The brush type, or Pirkle type chiral stationary phases  are also called π-π Donnor-Acceptor columns. According to some theoretical models separation on these CSPs is based on a three-point attachment between the solute and the bonded chiral ligand on the surface of the stationary phase. These interactions may be attractive or repulsive in nature, depending on the mutual properties. Pirkle columns discriminate enantiomers by binding of one enantiomer with the chiral stationary phase, thereby forming a diastereomeric complex through π-π bonding, hydrogen bonding, steric interactions, and/or dipole stacking. Pirkle CSP can be categorized into three classes:
(i)                 π-electron acceptor
(ii)               π-electron donor
(iii)             π-electron donor-π-electron acceptor.