An aqueous solution of L-glutamine (50 mmol/L) and methylglyoxal (100 mmol/L) was incubated at 120 degrees C for 3 h in a 200 mmol/L phosphate buffer (pH 7.4). Four major fluorophores were revealed on the HPLC chromatogram. The same four fluorophores were obtained from the heating of a mixture of ammonia and methylglyoxal. After purification and concentration, they were structurally characterized by electrospray ionization mass spectrometry (ESI-MS) using the high resolution and tandem mass spectrometry capabilities of a quadrupole time-of-flight MS. The accurate mass measurement of their [M+H]+ ions, the MS fragment patterns, and the presence of one to two nitrogen indicate the formation of fluorophores with molecular formulas of C7H7NO3, C8H9NO3, C12H14N2O4, and C12H14N2O5. These results show that, in an aqueous solution, free glutamine undergoes a rapid degradation, leading to the formation of ammonia which reacts with methylglyoxal to form fluorescent heterocyclic Maillard products.