Capillary isoelectric focusing with free or immobilized pH gradient in silica particles packed column

The use of immobilized pH gradient (IPG) capillary isoelectric focusing (CIEF) was confirmed to be possible with packed capillaries. The success of this experiment was due to two key factors: first, the use of surface-confined atom transfer radical polymerization method led to an increase in active reaction sites on the surface of silica particles; second, the subsequent free radical reaction caused carrier ampholytes (CAs) to bond faster and firmer. Based on this scheme, both CIEF with free pH gradient and CIEF with IPG were performed in capillaries packed with 3 μm modified silica particles. In our online CIEF-UV platform, both reversible and irreversible adsorption of proteins was shown to be negligible. Four proteins were focused: cytochrome c (pI 10.2), myoglobin (pI 7.3), carbonic anhydrase (pI 5.9) and trypsin inhibitor (pI 4.5). The comparison of the two CIEF columns showed that the time required for focusing in the packed capillary with IPG is only increased by a factor of 1.5 compared to the packed capillary, giving complete focusing in less than 12 min at 400 V/cm. With the increment of the electric field (the maximum at 600 V/cm), the run time was continually decreasing in these packed capillaries while the peak shape was improving. The four proteins (pH 4.5–10.2) could be successfully separated in our online CIEF platform. Moreover, for the newly online CIEF platform, pressure-driven mobilization without an applied electric field was achieved in the packed capillary with immobilized pH gradient.