Ind. Eng. Chem. Res. 1996, 35, 169-175
169
SEPARATIONS Characterization of Chiral Adsorbents on the Chromatographic Separation of Praziquantel Enantiomers Lim Bee-Gim and Ching Chi-Bun* Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore
Various chiral stationary phases (CSPs) are characterized for the preparative scale chromatographic separation of the enantiomers of an anthelmintic drug, praziquantel. The separation factors achieved for the intended separation in an analytical scale for various CSPs are evaluated. Other criteria for the choice of CSP for the separation are also analyzed. The magnitudes of the equilibrium and kinetic coefficients are evaluated for the selected CSP. Other system properties required as input parameters for the modeling and simulation of the process under consideration are also determined. Introduction The fundamental problem involved in the separation of enantiomers is that these nonsuperimposable mirrorimage forms have identical chemical and physical properties in a symmetrical or nonchiral environment. Therefore, enantiomers cannot be separated using conventional analytical or preparative methods. It has long been recognized that chromatographic methods could offer distinct advantages over classical techniques in the separation and analysis of enantiomers. The chromatographic resolution of enantiomers requires the introduction of an asymmetric or chiral environment. There are basically two approaches to the separation of an enantiomeric pair by chromatography. In one approach (indirect), the enantiomers are converted into diastereomeric compounds by a reaction with a homochiral reagent. Since diastereomers have different physicochemical properties in an achiral environment, they can be separated on a routine, achiral stationary phase. The other approach (direct) can be achieved by passing the enantiomers through either a column containing a chiral stationary phase (CSP) or an achiral column using a mobile phase containing a chiral additive. In either variant of the second approach, one depends on differential, transient diastereomer formations between the solutes and the selector to effect the separation. Among these methods, direct separation of racemates using a CSP has been a target of many research workers for the last 2 decades. Since then, the development of CSP for the direct chromatographic separation of enantiomers has become an area of intense research. In this paper, the characterization of several chiral adsorbents for the separation of praziquantel enantiomers (Figure 1) will be presented. This separation is essential for pharmaceutical application of the enantiomer. (-)-Praziquantel has the advantage of high efficacy and low toxicity compared with racemic praziquantel for population-based chemotherapy of a broad range of parasitic infections (Liu et al., 1988; Wu et al., 1991). The equilibrium and kinetic parameters for the * Corresponding author.
0888-5885/96/2635-0169$12.00/0
Figure 1. Chemical structure of praziquantel.
separation of the enantiomers using several CSPs will be determined by a chromatographic method. Theoretical Section The hydrodynamics of a chromatographic column can be described through analysis of the residence-time distribution of the eluent that is classically derived from the response curve to a Dirac injection. This response curve of the column to a Dirac injection contains information connected to the properties of the column. These properties, which include the equilibrium and mass-transfer parameters, can be extracted by the method of moments. Prior to the modeling of retention time for any solute, the zero retention time, t0 or t0R, has to be defined. Two different definitions are often taken because of the two levels of porosity existing in the column; the internal porosity, i, and the external porosity, . The usual (chromatographer’s) definition of zero retention time is
t0R ) TL/u
(1)
Total porosity, T, is defined as:
T ) + (1 - )i
(2)
For a retained solute, the capacity factor, k′ is defined as:
tR ) t0R(1 + k′)
(3)
Another definition of zero retention time which is more © 1996 American Chemical Society
170
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996
i
ii
iv
iii
Figure 2. Plots of first moment versus inverse volumetric flowrate on (i) Chiralcel OD, (ii) Chiralcel OJ, and (iii) Chiralpak AD columns and (iv) a column packed with swollen MCTA (9, (+)-praziquantel; 2, (-)-praziquantel; (, TTBB). Table 1. Details of CSPs Used for the Study of Performance in Separating Praziquantel Isomers particle size (µm)
Chiral stationary phase Chiralcel OD: cellulose tris(3,5-dimethylphenylcarbamate) on silica gel Chiralcel OJ: cellulose tris(4methylbenzoate) on silica gel Chiralpak AD: amylose tris(3,5-dimethylphenylcarbamate) on silica gel MCTA: microcrystalline cellulose triacetate
column dimension (L × i.d.) (mm)
10
250 × 4.6
10
250 × 4.6
10
250 × 4.6
25-40
300 × 6.5
propitious for modeling is given by
t0 ) L/u ) L/v
(4)
Therefore, for a retained solute,
(
tR ) t0 1 +
1- K
)
(5)
For the simple rate model, the first moment and second moment equations can be combined to give the following expression which defines the chromatographic plate height, HETP:
remark commercially packed recommended solvent: hexane, propanol commercially packed recommended solvent: hexane, propanol commercially packed recommended solvent: hexane, propanol CSP packed into column after being swollen in boiling ethanol for 30 min recommended solvent: methanol, ethanol
σ2L ) HETP ) µ12 2DL 1 + 2v 1+ v 1 - kK (1 - )K
( ) [
]
-2
(6)
It is evident that the contributions of axial dispersion and the mass transfer resistance are linearly additive. Gunn (1987) suggested that DL can be expressed as:
DL ) ηDm + λv
(7)
Since molecular diffusivities are small in liquid system, eq 7 can be approximated to consist of only the second term, the eddy diffusion term. Therefore, eq 6 can be simplified as:
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 171
σ2L ) HETP ) µ12
( ) [
2λ + 2v
]
1 1+ 1 - kK (1 - )K
-2
(8)
Thus, by calculating the first and second moments of a series of elution curves and plotting HETP versus fluid velocity, a straight line should result, with the intercept giving the axial dispersion contribution and the slope directly related to the mass-transfer resistance. Characterization of Various CSPs Experimental Section. Several adsorbents were explored in our study of the separation of praziquantel enantiomers. These included Chiralcel OD, Chiralcel OJ, Chiralpak AD (Daicel Industries, Tokyo, Japan), and microcrystalline cellulose triacetate (MCTA) (Merck, Darmstadt, Germany). The details of these chiral stationary phases (CSP) used are shown in Table 1. The experiments were carried out using a singlecolumn liquid chromatographic system. The system consisted of a Shimadzu Model LC-9A solvent delivery unit (Tokyo, Japan), a Rheodyne Model 7125 syringe loading valve fitted with a 10 µL sample loop (Cotati, CA), a Varian Model 2070 spectrofluorometer (Palo Alto, CA) with excitation and emission wavelengths set at 270 and 300 nm, respectively, and a data acquisition/control unit which was interfaced with a microcomputer for data storage and processing. The solvents used were of HPLC grade and purchased from Fisher Scientific (Pittsburgh, PA). The optically pure (-)- and (+)praziquantel were kindly provided by Chongqing Medical University (Chongqing, China), and 1,3,5-tri-tertbutylbenzene (TTBB) was purchased from Aldrich Chemical Co. (Milwaukee, WI). Results and Discussion. Determination of the Separation Factor for Various CSPs. The capacity factor, k′, was calculated from the following expressions:
k′ )
tR/Q - t0R/Q t0R/Q
(9)
The exact determination of t0R is important, since the capacity factors of the solutes refer to this quantity. Moreover, this value is necessary for calculation of the total porosity of the packing, T. Koller et al. (1983) have investigated the retention of a variety of compounds, such as phenol, di- and trihydroxybenzene, cresols, toluene, and trialkylbenzenes on MCTA. Among these compounds, TTBB elutes first. TTBB has also been used widely for determination of t0R for other cellulose or amylose derivative CSPs (Ching et al., 1992; Katti et al., 1992; Mannschreck et al., 1985). Although the sorption of solutes for the cellulose or amylose derivative is strongly supported by a phenyl group (Francotte et al., 1985), the latter is shielded by the tertbutyl groups in the case of TTBB, which would explain why this compound is not retained on these CSPs. On the other hand, an exclusion mechanism is not likely, due to the relatively small molecular size of TTBB. In this study, TTBB was therefore used to determine the t0R. By plotting the mean retention time of the praziquantel enantiomers and the unretained compound against various volumetric flowrates, the capacity factors can be evaluated from eq 9. These plots for various
Figure 3. Structure of (a) Chiralcel OD and (b) Chiralpak AD. Table 2. Capacity Factors and Separation Factors Achieved with Different Chiral Stationary Phases and Mobile Phase Composition CSP
mobile phase
k′(-)
k′(+)
R
Chiralcel OD
hexane:propanol ) 9:1 hexane:propanol ) 8:2 hexane:propanol ) 7:3 hexane:propanol ) 9:1 hexane:propanol ) 8:2 hexane:propanol ) 7:3 hexane:propanol ) 9:1 hexane:propanol ) 8:2 hexane:propanol ) 7:3 methanol
1.878 2.826 6.385 0.955 1.917 3.805 1.900 4.489 8.304 0.805
2.369 3.531 8.103 0.955 1.917 3.805 1.615 3.826 7.127 2.347
1.261 1.250 1.269 1.000 1.000 1.000 1.177 1.173 1.165 2.916
Chiralcel OJ Chiralpak AD MCTA
CSPs, at one of the mobile phases studied, are given in Figure 2i-iv. The separation factor, R, can be calculated from eq 10:
R ) k2′/k1′
(10)
where k1′ and k2′ are the capacity factors for the first and second eluting enantiomers, respectively. The capacity factors and separation factors obtained using different CSPs and mobile phase compositions are shown in Table 2. In Table 2, it is observed that both Chiralcel OD and Chiralpak AD are able to resolve the isomers of praziquantel efficiently. However, the elution order of the two isomers differs for the cellulose and amylose CSPs. Cellulose and amylose differ only in the configuration on the 1-position of each glucose unit (Figure 3). Different chiral recognition observed suggests that two or more carbamate groups of adjacent glucose units are involved for chiral recognition. If chiral recognition were attained only on a glucose unit, OD and AD CSPs would have exhibited similar chiral recognition. The different arrangement of glucose units along the polysaccharide chain between triphenylcarbamates of cellulose and amylose (Bittiger and Keilich, 1969; Vogt and Zugenmaier, 1985) could have led to the different arrangement of the carbamate groups that are responsible for chiral recognition. In Chiralcel OD, a carbamate group at the 6-position is close to the carbamate group at the 2- and 3-positions on two neighboring glucose units. However, in Chiralpak AD, a carbamate group at the 6-position is close to the carbamate group at the 6-position on two neighboring glucose units (Vogt
172
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996
and Zugenmaier, 1985). Therefore, the different chiral recognition of Chiralcel OD and Chiralpak AD seems to be mainly attributable to the different chiral cavity or space built by two or more carbamate groups on adjacent glucose. The Chiralcel OJ column was not able to resolve praziquantel enantiomers due to the lack of interactions with the solute. This can be deduced from the low retention time (capacity factor) of the enantiomers on this CSP, although a long retention time may not necessarily lead to high chiral recognition. By changing the mobile phase composition to contain a higher proportion of alcohol content, retention time decreases. However, there was no significant difference in the separation factor for all three Daicel columns. There can be a great difference between the main interaction(s) responsible for retention and the interactions responsible for chiral recognition. Therefore, the increase of k′, observed in this case, may not always lead to an increase in the R value. Among the CSPs studied, MCTA provided the highest enantioselectivity. The mechanisms responsible for chiral recognition on this CSP are not yet well understood. It has been postulated that the fiber structure of cellulose with a crystalline area remains unchanged by the heterogeneous acetylation. The lamellar arrangement of the cellulose in the manner of a crystalline lattice acts as a molecular sieve, making possible the inclusion of enantiomer. The inclusion of molecules in these cavities is mainly governed by the shape of the molecules and only to a minor extent by other factors such as electrostatic interactions involving the functional groups of the molecules (Hesse and Hagel, 1976). Therefore, the enantioseparation seems to be due to inclusion into the asymmetric cavities. Criteria for CSP Choice. The criteria for packing material choice in analytical scale chromatography are, namely, performance and selectivity. However, in preparative-scale systems, cost and loadability also need to be considered. It was deduced from the results that MCTA provided the highest separation factor. However, the chromatogram showed that the peaks were more dispersed. This could be due to the nonuniform shape and size of the packing materials and a higher mass-transfer resistance involved in the separation. Among the few CSPs characterized, only MCTA is available commercially, in quantities necessary for preparative-scale separation. MCTA could be synthesized from a relatively low-cost natural product by industrial transfer to microcrystalline cellulose, acetylation, and classification into the desired particle sizes (Mannschreck et al., 1985). Sorbent availability at low cost will be important for future large-scale separations design. The other Daicel CSPs are only available in prepacked columns, and the price is 100-fold more expensive than MCTA for the same quantity of the CSP. The choice of the mobile phase used for the separation is also of great importance. Solvent cost can account for over 50% of the total purification cost of the chromatographic method. With Chiralcel OD and Chiralpak AD columns, a mixed solvent is necessary to achieve high enantioselectivity, durability, and reproducibility of the column. The recommended solvents are hexane and propanol. As solvent composition has a great effect on the retention time, mixing of solvent with the reused/ recycled solvent to attain the initial composition accurately becomes critical. In the case of MCTA with
methanol as the solvent, it provides an additional advantage in the separation of praziquantel enantiomers. Besides its low cost of the solvent compared to other commonly used solvents, it also provides ease of solvent reuse/recycling. This is due to the use of a single solvent and the adequate volatility of the solvent. The solubility of rac-praziquantel in methanol and hexane-propanol (8:2) at room temperature was also tested and was found to be 127 and 15 mg/mL, respectively. The solubility has a significant effect on the throughput of a separation process. In the study of separation of β-blockers with Chiralcel OD columns using a simulated moving bed system, it has been reported that the low solubility of the solute in the solvent, hexane-propanol (8:2), has prevented the use of a higher feed concentration or feed rate (Ikeda et al., 1993). The high solubility of praziquantel in methanol thus provides a further advantage of resolution on MCTA. Therefore, despite the more dispersed peaks obtained in the study with MCTA, the high enantioselectivity, the availability of this CSP at much lower cost, the high solubility of the enantiomers in the mobile phase used, the ease of solvent reuse/recycling, and a very low operating pressure incurred make this CSP a very attractive choice for preparative-scale separation. The high loadability with this CSP has also been reported (Francotte and Wolf, 1990; Koller et al., 1983). The large peak broadening usually observed results in reduced efficiency. In spite of the given enantioselectivity, there is insufficient chromatographic resolution. The high dilution and the reduced peak capacity in the column also lead to reduction in the detection limits. These disadvantages discourage the use of this material for analytical purposes. On the other hand, baseline resolution could be obtained with both Chiralcel OD and Chiralpak AD columns. Therefore, they were employed in the subsequent analysis of the concentration and purity of the praziquantel enantiomers. The mobile phase composition 80:20 (hexane-propanol) was used as this allows a fast baseline separation of the two enantiomers at a reasonable column pressure. Determination of Equilibrium and Kinetic Data for MCTA Experimental Section. Eight chromatographic columns to be used in the preparative-scale simulated countercurrent chromatographic (SCC) separation were fabricated. Each column comprises a chromatographic tube (dimensions: 445 × 12.5 mm i.d.) with top and bottom flanges and a thermostatted jacket which kept the column temperature at 24 °C. Prior to packing, the adsorbent, MCTA (particle size 25-40 µm), was allowed to swell in boiling ethanol for 30 min (Mannschreck et al., 1985). The ethanol-moist material was transferred to a beaker and stirred with an equal amount of ethanol treated in an ultrasonic bath for 2 min and subsequently degassed under vacuum. The slurry was filled into the column via a reservoir and at a constant flowrate of 6 mL/min using an HPLCpump (Model 2510, Varian). The eluent was HPLCgrade methanol. The setup of the chromatographic study was the same as previously described except that each of the chromatographic columns in the SCC setup was used instead of the analytical column. Results and Discussion. Determination of Bed Voidage and Equilibrium Data. The correction for dead volume was determined from a pulse response
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 173
Figure 4. Plot of first moment versus inverse superficial velocity on preparative columns packed with swollen MCTA (9, (+)praziquantel; 2, (-)-praziquantel; (, TTBB).
Figure 5. Plot of HETP versus interstitial velocity on preparative columns packed with swollen MCTA (9, (+)-praziquantel; 2 (-)praziquantel; (, TTBB).
measurement, with the column removed from the system and the injector connected directly to the detector. The total porosity of the column, T, was determined from retention time measurement with TTBB according to eq 5. From the plot of first moment against inverse superficial velocity (Figure 4), T was found to be 0.70. From the correlation suggested by Suzuki (1990), Wakao and Smith (1962), and Wilke (in Ruthven, 1984), the following expression which relates total porosity and bed voidage, , was derived:
independent of liquid velocity. This condition can be achieved by injecting either a compound which penetrates the sorbent rapidly without influence of interphase mass transfer on dispersion or a compound which does not penetrate the pores. The problem of peak broadening of MCTA has been stressed by many workers (Hesse and Hagel, 1976; Koller et al., 1983; Krause and Galensa, 1988; Mannschreck et al., 1985; Rizzi, 1989a,b). From the study of a series of analytes, it was suggested that the reduced efficiency was attributed to the steric structure of the solute (Rizzi, 1989a,b), and the mass-transfer term in the chromatographic bed is the main source of the increase in plate height. Rizzi further suggested that two main types of adsorption sites are operative in MCTA. They differ essentially in the accessibility for the analytes and in the type and strength of the interaction with the analyte. The “quick”-type sites can be assessed easily and the adsorption or desorption is rapid. The “slow”-type sites have a narrow environment, and the adsorption or desorption at these sites is hindered for bulky analytes. Hence, the HETP of a solute is determined by the relative contribution of the narrow slow-type sites to the total retention and by the individual diffusion velocity of the solute at these sites. In this study, the HETP data for TTBB were used to determine the DL value. Due to the tert-butyl groups, TTBB is not able to access to the slow-type sites and is able to penetrate the sorbent rapidly. The HETP results can therefore be assumed to be contributed by the axial dispersion. The plot of HETP against the interstitial velocity of TTBB shows essentially a straight line with very little variation with the flowrate (Figure 5). The slight variation could be due to the adsorption to the quick-type sites or due to the film mass-transfer resistances which are dependent on the flowrate. The DL could still be determined from the HETP corresponding to v ) 0 which gives
T ) 0.45 + 0.55
(11)
From this expression, was estimated to be 0.45. In order to apply the moment analysis, all pulse experiments need to be carried out under linear conditions. Therefore, dilute praziquantel samples were used in the chromatographic studies. With 10-fold variation in the amount of solute injected, there was no difference in the first moment obtained. Hence, linear equilibrium isotherms were assumed under this condition. The first moment data for both praziquantel enantiomers were plotted against inverse superficial velocity (Figure 4), and the equilibrium constants were determined from eq 9. The equilibrium constants obtained were found to be 3.76 and 1.67 for (+)- and (-)praziquantel, respectively. These values were the average of the values obtained for all the eight columns. Due to the different batch of sorbent used, there was variation in the values obtained for different columns (approximately 5%). It has been reported that the retention time is very much dependent on the crystalline state or amorphous state of the CSP. The state of CSP can be influenced by the acetylation of the cellulose as well as the swelling process of the CSP (Francotte et al., 1985). This could have given rise to the difference in the retention value among different columns. Nevertheless, the equilibrium constants obtained were comparable to that obtained in the analytical column. Determination of Axial Dispersion and Kinetic Data. Equation 11 indicates that the HETP can be contributed by both the axial dispersion and the masstransfer resistances. Under conditions of axial dispersion control, one would expect to find a constant HETP,
2λ ) 2DL/v ) 0.32 cm
(12)
As axial mixing in a liquid system is determined by the flow pattern in the bed rather than by molecular diffusion, this contribution to HETP should therefore be approximately the same for all sorbates. It therefore
174
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996
implies that all solutes should give the same HETP ) 0.32 cm {)(2DL/v)}, at zero velocity. Plots of HETP against the interstitial velocity for the praziquantel enantiomers (Figure 5) show that HETP increases approximately linearly with velocity. The overall mass-transfer coefficient, k, can be determined according to eq 8; k’s were found to be 1.5 and 6.0 min-1 for (+)- and (-)-praziquantel, respectively. The low mass-transfer coefficient for (+)-praziquantel is essentially due to the slow diffusion and orientation at narrow sites (i.e., slow-type sites), in narrow channels or in cavity-like structures. It is postulated that the contribution of the film mass-transfer resistance to the HETP will be much smaller than the contribution by the slow transport at the slow-type sites, which is found usually with a porous silica particle (Ching and Chu, 1989; Huber and Rizzi, 1987). In general, the precise nature of the dispersive effects (external film resistance, intraparticle mass-transfer resistance) has only a very modest effect on the transient response curves of a reasonably long chromatographic column (Raghavan and Ruthven, 1985). For an approximate calculation, a lump mass-transfer resistance parameter can therefore be used quite effectively. The expression assumes that the dominating resistance is in the particle diffusion step which is described by an overall mass-transfer coefficient. This rate expression is used as it is difficult to make accurate estimates of the various mass-transfer resistances from presently available experiments. Furthermore, the parameters estimated in this study will be used in the modeling of a simulated countercurrent chromatographic system. In such a complex situation, a model which has sufficient accuracy and simplicity is preferred. Conclusions Liquid chromatography was used to determine the capacity factor for various chiral stationary phases (CSPs) on separation of praziquantel enantiomers. From the first moment analysis of the eluted peaks, microcrystalline cellulose triacetate (MCTA) showed higher enantioselectivity compared to other CSPs studied. Together with other advantages associated with the use of this CSP, MCTA was selected to be the CSP engaged in the preparative-scale chromatographic separation of the enantiomers. With the adsorbent packed in the preparative-scale columns, the column parameter, together with the equilibrium and kinetic parameters for the separation of praziquantel enantiomers, were determined from the first and second moment analysis of the eluted peaks. These parameters will be utilized in the mathematical modeling of the preparative-scale chromatographic separation process in a simulated countercurrent system. Nomenclature DL ) axial dispersion coefficient (min-1) Dm ) solute molecular diffusivity (cm2 min-1) HETP ) height equivalent to a theoretical plate (cm) IPA ) propanol k ) overall mass transfer coefficient (min-1) k′ ) capacity factor K ) equilibrium distribution constant L ) length of a chromatographic column (cm) Q ) volumetric flow rate (mL min-1) t ) time (s) t0 ) mean retention time for an unretained compound (s)
t0R ) mean retention time for an unretained compound (chromatographer’s definition) (s) tR ) mean retention time (s) u ) superficial velocity (cm s-1) v ) interstitial fluid velocity (cm s-1) Greek Symbols R ) separation factor ) external porosity or bed voidage i ) internal porosity T ) total porosity η ) tortuosity factor for a packed column λ ) flow-geometry dependent constant µ1 ) first moment (s) µ2 ) second moment (s2) σ2 ) variance (s2)
Literature Cited Bittiger, H.; Keilich, G. Optical rotatory dispersion and circular dichroism of carbanilyl polysaccharides. Biopolymers 1969, 7, 539. Ching, C. B.; Chu, K. H. A chromatographic analysis of hindered diffusion of saccharides in silica gels. Chromatographia 1989, 28, 369. Ching, C. B.; Lim, B. G.; Lee, E. J. D.; Ng, S. C. Chromatographic resolution of the chiral isomers of several β-blockers over cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phase. Chirality 1992, 4, 174. Francott, E.; Wolf, R. M. Preparation of chiral building blocks and auxiliaries by chromatography on cellulose triacetate (CTA I): indications for the presence of multiple interaction sites in CTA I. Chirality 1990, 2, 16. Francotte, E.; Wolf, R. M.; Lohmann, D.; Mueller, R. Chromatographic resolution of racemates on chiral stationary phases I. influence of the supramolecular structure of cellulose triacetate. J. Chromatogr. 1985, 347, 25. Gunn, D. J. Axial and radial dispersion in fixed beds. Chem. Eng. Sci. 1987, 42, 363. Hesse, G.; Hagel, R. Die chromatographische Racemattrennung. Liebigs Ann. Chem. 1976, 996. Huber, J. F. K.; Rizzi, A. Influence of the accuracy of the extracolumn peak-width determination on the verification of theoretical plate-height equations. J. Chromatogr. 1987, 384, 337. Ikeda, H.; Negawa, M.; Makino, N.; Murata, K. Optical resolution of β-blockers by HPLC and a simulated moving-bed adsorption technology. Presented at the Fourth International Symposium on Chiral Discrimination, Montreal, Quebec, Canada, Sept 1993. Katti, A.; Erlandsson, P.; Da¨ppen, R. Application of preparative liquid chromatography to the isolation of enantiomers of a benzodiazepinone derivative. J. Chromatogr. 1992, 590, 127. Koller, H.; Rimbo¨ck, K. H.; Mannschreck, A. High-pressure liquid chromatography on triacetylcellulose: Characterization of a sorbent for the separation of enantiomers. J. Chromatogr. 1983, 282, 89. Krause, M.; Galensa, R. Direct enantiomeric separation of racemic flavanones by high performance liquid chromatography using cellulose triacetate as a chiral stationary phases. J. Chromatogr. 1989, 441, 417. Liu, Y. H.; Wang, X. G.; Qian, M. X.; Yan, X. H.; Chen, B. Y.; Li, J. S.; Jin, J. M.; Zhang, G. S. A comparative trial of single dose treatment with praziquantel and levopraziquantel in human schistosomiasis japonica. Jpn. J. Parasitol. 1988, 37, 331. Mannschreck, A.; Koller, H.; Wernicke, R. Microcrystalline cellulose triacetate, a versatile stationary phase for the separation of enantiomers. Kontakte 1985, 40. Raghavan, N. S.; Ruthven, D. M. Simulation of chromatographic response in columns packed with bidiperse structured particles. Chem. Eng. Sci. 1985, 40, 699. Rizzi, A. M. Band broadening in high-performance liquid chromatographic separations of enantiomers with swollen microcrystalline cellulose triacetate packings I. Influence of capacity factor, analyte structure, flow velocity and column loading. J. Chromatogr. 1989a, 478, 71. Rizzi, A. M. Band broadening in high-performance liquid chromatographic separations of enantiomers with swollen micro-
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 175 crystalline cellulose triacetate packings II. Influence of eluent composition, temperature and pressure. J. Chromatogr. 1989b, 478, 87. Ruthven, D. M. Principles of adsorption and adsorption processes; John Wiley & Sons: New York, 1984; p 209. Suzuki, M. Adsorption Engineering; Elsevier Science Publishers B.V.: Amsterdam, The Netherlands, 1990; p 65. Vogt, U.; Zugenmaier, P. Structural models for some liquid crystalline cellulose derivatives. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 1217. Wakao, N.; Smith, J. M. Diffusion in catalyst pellets. Chem. Eng. Sci. 1962, 17, 825. Wu, M. H.; Wei, C. C.; Xu, Z. Y.; Yuan, H. C.; Lian, W. N.; Yang, Q. J.; Chen, M.; Jiang, Q. W.; Wang, C. Z.; Zhang, S. J.; Liu, Z.
D.; Wei, R. M.; Yuan, S. J.; Hu, L. S.; Wu, Z. S. Comparison of the therapeutic efficacy and side effects of a single dose of levopraziquantel with mixed isomer praziquantel in 278 cases of schistosomiasis japonica. Am. J. Trop. Med. Hyg. 1991, 45, 345.
Received for review August 10, 1995 Accepted September 1, 1995X IE9500883
X Abstract published in Advance ACS Abstracts, November 15, 1995.
