How Forensic Lab Techniques Work

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Forensic Drug Testing

Forensic labs are often called in to identify unknown powders, liquids and pills that may be illicit drugs. There are basically two categories of forensic tests used to analyze drugs and other unknown substances: Presumptive tests (such as color tests) give only an indication of which type of substance is present -- but they can't specifically identify the substance. Confirmatory tests (such as gas chromatography/mass spectrometry) are more specific and can determine the precise identity of the substance.

Forensic technicians are often called to identify unknown drugs.
Australian Federal Police via Getty Images
Forensic technicians are often called to identify unknown drugs. A beauty student allegedly tried to smuggle more than 10,000 amphetamine tablets into Australia.

Color tests
expose an unknown drug to a chemical or mixture of chemicals. What color the test substance turns can help determine the type of drug that's present. Here are a few examples of color tests:

Type of Test
ChemicalsWhat the Results Mean
Marquis Color
Formaldehyde and concentrated sulfuric acid
Heroin, morphine and most opium-based drugs will turn the solution purple. Amphetamines will turn it orange-brown.
Cobalt thiocyanate
Cobalt thiocyanate, distilled water, glycerin, hydrochloric acid, chloroform
Cocaine will turn the liquid blue.
Dillie-KoppanyiCobalt acetate and isopropylamine
Barbiturates will turn the solution violet-blue.
P-dimethylaminobenzaldehyde, hydrochloric acid, ethyl alcohol
LSD will turn the solution blue-purple.
Duquenois-Levine Test
Vanillin, acetaldehyde, ethyl alcohol, chloroform
Marijuana will turn the solution purple.

Other drug tests include ultraviolet spectrophotometry, which analyzes the way the substance reacts to ultraviolet (UV) and infrared (IR) light. A spectrophotometry machine emits UV and IR rays, and then measures how the sample reflects or absorbs these rays to give a general idea of what type of substance is present.

A more specific way to test drugs is with the microcrystalline test in which the scientist adds a drop of the suspected substance to a chemical on a slide. The mixture will begin to form crystals. Each type of drug has an individual crystal pattern when seen under a polarized light microscope.

Gas chromatography/mass spectrometry isolates the drug from any mixing agents or other substances that might be combined with it. A small amount of the substance is injected into the gas chromatograph. Different molecules move through the chromatograph's column at different speeds based on their density. For example, heavier compounds move more slowly, while lighter compounds move more quickly. Then the sample is funneled into a mass spectrometer, where an electron beam hits it and causes it to break apart. How the substance breaks apart can help the technicians tell what type of substance it is.

What methods do technicians use to help track down hit-and-run vehicles or arsonists? Find out next.