Choosing a Mode

Most commonly, the Octave System is operated in one of two continuous SMB chromatography modes: Isocratic and Step. Isocratic mode is the “classical” SMB chromatography method that utilizes true countercurrent separation. It is characterized by continuous flow of a single solvent between (usually 3 or 4) separation zones defined by Feed and Desorbent inlets and Extract and Raffinate outlets. In contrast, Step mode is characterized by operation of (usually) four or more distinct zones having different solvent conditions in a “bind-wash-elute-clean-equilibrate” sequence. A combination of these modes is also possible; for example, a 6-column Isocratic section can be set up in conjunction with a 2-column Step section in which a stronger-eluting solvent can be used to “purge” a column followed by re-equilibration in the solvent used for the Isocratic section.

Isocratic and Step modes employ fundamentally different approaches for method development. The table below lists some factors to be considered and is intended as a general guideline for choosing which continuous SMB chromatography mode to use.

Isocratic

Step

Preferred when selectivity is

< 5

> 5

Typical product dilution
factor

2 – 5

< 2 (often possible to concentrate product relative to Feed)

Method development
drivers

Triangle Theory
Adsorption isotherms

Adsorbent capacity
Feed concentration
Process steps & volumes

Example separation
chemistries

Chiral
Normal phase
Reversed phase
Size exclusion
Ion exchange (low selectivity)
Ion exclusion

Any when selectivity is high, e.g.:
Affinity
Solid phase extraction
Ion exchange
Hydrophobic interaction

Example configurations 3-zone 3-2-3
4-zone 2-2-2-2
3-zone 2-2-2 with column purge
4-step 2-2-2-2
4-step 1-1-1-1
4-step 3-2-1-2
5-step 3-2-1-1-2
5-step 3-2-1-1-2 with Feed flush
7-step 3-1-1-1-1-1-1
7-step 3-1-1-1-1-1-1 with Feed flush

In both Isocratic and Step operating modes, parameters for continuous chromatography are determined using data obtained from single column experiments. Semba Biosciences provides simple software tools for each mode to convert the single column data into starting conditions for continuous separation on the Octave System.