Patent ID: 7736625

Claim:
A method for reducing N-demethylation reactions in a thiazine dye solution, wherein the thiazine dye comprises: (1) the compounds having the structures: (2) the compounds having the structures: the ratio of the combined areas of the 254 nm HPLC peaks representing the compounds in (1) to the combined areas of the peaks representing the N-demethylation derivatives in (2) being at least 6:1; wherein molecules of the dye act as both the sensitizers and substrates in photochemical oxidative N-demethylation reactions in a thiazine dye solution, in which reactions at least a portion of the dye molecules absorb light and are converted to the singlet state, at least a portion of the singlet-state dye molecules form triplet state molecules by intersystem crossing, at least a portion of the triplet-state molecules react with ambient ground-state oxygen to produce singlet-state oxygen molecules, and at least a portion of the singlet-state oxygen molecules N-methylate ground-state dye molecules, the method comprising adding, after the thiazine dye solution is formed, a metal ion to the solution, wherein the metal ion quenches some of the triplet state dye molecules and returns them to the unreactive ground state.