Effect of Ionic Liquids as Mobile-Phase Additives on Chromatographic Parameters of Neuroleptic Drugs in Reversed-Phase High-Performance Liquid Chromatography

Abstract

Ionogenic basic compounds belonging to phenothiazine derivatives were analyzed in the reversed-phase system and were modified with the addition of three ionic liquids: 1-ethyl-3-methyl-imidazolium hexafluorophospate (EMIM PF₆), 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIM PF₆), and 1-butyl-3-methyl-imidazolium chloride (BMIM Cl). The effects of the concentration and the type of ionic liquid on the analytes' retention, peak symmetry, and efficiency were examined. The following trends increase analytes' retention factor and improve system efficiency: BMIM PF₆ > EMIM PF6 > BMIM Cl. With its asymmetric cation enlarged with hydrophobic substituents and a chaotropic anion, BMIM PF₆ appeared to be the most advantageous one. The isotherm of adsorption of this reagent presents a typical Langmuir course. By the application of high-performance liquid chromatography, lipophilicity parameters were established (lnk_w, S, ϕ ₀) for the investigated compounds. Chromatographic systems modified with ionic liquids were compared to buffered organic–aqueous mobile phase and eluent containing chaotropic salt additive.

Keywords:

• Ionic liquids
• phenothiazine derivatives
• retention behavior
• RP-HPLC

Notes

Notes. All calculations were performed using the HSM program. The following equation is used to calculate the number of theoretical plates according to USP standards: N = 16 (RT/W)² where RT is the actual full retention time of the appropriate peak and w is the peak width obtained by drawing tangents to each side of the peak and calculating the distance between the two points where the tangents meet the baseline. The HSM program uses the following equation to calculate asymmetry: A_s = 1/2 (1 + B/A), where A and B are evaluated at a 5% peak height of an appropriate peak. The capacity factor (k) is calculated as follows: k = t_R/t₀ − 1, where t_R is the actual retention time of the individual peak, t₀ is the elution time of the unretained sample (thiourea). The criterion for asymmetry or theoretical plates is not satisfied.