Segmented flow and curtain flow chromatography: Overcoming the wall effect and heterogeneous bed structures

• The wall effect seriously limits separation efficiency.
• End column detection greatly enhances separation efficiency.
• Parallel segmented flow emulates end column detection.
• Curtain flow chromatography maximises efficiency and sensitivity.
• A virtual column is established that is wall-less.

The variation in mobile phase velocity as a function of the column radius has been shown to be a major limitation in the efficiency of HPLC columns. One contributing factor to the variability in the flow velocity stems from the heterogeneity in the radial packing density, leading to what has been described as the ‘wall-effect’. The wall-effect generates parabolic-type elution profiles, which dilutes the sample and creates tailing bands. In this communication a new column technology is discussed that has been designed to overcome the wall effect, minimising the limitations associated with packing heterogeneity. This technology has been referred to as active flow technology and consists of two types of column designs, parallel segmented flow and curtain flow. In both these column designs sample that elutes through the column in the radial central region of the bed is separated from the flow that elutes along the wall region. Hence, the sample that elutes through the most efficiently packed region of the bed is collected to the detector. As a consequence more theoretical plates are obtained, and sensitivity is increased since the sample is not diluted by the diffuse tail. Sensitivity is enhanced further in the curtain flow design. The benefits of these new columns are discussed