Technology, School of Agriculture, Aristotle University of Thessaloniki, P.O. Box 235, 54124 Thessaloniki, Greece
* _*_ Correspondence: irakli@ipgrb.gr  
Medicinal and aromatic plants play a vital role in people's health, and recently they have been exploited as promising ingredients to develop novel products in sectors such as the pharmaceutical, cosmetic, food, and pesticide industries [1,2]. A feasible use can be the production of essential oils, by steam distillation processes, to obtain quality products in a traceable supply chain. However, a considerable number of solid residues which remainafter their essential oil distillation are currently unexploited, as they represent a major disposal problem for the industry concerned. Nonetheless, they are also promising sources for the recovery of health-promoting phenolic compounds which may be a profit in a sustainable way from low-cost raw material. These compounds may be used as natural antioxidants and functional food ingredients.