Module 1 - Forensic Toxicology

Colour Testing

Colour testing is a fast and simple technique used to determine if an individual has drug or poison in his or her system. This technique can identify the type of drug or poison present. However, colour tests exist for only certain substances, and they cannot indicate the quantity of the suspected substance. Because of this, colour testing is always followed by a confirmation test.

In one type of colour testing, a small test strip is dipped into a urine sample. The strip changes to a specific colour when exposed to a certain drug or poison. In another colour testing technique, certain chemicals are combined with an isolated sample. A reaction that causes a colour change indicates the presence of a certain drug or poison. In the Marquis colour test, an isolated sample is combined with formaldehyde and sulfuric acid. If the resulting solution turns purple, this indicates that the sample contains opiates.

Microcrystalline Testing

In this technique, a small amount of an isolated sample is combined with a specific chemical reagent. If a certain drug or poison is present, a chemical reaction occurs, producing a crystalline precipitate. The crystalline structure and colour vary according to the drug or poison being tested. After the precipitate has formed, it may be analyzed under a microscope to confirm its identity.

Microcrystalline testing can be more accurate than colour testing. However, like colour testing, it does not indicate the quantity of the suspected substance. Also, microcrystalline tests can test for only certain substances such as cocaine and methamphetamines. Because of this, microcrystalline testing is always followed by a confirmation test.

Immunoassay Testing

Immunoassay testing identifies and measures the level of a drug or poison in an isolated sample. It uses the chemical reactions of antibodies to their specific antigens. Immunoassay testing is common because it is able to detect and accurately determine the concentration of the drug or poison in an isolated sample.

Antibodies for drugs and poisons are produced in animal test subjects by combining the drug or poison with a protein to produce a drug-protein complex. Then, this is injected into the animal where it is perceived by the animal’s immune system as an antigen. Consequently, the animal’ immune systems produces specific antibodies against this complex. Then, these antibodies are collected from the blood of the animal and used in immunoassay testing.

For example, the breakdown-products of marijuana are combined with a protein and then injected into an animal test subject. Antibodies of this THC-complex are created and collected from the animal’s blood. These THC antibodies are then added to an isolated sample. If marijuana is in the sample, the THC antibodies react. If no marijuana is in the sample, the antibodies do not react. If THC antibodies do react, then estimating the number of THC antibodies that react determines the quantity of marijuana in the sample. To determine the quantity of antibodies that react, they must be labelled either with an enzyme (enzyme immunoassay technique, or EIT) or a radioactive isotope (radioimmunoassay, or RIA).

Some organic substances have similar chemical structures to certain drugs and poisons. As a result, these may react with the immunoassay antibodies to produce a false positive result. Because of this, immunoassay testing is always followed by a confirmation test.

Gas Chromatography

Gas chromatography separates an isolated drug or poison sample into its distinctive component chemical parts. Gas chromatography is common because it is accurate and fast. The basic steps involved in the gas chromatography technique include the following.

1. The isolated sample is placed in a heated injection chamber.
2. Small amounts of the isolated sample and some nitrogen gas are injected into narrow, coiled glass or stainless steel tubing that is 2 to 6 metres long. The inside of the tubing contains a thin film of liquid.
3. As the isolated sample passes through the tubing, its components are separated because they diffuse at different rates into the liquid.
4. By the time the isolated sample reaches the end of the tubing, its components are completely separated.
5. The individual components of the isolated sample enter a detector. This detector generates a series of electrical signals that produce a chromatogram.

A standard chromatogram is a graph with a series of peaks that correspond to the individual chemical components of a substance (see diagram below). Each drug or poison creates a predictable and distinctive peak or series of peaks that emerge at predictable times. Therefore, each can be identified easily in a chromatogram. The quantity of the individual drug or poison corresponds to the height of the peak(s) on the chromatogram. Thus, the higher the peak(s), the higher the concentration of drug or poison within the individual sample.

Example Chromatogram of Three Types of Barbituates

Because some organic substances have similar chemical structures to certain drugs and poisons, similar chromatograms may be produced by organic substances and the drugs. Therefore, false positive results are possible. Because of this, gas chromatography is always followed by confirmation testing as is immunoassay testing.

Both cocaine and marijuana can be detected in the bloodstream up to three days after a single dose has been ingested. Fact Sheet: Drug Testing in the Criminal Justice System. US Department of Justice: Drugs and Crime Data, March 1992.

Mass Spectrometry

In a mass spectrometer, a beam of high-energy electrons is directed towards the isolated sample. The electrons collide with the molecules in the isolated sample, causing them to lose electrons and become positively charged ions. These positively charged ions are unstable and break into fragments that pass through a magnetic field where they produce a unique ‘fragmentation pattern’ according to their individual masses. No two substances break into fragments in the same way because their chemical composition is unique. These results are then recorded and stored by a computer. Each fragmentation pattern is then compared to a database of known drug, toxin, and poison fragmentation patterns, and a match is determined.

Every drug or poison produces a distinct fragmentation pattern according to its unique individual chemical structure. No two patterns are exactly alike. Because of this, the results of mass spectrometry are highly accurate. Mass spectrometry is an excellent way to confirm the presence of a particular drug or poison.

"Of all the toxic agents encountered by forensic toxicologists, alcohol and cocaine account for 90% or more in a typical toxicology laboratory." Richard Saferstein, Ph.D: Criminalistics – An Introduction to Forensic Science. New Jersey: Prentice-Hall, Inc., 1998. (p. 313).