|Considerations for the use of ultra-high pressures in liquid chromatography for 2.1mm inner diameter columns
|Fundamentals in Separation Science (KVCV)
| Gert Desmet
|Vrije Universiteit Brussel
Abstract Information :
The reduction of particle size is a time-proven method to decrease analysis time and improve the quality of chromatographic separations. Sufficiently long columns packed with small particles (sub-2µm) can however only be operated at their optimal velocity if sufficiently high operating pressures are available. This requires to consider the thermal effects that result from pumping a liquid through a porous medium. This so-called viscous heating or viscous dissipation of the mechanical energy increases the temperature of the mobile phase, column bed and hardware (wall, fittings, frits). The heat can either be removed at the column outlet (leading to an axial gradient in mobile phase temperature) or through the column walls (leading to a radial temperature gradient). This contribution gives an overview of the effects of viscous heat dissipation in chromatographic columns (with an emphasis on so-called narrow bore columns with an inner diameter of 2.1mm) using numerical simulations of the temperature and velocity profiles and the resulting band broadening, for the first time at operating pressures up to 2000 bar, i.e. the expected operating pressure of LC instruments of the future. When operating columns under well-thermostatted conditions to maintain a constant temperature of the mobile phase, a dramatic increase in plate heights can be observed that voids any advantage one could expect from the possibility to use smaller particles offered by the increased pressure limit. In practice, most columns are placed in thermostatted column compartment (still-air oven), where, due to natural convection, a significant loss of heat will take place, resulting in local radial temperature and velocity gradients in the column. Although these gradients have an opposite sign at the front and the back of the column, these effects do not entirely compensate each other, especially when the bulky column endfittings are taken into account. Thus even when the column is not actively temperature controlled a significant loss in performance under standard operating conditions can be expected for operating pressure above 1250 bar, which increases with increasing temperature dependency of the retention factor. In addition, unprecedented experimental measurements of the temperature effects at an operating pressure up to 2600 bar were performed on a 10cm long, 2.1mm ID column showing a dramatic temperature increase up to 60°C relative to the inlet temperature when using methanol as a mobile phase. These results make it clear that drastic changes in column or instrument hardware are required before a further increase in operating pressure up to 2000 bar and beyond can become possible when using single, short (5-15cm) columns, such as an even further reduction in column ID (down to and below 1mm) with a concomitant decrease in instrument dispersion, the development of pressure capable column hardware with a very low wall conductivity or the availability of a cheap and user friendly vacuum housing for columns.