Guidelines for tuning the macropore structure of monolithic columns for high-performance liquid chromatography

The ability to control the external porosity and to tune the dimensions of the macropore size on multiple length scales provides the possibility of tailoring the monolithic support structure towards separation performance. This paper discusses the properties of conventional polymer-monolithic stationary phases and its limitations regarding the effects of morphology on kinetic performance. Furthermore, guidelines to improve the macropore structure are discussed. The optimal monolithic macropore structure is characterized by high external porosity (while maintaining ultra-high-pressure stability), high structure homogeneity, polymer globule clusters in the submicron range, and macropores with a diameter tuned toward speed (small diameter in the 100-500 nm range using short beds) or efficiency (larger macropores in the range of 500 nm-1 μm allowing the use of longer column formats). Finally, promising approaches to control the morphology are discussed.