A method for analyzing aromatic compounds, and a reagent kit for LC-MS analysis of aromatic compounds. The method includes preparing a diazonium reagent, contacting aromatic compounds in a sample with the diazonium reagent to form an analyte; and measuring an amount or ratio of the analyte. The reagent kit includes a diazonium reagent, wherein the diazonium reagent includes (i) a diazonium salt that contains a diazonium ion; (ii) an amine and nitrous acid; and/or (iii) a nitrite and an acid.