Analysis of Short Amines without Using Ion Pair Reagent

For the HPLC analysis of basic compounds such as short amines, ion exchange columns or ODS columns with an ion pair reagent are commonly used. However, for the analysis using ODS columns with an ion pair reagent, it is problematic to control the separation because the following two modes contribute to the separation.

1) Mode 1
With this mode, the sample and the ion pair reagent generate an ion pair compound, and at the same time, the sample hydrophobicity increases due to the presence of an alkyl chain coming from the ion pair reagent. Therefore the sample is retained by the column.
R-NH₃(+) + ion pair reagent(-) = [R-NH₃(+)(-) ion pair reagent]

2) Mode 2
With this mode, the ODS column works as an ion exchange column. The strongly hydrophobic alkyl chain coming from the ion pair reagent is adsorbed by the packing material.
Compared with ODS columns, polymer-based reversed phase ODP2 HP, Asahipak ODP and RSpak DE series columns have the following advantages:
a) Reversed phase analysis of short amines can be performed with an eluent at pH10-11 without using an ion pair reagent.
b) Under alkaline conditions, the peak shapes of short amines are very sharp.
c) ODP and DE columns are stable over a wide pH range from pH2 to pH11.