Patent ID: 8592213

Claim:
A method comprising: adding a first marker to a fuel; obtaining a sample of fuel having an unknown number of markers; testing the fuel for a presence of the first marker in the fuel in a predetermined concentration range; and testing the fuel for a presence of a second marker in the fuel wherein: the presence of the first marker in the fuel in the predetermined concentration range and an absence of the second marker in the fuel is indicative that the fuel is unaltered, the presence of the first marker in the fuel in a concentration less than the predetermined concentration range is indicative that the fuel is altered, and the presence of the second marker in the fuel is indicative of an altered fuel, wherein the testing the fuel for the presence of the first marker and testing the fuel for the presence of the second marker are performed a substantial period of time subsequent to the adding of the first marker to the fuel; the first marker is a quantitative marker, the second marker is a binary marker, the first marker and the second marker are each configured so that each can be at least partially removed from the fuel by the same laundering agent and testing of the fuel occurs on-site.