The CE separation mechanism is based on the differences
between the electrophoretic mobilities of charged species in
the presence of an electric field. In a conventional CE system,
the EOF is toward the cathode; anions with an electrophoretic
mobility higher than the electroosmotic mobility of the bulk
electrolyte cannot reach the detector, and the polarity of the
potential applied must be reversed in order to detect these
anions. However, under these conditions, anions with mobilities
lower than the electroosmotic mobility would never
reach the detector. Therefore, for most anion separations, it is
necessary to use a modifier to reverse the direction of the EOF
(Paull and King 2003).