The cops pull over a car doing 75 in a school zone. The driver is behaving errratically, so they pull him out of the car for a search and bingo: a small bag of white dust in the perp's pocket. But what is it—baking soda, cocaine, arsenic? To find out, they rely on this series of chemical tests.
Identifying the contents of pills and powders is no easy feat because of the sheer volume of potential chemicals present, from byproducts of the manufacturing process to adulterants. Cocaine, for example, is commonly cut with Ritalin, Dexedrine, or amphetamine. Ecstasy is a veritable slag pit, often containing methamphetamine, piperazine, caffeine, or benocyclidine in addition to the MDMA. So, before they attempt to positively identify the specific compound present, known as confirmatory testing, officers will first conduct "presumptive testing" in the field to narrow down the chemical suspects through the practice of color testing.
Color tests leverage specific reagents to interact with and test for specific chemicals in a batch of unknown drugs. If the illicit chemical is present the test will change colors, like a pregnancy test except you get a federal prison sentence instead of a bouncing baby boy. The type of test depends on what kind of drugs the police are looking for, which depends on what they think you're on. If, for example, you've been picked up for blocking traffic while attempting squash invisible gnomes, they'll use a Van Urk test to see if that vial in your pocket contains LSD. Or, if you've just broken out of your second pair of handcuffs and are attempting to headbutt your way out of the back of a patrol car, they'll employ a Marquis test to check for the presence of PCP in your cigarettes.
The Marquis Color test is among the most common presumptive tests. First developed in the late 19th century to differentiate between opiate alkaloids, it uses a combination of 40-percent Formaldehyde and 3ml concentrated sulfuric acid to test for and identify them based on both their initial color change and any subsequent color shifts. Opiates turn the test precipitate purple, amphetamines turn it brown-orange. The Marquis Test is commonly used for Ecstasy testing as MDMA turns the precipitate purple-black.
Cobalt thiocyanate is used to test for cocaine. The Cobalt is mixed with the questionable substance, then drowned in hydrochloric acid, doused in chloroform, and shaken. If cocaine is present it will precipitate out, bind with the cobalt and turn bright blue. The Dillie-Koppanyi test, on the other hand, uses Cobalt acetate and isopropylamine to test for barbituates, which turn the solution violet-blue.
Oddly enough, there is even a field test for marijuana—handy when testing bricks to see if they're made of hash or black tar heroin. Known as the Duquenois-Levine test, it uses Vanillin—vanilla's chemical flavor base—in the same manner as the Marquis test, however it uses hydrochloric acid rather than sulfuric. If there's THC present, the solution turns purple. The problem with the Duquenois-Levine test however is its high rate of false positives. THC isn't the only compound that turns purple in the test solution. Chinese motherwort and 25 other species of plants also turn violet.
Once the range of potential culprits is sufficiently narrowed, the drugs can then undergo confirmatory testing. This can include ultraviolet spectrophotometry, microcrystalline tests in which a reagent on a slide will form different crystal structures based on the illicit chemical added, and even gas chromatography which shatters molecules with an electron beam for analysis.