Field Testing
Law enforcement officers use presumptive color tests in the field to provide probable cause for arrest and determine if a substance warrants laboratory submission. The Marquis reagent turns purple-black with heroin/morphine and orange-brown with amphetamines. The Duquenois-Levine test produces a purple color that transfers to chloroform when cannabis is present. The Scott test (cobalt thiocyanate) turns blue for cocaine. The Mandelin reagent identifies various substances by distinct color reactions. However, field tests have significant limitations: false positives occur frequently (the Marquis reagent reacts with some over-the-counter medications, and cobalt thiocyanate reacts with some local anesthetics). Field test results alone are insufficient for prosecution—laboratory confirmation is always required for criminal charges.