Development of a simple field test for vehicle exhaust to detect illicit use of dyed diesel fuel

Tax-free diesel fuel is intended for off-road uses such as agricultural operations, but illicit use of this fuel does occur and is a convenient way of evading payment of excise taxes. Current enforcement to prevent this practice involves visual inspection for the red azo dye added to the fuel to indicate its tax-free status. This approach, while very effective, has shortcomings such as the invasive nature of the tests and/or various deceptive tactics applied by tax evaders. A test designed to detect illicit dyed-fuel use by analyzing the vehicle exhaust would circumvent these shortcomings. This paper describes the development of a simple color spot test designed to detect the use of tax-free (i.e., dyed) fuel by analyzing the engine exhaust. Development efforts first investigated the combustion products of C.I. Solvent Red 164 (the azo dye formulation used in the United States to tag tax-free fuel). A variety of aryl amines were identified as characteristic molecular remnants that appear to survive combustion. A number of micro-analytical color tests specific for aryl amines were then investigated. One test that detected aryl amines by reacting with 4-(dimethylamino)benzaldehyde seemed to be particularly applicable and was used in a proof-of-principle experiment. The 4-(dimethylamino)benzaldehyde color spot test was able to clearly distinguish between engines that were burning regular fuel and those that were burning dyed diesel fuel. Further development will refine this color spot test to provide an easy-to-use field test.