Discovery of invisible extra fronts in single-component frontal analysis in liquid chromatography

Frontal analysis (FA) is often used in the "staircase mode", in which the solute concentration in the eluent increases stepwise. We demonstrate here in the single-component case, that all eluted breakthrough curves (fronts) for the second and subsequent steps consist solely of displaced plateau molecules. The newly introduced molecules (i.e., the introduced mass) instead elute later, in a breakthrough front hidden from detection, i.e., the mass front. These effects were studied using experimentally verified numerical calculations, the mass fronts being visualized using an enantiomer pair in an achiral separation system. Notably, the mass front displays no self-sharpening effects, even under strongly nonlinear conditions. Instead, the front is sigmoidal in shape.