Physical Evidence Handbook

Drug Identification Unit

The drug identification unit analyzes evidence for the presence (or absence) of controlled substances. Controlled substances specifically refer to those compounds listed in the Uniform Controlled Substances Act, Chapter 961 of the Wisconsin statutes. Examples of just a few of the over 200 controlled substances include cocaine, heroin, methamphetamine, LSD, and tetrahydrocannabinol (THC), the active ingredient in marijuana.

Controlled substance evidence may be in many forms. Typical samples include powders, plant material and pharmaceutical preparations, both licit and illicit. Other sample types are occasionally encountered, such as clothing, paper, clandestine labs, and various drug paraphernalia.

A combination of different tests are performed on an unknown material until the analyst can identify or eliminate the presence of any controlled substance. To identify the presence of a controlled substance, generally, the analyst must perform a combination of preliminary-or indicative-tests and confirmatory test(s). Some of the more common tests used by the Crime Laboratory are outlined below.

Color Tests
Color tests simply involve adding a reagent or reagents to the unknown material and observing a color change. The development of a color may indicate the presence of a drug or a class of drugs. Since more than one compound can give the same results, color tests are not specific tests and cannot conclusively identify the presence of a compound. However, they are a good preliminary screening tool. For this reason and the fact that they are simple to perform, color tests are commonly used by law enforcement agencies as an initial screening test when encountering a suspected drug. Some of the common color tests used and the drugs or class of drugs they test for are listed in the table below.

Test Drug
Marquis and Mecke Opiates/MDA-type
Cobalt Thiocyanate Cocaine
PDMAB Tryptamine type
Duquenois-Levine Marijuana

Gas Chromatography
Gas Chromatography Gas chromatography (GC) is an analytical technique used to separate the individual components of a sample. The gas chromatograph consists of an injection port, an oven containing a column, a detector, and a recorder. The column is a long, thin tube containing a stationary liquid phase with a mobile gas phase constantly flowing through it. The unknown sample is dissolved in a suitable solvent and injected into the instrument. The gas carries the sample components through the heated column. Different components move through the column at different rates, resulting in separation of the components. When a compound exits the column, it is detected and recorded. The time required to move through the column from the time of injection to a peak apex is referred to as the retention time. If the retention time of a questioned substance compares to the retention time of a known reference standard, it indicates the presence of that substance. Since it is possible for more than one compound to have the same retention time, this test is not specific. It can be used to indicate that substance but does not conclusively identify it.

Example of Gas Chromatography Output
Example of GC output. Time is measured on the vertical scale

Infrared Spectrometry
Infrared Spectrometry (IR) is a very important analytical tool used for the identification of controlled substances. It involves passing infrared radiation ("light") through the unknown sample. The sample will absorb the infrared radiation resulting in a series of valleys and peaks referred to as an infrared spectrum. Each compound gives a highly specific infrared spectrum. By comparing the spectrum of an unknown sample to the spectrum of a known reference standard an identification of that compound can be made.

Infrared Spectrometry Output
Example of IR output.

Mass Spectrometry
Mass Spectrometry (MS) like IR is a highly specific analysis used for identification of controlled substances. Often, MS is used in combination with gas chromatography and referred to as gas chromatography/mass spectrometry (GC/MS). Gas chromatography, as discussed perviously, separates a sample into its individual components. When coupled to a mass spectrometer, the sample exits the gas chromatograph and is passed to the mass spectrometer. As each component enters the mass spectrometer it is bombarded with a beam of electrons, causing the compound to fragment in a characteristic manner. This fragmentation pattern is referred to as the mass spectrum. Each compound gives a highly specific mass spectrum. By comparing that spectrum to the spectrum of a known reference standard, an identification can be made.