Forensic science can involve determinations of the presence or absence of compounds or materials. For example, if a victim has died of a stab wound, then noting that a knife was found near the body can be an important piece of evidence in trying to decipher the details of the death.
This sort of presence or absence level of detail produces qualitative information. The data does not have an amount associated with it or information concerning the composition of a sample.
In contrast, other useful information can be gained by quantitative examinations; examinations that tell how much of a material is contained within a sample. For example, if the knife noted above has blood on the blade, a blood sample can be vital to learning the blood type and the composition of the DNA carried in the blood, as well as indicating the presence and amount of any toxins or chemical poisons.
For quantitative forensic analyses, specialized instruments are used. A variety of analytical instruments exist, which have their respective advantages depending on what is being examined and the potential target molecules that could be contained within it.
Another area of forensic analysis that uses analytical instrumentation is gunshot residue analysis. When a rifle or a handgun is fired, the residue that propelled the bullet out of the barrel is also propelled outward. The residue can attach to exposed skin or the clothing of the person who fired the gun, or on a nearby person or surface.
The residue contains spherical particles that are comprised of lead, antimony, and barium. The particles' shapes and elemental compositions mean that they can be detected using a scanning electron microscope equipped with an energy dispersive x-ray analyzer. The analyzer operates by trapping electrons from the scanning microscope that have bounced off of the sample surface. The surface interaction causes an energy loss. Depending on the nature of that surface, the electrons will lose a certain amount of energy. As well, some of the electrons that were part of the object's surface can be dislodged by the force of the bombardment. By analyzing the pattern of energy levels of the reflected and dislodged electrons, the instrument can be used to determine the elements that are present in the sample and even how much of each element is present. Also, the scanning electron microscope allows the spherical residue particles to be directly visualized. Some instruments are equipped to visually display the elemental pattern on the sample image.
The elemental pattern that is obtained consists of various peaks rising above the background signal. Each peak represents a single element. If the pattern obtained from a sample recovered from a person matches the pattern of a sample obtained from a suspect, then it can be powerful evidence linking the suspect to the fired weapon.
Another important facet of forensic science is the use of DNA typing to identify individuals. The subtle differences in the arrangement of DNA that exist from person to person are every bit as unique as a fingerprint , and so have the potential to identify a DNA sample as belonging to a particular individual.
An especially powerful DNA examination technique is known as the polymerase chain reaction (PCR ). The technique uses an enzyme (polymerase) to make many copies of a minute amount of target DNA. The amount of DNA that can be made permits other analyses to be done on material that otherwise would have been present in too low a quantity.
PCR allows DNA to be recovered and analyzed from samples such as cigarette butts, the sealing flaps of envelopes, or pieces of hair and bone, even if the samples have been exposed to the environment or are contaminated with other compounds or micro-organisms.
Another analytical instrument, the gas chromatograph-mass spectrometer , is adept at analyzing fluid samples. Separation of the various compounds that make up the fluid is accomplished by the gas chromatograph. The sample is injected into the machine and is immediately vaporized. The now-vaporized chemicals are carried through the chromatography column by a non-reactive gas such as helium. Depending on the chemical properties of the column, different compounds move at different rates of speed and so appear at the other end of the column at different times. This allows the different compounds to be separated from each other.
The separated compounds are then analyzed by the mass spectrometer. The sample's molecules are hit by a beam of electrons, which causes some of the sample's electrons to be dislodged (ionization). The ionization pattern can be used to identify the molecules and even to determine the mass of the compounds.
Databases that contain the mass spectrometric information on thousands of compounds exist in various state and federal law enforcement agencies. This information can be accessed to help identify the composition of a sample mixture with great precision.
see also Air plume and chemical analysis; Biodetectors; Chemical and biological detection technologies; DNA fingerprint; Gas chromatograph-mass spectrometer; Laser ablation-inductively coupled plasma mass spectrometry; Micro-fourier transform infrared spectrometry; Mitochondrial DNA analysis; PCR (polymerase chain reaction); Visible microspectrophotometry.