Real Life CSI

The Chemistry of Crime

Taking evidence DNA sample.
Peter Dazeley / Getty Images

Blood splatter wiped hastily from a wall. Fingerprints on the fireplace mantle. When someone commits a crime, they leave evidence of their wrongdoing. Tests based on chemistry and other sciences can help crime experts collect and analyze such evidence to unmask specifics of a case.

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Hidden Blood

Luminol

Someone has been murdered in a living room, and you, the investigator, must figure out how it happened. The criminal has tidied up, making sure the room looks spotless. With a few tests, you can quickly check for that invisible blood.

Kastle-Meyer Test 

In the Kastle-Meyer test, you touch a cotton swab to a place where there might be blood, drop the Kastle-Meyer solution on it, and watch how quickly your swab turns pink. If it turns pink within seconds, you have blood. 30 seconds or more, and you don’t.

This test works because iron in the blood protein hemoglobin acts as a catalyst,​ speeding up how quickly the chemical phenolphthalein turns from colorless to pink as a result of losing electrons to other chemicals

Animal blood and some vegetables can also make phenolphthalein pink. You should confirm your results with tests that only react with human blood.

Luminol​

The Kastle-Meyer test is effective for blood on small spots, but not over a large area. For this, you can use luminol, which is sprayed on blood so that it glows in the dark. Afterwards, you can photograph the blood pattern to figure out how a victim was killed.

The reaction works like phenolphthalein’s. The iron in hemoglobin speeds up how quickly luminol loses electrons to other chemicals. This yields another chemical that has a lot of extra energy, which the chemical shakes off as light. The glow doesn’t last. After about 30 seconds, luminol no longer lights up. 

Like the Kastle-Meyer test, luminol can give false positives when reacted with metals, vegetables, and other things. Luminol may also make the bloodstain harder to analyze or destroy the blood’s genetic markers that help to identify the victim, making other tests preferable.

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Hidden Fingerprints

Close up detail of fingerprint on screen in forensic laboratory
Monty Rakusen / Getty Images

 A thief who opened a window to escape left you perfectly shaped fingerprints—that oil, sweat, and other stuff like dirt that together trace the ridges of your finger. You collect it for further analysis.

Normal fingerprint powders will easily stick to fingerprints if they’re on a smooth surface. But they don’t work as well on some plastics, on textured surfaces like cardboard, or on wet and sticky surfaces.

For these circumstances, there are other methods that take advantage of how different chemicals react with your fingerprint and its chemical components. For example, you can expose a fingerprint to superglue vapors, which will stick to your fingerprint and solidify. 

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Drugs

In a laboratory
Dr. Heinz Linke / Getty Images

You’re searching a known drug smuggler’s house, having obtained a warrant. The suspect is gone, but you find a mysterious powder. You send it to the lab for further analysis.

Color tests

When you mix certain drugs with certain chemicals, you get another chemical that has a characteristic color. You can perform these “color tests” quickly to screen for potential drugs.

For example,

  • The Marquis test turns purple in the presence of heroin, morphine, or opium, and orange brown with amphetamines.
  • The Scott test turns blue with cocaine
  • The Van Urk test turns purple with LSD. 

These tests work well for pointing you in the right direction. If you see the color you want, you can be more confident it’s the drug you’re looking for. If you don’t, you’ve crossed out one of several possibilities. However, the tests are not bulletproof since they are not specific to one drug compound. You should confirm your results with more analytical methods like chromatography.

Chromatography 

When you have a mixture of different things, how do you know what’s in it? It’s easy when it’s a handful of blue and yellow M&Ms, but not so much when you have a mysterious white powder.

With chromatography, you can separate that powder into its component chemicals. There are several types of chromatography that work via the same underlying principle. Like runners who sprint along a racetrack at different speeds, different chemicals can be made to run down a surface, like a strip of paper or through a column with the consistency of Jell-O, at different rates. This can happen for various reasons, like how small your chemical particles are and their composition.

Afterwards, you see how far each chemical has traveled and check if they match the expected results for a known drug.

For the crime expert, chromatography isn’t just useful for identifying drugs. You can also use it to break down ink, poisons, clothing dyes, and other suspicious items.

Putting it All Together

Using these tests, both investigators and scientists work together to unfold the story of a crime. Some tests, like the Kastle-Meyer test and applying fingerprint powder, are done by investigators right at the scene itself. Others, like chromatography, can only be performed by scientists in a crime lab. Furthermore, quick tests like the ones listed for bloodstains and drugs should be corroborated with results from more conclusive techniques. Whichever one you use, these methods, and many others in crime scene investigation are possible because of the application of scientific principles.