Approaches to find complementary separation conditions for resolving complex mixtures by high-performance liquid chromatography

Chromatographic problems are usually addressed trying to find out a single experimental condition aimed to resolve all compounds in the sample. However, very often, the chromatographic system is not able to provide full resolution. When a separation fails, the usual choice is introducing a drastic change in the chromatographic system (e.g. column, solvent, pH). There are, however, other possibilities that take advantage of the gathered information in the failed separation, without the need of new experiments, based on the concept of complementary separations (e.g. isocratic mobile phases, gradients, columns, chromatographic modes). One separation condition will focus on the resolution of some compounds in the sample, while the other compounds will be resolved using a second (or subsequent) condition(s). Complementary separations, being a simple and attractive idea, present, however, challenges in terms of computation volume and complexity of the required algorithms. This work describes in detail different approaches that have been developed up-to-date for this purpose, and introduces a new approach based on the peak count concept that is benefited of the best features of the previous approaches: high reliability in finding the solution, accessibility to analysts without specialised programming skills and short computation time.