Towards a solution for viscous heating in ultra-high pressure liquid chromatography using intermediate cooling.

A generic solution is proposed for the deleterious viscous heating effects in adiabatic or near-adiabatic systems that can be expected when trying to push the column operating pressures above the currently available range of ultra-high pressures (i.e., 1200 bar). A set of proof-of-principle experiments, mainly using existing commercial equipment, is presented. The solution is based on splitting up a column with given length L into n segments with length L/n, and providing an active cooling to the capillaries connecting the segments. In this way, the viscous heat is removed at a location where the radial heat removal does not lead to an efficiency loss (i.e., in the thin connection capillaries), while the column segments can be operated under near-adiabatic conditions without suffering from an unacceptable rise of the mobile phase temperature. Experimental results indicate that the column segmentation does not lead to a significant efficiency loss (comparing the performance of a 10 cm column with a 2 cm x 5 cm column system), whereas, as expected, the system displays a much improved temperature stability, both in time (because of the shortened temperature transient times) and in space (reduction of the average axial temperature rise by a factor n). The method also prevents a large backflow of heat along the column wall that would lead to large efficiency losses if one would attempt to operate columns at pressures of 1500 bar or more. A real-world pharmaceutical example is given where this improved temperature robustness could help in moderating the changes in selectivity during method transfer from a low to a high pressure operation, although the complex non-linear behavior of the viscous heating and high pressure effects result in lower than expected improvement.