Analysis of Anions by Capillary Electrophoresis and Ion Chromatography for Forensic Applications

Article excerpt


Anion analyses can provide important leads in the forensic investigation of bombings and poisonings. The determination of these negative ions by ion chromatography or capillary electrophoresis provides information on the chemical species of the anions. In addition, these techniques provide a means for the separation of complex mixtures. Separating mixtures and providing speciation are two major advantages of ion chromatography and capillary electrophoresis over elemental analysis. The forensic application of analytical methods for anions found in explosives and poisons is discussed, and relevant examples are presented in this article.


Inorganic salts and acids may be components or residue of explosives and poisons. The identification and characterization of the anions from compounds are of major significance for forensic applications. The advancement of chromatographic techniques such as ion chromatography and capillary electrophoresis has led to the ability to quickly profile and quantitate anionic species. This information can be used in forensic science to determine what substances may have been used to commit crimes and to associate evidence from criminal acts to suspects. Ion analysis has become prominent in forensic casework involving explosives residue (Doyle and McCord 1998; Kishi et al. 1998; Ruetter et al. 1983; Smith et al. 1999), illicit drug samples (Krawczeniuk and Bravenec 1998; Walker et al. 1996), tobacco products (Lu and Ralapati 1998), and biological specimens (Hortin et al. 1999; Wildman et al. 1991). Analysis techniques for anions of forensic interest are discussed in this article, and relevant examples are provided. This article presents past and current work on anion analysis at the Forensic Science Research Unit of the FBI Laboratory in Quantico, Virginia, and is meant to demonstrate the potential of ion determinations to practicing forensic scientists. The current research methods portion describes work in progress and is not to be viewed as finalized or validated operating procedures.


The analysis of anions found in explosives and explosives residue has been of particular interest with the increase in terrorist bombings in recent years. The bombings of the World Trade Center in New York City, New York, and the Federal Building in Oklahoma City, Oklahoma, deserve mention because ion analysis played a significant role in both investigations. The proliferation of information in the public domain and its destructive use are of major concern to law enforcement. The recipes for explosive materials are readily obtained from sites on the Internet or from the literature. Obtaining the materials necessary to make improvised explosives devices is also easy. As an example, fertilizers and fuel oils used to formulate an ammonium nitrate/fuel oil explosive are commonplace and may be purchased without causing any suspicion to the sellers.

The detection of explosives residue after a bombing may be possible because not all of the explosive material is consumed during an explosion. Additionally, characteristic by-products may be formed by the chemical reactions that occur during an explosion. Inorganic explosives, mixtures of strong oxidizers and fuels, are used in the majority of improvised explosive devices in the United States (FBI Bomb Data Center 1997). Inorganic salts of chlorate, nitrate, and perchlorate are frequently used as oxidizers in low explosives. Common fuels include sources of carbon (e.g., charcoal, sugar, hydrocarbons), aluminum, and sulfur. Examples of anions of interest in pre- and post-blast materials include azide, chlorate, chloride, nitrate, nitrite, perchlorate, sulfate, and thiocyanate. However, samples containing low concentrations of these ions may be difficult to isolate and interpret in a massive pile of rubble after a bombing. The presence of high levels of the relevant anions, either alone or in combination, may be of significance when detected in post-blast residue. …