Effect of acidic mobile phase additives on chiral selectivity for phenylalanine analogs using subcritical fluid chromatography

Analysis of flavonoids with unified chromatography-electrospray ionization mass spectrometry-method development and application to compounds of pharmaceutical and cosmetic interest

In this project, we aimed at analysing flavonoid-type compounds with unified chromatography (joining supercritical fluid chromatography and enhanced fluidity liquid chromatography with carbon dioxide-methanol mobile phases covering a wide range of compositions) and diode-array and electrospray ionization mass spectrometric detection (UC-DAD-ESI-MS). First, the chromatographic method was developed for 9 standard flavonoid molecules from three different families (flavanols, flavanones and flavonols, glycosylated or not), with a strong focus on mobile phase composition to achieve the elution of a wide range of flavonoids with good chromatographic quality (efficiency and resolution). For this purpose, two stationary phases were selected (ACQUITY UPC² DEA and Diol), and five different additives (formic acid, citric acid, phosphoric acid, methanesulfonic acid and ammonium hydroxide) were successively introduced in the methanol co-solvent. The composition containing 0.1% methanesulfonic acid in methanol was retained as it provided the best chromatographic quality together with the possibility of hyphenating the chromatography to mass spectrometry. The DEA column appeared to provide the best efficiency and was retained for further method development. The gradient method was then optimized to achieve a fast analysis, which involved elution with a wide range of mobile phase composition (from 20 to 100% co-solvent in methanol) together with reversed flow rate and reversed pressure gradients at fixed temperature. The final gradient lasted 10 min, followed by 2.5 min of re-equilibration. Then, ESI-MS detection was optimized. Because the single-quadrupole mass spectrometer employed (ACQUITY UPC² QDa) allowed the variation of only a few parameters, a design of experiments was used to define the best compromise for three parameters (probe temperature, cone voltage and capillary voltage). The make-up fluid introduced before entering the MS was also varied: different compositions of methanol-water containing either formic acid, ammonium hydroxide or sodium chloride were tested. The best results in terms of signal-to-noise ratio were obtained with methanol containing 20 mM ammonium hydroxide and 2% water. The optimal UC-DAD-ESI-MS method was then applied to two different flavonoid formulation ingredients. The first one, hidrosmin (5-O-(β-hydroxyethyl)diosmin), is known for its vasoprotective properties and therefore employed in pharmaceutical formulations. The second one, α-glucosyl-hesperidin (sometimes referred to as vitamin P), is employed in cosmetic formulations. Identification of the major compounds in each sample was achieved with the help of MS detection. Graphical abstract.

The effect of acidic and basic additives on the enantioseparation of basic drugs using polysaccharide-based chiral stationary phases

The enantioseparation of nine commercially available basic drugs was achieved on polysaccharide-based chiral stationary phases with the acidic additive ethanesulfonic acid and the basic additive butylamine. Seven different commercially available CSPs were used for the study (AD, AS, OD, OJ, OG, OB, and OC). Mobile phase additives have been proven to be essential in obtaining satisfactory enantio-resolution in terms of both efficiency and selectivity. Significantly improved selectivities were obtained for the basic probe drugs with the acidic additive, ethanesulfonic acid, rather than the basic additive, butylamine. This is best seen with Chiralpak AS CSP. No enantioseparation for the nine drugs was observed when butylamine was used as an additive; however, satisfactory enantioseparation for the nine drugs was achieved using ethanesulfonic acid. Higher column efficiencies were observed with the acidic additive, especially when isopropanol was used as a modifier. Higher sensitivity was also achieved with ethanesulfonic acid because of the significantly lower background at the UV detection wavelength. The acidic additive was demonstrated to be superior to the basic additive for the enantioseparation of basic drugs using seven different polysaccharide-based CSPs. These results are counterintuitive to the common "rule of thumb" in enantioseparation that states acidic additives work best for acidic analytes and basic additives work best for basic analytes. The beneficial effects of acidic additive in enantioseparations observed in this study could significantly improve the applicability of polysaccharide-based CSPs for the enantioseparation of basic analytes.

Chiral separation of amines by high-performance liquid chromatography using polysaccharide stationary phases and acidic additives

A dramatic and beneficial effect of ethanesulfonic acid (ESA) on the chiral HPLC separation of basic compounds was found. Using a single chiral column and a starting mobile phase, more than half of a diverse set of amines was baseline separated. Changing alcohol content and alcohol type increased the success rate. Methanesulfonic acid (MSA) proved even more successful. The mechanism of this unexpected finding appears to be a combination of ion-pair salt formation in the mobile phase and increased binding with the chiral stationary phase (CSP) arising from a localized decrease in pH.

Origin of enhanced chiral selectivity by acidic additives for a polysaccharide-based stationary phase

The effects of ethanesulfonic acid (ESA) and n-butylamine as additives were studied for a wide variety of chiral compounds using the polysaccharide chiral stationary phase (CSP), Chiralpak AD. The mobile phase consisted of hexane-ethanol (90:10, v/v). The additives typically had small effects, with one exception: the acidic additive had an enormous effect on the chiral selectivity of amino acid esters. The improved chiral selectivity was largely due to the longer retention of the later eluting enantiomer. Retention behavior of amines indicated that the higher selectivity for amino acid esters owes to increased hydrogen-bonding donation by the amine group of the analyte. Computation establishes the feasibility of a planar complex between the analyte and the cliral stationary phase, involving a pair of complementary hydrogen-bonding groups on each species, enabled by protonation of the analyte. Retention behaviors for a range of structures point to steric hindrance as the third interaction to comprise the requisite three interactions in chiral recognition.

Application of molecularly imprinted polymers in supercritical fluid chromatography

Molecularly imprinted polymers (MIPs), for the templates free base racemic propranolol and the L-enantiomer of phenylalanine anilide (L-PA), were investigated as stationary phases in supercritical fluid chromatography (SFC). Large retention differences were observed on the propranolol MIP for both the template molecule and the structural analogue metoprolol compared to that observed on the corresponding blank polymer. Mobile phase composition and solute concentration were found to affect this retention behaviour. The phenylalanine anilide MIP (L-PA MIP) was found to be enantioselective in SFC with stronger retention observed for the template enantiomer. Throughout the study, characteristic imprinting peak shapes for the stronger retained template molecule were observed for both MIPs examined. After a number of days under supercritical fluid conditions, the performance of the photochemically initiated L-PA MIP was found to significantly deteriorate whereas the thermally initiated propranolol MIP revealed only small changes in its separation performance after a long term of operation. The separation behaviour of these two MIPs in SFC was compared with results obtained on the same columns in high-performance liquid chromatography (HPLC) both before and after their application in SFC.