An LC-MS is an HPLC system with a mass spec detector. The HPLC separates chemicals by conventional chromatography on a column. Usually the method will be reverse phase chromatography, where the metabolite binds to the column by hydrophobic interactions in the presence of a hydrophilic solvent (for instance water) and is eluted off by a more hydrophobic solvent (methanol or acetonitrile). As the metabolites appear from the end of the column they enter the mass detector, where the solvent is removed and the metabolites are ionised. The metabolites must be ionised because the detector can only work with ions, not neutral molecules. And ions only fly through a very good vacuum, so removal of the solvent is a vital first step. The mass detector then scans the molecules it sees by mass and produces a full high-resolution spectrum, separating all ions that have different masses.