Microcrystalline tests
Microcrystalline tests are rapid analytical methods used primarily in forensic laboratories to identify substances, particularly suspected drugs, by observing the characteristic crystals formed when these substances react with specific reagents. These tests play a crucial role in the initial identification process, providing a quick assessment of the likely drug present before more complex confirmatory analyses are conducted. The procedure involves placing a small sample alongside a reagent on a microscope slide, and then examining the resulting crystals under a microscope to determine their size and shape, which are unique to certain drugs.
Microcrystalline tests are advantageous as they are simple to perform and utilize readily available materials, making them accessible in laboratory settings. Additionally, they can differentiate isomers, which are compounds with identical molecular formulas but different structural orientations—a capability not typically achievable with more complex techniques like mass spectrometry. While these tests are effective for many commonly investigated drugs, they may not be available for every substance, and the crystals formed can vary with different reagents. Therefore, analysts conduct tests alongside known standards to enhance the reliability of their findings. Overall, microcrystalline tests serve as a valuable preliminary tool in forensic drug analysis, guiding further investigative procedures.
On this Page
Subject Terms
Microcrystalline tests
DEFINITION: Rapid tests in which questioned substances are mixed with reagents and identified based on the characteristic crystals formed.
SIGNIFICANCE: Microcrystalline tests are widely used in forensic laboratories in the initial identification of various substances in evidence samples, particularly suspected drugs. These simple and rapid tests allow scientists to determine the likely presence of particular drugs before going ahead with more complex confirmatory testing.
Although microcrystalline tests can be used in the identification of blood, they are more commonly used in forensic drug analysis. When a seized drug sample is submitted for analysis, the forensic scientist is tasked with identifying the actual drug present, as this will determine subsequent penalties faced by the suspect. Often, rapid and simple tests are used first to determine the likely drug present before more detailed instrumental analysis is conducted to confirm the identity of the drug. When used as part of an analytical scheme for drug identification, microcrystalline tests are considered to be selective tests; that is, they are used to indicate the likely drug present, but these tests alone cannot definitively identify the drug.
The microcrystalline test is rapid and simple to perform. A small amount of the questioned sample is placed on a microscope slide, and a few drops of the microcrystalline test are placed alongside the sample material. The analyst then views the slide under a microscope. Using a wooden stick, the analyst draws the reagent to the sample and observes the crystals that are formed when the two substances meet.
Particular drugs form crystals of characteristic sizes and shapes with different test reagents. For example, gold chloride can react with amphetamine to give crystals that are yellow, long, and rod-shaped. Similarly, reacts with platinic chloride to form crystals that are branched with pointed blades. It is important that the analyst test a known standard at the same time with the same reagent as the questioned sample. In this way, the analyst can directly compare the sizes and shapes of the crystals formed by the known standard and the questioned sample under identical conditions.
In addition to the fact that microcrystalline tests are simple to perform and give rapid results, the materials required for such tests are readily available in laboratories. Microcrystalline tests can also be used to differentiate isomers, which are molecules that have the same molecular formula but different orientations in space. This is a great advantage for microcrystalline tests, because more complex techniques used to identify the chemical components of samples, such as mass spectrometry, are not capable of differentiating isomers.
Microcrystalline tests have some perceived disadvantages, but these are not necessarily limitations of the tests. For example, although microcrystalline tests are available for the drugs most commonly of interest in law-enforcement investigations, such tests are not available for all drugs. A second disadvantage often stated is that given drugs may form crystals of different shapes with different reagents. This is not necessarily a limitation, however, as the analyst should always compare a known standard with the questioned sample under similar conditions. If the known standard gives similar shapes as the questioned sample with the different reagents, then this increases confidence in the likely identity of the drug present in the questioned sample.
Bibliography
Bell, Suzanne. Forensic Chemistry. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006.
Johll, Matthew. Investigating Chemistry: A Forensic Science Perspective. New York: W. H. Freeman, 2007.
"Microcrystal Testing in Forensic Analysis for Cocaine." American Academy of Forensic Science, Apr. 2023, www.aafs.org/sites/default/files/media/documents/SeizedDrugs%20ANSI%20ASTM%20E1968-19%20.pdf. Accessed 15 Aug. 2024.
Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007.
"Standard Practice for Microcrystal Testing in Forensic Analysis for Cocaine." ASTM International, 12 May 2024, www.astm.org/e1968-19.html. Accessed 15 Aug. 2024.