None! LCMS is HPLC with an MS detector. Instead of phosphate buffer in the mobile phase for HPLC-PDA (for pH adjustment) you use formic acid (in order to prevent MS contamination). The chromatographic conditions are about the same. MS is about 1000X more sensitive than UV (PDA-DAD) and can detect concentrations of the molecular target commonly found in biological fluids (~pg/mL).