EVALUATION OF EXPERIMENTAL PARAMETER INFLUENCE ON HPLC SEPARATION OF SOME AMINES AND

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1 ACTA UNIVERSITATIS PALACKIANAE OLOMUCENSIS FACULTAS RERUM NATURALIUM 1999 CHEMICA 38 EVALUATION OF EXPERIMENTAL PARAMETER INFLUENCE ON HPLC SEPARATION OF SOME AMINES AND PYRETHROIDS USING TWO b-cyklodextrin COLUMNS Karel Lemr 1, Juraj Ševčík 1, David Friedecký 1, Alena Jonáková 1 and David Jirovský 2 1 Department of Analytical Chemistry, Palacký University, Třída Svobody 8, Olomouc, Czech Republic 2 Laboratory of Bioanalytical Research, Palacký University, Třída Svobody 8, Olomouc, Czech Republic Received May 31, 1999 Abstract Similarly to earlier described procedure the separation of enantiomers of ephedrine, methamphetamine, selegiline, alphametrin and cypermethrin using b-cyclodextrin column Cyclobond I 2000 was optimized. The effect of different parameters (mobile phase composition, flow-rate, temperature and injected amount) on the separation was evaluated. The acquired results were used for comparison of the importance of experimental parameters in optimization of mentioned compound separations on two b-cyclodextrin columns (ChiraDex and Cyclobond). On studied columns differences in cation, organic solvent, injected amount influence on amine separation and differences in ph, temperature, injected amount influence on pyrethroid separation, respectively, were observed. The results show that diversities in the chromatographic behavior of nominally the same columns can be expected. These diversities can be ascribed to the influence of b-cyclodextrin bonding to silica support (the presence of nitrogen atom in ChiraDex stationary phase) and/or other possible varieties in properties of stationary phases (different surface concentration of chiral selector, accessibility of silanol groups etc.). 41

2 Key Words: HPLC, cyclodextrin, enantiomer, amines, pyrethroids Introduction Diverse influence of enantiomers on living organisms has been pointed out many times and for example in pharmacological studies has to be carefully evaluated. The chiral discrimination is all the time interesting phenomenon for chemists. The asymmetric syntheses are important for production of many compounds in pharmaceutical and chemical industry. The production processes (especially the final products) have to be controlled. That is one of the fields of application of analytical methods being able to recognize individual enantiomers. The other ones are research works in the different areas of interest (biochemistry, toxicology, pharmacology etc.). Among other analytical techniques the separation ones offer many possibilities in analyses of optical active compounds. For the enantiomeric separation of some amines, pyrethroids and methamphetamine metabolites we used high performance liquid chromatography (HPLC) 1 3 and capillary electrophoresis 3 5, respectively. In HPLC we employed the chiral stationary phase with b-cyclodextrin bonded to silica. The use of cyclodextrins for chiral discrimination by HPLC has been well known for a longer time 6,7. With mentioned chiral selector we were able to attain good enantiomeric separation of ephedrine, methamphetamine and selegiline in one run 1. We systematically optimized different experimental parameters (ph, organic solvent, salt nature and concentration, flow rate, injected amount and temperature). We stated that each parameter contributes to the final result of course in the different extent for individual studied compounds. The similar procedure was used for the separation of stereoisomers of some pyrethroids 3. We observed for example the change of separation with the change of ph although they are neutral. For amines as well as pyrethroids we used b-cyclodextrin column, similarly for both groups Pirkle type stationary phases were employed In general one type of stationary phase can be used for many compounds differentiating in their structure. On the contrary in HPLC nominally equivalent columns (not only chiral) can give much unlike results. In practical applications we can meet this situation very often for the most widely spread stationary phase, octadecyl chemically bonded to silica. In the presented work we are comparing the influence of different experimental parameters on the chiral separation of ephedrine, methamphetamine, selegiline, alphametrin and cypermethrin (structures see Fig. 1) using two nominally the same stationary phases with b-cyclodextrin bonded to silica. The goal of this work is not to search advantages of one column in comparison to second one. Regarding to limited number of separated compounds it is not possible. We want only to direct attention to the different behavior of used columns what can be interesting for chromatographers working in chiral separation. 42

3 Fig. 1: Structures of studied compounds. Alphamethrin two enatiomers from a cypermethrin stereoisomer mixture, 1 R cis a S and 1 S cis a R. Experimental The chromatographic experiments were performed using a liquid chromatograph Spectra Physics (pump SP 8700, UV/VIS detector SP 8440, all Spectra-Physics, San Jose, CA, USA) equipped with a manual 7125 injector (10 µl loop) (Rheodyne, Cotati, CA, USA) and Chromatography Station for Windows CSW version 1.0 (DataApex, Prague, Czech Republic). UV absorption chromatograms were recorded at 206 and 258 nm, respectively, for amines or at 210 nm in the case of pyrethroids. For the separation of studied compounds two b-cyclodextrin columns were used, ChiraDex 250 x 4 mm I.D., 5 µm, LiChroCart (E. Merck, Darmstadt, F.R. Germany) 1,3 and Cyclobond I 2000, 250 x 4.6 mm I.D., 5 µm (Astec, Whippany, NJ, USA). The temperature of the column was controlled with the precision ±0.1 C using a glass water jacket and a laboratory water thermostat equipped with a freon cooler. Hold-up volumes were determined by at least triplicate injections of water (10 µl) with the detection wavelength 200 nm and mobile phase methanol : water = 40 : 60 (v/v) or mobile phase with higher elution strength. As described previously the mobile phases were prepared by volume by volume mixing of components using HPLC-grade (E. Merck, Darmstadt, F.R. Germany) or UV-grade (Lachema, Brno, Czech Republic) solvents and redistilled water. The other chemicals were of analytical grade 1,3. The hydrochlorides of enantiomers of ephedrine, methamphetamine and selegiline were gift of Farmakon, Olomouc, Czech Republic. The contents of the minor enantiomer in the major one were undetectable using evaluated methods. Cypermethrin and alphamethrin (two enatiomers from a cypermethrin stereoisomer mixture, 1 R cis a S and 1 S cis a R) were supplied by Shell Res. Ltd., Sittingbourne, Kent, U.K. 43

4 Results and discussion The separations of amines and selected pyrethroids were optimized using Cyclobond similarly to earlier described separation performed on ChiraDex 1,3. We followed the influence of mobile phase composition (ph, nature and concentration of added salt, organic solvent), flow-rate, injected amount and temperature. Three examples of separations under optimal conditions are shown on Figs The chromatograms of ephedrine, methamfetamine and selegiline are similar. Sufficient separation of racemates was achieved but also minor enantiomers in the content around 1% can be recognized. As regards pyrethroids the separation of alphamthrin was better on ChiraDex (Fig. 3). Enantiomers are separated to the baseline. Cyclobond allowed to recognize the presence of both enantiomers but to determine one in the excess of the second was not possible. On the contrary better separation of cypermethrin stereoisomers was attained using Cyclobond. Under tested conditions none of the used columns allowed to separate all eight stereoisomers of cypermethrin but using Cyclobond we could better distinguish more peaks in comparison to Chiradex (Fig. 4). We can see that the columns offer different separation of the stereoisomers of single compound, some of its stereoisomers were separated better on ChiraDex (alphamethrin) other on Cyclobond (compare Figs. 3 and 4). Bellow we are comparing the influence of some experimental parameters that showed evident differences in their effect on separation of studied compounds according to the used columns. In the case of amines the optimization on both columns was made with the goal to separate all three enantiomeric pairs in one run. The differences were observed during optimization of cation added to the mobile phase (see Fig. 5 and Tab. 1). The worst results for both columns were attained using Na +. The unsatisfactory situation we observed for ChiraDex where the peaks are very tailing and such separation can not be used for the practical purposes in comparison to Cyclobond. The use of ammonium reduced analysis time as well as tailing on ChiraDex, the change for Cyclobond was not too important. For both columns the best result was achieved using triethylammonium. Especially for ChiraDex the shape of peaks was markedly better. Tab. 1 shows the values of Kaiser s criterion of separation 12 (the explanation of criterion see Fig. 6). Whereas for Cyclobond its change with the change of cation is mild for ChiraDex the criterion shows that it was necessary to make cation optimization to attain required enantiomeric separation. This can be ascribed to the necessity to block the active nonstereoselective centers on stationary phase. Other observed difference between the columns was higher retention of compounds on Cyclobond what allowed to use organic modifier in the mobile phase. Whereas any addition of organic solvent decreases resolution on ChiraDex on Cyclobond could be used to improve resolution especially not between enatiomers but between methamphetamine and selegiline. For Cyclobond in purely water mobile 44

5 Fig. 2: Separation of racemic mixture of ephedrine (EP), methamphetamine (MAP) and selegiline (SEG). ChiraDex: stationary phase: ChiraDex, 5µm; column: LiChroCart 250 x 4 mm I.D.; mobile phase: 500 mmol triethylamine/l water with H 2 SO 4, ph = 3.5; flow-rate: 0.8ml/min; detection: UV absorption at 206 nm; temperature: ambient. Cyclobond: stationary phase: Cyclobond I 2000, 5 µm; column: 250 x 4.6 mm I.D.; mobile phase: 150 mmol triethylamine/l water with H 3 PO 4 (ph = 3.5) : acetonitrile = 99 : 1 (v : v); flow-rate: 1.0 ml/min; detection: UV absorption at 206 nm; temperature: 20 C. 45

6 Fig. 3: Chiral separation of alphamethrin. ChiraDex: mobile phase: 150 mmol triethylamine/l water with H 2 SO 4 (ph = 3.5) : methanol = 45 : 55 (v : v); detection: UV absorption at 210 nm; temperature: 20 C. Cyclobond: mobile phase: 150 mmol triethylamine/l water with H 3 PO 4 (ph = 3.5) : methanol = 50 : 50 (v : v); flow-rate: 0.8 ml/min; detection: UV absorption at 210 nm; temperature: 20 C. Other conditions see Fig. 2. phase the resolution of enantiomers was 1.17 (ephedrine), 1.26 (methamphetamine), 1.30 (selegiline) and the resolution between neighboring peeks of methamphetamine and selegiline was In the mobile phase with 5% of methanol the values of resolution were in the same order 1.18, 1.33, 1.29 and It means that enantiomeric resolution was not worsening and in the same time the resolution between methamphetamine and selegiline was significantly improving. Slightly better enantiomeric resolution was attained using 1% of acetonitrile (instead of methanol), resolutions in the same order were 1.19, 1.37, 1.56, For the enantiomeric separation of studied amines the last discussed parameter is the injected amount. Too high amount of analyte made the separation on both columns impossible. For analyzed amines we observed a higher loss of resolution with the increase of injected amount on Cyclobond. In percentage the loss of resolution 46

7 Fig. 4: Separation of cypermethrin stereoisomers. ChiraDex: flow-rate: 0.6 ml/min; temperature: 10 C. Other conditions see Fig. 3. with the change of racemate concentration in injected solution from 0.05mg/ml to 0.5 mg/ml was close to 40% for Cyclobond and around 10% for ChiraDex, respectively. For the optimization of studied pyrethroid separation we found ph, temperature and injected amount as the parameters with markedly different influence according to the used columns. Pyrethroids are neutral and that is why the influence of ph is not so expected as in the case of amines but the change of pyrethroid separation with the change of ph was observed earlier 3. For alphamethrin using ChiraDex the increase of ph (from 3.5 to 7.5) led to the decrease of the relative retention of its enantiomers from 1.53 to On the contrary on Cyclobond column this relative retention was practically constant (1.09). This can be explained similarly to the influence of cation on amine separation by the participation of other active centers on stationary phase in retention process. For ChiraDex we can expect that for higher ph the contribution of nonstereoselective centers of stationary phase is increasing. 47

8 Fig. 5: Influence of cations on separation of studied amines. ChiraDex: mobile phase: 500 mmol cation/l water with H 2 SO 4, ph = 3.5. Cyclobond: mobile phase: 150 mmol cation/l water with H 3 PO 4 (ph = 3.5) : methanol = 95 : 5 (v : v). Other conditions see Fig

9 Table 1: Influence of cations on value of Kaiser s criterion of separation. &KLUD'H[ &\FORERQG &DWLRQ (3 0$3 6(* (3 0$3 6(* 1D 1+ 7($ ChiraDex: stationary phase: ChiraDex, 5 µm; column: LiChroCart 250 x 4 mm I.D.; mobile phase: 500 mmol cation/l water with H 2 SO 4, ph = 3.5; flow-rate: 0.8ml/min; detection: UV absorption at 206 nm; temperature: ambient. Cyclobond: stationary phase: Cyclobond I 2000, 5 µm; column: 250 x 4.6 mm I.D.; mobile phase: 150 mmol cation/l water with H 3 PO 4 (ph = 3.5) : methanol = 95 : 5 (v : v); flow-rate: 1.0 ml/min; detection: UV absorption at 206 nm; temperature: 20 C. EP ephedrine, MAP methamphetamine, SEG selegiline. Fig. 6: Kaiser s criterion of separation (P = f/g)

10 Table 2. Influence of temperature on relative retention of alphamethrin enantiomers. 7Ãƒ& &KLUD'H[ &\FORERQG ChiraDex: stationary phase: ChiraDex, 5 µm; column: LiChroCart 250 x 4 mm I.D.; mobile phase: 150 mmol triethylamine/l water with H 2 SO 4 (ph = 3.5) : methanol = 45 : 55 (v : v); flow-rate: 0.8ml/min; detection: UV absorption at 210 nm. Cyclobond: stationary phase: Cyclobond I 2000, 5 µm; column: 250 x 4.6 mm I.D.; mobile phase: 150 mmol triethylamine/l water with H 3 PO 4 (ph = 3.5) : methanol = 50 : 50 (v : v); flow-rate: 0.8 ml/min; detection: UV absorption at 210 nm. Relative retention = capacity factor of later eluted enantiomer/capacity factor of earlier eluted enantiomer. Different influence of temperature was observed for alphamethrin (Tab. 2). In the case of ChiraDex its increase led to the worsening of separation but during the change from 25 C to 30 C the selectivity was improving again (it might be the positive influence of change of entropy on separation). For Cyclobond the effect of temperature was negligible. Similarly to amines the next important parameter affecting the chiral discrimination of pyrethroids in the different way was injected amount. The one order higher injection meant for ChiraDex approximately three times and for Cyclobond 1.5 times lower resolution, respectively. The situation is opposite to the case of amine separation and can be probably partially ascribed to the lower starting value of resolution on Cyclobond (0.8 in comparison to 2.4 on ChiraDex) and uncertainty in its calculation. For comparison Kaiser s criterion of separation showed decrease 2.5 times on Cyclobond. Unfortunately this decrease can not be calculated for ChiraDex due to achievement of the limit of this criterion (value 1) for lower injection. Conclusion In the presented work we discussed the experimental parameters that show different importance for the separation on the used columns. Of course the optimization of other parameters is also important to reach good enantiomeric separation but their effect is similar for both columns. In the case of amines we described differences for cations, organic solvent and injected amount, in the case of pyrethroids for ph, temperature and also injected amount. Observed facts can be ascribed to the differences in the stationary phases and column dimension. There can be diversities in surface concentration of chiral selec- 50

11 tor, accessibility of silanol groups, quality of silicagel, in the bonding of b-cyclodextrin to silica support (the presence of nitrogen atom in ChiraDex stationary phase) etc. The found results can not be in any case used for critical comparison of columns regarding to their applicability in chiral separations at least due to limited number of separated compounds. They should point out the problems that can be connected with the transfer of method between two columns with the same chiral selector but from different producers. Acknowledgments We are grateful for support of this work by grant from the Ministry of Education of Czech Republic (grant. No. VS 96021). References 1. Lemr, K., Jirovský, D., Ševčík, J.: J. Liq. Chromatogr. 19, 3173 (1996). 2. Jirovský, D., Lemr, K., Ševčík, J., Smysl, B., Hlaváč, J.: Acta Univ. Palacki. Olomouc., Chemica 35, 85 (1996). 3. Ševčík, J., Lemr, K., Stránský, Z., Večeřa, T., Hlaváč, J.: Chirality 9, 162 (1997). 4. Ševčík, J., Stránský, Z., Ingelse, B. A., Lemr, K.: J. Pharm. Biomed. Anal. 14, 1089 (1996). 5. Ševčík, J., Lemr, K., Smysl, B., Jirovský, D., Hradil, P.: J. Liq. Chromatogr. 21, 2473 (1998). 6. Krstulovic, A. M., (Editor): Chiral Separation by HPLC, Ellis Horwood Limited, Chichester, Pirkle, W. H., Pochapsky, T. C.: Chem. Rev. 89, 347 (1989). 8. Doyle, T. D., Adams, W. M., Fry Jr., F. S., Wainer, I. W.: J. Liq. Chromatogr. 9, 455 (1986). 9. Cayley, G. R., Simpson, B. W.: J. Chromatogr. 356, 123 (1986). 10. Chapman, R. A.: J. Chromatogr. 258, 175 (1983). 11. Lisseter, S. G., Hambling, S. G.: J. Chromatogr. 539, 207 (1991). 12. Schoemakers, P. J.: Optimization of Chromatographic Selectivity A Guide to Method Development, Elsevier, Amsterdam, 1986, p

1Determination of optical purity of N-acetyl-1-naphthylethylamine by chiral chromatography and NMR spectroscopy

Printed in the Republic of Korea "/"-:5*$"- 4$*&/$& 5&$)/0-0(: Vol. 23, No. 1, 97-101, 2010 1Determination of optical purity of N-acetyl-1-naphthylethylamine by chiral chromatography and NMR spectroscopy