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 have been 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 reagent are placed alongside the sample material. The analyst then views the slide under a microscope. Using a wooden stick or a similar applicator, 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, heroin reacts with platinic chloride to form distinctive microcrystals that are branched and pointed. 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 forensic science laboratories. Microcrystalline tests can also be used to differentiate some isomers, which are molecules that have the same molecular formula but different arrangements in space. This is a great advantage for some microcrystalline tests, because more complex techniques used to identify the chemical components of samples, such as basic mass spectrometry, may not differentiate certain isomers without additional methods.

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, and have expanded to include some synthetic drugs and pharmaceuticals, such tests are not available for all drugs. The additional controlled substances for which microcrystalline tests exist include synthetic cathinones and certain pharmaceuticals, such as alprazolam, mephedrone, α-PVP, tramadol, and zolpidem. 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. Raman spectroscopy is used as a complementary analytical technique to support microcrystalline test results by providing molecular confirmation of suspected drug identities. If the known standard gives similar shapes to 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. Pearson Prentice Hall, 2006.

Elie, Leonie, et al. “Microcrystalline Testing Used in Combination with Raman Micro-Spectroscopy for Absolute Identification of Novel Psychoactive Substances.” School of Chemistry, University of Lincoln, n.d., irep.ntu.ac.uk/id/eprint/28139/1/5667_Cave.pdf. Accessed 4 Feb. 2026.

Johll, Matthew. Investigating Chemistry: A Forensic Science Perspective. W. H. Freeman, 2007.

Laughlin, Gary J. Development of Modern Microcrystal Tests for Controlled Drugs, Diverted Pharmaceuticals and Bath Salts. National Institute of Justice, Apr. 2020, www.ojp.gov/pdffiles1/nij/grants/254676.pdf. Accessed 4 Feb. 2026.

“Microcrystalline Test in Forensic Drug Analysis.” Courseware, Centurion University of Technology and Management, 2020, courseware.cutm.ac.in/wp-content/uploads/2020/07/microcrystalline-test-in-forensic-drug-analysis.pdf. Accessed 4 Feb. 2026.

“Microcrystal Testing in Forensic Analysis for Cocaine.” American Academy of Forensic Sciences, Apr. 2023, www.aafs.org/sites/default/files/media/documents/SeizedDrugs%20ANSI%20ASTM%20E1968-19%20.pdf. Accessed 4 Feb. 2026.

Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 9th ed., 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 4 Feb. 2026.