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Reports: UR951984-UR9: Fluid Flow and Solute Migration in Supercritical Fluid Chromatography

Donald P. Poe, PhD, University of Minnesota (Duluth)

A major objective of this research is to develop the theoretical framework for understanding how the hydrodynamics of supercritical fluids percolating through a packed column affect the chromatographically important properties related to retention and efficiency in supercritical fluid chromatography (SFC).  During the first year we examined compressibility effects of carbon dioxide-based mobile phases on peak dispersion, on modeling retention and efficiency in SFC, and on development and characterization of instrumentation needed for these studies.  We developed explicit approaches to predict the temperature changes along SFC columns during normal operation, and examined the effects of pressure drop and temperature profiles on retention and efficiency in SFC columns.  The temperature profiles, and the overall temperature change along the column, are determined by the pressure drop, the Joule-Thomson coefficient of the mobile phase, and by the thermal properties of the column and its environment.  In particular, we showed that the temperature drop along a packed column operated under near adiabatic conditions can be calculated from the Joule-Thomson coefficient of the mobile phase and the total pressure drop along the column.  These temperature drops have an important impact on column efficiency and peak resolution. During the second year we performed extensive experimental studies to establish the utility of the Joule-Thomson coefficient as a guide for efficiency in SFC separations.  We made experimental measurements of the column efficiency for elution of model solutes over a wide range of temperatures and outlet pressures relevant to SFC and related the observed efficiency to the Joule-Thomson coefficient at the column outlet.  We used a mixture of normal alkanes 10 to 18 carbon atoms in length as the test mixture, eluted with carbon dioxide mobile phase containing 5, 10 and 20% methanol.  The column was 250 x 4.6 mm ID packed with porous, spherical 5-micron Phenomenex Luna-C18 octadecylsilica particles.  Results for octadecylbenzene are shown in Figure 1.  The operating conditions are defined as the oven temperature and the pressure at the column outlet.  The heavy dashed red line shows the zero heat balance condition where the Joule-Thomson coefficient is equal to zero.  At this condition the viscous heating and the enthalpic cooling due to expansion of the mobile phase are in balance, and no net heating or cooling of the mobile phase occurs.  At larger values of the J-T coefficient net cooling occurs, leading to axial and radial temperature gradients and loss of column efficiency.  Because the pressure is lowest at the column outlet, the J-T coefficient is greatest near the column outlet.  The estimated J-T coefficient at the column outlet can therefore serve as a guide to safe operating conditions.  The boundaries between the operating zones depend somewhat on the solute retention as well.  For moderately retained solutes with retention factors between 2 and 5, efficiency generally deteriorates when the J-T coefficient exceeds 0.1 K/bar, leading to poor separations.  Changes in the mobile phase composition result in changes in the positions of the J-T isopleths.  Increasing the percent methanol shifts these isopleths to lower pressures and higher temperatures, and the operating temperatures and pressure corresponding to the different operating zones shift as well, according to changes in the J-T coefficient.  Changes in the column diameter and thermal conditions (e.g., forced air vs still air) also should have a significant impact of the boundaries of the operating zones.

Figure 1. Operating zones in SFC using 95% CO2 / 5% methanol as mobile phase.  Column: 250 x 4.6 mm ID with 5-micron Phenomenex Luna-C18 particles; solute: octadecylbenzene; thermal environment: forced air. Dashed red lines show constant Joule-Thomson coefficient value in K/bar.