However, unlike cation-exchange, a particularly hydrophilic peptide can be retained more strongly than a hydrophobic peptide with more basic residues. Thus, the selectivity of the two methods is complementary. No salt is necessary unless the carbohydrate is charged. Anomeric forms of reducing sugars are resolved. C and G are retained much more than A and T, and may necessitate lower levels of acetonitrile.
Selectivity depends upon the pH and ionic strength.
Some experimentation with the salt level and pH will be necessary in each case. Volatile Mobile Phases and Sequencing The presence of 5-10 mM nonvolatile buffer salt does not interfere with many sequencing techniques for peptides. If a completely volatile mobile phase is needed, as for mass spectroscopy, then ammonium formate can be used as the buffer salt, with a descending acetonitrile gradient. Unfortunately, formate absorbs and gives baseline artifacts in gradient elution at low wavelengths. No such problems are encountered at 254 or 280 nm.