A diesel oxidation catalyst (DOC) is a flow through device that consists of a canister containing a honeycomb-like structure or substrate. The substrate has a large surface area that is coated with an active catalyst layer. This layer contains a small, well dispersed amount of precious metals such as platinum or palladium. As the exhaust gases traverse the catalyst, carbon monoxide, gaseous hydrocarbons and liquid hydrocarbon particles (unburned fuel and oil) are oxidized, thereby reducing harmful emissions. About 30% of the total particulate matter (PM) mass of diesel exhaust is attributed to liquid hydrocarbons, or soluble organic fraction (SOF). Under certain operating conditions, DOCs have achieved SOF removal efficiencies of 80 to 90%. (As a result, the reduction in overall PM emissions from DOC use is often cited at 20 to 50%. Actual emission reductions vary however, as a result of engine type, size, age, duty cycle, condition, maintenance procedures, baseline emissions, test procedure, product manufacturer and the fuel sulfur level.