column If you would like a subscription, or more information, please contact your nearest Waters Office.

Transcription

1 column Waters A Reprint from Spring 995 If you would like a subscription, or more information, please contact your nearest Waters Office. Check the Waters website for up-to-date information: Published by Waters Corporation 34 Maple Street Milford, MA 0757 USA Tel: Toll-free: Fax: Editor: Uwe D. Neue, Ph.D. ISSN # Waters Corporation

2 Waters is giving a little something extra to the customer who purchases the 0,000th Symmetry column. Spring 995 Separation of Enantiomers by Micellar Electrokinetic Capillary Chromatography with Novel Chiral Surfactants Jeffrey R. Mazzeo, Edward R. Grover and Michael E. Swartz through the use of a chiral stationary phase, or through the use of a chiral mobile phase additive. Many chiral selectors have been described for the separation of enantiomeric mixtures by LC. There are more than fifty commercially available chiral LC columns, which include Pirkle-type, cyclodextrin, protein, carbohydrate, crown ether, and ligand exchange (). Table of Contents Novel Chiral Surfactants Interconverting Conformers Summary This paper describes the separation of enantiomers using a novel chiral surfactant and capillary electrophoresis (CE). Before describing the results, theoretical background is given that illustrates the advantage of using CE instead of LC in achieving the resolution of enantiomers. Enantioselectivity data obtained with a chiral stationary phase used in LC shows that the applicability of a given chiral selector is significantly increased when used in CE. Experimental results using chiral surfactants for the CE separation of enantiomers are consistent with this hypothesis. Introduction The separation and quantitation of enantiomeric mixtures is important because many drugs, agrochemicals, food additives and fragrances are chiral, and their enantiomers have been shown to exhibit different bioactivity. As a result, the FDA has published conservative guidelines to ascertain enantiopurity. A mixture of two enantiomers can be separated through the use of a chiral selector. Liquid chromatography (LC) is currently the predominant method for separating enantiomers. In LC, separation of enantiomers may be achieved Anionic Ion Pairing Literature Corner Ion Analysis by CE Application Notebook Polyolefin Separation Antisense Therapeutic Amino Acid Analysis In This Issue: Look for our NEW Pull-out Applications Notes

3 A researcher interested in separating a chiral compound into its two enantiomers and determining enantiomeric purity by LC is faced with a daunting task. Frequently, five or more chiral stationary phases must be screened before partial resolution is achieved. To optimize partitioning, the mobile phase can be altered; but in many cases, simple changes in the mobile phase can lead to complete loss of enantioselectivity. After considerable trial and error, the desired resolution may be obtained. To make matters worse, chiral stationary phases often suffer from reproducibility and/or stability issues. As a result, LC separation of enantiomers is generally an expensive (columns, mobile phases, method development time), labor intensive process. To understand why separation of enantiomers is difficult by LC, the resolution equation for chromatography must be examined: Figure a: A Graph of Pirkle Data (2) Showing Success Rate with N=5,000 % of Compounds Alpha 32% Resolved Data from reference 2 was plotted as alpha versus percent of compounds exhibiting at least this alpha. Shaded area represents the percent of the compounds examined which could be baseline resolved assuming a capacity factor of and 5,000 theoretical plates. Figure b: Graph of Pirkle Data (2) Showing Success Rate with N=00, Rs = N 2 4 ( ) α α A fundamental problem exists because enantioselectivity values (α) are not great enough to compensate for the low plate counts (N) obtained in LC. The plate counts of LC chiral columns are limited to a maximum of about 5,000. Partitioning can be optimized by increasing or decreasing the strength of the mobile phase. Unfortunately, these modifications can often influence selectivity in an unpredictable way. Given these limitations, the goal has been to develop phases with higher α. However, because pharmaceutical and agrochemical compounds are diverse in structure, a chiral phase which achieves high alphas for many structures has been elusive. Since a limit has apparently been reached in developing chiral LC phases with higher enantioselectivity, it is logical to try existing chiral selectors in a separation system which exhibits significantly higher plates (N) than LC. Capillary electrophoresis is an ideal system for chiral separations since typical plate counts for separation of k 2 k 2 + % of Compounds Alpha 98% Resolved Data from reference 2 was plotted as alpha versus percent of compounds exhibiting at least this alpha. Shaded area represents the percent of the compounds examined which could be baseline resolved assuming a capacity factor of and 00,000 theoretical plates. small molecules are 00,000 or greater. Referring to the resolution equation above, assume a capacity factor value of (k 2 = ) for a given separation. Under these conditions, an LC chiral column exhibiting 5,000 theoretical plates would require an alpha value of.20 to achieve baseline resolution (Rs =.5). A chiral selector in CE exhibiting 00,000 theoretical plates would require an alpha value of.04 to achieve baseline resolution. (An alpha value of.04 in LC would lead to a resolution of 0.3.) Therefore, a given chiral selector will resolve more enantiomeric mixtures when used in CE compared to LC. Another way to illustrate the advantages of high efficiency in the separation of enantiomers is to examine the enantioselectivity data published by Pirkle on a 2,2,2-trifluoro--(9-anthryl)- ethanol LC phase (2). A total of 52 racemates were screened with this phase. The enantioselectivity data for 2

4 these compounds can be plotted as alpha vs. % of the compounds exhibiting at least this alpha. For instance, 00% of the compounds had an alpha of.02 or greater, 98% had an alpha of.04 or greater, etc. Assuming a capacity factor value of and a desired resolution of.5, this phase could resolve 32% of the compounds screened when used in an LC mode exhibiting 5,000 theoretical plates (Figure a). The same phase, when used in a CE format exhibiting 00,000 theoretical plates, could resolve 98% of the compounds (Figure b). Exploiting the phase s enantioselectivity in CE could increase its applicability threefold. There are several desirable characteristics of a chiral selector for use in CE. First, the ideal selector should lead to high efficiencies, otherwise it will not be broadly applicable. Since the selector is dissolved in the buffer, which will constantly move through the detector, it should have low UV absorbance. It should also be usable over a wide ph range, since ph is a critical selectivity variable in CE. It should be easily implemented, and available in high chemical and optical purity. Finally, both enantiomers of the selector should be available. This will allow for exact enantiomer migration order reversal. The ability to reverse enantiomer migration order is important to improve quantitation of enantiomeric purity and to determine if a resulting separation is enantiomeric in nature. Many different chiral selectors have been described in CE, including proteins, crown ethers, cyclodextrins and bile salts. Proteins suffer from several disadvantages, including high UV absorbance, low efficiency, and finite ph range. Crown ethers have been applicable only to primary amines. Bile salts have been useful only for hydrophobic analytes and, in general, exhibit low enantioselectivity. Cyclodextrins currently enjoy the most popularity as chiral selectors for CE, but have not been widely applicable. None of these selectors allows for exact enantiomer migration order reversal. We have focused our efforts on the development of novel chiral surfactants for the separation of enantiomers by micellar electrokinetic capillary chromatography (MECC). (R)- and (S)-Ndodecoxycarbonylvaline, chiral surfactants synthesized in our laboratories, meet the required characteristics of a chiral selector for CE (3). The rest of this article will overview the performance of these novel chiral surfactants. Experimental All CE separations were performed on a Quanta 4000E Capillary Electrophoresis System (Waters, Milford, MA). Buffers were prepared with surfactant and disodium phosphate and/or disodium tetraborate. The ph was then adjusted with either sodium hydroxide or phosphoric acid ( J.T. Baker). Prior to use, AccuSep (Waters) capillaries, 50 µm I.D. x 60 cm (52.5 cm effective distance), were rinsed for five minutes with 0.5 mol/l NaOH. Between injections, the capillaries were rinsed with 0. mol/l NaOH (3 minutes) and buffer (3 minutes). Unless indicated, separations were performed with UV detection at 24 nm and a voltage of + 2 kv. Hydrostatic injection times ranged from 2 to 20 seconds. All buffers and samples were filtered through a 0.45 µm filter (Millipore, Bedford, MA). Separations were performed at ambient temperature. Samples were prepared as 0 mg/ml stock solutions in methanol and then diluted to 0. mg/ml in buffer. The micelle migration time was measured with the hydrophobic compound sulconazole. Racemates and the individual enantiomers (when possible) were purchased from Sigma (St. Louis, MO) and Aldrich (Milwaukee, WI). Figure 2: Structure of (S)-N-dodecoxycarbonylvaline Results and Discussion In MECC, anionic surfactant molecules are added to CE buffers at concentrations above their critical micelle concentration (cmc), resulting in aggregates of surfactant monomers known as micelles. These micelles consist of a hydrophobic core and a hydrophilic surface. Analytes partition between the aqueous CE buffer and the micelle, resulting in separation based on hydrophobicity, hydrogen bonding and ionic interaction. Incorporation of a chiral selector into the surfactant molecules affords micelles with chiral selectivity. The structure of (S)-N-dodecoxycarbonylvaline is shown in Figure 2. In this surfactant, the amino acid valine is the chiral selector. Under typical operating conditions, MECC operates with a finite migration window (unlike LC where there is an infinite elution window). In uncoated fused silica capillaries with cathodic electroosmotic flow, anionic micelles move slowly toward the cathode, while unpartitioned analytes move rapidly toward the cathode. This condition results in a migration window, the boundaries of which an analyte must migrate within. Because of this finite migration window, the resolution equation for MECC is different from the resolution equation for LC (4): Rs = N 2 4 ( ) α α k 2 k + 2 k 2 t aq t mc + t aq where t aq is the migration time of the analyte if it does not interact with the micelles and is the migration time if it completely partitions into the micelle. O (S) N dodecoxycarbonylvaline O H N H O OH 3

5 The equation for calculation of the capacity factor value is also different from the one used in LC (4): k = t r t aq t aq t r k optimum = t aq where t r is the observed migration time of the analyte. Note that if approaches infinity, both the resolution equation and the capacity factor equation are identical to those used in LC. The limited migration window also leads to an optimum value for the capacity factor, which is determined by the values of t aq and (5): The separation of pseudoephedrine enantiomers using (S)-N-dodecoxycarbonylvaline in Figure 3 demonstrates an MECC migration window. The baseline disturbance at minutes is the marker for the electroosmotic flow (methanol in this case, formamide may also be used), the migration time of a neutral compound which does not partition into the micelles. The peak at minutes is the micelle marker, the migration time for sulconazole, which is completely partitioned into the micelles. The peaks at and minutes are the two pseudoephedrine enantiomers, which are separated with a resolution value of 5.2. In this case, the alpha value was.24, and the average plate count was 04,000. Since in MECC the selective phase is dissolved in the buffer, k can be linearly changed through the surfactant concentration (phase ratio) (4), with no change in alpha. Figure 4 shows the separation of atenolol enantiomers at two surfactant concentrations. At 25 mm, k was 0.6, less than the optimum value of 2.8, and the resolution was 0.8. Increasing the surfactant concentration to 00 mm led to a capacity factor of 2.5 and a resolution of 2.7. In both cases, α was.04. This ability to optimize partitioning independent of selectivity is a significant advantage of MECC versus LC. 2 Figure 3: Separation of Pseudoephedrine Enantiomers The baseline disturbance at minutes is the electroosmotic flow marker (methanol) and the peak at minutes is the micelle marker (sulconazole). The resolution value of the two enantiomers, was 5.2, the alpha value.24 and the average plate count 04,000. Figure 4: Influence of Surfactant Concentration on the Separation of Atenolol Enantiomers mau mau mau Sample: 00 µg/ml racemic pseudoephedrine and 50 µg/ml sulconazole dissolved in buffer Buffer: 50 mm (S)-N-dodecoxycarbonylvaline 25 mm Na 2 HPO 4 /25 mm NaB 4 O 7 Reproduced from reference 3, with permission of Elsevier Science Publishers t os Minutes Sample: 00 µg/ml racemic atenolol dissolved in buffer Buffer: 25 mm or 00 mm (S)- N-dodecoxycarbonylvaline 25 mm Na 2 HPO 4 /25 mm NaB 4 O 7 Reproduced from reference 3, with permission of Elsevier Science Publishers. 25 mm SURFACTANT 00 mm SURFACTANT Minutes At 25 mm surfactant, the capacity factor for the two enantiomers was 0.6, less than the optimum value of 2.8, and the resolution was only 0.8. At 00 mm surfactant, k was 2.5, closer to the optimum of 2.8, and resolution was

6 (R)- and (S)-N-dodecoxycarbonylvaline can be used to exactly reverse the migration order of a separated enantiomeric pair. This ability is shown in Figure 5, where the two enantiomers of the surfactant are used to separate a 3: ratio of (S):(R) benzoin. With the (R)-surfactant, the (S)-enantiomer of benzoin was more highly retained and migrated second. With the (S)-surfactant, the (S)-enantiomer was less retained and migrated first. This ability to exactly reverse migration order is important to improve quantitation of enantiomeric purity and to determine if a resulting separation is enantiomeric. Peaks which represent the separation of enantiomers will be reversed in migration order when run with the two enantiomers of the surfactant, while achiral constituents of the sample migrate at the same time with both enantiomers of the surfactant. A general advantage of CE over LC is the ability to perform fast separations. Fast chiral separations can be obtained with (R)- and (S)-N-dodecoxycarbonylvaline when a low ionic strength buffer and high field strength are employed. Figure 6 shows the separation of N- methylpseudoephedrine enantiomers in less than 90 seconds under such conditions. Figure 5: Migration Order Reversal of Benzoin Enantiomers mau mau Sample: 3: ratio of (S)- to (R)-benzoin (200 µg/ml) dissolved in buffer Buffer: 25 mm (R)- or (S)-N-dodecoxycarbonylvaline 25 mm Na 2 HPO 4 /25 mm NaB 4 O 7 Reproduced from reference 3, with permission of Elsevier Science Publishers. (R) SURFACTANT (S) SURFACTANT 2 (R) (S) (S) (R) Minutes (R)-Benzoin migrated first with (R)-N-dodecoxycarbonylvaline, while (S)-benzoin migrated first with (S)-N-dodecoxycarbonylvaline. The migrate times of the peaks were the same in the two cases. Figure 6: Fast Separation of N-methylpseudoephedrine Enantiomers Sample: 00 µg/ml racemic N-methylpseudoephedrine dissolved in buffer Buffer: 25 mm (S)-N-dodecoxycarbonylvaline 50 mm CHES Capillary: 50 µm i.d. x 35 cm Voltage: + 30 kv Reproduced from reference 3, with permission of Elsevier Science Publishers Seconds The enantiomers of N-methylpseudoephedrine were separated in less than 90 seconds using a low ionic strength buffer, high field strength and short capillary. 5

7 (R)- and (S)-N-dodecoxycarbonylvaline have been successful in separating neutral and basic enantiomeric mixtures which contain at least two hydrogen bonding sites near the chiral center. Compounds which are acidic will separate if they are reasonably hydrophobic. Hydrophilic acids are generally not separable due to low partitioning, most likely due to charge repulsion. Performing separations at acidic ph would minimize any charge repulsion, however, (R)- and (S)-N-dodecoxycarbonylvaline are insoluble below ph 6.5. A list of chiral compounds which have been separated with (R)- and (S)-N-dodecoxycarbonylvaline to date is given in Table I. Compounds which have structures similar to those in Table I should be separable. We continue to add to this list as improved methods of implementing the chiral surfactants are developed. The novel chiral surfactants described in this article are currently under development at Waters. If you would like to be kept informed on new developments in this area, please check box number 3 on the reply card. Conclusion Capillary electrophoresis promises to play a major role in the analytical scale separation of enantiomeric mixtures. CE separations of enantiomers have the potential to replace LC methods, due to higher efficiency, faster methods development time, reduced solvent consumption, and ease of use. MECC with synthetic chiral surfactants represents the ideal method for separation of enantiomers in CE. The advantages of this approach include exact enantiomer migration order reversal, simultaneous Table I: Compounds Baseline Resolved with (R)- and (S)-N-dodecoxycarbonylvaline Acebutolol AQC-Amino Acids Aminoglutethimide Atenolol Benzoin Bupivacaine CBZ-Amino Acids Disopyramide Ephedrine Epinephrine FMOC-Amino Acids Glutethimide Homatropine Isoproterenol Ketamine Laudanosine chiral and achiral separations, straightforward methods development, and high purity. Waters synthetic chiral surfactant technology offers the ability to perform controlled surfactant design, so that those structural features which influence enantioselectivity can be studied and combined into new surfactants. Our goal is to develop 4-6 complementary chiral surfactants which will separate all chiral compounds. In the next article, method development with the surfactants will be described. Methadone Metoprolol Nadolol N-Methylpseudoephedrine Norephedrine Norphenylephrine Octopamine Oxprenolol Phenylglutethimide Pindolol Propranolol PTH-Amino Acids Terbutaline Tetramisole Tolperisone Warfarin References () M. Zief and L.J. Crane (Editors), Chromatographic Chiral Separations, Marcel Dekker, Inc., New York, 988. (2) W. Pirkle and D. House, J. Org. Chem., 44 (979) 957. (3) J. Mazzeo, E. Grover, M. Swartz and J. Petersen, J. Chromatogr., in press. (4) S. Terabe, K. Otsuka and T. Ando, Anal. Chem., 57 (985) 834. (5) J. Foley, Anal. Chem., 62 (990) 302. Special Offer If you are using Bondclone, Whatman s Equivalent or any other brand product that claims to be similar to a Waters µbondapak C 8, we have a special offer for you. Please call , ext for information on this special offer. µbondapak C 8 is a trademark of Waters Corporation. Bondclone is a trademark of Phenomenex, Inc. 6