Influence of phase type and solute structure on changes in retention with pressure in reversed-phase high performance liquid chromatography

• We studied the effect of pressure on retention for different reversed-phase columns.
• Solute molecular size has the greatest influence on retention increases produced.
• C18 column nature (monomeric or polymeric) has relatively little influence.
• Solute polarity and planarity influence pressure effect on retention.
• Increased pressure increases shape selectivity of polymeric columns.

The influence of pressure on the retention of several types of solute, including acids, bases and neutrals, was studied by the use of restriction capillaries added to the end of various monomeric and polymeric octadecylsilyl-modified 5 μm particle size columns. Although it appeared that certain polymeric columns could give somewhat greater increases in retention with pressure, differences in behaviour between these different C18 columns were rather small. Differences in solute molecular size were most important in determining increases in retention with pressure. However, solute structure such as polarity and planarity were also influential. A prototype C30 column gave interesting selectivity changes between planar and non-planar solutes as a function of pressure. Considerable selectivity differences with pressure were shown when diverse mixtures of solutes were analysed. For the solutes studied, only minor effects of increased pressure on column efficiency and peak shape were noted.