Patent ID: 7915048

Claim:
A method for marking a liquid petroleum hydrocarbon; said method comprising adding to said liquid petroleum hydrocarbon at least one anthraquinone dye having formula (I) wherein X is O or S; Y is O or S; R 1 and R 2 independently are hydrogen, alkyl, aryl, aralkyl, heteroalkyl, heterocyclic or alkanoyl; R 3 and R 5 independently are alkyl, aryl, aralkyl, heteroalkyl or heterocyclic; R 4 and R 6 independently are hydrogen or alkyl; and wherein said at least one substituted anthraquinone dye has an absorption maximum in the range from 600 nm to 750 nm; wherein said liquid petroleum hydrocarbon is selected from the group consisting of lubricating oil, hydraulic fluid, brake fluid, gasoline, diesel fuel, kerosene, jet fuel and heating oil; and wherein each anthraquinone dye of formula (I) is present in an amount from 0.01 ppm to 2 ppm.