6-Amidino-2-naphthol methanesulfonate is a chemical compound primarily known as a precursor to the fluorescent dye, 6-Aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC). AQC is a highly sensitive and widely used derivatization reagent for the pre-column fluorescence labeling of amino acids in high-performance liquid chromatography (HPLC) analysis. The compound itself features a naphthalene core substituted with an amidino group and a hydroxyl group, which is stabilized as the methanesulfonate salt. Its main industrial and research application is in the field of analytical biochemistry and life sciences. After appropriate synthetic conversion, the derived AQC reagent reacts rapidly and quantitatively with primary and secondary amines under mild conditions to form stable, fluorescent urea derivatives. This enables the sensitive detection and quantification of amino acids, peptides, and other amine-containing biomolecules in complex samples such as physiological fluids, hydrolysates, and food products. Beyond its role as a dedicated precursor for a specific fluorescent labeling reagent, the structural motif of a substituted naphthol with an amidine group could potentially serve as a specialized building block in organic synthesis for constructing more complex fluorescent probes or heterocyclic systems. However, its use is predominantly tied to the production and application scope of AQC-based analytical methodologies.