Dr. Jennifer Jamison
Separations are important techniques in chemistry that are used to separate various components of a mixture. They are carried out by mixing two immiscible liquids containing certain solutes together in a separatory funnel, allowing them to separate, then extracting the distinct layers that form. The ratio of the concentration of solute present in the upper layer to the concentration in the lower layer is called the partition coefficient. The efficiency of a separation is described by this partition coefficient. If the coefficients for the two layers are largely different, then the separation can be carried out in a single step. If they aren’t, a more complex process is necessary.1,2 Countercurrent chromatography is a technique used carry out separations in these kinds of cases. It uses a continuous liquid-liquid partitioning process to streamline the usual extraction procedure.
The technique of countercurrent chromatography is based off of an older technique called countercurrent distribution.3 This method is a multi-step extraction technique that uses something called a Craig apparatus. It works by first adding a specific quantity of one solvent containing a solute of interest to each of the specially designed as in Figure 1, interconnected tubes in the series, then adding a second, lighter solvent that provides a partition coefficient greater than 1 to the first tube only. The two solvents are then mixed together, then allowed to separate into layers. The lighter top solvent is then transferred to the second tube in the series by tilting the apparatus, while the heavier bottom layer is retained. A fresh quantity of the lighter sol...
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...d to mix the contents of the column. Aside from these commonalities, the instrumentation used varies greatly depending on the type of countercurrent chromatography used.
The first type of countercurrent chromatography ever designed was helix countercurrent chromatography. In this form, a helical column is first filled with the stationary phase, then the second phase is continuously injected into one end of the tube. The mobile phase passes through the stationary phase, which is trapped at the bottom of each turn of the coil by gravity. The two phases equilibrate in the stationary segments to varying degrees. The degree of equilibration is controlled by a few factors, including the thoroughness of mixing and physical properties of the two, such as surface tension. At the end the mobile phase is eluted, usually along with a small amount of the stationary phase.