Separation tests
Separation tests are techniques used to isolate and identify the chemical components in sample mixtures. They play a crucial role in forensic science, where accurate identification of substances is essential, especially when samples may appear similar at first glance. Various methods are employed to achieve separation, including filtering, extraction, and thin-layer chromatography (TLC).
Filtering is a straightforward method that involves passing a solvent over a solid sample to separate soluble components from insoluble ones. Extraction, particularly liquid-liquid extraction, allows for the selective transfer of a target component into a solvent, based on solubility and immiscibility. TLC is a more nuanced technique used extensively in narcotics and toxicology, where components are separated based on their interactions with a solid stationary phase and a liquid mobile phase. As the mobile phase moves up a coated plate, different components travel varying distances, revealing their identities upon analysis.
These methods are vital for forensic investigations, enabling the differentiation of substances such as inks, fibers, and illicit drugs, which may contain various chemical additives or cutting agents. Overall, separation tests are indispensable tools for forensic scientists in their pursuit of accurate and reliable evidence analysis.
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Separation tests
DEFINITION: Techniques used to separate the chemical components in sample mixtures so that they can be identified.
SIGNIFICANCE: Forensic scientists often need to separate evidence sample mixtures into their individual chemical components to determine precisely what the substances are and to distinguish among samples that may initially look very similar.
Forensic laboratories use a number of different techniques to separate samples into their individual chemical components. These include filtering methods, extraction methods, and thin-layer chromatography. Filtering is a simple and rapid method of separating the components that make up sample mixtures. A solid sample is placed in a filter funnel lined with filter paper that is positioned over a beaker. A solvent is poured over the sample and collected in the beaker after it passes over the sample. Sample components that are soluble in the solvent pass through the filter paper in the solvent and are collected with the solvent in the beaker. Components that are insoluble remain in the filter paper. Although this method is simple to perform, the success of filtering depends on the solubility of sample components in the solvent.
Another procedure used to separate sample components is extraction. One of the most routinely used of the many different extraction methods is liquid-liquid extraction. In this case, the sample is in solution form and an extraction solvent is added. The extraction solvent is chosen based on the chemistry of the target component; the component should be soluble in the extraction solvent and should prefer to exist in that solvent. In addition, the extraction solvent should not mix with the original solvent; that is, the solvents should form two separate layers. With the correct choice of extraction solvent, the target component moves from the sample solution into the extraction solvent, which is then removed. The component has thus been extracted into this solvent and can be analyzed further.
Thin-layer (TLC) provides another means to separate complex samples that contain numerous components of interest. This method is primarily used in narcotics and toxicology investigations. As with all chromatography techniques, separation is achieved based on differences in the interaction of sample components with a mobile and a stationary phase. In TLC, the stationary phase is typically a solid adsorbent coated onto a thin plate of silica, and the mobile phase is a liquid. The sample is applied to the TLC plate, which is then introduced into a small volume of the mobile phase. The mobile phase travels up the plate by capillary action, dissolving the sample and hence carrying the sample up the plate.
Separation of components occurs because of differences in attraction for the stationary and mobile phases. Components that have stronger attraction for the stationary phase spend more time in that phase and therefore do not travel as quickly as components with less attraction for the same phase. In TLC, this is seen as component spots on the plate—spots close to the bottom of the plate indicate components that did not travel far and hence had greater attraction for the stationary phase. The plate is removed from the mobile phase and dried, and then the distance that the components have traveled is determined. The numbers of components present in a known standard and in the questioned sample are compared, as are the distances corresponding components in both samples traveled.
TLC can be used in the analysis of a variety of different types of evidence. For example, two inks or two fibers may both appear blue to the naked eye when in fact they are composed of different dyes. Using TLC, dyes in the inks or fibers may be separated into the individual dye components and compared. Illicit drug samples may contain cutting agents that may or may not be illegal. TLC can be used to separate the illegal drug from these other substances before further analysis is performed to confirm the identity of the drug.
Bibliography
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Bogusz, M. J., ed. Handbook of Analytical Separations. Vol. 6 in Forensic Science, edited by Roger M. Smith. 2d ed. New York: Elsevier, 2007.
"DNA Separation." National Institute of Justice, 20 June 2023, nij.ojp.gov/nij-hosted-online-training-courses/principles-forensic-dna-officers-court/04-introduction-forensic-dna-laboratory-protocol/laboratory-processes-used-forensic-dna-analysis/dna-separation. Accessed 16 Aug. 2024.
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