The effect of ultraviolet (UV) light (240–260 mμ range) on 6, 7-dihydrodipyrido (1, 2-a:2’,1′-c)-pyrazidiinium salt (diquat) and 1,1′-dimethyl-4,4′-bipyridinium salt (paraquat) was studied. Both dipyridyls were very stable in dark but were altered rapidly in the presence of UV light. Radioassay data collected from planchets containing solidified diquat or paraquat indicated a loss of more than 50% of the C14 from both compounds after 48 hr and more than 75% after 96 hr. When radioactive aqueous solutions were placed under UV lights, little change in the radioactivity occurred during an 8-day period as determined by liquid scintillation. When the concentration of diquat or paraquat in these solutions was determined by chemical analysis, degradation of the original molecules was observed.
Information obtained by autoradiography of chromatograms, spotted with radioactive aqueous solutions exposed to UV light, indicated that two or possibly more C14-containing degradation products were formed from both diquat and paraquat. Co-chromatography indicated that one degradation product from paraquat was 1-methyl-4-carboxy-pyridinium ion.