Simulated moving bed
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In chromatography, the simulated moving bed (SMB) technique is a a variant of high performance liquid chromatography; it is used to separate particles and/or chemical compounds that would be difficult or impossible to resolve otherwise. This increased separation is brought about by a valve-and-column arrangement that is used to lengthen the stationary phase indefinitely.
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[edit] Construction
Specifically, an SMB system has two or more identical columns, which are connected to the mobile phase pump, and each other, by a multi-port valve. The plumbing is configured in such a way that:
- a) all columns will be connected in series, regardless of the valve's position;
- b) each different position of the valve will reconnect the columns to one another in one possible sequential arrangement of the columns;
- and
- c) all possible positions of the valve will arrange the columns in every possible sequential order.
For example, consider a case where two HPLC columns, A and B, are connected to one another, and the mobile-phase pump, via a six-port, two-position valve (e.g. a Rheodyne 7000). One valve position will distribute the the flow in the manner
- Pump-->Column A-->Column B-->Waste,
while the other position will distribute the flow in the manner
- Pump-->Column B-->Column A-->Waste.
Consequently, switching of the valve will "leapfrog" the columns over one another. If elution across two columns in series is not adequate to resolve two compounds in a given run, the eluent can then be made to go through 3, 4, 5... columns in additional runs by carefully timed switching. This increases the number of theoretical plates until separation can be attained.
[edit] Advantages
When affinity differences between molecules are very small, it is sometimes not possible to improve resolution via mobile- or stationary-phase changes. In these cases, the multi-pass approach of SMB can separate mixtures of those compounds by allowing their small retention time differences to accumulate.
[edit] Drawbacks
The most obvious drawback of SMB is that solvent must continuously flow through the system, from reservoir to waste. Over the course of an extended separation, this can result in very large quantities of mobile phase being used, increasing solvent and waste-disposal expenses dramatically.
Another difficulty with the SMB technique is termed "pre-elution." If, over the course of a separation, a series of peaks extends over more than one column length, switching the valve will cause the later peaks to elute before the earlier ones.
[edit] Applications
In size exclusion chromatography, where the separation process is driven by entropy, it is not possible to increase the resolution attained by a column via temperature or solvent gradients. Consequently, these separations often require SMB, to create usable retention time differences between the molecules or particles being resolved. SMB is also very useful in the pharmaceutical industry, where resolution of molecules having different chirality must be done on an very large scale.
