Analytical Chemistry

Table 4.24 Some additives used in capillary electrophoresis

Additive Example Use

surfactants (anionic, cationic, or

neutral)

SDS, CTAB, BRIJ, TWEEN • EOF modification

• solubilize hydrophobic solutes


• ion pairing


• MEKC above CMC
  zwitterionic substances MES, TRIS, CHAPS, CHAPSO • increase ionic strength without increasing
  conductivity


• affect selectivity of proteins
  linear hydrophilic polymers methyl cellulose, polyacrylamide,
  PEG, PVA


• reduce EOF


• minimize sample adsorption at low
  concentrations


• CGE at high concentrations
  organic modifiers methanol, acetonitrile, TFA • alter EOF (generally reduce)


• change selectivity in MEKC and chiral analyses
  chiral selectors cyclodextrins, crown ethers, bile
  salts


• chiral separations


• solubilization of hydrophobic solutes
  metal ions K+, Na+, Cu2+, Li+ • alter selectivity in MEKC and CGE

hydrogen bonding/solubilizing

agents

urea • melt double stranded DNA in CGE

• solubilize proteins


• alter selectivity in MEKC
  complexing buffers borate • carbohydrate and catechol separations
  quaternary amines diaminopropane • ion pairing


• EOF reversal

or micelles that act as a second or pseudo-stationary phase with which solute species can interact. The
interaction is hydrophobic and/or electrostatic and is analogous to a sorption mechanism in
chromatography. The hydrophobic ends of the surfactant molecules are oriented inwards towards the
centres of the micelles and the polar or ionic hydrophilic ends point outwards forming a charged surface
which is in contact with the buffer solution (Figure 4.56). As the micelles are usually either cationic or
anionic they migrate during electrophoresis, but if the buffer solution is neutral or basic the EOF carries
all species towards the cathode and past the detector, as in CZE. Neutral solutes interact with the
micelles to varying degrees, not unlike partitioning in chromatography, and thus have different
electrophoretic mobilities. The more hydrophobic a solute species is the more strongly it interacts with
or partitions into the micelles. Consequently, for an anionic surfactant such as SDS (sodium dodecyl
sulphate), which is widely used and which migrates against the EOF, the most hydrophobic neutral
solutes have the longest 'retention times' migrating