Multimodal liquid chromatography of small molecules†
Multimodal chromatography (MMC) is a family of chromatographic techniques that has been used when single separation modes fail. Multimodal liquid chromatography (MMLC) is a special type of MMC that is applied to increase the number of compounds that can be separated in a single run and to widen the applicability of LC methods. Employment of various modes for chromatographic separations is possible by utilizing different stationary phases, different mobile phases, or different chromatographic conditions in the same run. The selected stationary phases represent the most important variable in MMLC systems. These stationary phases can be classified as mixed or discrete where the former can be either homogeneous or heterogeneous in nature, and the latter can be either tandem or biphasic. Common examples are ion exchange phases with hydrophobic or hydrophilic linkages. Choosing the right mobile phase depends on the compatibility with the recruited modes. A sequential pH and organic solvent gradient is effective for a mixture of charged and neutral compounds. The contributing modes in MMLC can be determined by changing the stationary phase, changing the mobile phase variables, investigating the retention order or comparing the retention patterns to those obtained by single mode methods. The different applications of MMLC in medical, pharmaceutical, and industrial analysis of small molecules are discussed. Representative sample mixtures are water soluble/fat soluble vitamins, acidic/neutral aromatics, and inorganic anions/cations. A table that summarizes the separation modes, the employed stationary phases, the mobile phases, and the detection methods is provided.