Mechanisms of Solute Partitioning in Various Types of CPC

Centrifugal counter-current chromatography is a liquid-liquid chromatographic technique resembling to some extent droplet counter-current chromatography (DCCC) (Fig. 2) [20], In DCCC, the stationary phase is retained in a large number of vertical narrow-bore tubes, while the mobile phase, depending upon its density, is pumped through the system in an ascending or descending mode in the form of small droplets. CPC differs from DCCC in that centrifugal and/or Archimedean screw forces maintain the stationary phase, while the mobile phase is pumped through [21]. These features allow high partition efficiency and large retention capacity of the stationary phase under a high flow-rate of mobile phase. .

In a recent review, Ito presented the historical background, development and mechanisms of distribution of the stationary and mobile phases in the CPC coil [21], classifying the mechanisms of CPC systems into two forms, namely hydrostatic equilibrium systems (HSES) and hydrodynamic equilibrium systems (HDES).

Flow in

1—1

—1

—O-

oZ

—0-

IZo

_0_0

u-

—o-

_0_

Flow out

Figure 2. Schematic illustration of the principle of DCCC in the ascending mode (lighter phase as mobile phase). 1, mobile phase; 2, stationary phase.

6.3.1 Hydrostatic Equilibrium Systems

The hydrostatic equilibrium systems use stationary coils, for example PTFE tubing such as those used in the toroidal Coil CPC (Fig. 3), or multichannel cartridges such as those used in the Sanki CPC (Fig. 4) [22]. The coils are so-called stationary with respect to the rotating frame towards the central axis of the centrifuge. Measurements begin with filling the coil with the stationary phase. The mobile phase is then pumped into the rotating centrifuge and starts to percolate through the stationary phase segments on one side of the coil, displacing nearly half the volume of stationary phase plastic protection

CD pulley

IBM bell

Figure 3. Schematic illustration of toroidal coil planet centrifuge.

column

Figure 3. Schematic illustration of toroidal coil planet centrifuge.

plastic protection column

CD pulley

IBM bell in the coil. Hence, solutes introduced at the inlet of the coil are subjected to a continuous partitioning process between the two phases. In this system, the retention of the stationary phase and the distribution of stationary and mobile phases in the coil are governed mainly by the centrifugal force. As it appears, the pressure in the system is relatively high and the leakage of phase solutions becomes one of the major problems for the measurement. Moreover, for the measurement of very lipophilic (log P > 2) or very hydrophilic compounds (log P < -2), these systems appear impractical due to the low retention volume of the stationary phase (c. 50 %).

Figure 4. Centrifuge of the multichannel cartridge CPC. 1, rotor of the centrifuge; 2, rotary seal joint; 3, column cartridge; 4, tubing connecting rotary seal joint and column cartridges; 5, tubing connecting cartridges.

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