Analytical instrumentation (Forensic Science)
Advances in analytical instrumentation have significantly changed how forensic investigations are completed. Forensic scientists use many different types of analytical instruments, but all these tools serve the purpose of enabling the scientists to obtain more information on forensic samples. Analytical techniques have the ability to change a sample from one that was thought to have only class characteristics to one that has individual characteristics, making it more valuable in an investigation. This ability to detect individual characteristics is one reason analytical instrumentation has become an important part of forensic investigations.
Analytical instruments can be grouped according to the types of chemical and physical properties they measure. The analytical techniques most commonly used by forensic scientists are microscopy, chromatography, electrophoresis, spectrometry, and spectroscopy.
(The entire section is 122 words.)
Microscopy (Forensic Science)
Light microscopy, or the use of light microscopes, allows forensic analysts to magnify samples so the fine details can be viewed and evaluated. Light microscopes have the ability to magnify up to around 1,500 (that is, 1,500 times normal size). Light microscopy is useful for comparisons of samples and in the evaluation of specimens for similarities and differences. Common light microscopes used in forensic science include the compound microscope, the stereo microscope, and the comparison microscope. A comparison microscope allows an analyst to view two samples side by side, so they can easily be compared; fiber samples and bullets are among the kinds of forensic evidence often compared in this way.
An electron microscope uses a beam of electrons to probe a sample and allows a forensic scientist to view a sample at a greater magnification than is possible with a light microscope. A common type of electron microscope used in forensic applications is the scanning electron microscope (SEM), which can reach a magnification of 100,000 or greater. Another advantage of the SEM is that it enables the scientist to probe the elemental composition and elemental distribution of specimens using the X-ray fluorescence property of the microscope.
(The entire section is 194 words.)
Chromatography and Electrophoresis (Forensic Science)
Forensic scientists use chromatography and electrophoresis to analyze complex mixtures of chemicals. The term “chromatography” is used to refer to a range of techniques that allow the separation of the individual components of chemical mixtures through the use of either a gas or a liquid moving phase. Chromatographic analysis can be used to determine all the different chemical components that make up a sample and how much of each component is present.
The main types of chromatography used in forensic investigations are gas chromatography (GC) and high-performance liquid chromatography (HPLC). GC separates, detects, and quantifies volatile species (atoms, molecules, or ions) or chemical compounds that can be converted to the gas phase by heating. Once in a gas phase, species move at different rates through a column, which results in a physical separation between components. This technique is very useful for arson investigations, in which fire accelerants often need to be evaluated. HPLC involves the analysis of mostly organic samples (molecules containing carbon) in a liquid state. The samples are dissolved in a suitable liquid solvent, such as water or an alcohol. This technique can be used to identify and determine the amounts of different drugs in samples collected at crime scenes.
Capillary electrophoresis is a technique used by forensic scientists to separate charged chemical species such as...
(The entire section is 252 words.)
Spectrometry and Spectroscopy (Forensic Science)
Forensic scientists use molecular spectrometry and spectrophotometry to look at the molecular or organic structure of chemical compounds. Techniques such as Fourier transform infrared (FTIR) spectrometry, ultraviolet and visible spectrometry (UV-Vis), and mass spectrometry (MS) allow analysts to classify and identify chemicals by their molecular spectra. FTIR spectrometry uses infrared light, and UV-Vis uses visible and ultraviolet light. A forensic scientist might compare an FTIR spectra of a forensic sample such as a white powder found at a crime scene with a spectral library of known compounds in order to identify the powder. FTIR can also be attached to a microscope to create a microspectrophotometer, which enables examination of the molecular structure of a sample. MS is often carried out in conjunction with gas or liquid chromatography to provide more detailed identification of components in a forensic sample.
Elemental spectroscopy is accomplished by techniques that measure the elemental composition and concentration in a sample. Atomic absorption (AA), inductively coupled plasma (ICP), X-ray fluorescence (XRF), X-ray diffraction (XRD), and neutron activation analysis (NAA) are typical instruments used in inorganic analysis. XRF can be used to determine the presence of lead and barium in gunshot residue. ICP can be used in finding out what elements are in a metal sample, such as a bullet; this allows the...
(The entire section is 229 words.)
Further Reading (Forensic Science)
Bell, Suzanne. Forensic Chemistry. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006. Chemistry-focused text presents discussion of the use of analytical instrumentation.
Girard, James E. Criminalistics: Forensic Science and Crime. Sudbury, Mass.: Jones & Bartlett, 2008. Textbook includes sections in which instruments and their uses are described.
Houck, Max M., and Jay A. Siegel. Fundamentals of Forensic Science. Burlington, Mass.: Elsevier Academic Press, 2006. Good general textbook includes a well-presented section on analytical tools.
James, Stuart H., and Jon J. Nordby, eds. Forensic Science: An Introduction to Scientific and Investigative Techniques. 2d ed. Boca Raton, Fla.: CRC Press, 2005. Covers analytical instrumentation in a section on forensic science in the laboratory.
Johll, Matthew. Investigating Chemistry: A Forensic Science Approach. New York: W. H. Freeman, 2007. Textbook designed for nonscience majors presents simple explanations of analytical instruments and their uses.
Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007. Introductory text describes and explains the uses of various analytical instruments and techniques.
(The entire section is 166 words.)
Analytical Instrumentation (World of Forensic Science)
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.