Aqueous solutions of the calcium and sodium salts of dipicolinic acid (DPA) were shown to have weak fluorescence when excited at wavelengths near 300 nm, but no fluorescence was observed from DPA alone. Upon UV irradiation at 254 nm the fluorescence of all three forms increases dramatically. The emission spectrum of calcium-DPA (CaDPA) has a maximum at 406 nm with full width at half-maximum of 70 nm. Changes in the absorption spectrum of the irradiated solution and the fact that the changes neither in absorption nor in fluorescence reverse after several days in the dark indicate that a photochemical reaction has taken place. The shapes of the emission spectra for all three DPA forms were very close to identical for excitation wavelengths in the range of 270 to 310 nm and for UV irradiation up to three hours, suggesting that one particular photoproduct dominates in producing the enhanced fluorescence.
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