Analytical instrumentation

DEFINITION: Tools used during the chemical and physical investigation of physical evidence to identify components and their associated concentrations.

SIGNIFICANCE: The scientific evaluation of forensic samples provides information that can be useful to law-enforcement investigators. The instruments employed by forensic scientists are designed to detect and measure small quantities and fine details, thus enabling comparisons of samples that can link suspects to crime scenes or eliminate persons from suspicion.

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. Analytical instrumentation is also particularly important for the fields of nanoscience and nanotechnology. As many researchers believe that these fields will play important roles in the technology of the future, the continued development of nanoscience-enabled analytical tools may become an industry priority.

Microscopy

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 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.

Chromatography and Electrophoresis

Forensic scientists use and 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 (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 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 proteins and peptides. It uses an electric potential to cause positive and negative charged species to migrate and separate into components. The main forensic application of this technique is in (deoxyribonucleic acid) analysis.

Spectrometry and Spectroscopy

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 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 (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 scientist to determine the alloy type, which then may be traced to a manufacturer.

Bibliography

Bell, Suzanne. Forensic Chemistry. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006.

Girard, James E. Criminalistics: Forensic Science and Crime. Sudbury, Mass.: Jones & Bartlett, 2008.

Houck, Max M., and Jay A. Siegel. Fundamentals of Forensic Science. Burlington, Mass.: Elsevier Academic Press, 2006.

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.

Johll, Matthew. Investigating Chemistry: A Forensic Science Approach. New York: W. H. Freeman, 2007.

Lopez-Lorente, Angela I., et al. "The Third Way in Analytical Nanoscience and Nanotechnology: Involvement of Nanotools and Nanoanalytes in the Same Analytical Process." TrAC Trends in Analytical Chemistry, 2016, www.sciencedirect.com/science/article/abs/pii/S0165993615002538. Accessed 13 Aug. 2024.

"Nanoscience Enabled Analytical Tools." Nature, 2024, www.nature.com/collections/hejecbagac. Accessed 13 Aug. 2024.

Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007.