The Discovery and Development of CPC

The invention of centrifugal liquid-liquid partition chromatography is not as new as it appears. In 1966, the year in which the first description of HPLC by Horvath and Lipsky [12] was published, Ito at the National Institutes of Health, Maryland, discovered this novel form of liquid-liquid partition chromatography for the separation of cell particles [13]. He subsequently developed various types of centrifugal partition chromatography with his rich imagination, a fine sense of fluid dynamics, and a good talent in engineering. This ingenious invention was recommended for the purpose of preparative separation with much shorter separation times as compared with the previous form of liquid-liquid partition chromatography and to avoid the problems of solidphase adsorption encountered in HPLC. As a shortcoming of CPC, its theoretical plate number is usually in the range of hundreds and much inferior to that of HPLC. The first commercialization of the prototype CPC did not begin until the 1980s, which is at least one of the factors accounting for its limited use given its attractive merits in separation technologies. To date, several commercialized chromatographs are still in a state of prototype and their many mechanical designs are to be improved.

6.2.2 From log P0ctanoi-hexane/Water to log Poc, using Multichannel Cartridge-type CPC

In view of the success of the log Poa parameter in drug research, attempts to use CPC for lipophilicity measurements are understandable. Terada and coworkers [14] were the first to determine lipophilicity using the CPC technique in 1987. They used the multichannel cartridge-type CPC (model CPC-LLN, Sanki Engineering, Nagaokakyo, Japan). Perhaps due to the viscous nature of octanol and the high pressure generated in the chromatograph, no direct measurements using octanol/water systems were reported. Instead, a mixture of octanol :hexane (20% :80%) was used as the mobile phase, and the calculated partition coefficients were correlated with log Poa values from the literature. A log P range of -0.5 to -2 was reported. The following publication in 1988 was from Armstrong and coworkers [15], in which both octanol: hexane (40 % : 60 %) and octanol (100 %) were used as the organic phase by employing a similar apparatus (model CPC-NMF, Sanki Laboratories Inc., Sharon Hill, Pennsylvania, USA). For the measurement of compounds with log P > 0, they recommended use of water as the mobile phase and reported measured log P values from 0.2 to 2.3. Two points are worth mentioning in this study; first, the continuous loss of the stationary phase (i. e., "bleeding") requires the frequent control of dead time; second, the retention time of lipophilic solutes in unacceptably long (8-11 hours for compounds of log P~ 2). A similar study from our laboratory [16] using the Sanki model CPC-LLN also showed that it is impossible to use octanol as the mobile phase, the pressure being too high in the chromatograph. It was thus concluded that the multichannel cartridge-type CPC is not appropriate for lipophilicity measurements.

6.2.3 From log Poct to log P (solvent "quartet") using Coil Planet-type CPC

The endeavor to use CPC for lipophilicity measurements was not stopped by the difficulties encountered in the multichannel cartridge-type CPC. Our laboratory changed to the coil planet-type CPC as an alternative for this purpose. Log Pna values from -1.3 to 1.3 were obtained using octanol as the mobile phase with the Ito Multilayer Coil Separator-Extractor (P. C. Inc., Potomac, Maryland, USA) [17]. Soon after that, we have further extended the log P„a range from -3 to 3 using "rapid mode" operation procedures [18] (see section 6.4.3 for details).

Since partition coefficients in various solvent systems encode different structural information [19], it would be desirable to determine partition coefficients in the four model solvent systems: amphiprotic (n-octanol/water), protic (chloroform/water), aprotic (di-n-butyl ether/water) and inert (alkane/water) systems. The progress made in our laboratory in the past few years has enabled the measurements to be carried out in «-octanol/water, di-n-butyl ether/water and w-dodecane/water and allowed important structural properties such as hydrogen-bonding capacity to be determined from the results.

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