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Articles  »  Forensic Toxicology

Gas Chromatography / Mass Spectrometry

     The techniques are widely used because it can distinguish the presence of a certain chemical in a highly complex mixture. Once the sample solution is inserted into the Gas Chromatography inlet, it is vaporized immediately because of the high temperature. This area is the heated injection stream. From here, it is flown to the column by a carrier gas. The carrier gas used most often is Nitrogen or Hydrogen. Helium is also sometimes used.

     After the mixture of gases enters the packed column, the column containing a thin film of liquid, the sample is separated in the column, and the carrier gas and separated components exit the columns and enter the detector. The next column is called the capillary column. The capillary column interface is the connection between the Gas Chromatography Unit and the Mass Spectrometer. "Some interfaces are separators and concentrate the sample via removal of the helium carrier".

     The sample now has to be ionized. There are two different methods that exist for ion production. In a Toxicology lab, the method, which is used more frequently, is called the Electron Impact Method (EI). The alternative to this method is the Chemical Ionization Process (CI). "For electron impact ionization a collimated beam of electrons impact the sample molecules causing the loss of an electron from the molecule". A molecular ion is the molecule with an electron missing. It is denoted as M+. The process also forms negative ions but the detector only reacts to the positive "species".
Once ionization occurs, a very small positive charge is used to push the positive ions out of the chamber. The positive charge repels the ions out since the charge is the same on both members, and the same charges always repel each other.

     The next step in this process is the actual filtering based on their mass. The component that does this is the Mass Analyzer Filter. There are several techniques, which exist for the separating of these particles: "quadruple filter, ion traps, magnetic deflection, time-of-flight, radio frequency, cyclotron resonance, and focusing". The most commonly used techniques are quadruples and ion traps.

     The process of ionization generates an electrical signal. That electrical signal is graphed on a strip of paper, according to the time. Chromatogram refers to the graph of the function of chemical signals plotted against their respected time. Retention time is the time required for a chemical to exit the column from the time of its insertion.

     Gas chromatography/Mass Spectrometry can be used as a crime-fighting tool. It can be used to yield results for the presence of a certain substance in a mixture. A technique of gas chromatography is used in arson cases to test for the presence of accelerants. That process is called Pyrolysis Gas Chromatography.    

chromatography - resolution of a chemical mixture into
its component compounds by passing it through a system that retards each compound to a varying degree; a system capable of accomplishing this is called a chromatograph. The retarding system can be a surface adsorbant, such as silica, alumina, cellulose, or charcoal, capable of reversibly adsorbing the compounds.

spectrometer - an optical device for measuring wavelengths, deviation of 
refracted rays, and angles between faces of a prism, esp. an instrument (prism spectrometer) consisting of a slit  through which light passes, a collimator, a prism that deviates the light and a telescope through which the deviated light is viewed and examined.

pyrolysis - the subjection of organic compounds to very high temperatures or the resulting decomposition.

Saferstein, Richard: CRIMINALISTICS: An Introduction to Forensic Science
(Prentice Hall Publishings: New Jersey 1995)

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