Considerations for the use of ultra-high pressures in liquid chromatography for 2.1 mm inner diameter columns

A multiscale modelling study on the sense and nonsense of thermal conductivity enhancement of liquid chromatography packings and other potential solutions for viscous heating effects

“…(2) In which P is the local pressure and ux and ur the axial and radial velocity components. This pressure work term multiplied with T, in which  is the thermal expansion coefficient, gives the amount of energy absorbed by the thermal expansion and hence the remaining energy, available for heating is [7,18] :…”

“…This so-called viscous heating or viscous dissipation of the mechanical energy increases the temperature T of the mobile phase, column bed and hardware (wall, fittings, frits). The heat either exits the column at its outlet (giving rise to axial temperature gradients) or through its wall (giving rise to radial temperature gradients) [1][2][3][4][5][6][7] . In the limiting cases, all generated heat is either stored in the liquid (perfectly adiabatic conditions, large axial temperature gradient), or removed through the wall (perfectly isothermal column wall, large radial temperature gradient) [4,5,7] .…”

“…The heat either exits the column at its outlet (giving rise to axial temperature gradients) or through its wall (giving rise to radial temperature gradients) [1][2][3][4][5][6][7] . In the limiting cases, all generated heat is either stored in the liquid (perfectly adiabatic conditions, large axial temperature gradient), or removed through the wall (perfectly isothermal column wall, large radial temperature gradient) [4,5,7] . Under practical LC conditions, where the column hangs in a thermostatted compartment filled with air, the thermal boundary conditions will fall in between these limiting cases, resulting in both axial and radial temperature gradients, with their amplitude depending on the flow conditions around the column (still or forced air oven).…”

Methods to determine the kinetic performance limit of contemporary chromatographic techniques

“…If the column is operated in an isothermal mode by placing it in a water bath or in a forced air oven (near‐isothermal condition), the temperature does not increase significantly in the column and the pressure effects will dominate. Unfortunately, the removal of the generated heat through the column wall also creates large radial temperature gradient, which in turn lead to radial velocity gradients [100–106]. The trans‐column differences in velocity can cause significant band broadening for normal and narrow bore columns, and also affect the predictions made by the kinetic plot.…”

“…The trans‐column differences in velocity can cause significant band broadening for normal and narrow bore columns, and also affect the predictions made by the kinetic plot. When operating in near adiabatic conditions, which can be achieved in a vacuum jacket oven [107,108] or can be approximated by using an insulated column, the axial temperature gradient is much larger, but the radial gradient, at least at current available instrumental pressures [104,109], is more limited. In this case a much smaller effect on performance will be observed, but a more pronounced one on the retention factor.…”