SUPPLEMENTAL MATERIAL
for
“Burning Down the House:
A Case Study in Forensic Instrumental Analysis”

Introduction to GC-MS

In gas chromatography (GC), a sample is transported through a chromatography column by a gas mobile phase, called the carrier gas. When a volatile or semi-volatile liquid sample is injected into the heated injection port of the GC, the sample volatilizes into a gas and is swept out of the injection port and through the column via the carrier gas. Because the components of a mixture move through the column at different rates based on their affinity to the column material, they are said to have different retention times. These retention times can be used to help identify components of the sample.

Mass spectrometry (MS) is an analytical technique in which atoms or molecules from a sample are ionized, separated according to their mass to charge ratio, and then recorded. In mass spectrometry, molecules are bombarded with high energy electrons. When an electron with a sufficient amount of energy collides with a sample molecule, it dislodges an electron from the sample molecule and creates a molecular ion called a cation ion radical. The cation radical is positively charged and has an odd number of electrons but has basically the same mass as the uncharged parent compound because the mass of an electron is negligible. The positively charged ions are then directed into an analyzer tube surrounded by a magnet. Given that different fragments will be formed, and they will have different sizes, particular types of cationic species will have characteristic mass-to-charge ratios (m/z). A mass spectrometer scans all the m/z values and gives a distribution of positive ions, called a mass spectrum, which is characteristic of a certain compound. The most intense peak in the mass spectrum is called the base peak and is assigned a relative intensity of 100.

When these two methods are coupled together into the same machine, we perform gas chromatography / mass spectrometry or GC-MS analysis. Gas chromatography is used to separate a mixture and mass spectrometry is used to analyze it. If we can selectively monitor specific ions we can easily measure one component, for example an accelerant, in a complex chromatogram of poorly separated compounds.

Introduction to Headspace Analysis

To recover and identify accelerants from charred debris, forensic chemists typically use either headspace analysis or steam distillation. In headspace analysis, a portion of the matrix (which contains the analyte) is placed in a glass tube, which is covered with a rubber septum. When the tube is heated, volatile residue present in the debris will vaporize and will be enclosed in the air space above the sample in the tube. The vapor can be removed with a syringe and subsequently analyzed by gas chromatography. There are three different types of headspace analysis (Grob, 1995):

• Static Headspace
  
  • Sample is heated to increase vapor pressure of analyte and gaseous sample collected with syringe and analyzed via gas chromatography
  • (+) Detection limit: 5–10 microliters of petroleum accelerant in gallon can; ease of application
• Passive Headspace
  
  • Accelerant vapors are diffused from sample onto an absorbent placed inside the container of fire debris.
  • Accelerants are desorbed with carbon disulfide
  • SPME (Solid-Phase Microextraction) An alternative passive headspace sampling technique for gasoline in fire debris. SPME is a simple, solventless extraction procedure in which a phase coated fused silica fiber is exposed to the headspace above the fire debris packaged in a closed container. SPME has been able to detect .04 microliters of gasoline in a quart can.
  • (+) multiple samples can be taken from one sample matrix, Teflon strips can be frozen for later analysis
  • (-) carbon disulfide can interfere with FID (Flame Ionization Detector)
• Dynamic Headspace
  
  • Air or inert gas is passed over the sample (purge and trap technique if sample is liquid) and adsorbed onto a substance such as activated carbon. After the accelerant has been adsorbed onto the carbon, it is often extracted with carbon disulfide. Thermal desorption is not practical because of the high temperature needed to remove hydrocarbons from charcoal (950°C). In one version of this technique, chemists purged a can containing fire debris with heated nitrogen. The nitrogen then passed out a hole in the lid of the container and through a Pasteur pipette packed with activated charcoal. The accelerant was then desorped from the charcoal with carbon disulfide and subjected to GC analysis.
  • (+) more sensitive than static headspace, and not as cumbersome as distillation methods
  • (-) does not give good recoveries of high petroleum distillates, such as diesel fuel

Other Procedural Considerations

In the given scheme, the students are tasked with developing a methodology, determining if the fire was arson, and determining the innocence or guilt of Dr. Marie Stanforth. Depending on how the instructor wishes to arrange the experiment, Dr. Stanforth may be guilty or she may be innocent. Yet, both in planning the experiment, and allowing students to conduct their investigation, the instructor should keep in mind that some accelerants may have a logical explanation for their presence other than arson. The detection of isopropyl alcohol in the bathroom, for example, would not prove arson because it is not uncommon for people to store this product in the bathroom.

The instructor may want to remind students of this fact, or may want to hint at it, so that the detection of one accelerant does not force the students into automatically assuming the fire was arson or that the suspect in question is guilty. For more advanced classes, the instructor may want students to make this conclusion on their own.

Consider the environment from which the sample was taken—if an analyte is detected, is there any other logical explanation for its presence other than arson?

Inspect clothing for traces of accelerant.

Students should use headspace analysis on the clothing sample in the same fashion as the charred samples provided to them. Their conclusions about the guilt or innocence of Dr. Marie Stanforth should evolve from the correlation of any accelerants found on the clothing swatch recovered from Marie and the accelerant (if any) found near the point of origin (which would indicate the fire to be the result of arson).

Sources of Additional Information

Headspace Analysis & GC-MS Analysis

• Grob, R. 1995. Modern Practice of Gas Chromatography. New York: Wiley & Sons.
• James, R. E., C. Meloan, and R. Saferstein. 1980. Laboratory Manual for Criminalistics. New Jersey: Prentice-Hall.
• Newman, R., M. Gilbert, and K. Lothridge. 1998. GC-MS Guide to Ignitable Liquids. New York: CRC Press.
• Niessen, W.M.A., ed. 2001. Current Practice of Mass Spectrometry. New York: Marcel Dekker.

Arson & Arson Investigation

• Conklin, B., R. Gardner, and D. Shortelle. 2002. Encyclopedia of Forensic Science: A Compendium of Detective Fact and Fiction. Westport: Oryx Press.
• Hall, T.L., ed. Magill’s Legal Guide. Pasadena: Salem Press.
• James, S.H., and J.J. Nordby, eds. 2003. Forensic Science: An Introduction to Scientific and Investigative Techniques. New York: CRC Press.
• Owen, D. 2000. Hidden Evidence. New York: Firefly Books.
• U.S. Fire Administration, April 19, 2005, http://www.usfa.fema.gov/ (20 February 2005).
• Zonderman, J. 1990. Beyond the Crime Lab. New York: Wiley.

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