Quantitative and qualitative analysis of chemicals
Quantitative and qualitative analysis of chemicals (Forensic Science)
In chemical analysis, the components of a sample are identified and their concentrations determined. The main objective of qualitative analysis is to identify one or more substances or chemicals that may be present in a sample. The main goal of quantitative analysis is to determine how much of a particular chemical or substance is present in the sample. Most forensic analysis techniques currently in use are qualitative and are used to identify or confirm the presence or absence of certain materials. Subsequent analyses provide additional information, such as the amounts of component substances present in a sample. Forensics scientists conduct qualitative and quantitative analyses on chemical components of materials, objects, solutions, and biological specimens, including body fluids.
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Qualitative Analysis (Forensic Science)
Qualitative analysis is an integral part of forensic science, with the primary goal to identify unknown substances and chemical elements present in a sample. Qualitative analyses are often used to confirm the presence or absence of certain materials in a sample. Although qualitative analysis can be used to separate components into categories, it is not useful for determining the amounts of certain compounds that are present. Comparison of samples is qualitative and can be conducted to determine whether the chemical compositions of two or more samples are similar.
Qualitative analyses are often based on physical properties, such as melting point, color, texture, density, and other properties that are distinctive for specific elements or compounds. Qualitative analyses are often conducted to determine whether illicit drugs or poisons are present in a sample. After it is known which substances are present, quantitative analysis follows to determine the concentration of each substance present in the sample.
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Quantitative Analysis (Forensic Science)
When the presence of certain substances in a sample has been confirmed, the absolute or relative abundance of these compounds can be determined using quantitative techniques. Quantitative analysis is used to determine the quantity or concentration of a specific substance in a sample. For example, qualitative analysis is used to determine the presence of alcohol in the blood of a person suspected of driving under the influence; quantitative analysis is used to determine the amount of alcohol present in the blood (that is, the blood alcohol level).
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Qualitative and Quantitative Analyses in Combination (Forensic Science)
A more complex analytical situation occurs when both qualitative and quantitative data are required. Although qualitative analyses are generally conducted for identification, in some situations quantitative information is also important. For example, reliable identification of a compound present in a sample is not possible unless a minimum amount of material is also present, which is quantitative information.
If comparing samples requires that the amount of a specific substance present in a sample be known in addition to the chemical elements that are present, then comparative qualitative methods also have quantitative features. Chemical analysts in forensic laboratories conducting qualitative analyses may also need to be involved with quantitative analysis.
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Analytical Techniques (Forensic Science)
Many sophisticated laboratory techniques are used in forensic science for various types of chemical analyses. Some techniques are more suited for qualitative analysis, and others are more appropriate for conducting quantitative analysis. Some methods can be useful for both qualitative and quantitative analyses, depending on how the data are collected and analyzed. Techniques that are primarily used for qualitative analyses, such as substance identification, include “wet” chemical techniques to collect physical properties of sample components, some chromatographic techniques and spectroscopic methods, and DNA (deoxyribonucleic acid) analysis.
Although microscopy is generally qualitative in nature, it can be quantitative when used in combination with specific spectrophotometric measurements. Spectroscopy can be qualitative or quantitative or both, depending on the procedures used and the types of measurements collected. For example, ultraviolet and visible spectrophotometry is generally used as a screening tool to determine the presence or absence of suspected compounds, but it can also be quantitative in single-substance solutions or with appropriate standards. Other forms of spectroscopy, such as infrared (IR) or Fourier transform infrared (FTIR), are also recognized as useful for collecting quantitative measurements.
Chromatographic methods are used extensively in forensic science labs to analyze body fluids...
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Further Reading (Forensic Science)
Bogusz, M. J., ed. Handbook of Analytical Separations. Vol. 6 in Forensic Science, edited by Roger M. Smith. 2d ed. New York: Elsevier, 2007. Describes the applications of various separation methods used in forensic laboratories. Includes chapters devoted to methods of screening for various chemicals.
Harris, Daniel C. Quantitative Chemical Analysis. 7th ed. New York: W. H. Freeman, 2006. Widely used text covers all aspects of quantitative analytical chemistry.
Houck, Max M., and Jay A. Siegel. Fundamentals of Forensic Science. Burlington, Mass.: Elsevier Academic Press, 2006. Introductory textbook provides discussion of the basic methods of analysis used in forensic examinations.
Langford, Alan, et al. Practical Skills in Forensic Science. New York: Pearson Prentice Hall, 2005. Good introductory text addresses all of the various methods used in the forensic sciences. Includes several chapters on quantitative analyses, listed by technique.
Tebbett, Ian, ed. Gas Chromatography in Forensic Science. New York: Ellis Horwood, 1992. Provides an introduction to chromatography and then discusses specific issues regarding the uses of this technique in forensic chemistry, including in drug analysis, analysis of explosives, and toxicology.
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