Crime Library: Criminal Minds and Methods

Forensic Toxicology

Modern Detection Methods

For both alcohol and drugs, the most state-of-the-art analysis will involve spectrometry and often some form of chromatography.

"There are two ways to describe it," says Dr. Robert Middleberg, the Laboratory Director at National Medical Services.   "Either chromatography is an analytical technique that has a mass spectrometer as a detector, or mass spectrometry is an analytical technique that uses chromatography and other methods to get the samples into it."   

For example, toxicologists dissolve tissues for analysis in an acidic or alkaline solution and then use high-pressure liquid chromatography or gas chromatography (which replaced paper chromatography) with the mass spectrometer.   Thin layer chromatography is also used, where the sample is placed in a vertical gel film and then subjected to a liquid solvent that breaks it into its constituent parts.  Direct tests on substances could now be used rather than having to go through the tedious process of extraction.

Gas chromatograph with mass spectrometry
Gas chromatograph with mass spectrometry
 

The real workhorse of a crime lab is the gas chromatograph with mass spectrometry (GC/MS).   Most things encountered at a crime scene are complex mixtures and with this method can be separated into their purest components.    

A small amount of the suspect substance or unknown material is dissolved in a solvent and then injected by needle into a hollow tube.   A flow of inert gas (helium or nitrogen) propels the heated mixture through the coiled glass tube, where a highly sensitive, computerized detector identifies the separate elements at the other end.  Since each element moves at its own speed, as it crosses the "finish" line, it can be identified.  The amount of pure substance in the mixture is measured as well, producing a chart that offers a composite profile (via travel time measured in minutes).  Comparison---or control---substances are also put through the GC.  This helps to identify suspicious substances, such as an accelerant on a piece of charred wood when compared against wood without accelerant.  GC can be used to identify many things, from poisons to drugs to explosives.  It's also used for blood alcohol evaluations.

Linked with the mass spectrometer, the GC separates the sample into its component parts and the MS bombards the sample with electrons produced by a heated cathode, breaking it into electrically charged fragments.   These fragments pass through the spectrometer, accelerated by an electric field.  A magnetic field deflects them onto a circular path, the radius of which varies according to the mass of the fragment.  As the magnetic field is increased, a detector linked to a computer records the energy spectrums.  The position of each fragment on the spectrum measures its mass, and its intensity indicates its proportion in the sample.  This comes through as a printed readout.

In other words, no matter how the samples are divided according to molecular weight, the spectrometer can identify the smallest traces of individual chemicals.

More Chemistry and Crime
More Chemistry and Crime
 

These days, there are three basic situations, says Marina Stajic in More Chemistry and Crime, that require toxicological analysis.

  • The cause of death is known but drug findings are needed to clarify the circumstances
  • When drugs are the direct cause of death
  • When negative results permit the pathologist to rule out drugs or poison and concentrate on disease  

Middleberg adds "the unexplained death of someone with no obvious trauma and no medical history or trouble." 

While all of this multimillion dollar equipment is used every day, he notes, toxicologists still use tests that seem archaic by today's standards.   "When we want to confirm carbon monoxide poisoning, for example," he says, "we use a little white disk that we put chemicals in, mix, and come back later to look for a color change.  It's easy and it works."   

In some cases, time is of the essence.  In others, it becomes a question of justice.  Let's look at some of the drugs that killers have chosen for their nefarious purposes.

 

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