Derivatization reagents were added through the particle beam high-performance liquid chromatograph-mass spectrometer interface in order to improve the detectability of polar compounds separated by high-performance liquid chromatography and detected by mass spectrometry by formation of the corresponding derivative. Mainly qualitative aspects are considered in this work. Dynamic derivatization in the gas phase of several substances containing carboxylic, sulphonic, hydroxy and other acidic or basic functional groups is described.