Cyclodextrins as a chiral mobile phase additive in nano-liquid chromatography: comparison of reversed-phase silica monolithic and particulate capillary columns

Enantioseparations of racemic nonsteroidal anti- inflammatory drugs (naproxen, ibuprofen, ketoprofen, flur- biprofen, suprofen, indoprofen, cicloprofen, and carprofen) were performed by nano-liquid chromatography, employing achiral capillary columns and heptakis(2,3,6-tri-O-methyl)-?- cyclodextrin (TM-?-CD) or hydroxylpropyl-?-cyclodextrin (HP-?-CD) as a chiral mobile phase additive (CMPA). Working under the same experimental conditions (in terms ofmobile phase and linear velocity), the performance of a RP- C18 monolithic column was compared with that of a RP-C18 packed column of the same dimensions (100 ?mi.d. × 10 cm). Utilizing a mobile phase composed of 30% ACN (v/v) buff- ered with 50 mM sodium acetate at pH 3, and containing 30 mM TM-?-CD, the monolithic column provided faster analysis but lower resolution than the packed column. This behavior was ascribed to the high permeability of the monolithic column, as well as to its minor selectiv- ity. HP-?-CD was chosen as an alternative to TM-?-CD. Employing the monolithic column, the effects of different parameters such as HP-?-CD concentration, mobile phase composition, and pH on the retention factor and the chiral resolution of the analytes were studied. For the most of the analytes, enantioresolution (which ranged from Rs01.80 for naproxen to Rs00.86 for flurbiprofen) was obtained with a mobile phase consisting of sodium acetate buffer (25 mM, pH 3), 10% MeOH, and 15 mM HP-?-CD. When the same experimental conditions were used with the packed column, no compound eluted within 1 h. Upon increasing the percent- age of organic modifier to favor analyte elution, only suprofen eluted within 30 min, with an Rs value of 1.14 (20% MeOH). Replacing MeOH with ACN resulted in a loss of enantiore- solution, except for naproxen (Rs00.89).