Optimization of the thermal environment of columns packed with very fine particles

The apparent reduced HETP (height equivalent to a theoretical plate) of columns packed with very fine 1.7 μm BEH-C18particles were measured under isocratic conditions, for different thermal conditions applied to the surface wall (column left in still-air at laboratory temperature, column placed in the oven of the Acquity UPLC chromatograph heated at different temperatures, or column immersed in a water stream at different controlled temperatures). The sample was the low-molecular weight, highly diffusive compound naphtho[2,3-a]pyrene. The mobile phase was either pure acetonitrile or a mixture of acetonitrile and water (85/15, v/v). The plot of the measured HETPs versus the mobile phase flow velocity begins to deviate from the classical isothermal van Deemter curve or the advantages of using very fine particles begins to disappear when the heat power released by the friction of the stream percolating through the packed bed exceeds 4 W per unit length of column (m). These effects are minimum when (1) the temperature of the column wall is not controlled and the wall remains in contact with air; or (2) a suitable difference between the temperature of the column wall (hot) and the temperature of the entering liquid (cooler) is set by the analyst.