The invention relates to an activated form of procaine, and the use of the activated procaine, or salts or solvates thereof, to label amine-containing compounds, such as N-glycans, amine-containing amino acids, amine-containing peptides, amine-containing proteins, or other amine-containing compounds in a sample. Use of activated procaine as a label allows for sensitive detection of compounds labeled with it by both fluorescence and mass spectrometry.