Countercurrent Chromatography
From TheLiquidPhase
Countercurrent Chromatography or CCC, is a form of Liquid-Liquid Chromatography, which is a specific form of Chromatography. This site is dedicated entirely to this topic.
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A Brief Non-technical Summary
CCC is a modern technology that is perfectly suited to separate individual Chemical Compounds, or groups of compounds, from a Complex Mixture. For example, imagine that there is a specific compound in corn that has been proven to fight some disease. One could use CCC to extract only that target compound from a Crude Extract of corn. The pure compound could then be used for further research or the development of products based on it. CCC is especially useful in the separation of Natural Products, where achieving satisfactory separations is often a challenge.
Technical Summary
In general, CCC is a Chromatographic technique in which components are separated as a result of repeated partitioning between two phases of a bi-phasic solvent system. As indicated above, CCC is a “Liquid-Liquid” technique, which means that both the Stationary Phase and the Mobile Phase are liquids. This avoids many of the problems associated with other forms of chromatography. (See Advantages of CCC)
One of the major development challenges of CCC was the retention of stationary phase within the system, while still being able to pump a mobile phase through it. In other forms of Chromatography, this is much easier. For example, in Liquid-Solid chromatography the solid stationary phase can be held in place by simply packing it tightly into an inert cylinder. If packed properly a liquid mobile phase can be pumped through it without dislodging the stationary phase. CCC technology on the other hand, requires something a bit different to hold the stationary phase in place. The original machines relied heavily on gravity to achieve this, however modern CCCs use centrifugal forces which provide much faster separations. (See History of CCC)
Again, CCC works by creating repeated [[[Liquid-Liquid Extraction]]s continuously within the system. The two types of machines most commonly used today are HSCCC and CPC. HSCCC uses a helical coil(s) of inert tubing which rotates on a planetary axis and simultaneously rotates eccentrically about another solar axis. CPC uses a single rotor which spins on a single axis, this rotor is actually a stack of disks in which columns are cut, oriented out from the middle, and connected by channels. Both of these machines effectively create many zones of mixing, settling and separation, which progress along the internal liquid path through the system. These three important steps of mixing, settling and separation can be thought of as exactly the same thing that happens with an analyte in a Separatory Funnel.
It should be noted that, as with HPLC or other “Liquid-Solid” chromatographic methods, researchers are not limited by the composition of the columns commercially available for their instrument. Nearly any pair of immiscible solutions can be used in liquid-liquid chromatography, and most instruments can be operated in standard or reverse-phase modes, allowing you to use either liquid as the stationary phase.
The most comprehensive single book on CCC to date is Volume XXXVIII of Wilson and Wilson's "Comprehensive Analytical Chemistry"; the title of the volume is "Countercurrent Chromatography: The support-free liquid stationary phase"; and the Editor is Alain Berthod. The following figure helps to illustrate the complementarity of CCC as a preparative tool and HPLC as an analytical technique:
The figure to the right shows the chromatographic efficieny, given in number of theoretical plates, that are needed to obtain Rs = 1.5 (baseline separation) between a hypothetical solute with K(1) = 1 and another solute with K(2) as shown (1.1, 1.2, 2, or 5). The dotted area gives the CCC working range and the hatched area is the HPLC working range. This figure illustrates a fundamental difference between HPLC and CCC: users of HPLC are accustomed to rating performance in terms of theoretical plates, whereas CCC users are concerned primarily with efficiency. This is due to the relatively very large stationary phase volume in CCC compared with HPLC.
Adoption of CCC
The adoption of CCC in industry and academia has been somewhat slow through the last few decades. However, in recent years, accompanying several advances in the technology, the potential of CCC is being realized and its use is growing rapidly. (See Publications)
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