Adjustable column length using a water stationary phase in supercritical fluid chromatography

Abstract

A novel method for adjusting the column length during analysis in capillary supercritical fluid chromatography (SFC) is introduced. The approach is based on using a water stationary phase that can be partially ejected (or replenished) from the column as desired, without physically removing the supporting hardware elements. By flowing cool air through a sleeve surrounding the column in a heated oven, an axial thermal gradient along the length of the column was formed. This established a cooler region where the water stationary phase could be maintained and a hotter region where the coating was removed through dehydration. As such, the effective column length could be easily adjusted by changing the gradient via the air flow rate. Using this prototype arrangement, column lengths could be readily varied between 1.4 and 10 m. System response was also fairly rapid and changes took effect in under 1 min. Once a given length was established, retention times were highly reproducible with a relative standard deviation of 1.8% (n = 3). The method is cheaper and faster than the conventional method of storing numerous columns for manual switching. Further, it avoids the convolution of system pressure and flow rate that accompanies the pressure adjustments normally used to optimize capillary SFC separations. Results indicate that this approach could be a useful alternative for adjusting column length to optimize separation speed and resolution.