Evaluation of novel amylose and cellulose-based chiral stationary phases for the stereoisomer separation of flavanones by means of nano-liquid chromatography.

Three polysaccharide-based chiral stationary phases, Sepapak(®) 1, Sepapak(®) 2 and Sepapak(®) 3 have been evaluated in the present work for the stereoisomer separation of a group of 12 flavonoids including flavanones (flavanone, 4'-methoxyflavanone, 6-methoxyflavanone, 7-methoxyflavanone, 2'-hydroxyflavanone, 4'-hydroxyflavanone, 6-hydroxyflavanone, 7-hydroxyflavanone, hesperetin, naringenin) and flavanone glycosides (hesperidin, naringin) by nano-liquid chromatography (nano-LC). The behaviour of these chiral stationary phases (CSPs) towards the selected compounds was studied in capillary columns (100μm internal diameter (i.d.)) packed with the above mentioned CSPs using polar organic, reversed and normal elution modes. The influence of nature and composition of the mobile phase in terms of concentration and type of organic modifier, buffer type and water content (reversed phase elution mode) on the enantioresolution (R(s)), retention factor (k) and enantioselectivity (α) was evaluated. Sepapak(®) 3 showed the best chromatographic results in terms of enantioresolution, enantioselectivity and short analysis time, employing a polar organic phase mode. A mixture of methanol/isopropanol (20/80, v/v) as mobile phase enabled the chiral separation of eight flavanones with enantioresolution factor (R(s)) in the range 1.15-4.18. The same analytes were also resolved employing reversed and normal phase modes with mixtures of methanol/water and hexane/ethanol at different ratios as mobile phases, respectively. Loss in resolution for some compounds, broaden peaks and longer analysis times were observed with these last two chromatographic elution modes. Afterwards, a comparison with the other two CSPs was performed. A lower discrimination ability of Sepapak(®) 1 and Sepapak(®) 2 towards all the studied flavanoids was observed. However, Sepapak(®) 1 allowed the separation of naringenin enantiomers and naringin stereoisomers in polar organic phase which were not resolved with the other two CSPs. The nature of the chiral selector was found to be of utmost importance for the resolution of the selected compounds. Indeed, significant differences in enantioresolution among the three tested CSPs were observed. With regard to the only few data reported in the literature for the resolution of this class of compounds using polysaccharide-based CSPs by high performance liquid chromatography (HPLC), the results obtained in this study by means of nano-LC showed higher (R(s)) values and shorter analysis time.