Forensic Chemistry

Probably the most famous case of forensic chemistry can be seen in the works of Sir Arthur Conan Doyle. His character of Sherlock Holmes was able to identify the different types of soil found in London. Albeit the complexity of forensic chemistry has grown in the last one hundred years but the concepts laid down by Doyle have given the scientific community direction on fighting crime with chemistry.

Below are the most common of the forensic analytical techniques used today in combating crime.

Scanning Electron Microscope (SEM)

This instrument has the capability of producing micron (10^-6) size and smaller images. These images are useful when large scale photographs and analysis are not enough. It works by passing a beam of focused electrons across the surface of the specimen and intern the specimen emits secondary electrons which are collected by a detector and an image is produced. An additional result of this analysis is that it can provide an elemental analysis through x-rays that are produced when the initial electron beam bombards the sample. Because each element emits a unique wave pattern the amount of each element present can be determined. This technique has proven especially useful in gunshot residue analysis. This residue contains elements produced from the primer, propellant charge (gun-powder), bullet, cartridge case, and lubricants. Particles normally consist of Pb (lead), Sb (antimony) and Ba (barium). Each firearm produces a unique gunshot residue both in the size of the particles produced and their elemental composition.

Revolver Discharge Showing Clouds of Residue

SEM Analysis of Residue Particle (~50 microns)

Gas Chromatography - Mass Spectrometry (GCMS)

GCMS has been very useful in the analysis of narcotics. A sample is prepared and injected into the GC. A GC contains a long column in which the components are injected and travel through at different rates. Using a detector at the end of the column, part of the sample is analyzed for what compounds are present. The other part of the sample is sent to the mass spec where the carrier gas is separated. The sample is ionized (making it electrically charged) and the spectrum is recorded. Because many street level drugs are cut with different compounds such as caffeine, GCMS has given scientists a quick and easy way to analyze samples.

Typical GCMS Readout Showing Relative Abundance Along y-axis and Mass to Charge Ratio Along x-axis

Thin Layer Chromatography (TLC)

TLC is used in the separation of narcotics and dyes from fibers. Typical dyes used in the coloring of fibers are made from several different compounds. TLC makes use of the fact that these different compounds will travel different distances on a solid support (often silica gel) when a solvent is absorbed by the support. Using standards for comparison a dye sample can be analyzed and the different compounds can be compared.

Sample is Placed at "Starting Line" and the Compounds Separate as They Travel Along the Substrate

TLC Analysis of Narcotics Samples

Other techniques used in analysis include: Plasma Emission Spectroscopy (PES) - elemental analysis, Pyrolysis Gas Chromatography (PGC) - synthetic fiber and paint, and Fusion Microscopy - physical structure and optical properties of crystalline and pseudo-crystalline samples.

Luminol Test

The Luminol test is a technique that analyzes samples for the presence of blood. Many times criminals have attempted to wash away blood stains from the carpet and fabric and this test is a good indicator of whether blood was previously present. Hemoglobin is present in human blood and is the means by which oxygen is carried around the body. Hemoglobin contains an iron atom at the center of the structure as is shown below:

In the presence of a blue light the reaction between the luminol reagent and the iron present in the hemoglobin will glow. The reaction occurs as follows and the glowing results can be just below the reaction:

This a sensitive test in which minute traces of blood can be detected.

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