The basic principal of HPLC is the partitioning the analytes between the solid phase and the mobile phase. Since compounds will have different partition coefficients they will be separated on that basis.
HPLC (high performance liquid chromatography) is a chromatography technique where the mobile phase is a liquid and the stationary phase is packed into a stainless steel column at high pressure. It is usually silica particles mainly spherical nowadays. The efficiency is better when the particles are smaller typically 5um. A pump is used to push the solvent through the column and a detector with a flow-through cell used to measure the separated peaks. Usually a computer with integration software collects the data and help quantifying the components. There are huge variations in the sample injection, pump, columns, detectors and software.
HPLC works on the principle that some molecules take longer than others to pass through a chromatography column. This depends on the affinity of the molecule with the mobile phase (liquid or gas) and the stationary phase (solid or liquid). The ones that have more affinity with the stationary phase take longer to pass through and vice versa.
There are different types of columns to separate molecules by different criteria.
There’s molecular exclusion columns, that separate molecules by weight, these columns use tiny beads with canals in them, this causes small molecules to take a longer route and therefore take longer to pass through; normal phase liquid chromatography columns (NPLC), that separate molecules depending on their polarity, these columns are made out of silica with polar functional groups covalently attached, polar molecules take longer to pass through these columns because they are attracted to it through dipole-dipole interactions; reverse phase chromatography columns (RPLC), that do the opposite of NPLC columns, they’re made of silica with non-polar groups equivalently attached, here, non-polar molecules stay behind interacting with the stationary phase through Van Der Waals forces and the hydrophobic effect and take longer to pass through; and ionic exchange columns, that separate molecules by charge, in these columns molecules with formal charges take longer to pass through by electrostatic interactions.