THE MCSGP PROCESS
Figure 1
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The working principle of the MCSGP process can be explained using a conventional batch elution chromatogram, as shown in Figure
1. The x-axis in the figure indicates operating time. The following steps are taken sequentially: 1) load the column with
feed, 2) wash the column to elute nonadsorbing impurities, 3) gradient elution, 4) clean-in-place (CIP) or wash with a strong
eluent, and 5) wash with a weak eluent for regeneration. The next batch elution can then be started by loading the column
again.
The molecules eluted during the gradient operation can generally be classified into the following three categories: early
eluting impurities, product, and late eluting impurities. These categories will be abbreviated as follows: W for early eluting
or weakly adsorbing, P for product, and S for late eluting or strongly adsorbing. These three categories are indicated schematically
in Figure 1 by three triangles: the first triangle represents W and ends at time tB, P elutes between tA and tD, and S starts eluting at tC. In general, the elution time windows of W–P and P–S overlap in the time intervals tA to tB and tC to tD, respectively, as indicated in Figure 1. In conventional batch chromatography, the P eluted in the overlapping regions would
have to be discarded (or could be recycled in peptide purification) and only the P eluted between tB and tC is the collected product meeting specifications.
In the MCSGP process, the same elution scheme as shown in Figure 1 is maintained, but the process is operated with three small
columns instead of the one large column used in batch chromatography.
Figure 2
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The three columns used in the MCSGP process are operated either in the connected mode (upper flow path in Figure 2) or in
batch mode (lower flow path in Figure 2). Each flow path is operated for a fixed time, which can be the same for the batch
and the connected mode, but does not need to be. The arrows in Figure 2 show how a single column is switched through the different
positions.
Tracking a single column, the following tasks are carried out: in the first position in the lower right corner of Figure 2,
the column is loaded with the feed so that a first portion of the W is eluted from the column, but no P is eluted. This task
corresponds in batch chromatography to the elution of all molecules from t = 0 to t = tA. Before any product is eluted, the column is switched to the middle position in the connected mode.
At this position, the overlapping fraction W–P (i.e., tA to tB in Figure 1) is eluted. It should be noted that this fraction that is being eluted from the middle column in the connected
mode is not leaving the process, but gets mixed with the stream coming from a gradient pump before it enters the column in
the upper right position, where it adsorbs.
The column is then switched to the lower middle position, where the pure P can be eluted, equivalent to the time interval
from tB to tC in the batch chromatogram in Figure 1.
Before any S is eluted into the pure product outlet stream, the column is switched again to the upper left position where
the overlapping region P–S is eluted, but because of the connected mode, it stays in the process. This corresponds to the
elution time interval tC to tD in Figure 1. Now the remaining molecules that are still in the column all belong to the category
S.
Therefore, the column is switched to the lower left position, where it is treated with a strong eluent to wash the column.
If necessary, an additional, more rigorous treatment with another eluent can be considered.16
It is not in the scope of this paper to provide a detailed description of the principles or design of the MCSGP process. A
typical application for MCSGP will be shown in the following section. More details about the design of the MCSGP can be found
elsewhere.1–3
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