The Chemistry of Fingerprint Detection

Touch a glass, a door handle or a sheet of paper and you leave behind a copy of the ridges on your fingertip. Most of these marks are latent: invisible to the naked eye. Making them visible is a chemistry problem. The investigator needs a reagent that reacts with, sticks to or dissolves into the residue of a fingerprint, but not the surface around it.

This article looks at what a fingerprint is actually made of and the chemistry of the main methods used to reveal one.

What is a fingerprint made of?

A latent print is a thin film of residue transferred from the skin. It is about 98–99% water when fresh, and the water evaporates quickly. What’s left is a tiny amount of:

• Eccrine sweat components (from sweat glands on the fingertips): amino acids, urea, lactic acid and salts such as sodium chloride.
• Sebaceous components (oils picked up when people touch their face or hair): fatty acids, triglycerides, wax esters and squalene.
• Contaminants: food, cosmetics, dirt or blood.

The exact mixture varies with the person, their diet, what they touched and the age of the print. This matters, because each detection method targets a particular component.

Porous vs non-porous surfaces

The surface is the first thing an examiner considers.

• Porous surfaces (paper, cardboard, untreated wood) absorb the water-soluble parts of the print, including amino acids. These stay in the paper for a long time, sometimes decades.
• Non-porous surfaces (glass, plastic, painted metal) leave the residue sitting on top, where it’s fragile but easy to reach.

Method 1: Powders

The oldest method is still widely used on non-porous surfaces. A fine powder is brushed gently over the surface and sticks to the moist, oily residue.

• Black powders often contain carbon; light powders contain materials such as aluminium or titanium dioxide, for contrast on dark surfaces.
• Magnetic powders contain fine iron particles and are applied with a magnetic wand, so no brush touches the print.
• Fluorescent powders glow under particular wavelengths of light, which helps on patterned surfaces.

This is physical, not chemical, adhesion, so it works best on fresh prints.

Method 2: Ninhydrin (paper and cardboard)

Ninhydrin is the classic reagent for prints on paper. It reacts with the amino acids in sweat to give an intense purple product known as Ruhemann’s purple.

The reaction involves the amino acid’s –NH₂ group: two ninhydrin molecules and the nitrogen of one amino acid combine into the coloured product, while the rest of the amino acid is broken down, releasing carbon dioxide and an aldehyde. Because all common amino acids have that amino group, ninhydrin detects them all. (The same reaction is used in biochemistry labs to detect amino acids on chromatograms; see amino acids as acids and bases.)

In practice: the document is dipped in or sprayed with a ninhydrin solution, then kept warm and humid for a while to speed up the reaction. Purple ridges appear over hours to days.

Relatives of ninhydrin, such as DFO and 1,2-indanedione, also react with amino acids but give products that fluoresce strongly under a forensic light source. They’re often used before ninhydrin because they can reveal fainter prints.

Method 3: Superglue fuming (non-porous surfaces)

This method was discovered by accident when people noticed white fingerprints appearing near open superglue. Superglue is mostly ethyl cyanoacrylate.

When superglue is warmed in a closed chamber, it vaporises. The vapour polymerises on the fingerprint residue: water and other components in the print act as initiators, starting a chain reaction that forms a hard, white polymer (poly(ethyl cyanoacrylate)) along the ridges.

The white print is stable and can then be dusted with powder or stained with a fluorescent dye for photography. Superglue fuming is standard for plastic bags, bottles, weapons and other non-porous items. It’s also a good example of polymer chemistry in action.

Method 4: Iodine fuming

Iodine crystals are warmed to give violet iodine vapour, which dissolves in the oily components of a print and turns it yellow-brown. The colour fades quickly as the iodine sublimes away again, so prints must be photographed at once. It’s non-destructive, so other methods can follow. See halogens for more on iodine’s properties.

Method 5: Silver nitrate

Silver nitrate reacts with the chloride ions from salt in sweat, forming silver chloride:

Ag⁺ + Cl⁻ → AgCl(s)

When exposed to light, silver chloride slowly decomposes to dark metallic silver, the same chemistry used in traditional photography. The print appears as dark ridges. Because chloride is water-soluble, it spreads through paper over time, so this method is used less often today than ninhydrin.

Method 6: Physical developer and metal deposition

For paper that has got wet, the amino acids and salts may have washed away, but the fatty, water-insoluble components remain. A physical developer solution deposits fine silver particles on those fatty residues, revealing prints even on documents that have been soaked.

Doing things in the right order

Several methods can be applied to one item, but some destroy the residues needed by others. A typical sequence for paper:

1. Visual examination and photography under different lights.
2. A fluorescent amino acid reagent (such as DFO or indanedione).
3. Ninhydrin.
4. Physical developer (for the water-insoluble components).

For a non-porous item, superglue fuming is usually followed by a fluorescent dye stain and then powder.

Beyond the pattern: chemical information in a print

Modern research goes further than revealing ridges. Techniques such as mass spectrometry imaging can detect substances in the print residue: traces of drugs or their breakdown products, cosmetics, explosives or gunshot residue handled by the person. This can connect a print not just to a person but to an activity.

Why some prints can’t be recovered

Not every touch leaves a usable print. Dry hands leave very little residue; rough surfaces break up the ridge pattern; heat, rain and sunlight degrade the components; and handling an item smears what’s there. Examiners therefore collect evidence carefully, touching items only at edges and with gloves, and packaging them so nothing rubs against the surface.

Try a safe version at home

You can see the principle with simple materials: press an oily fingertip onto a clean glass, dust it lightly with cocoa powder or fine talc using a soft make-up brush, blow off the excess, and lift the print with clear sticky tape onto dark or light card. It’s a purely physical method, just like powder dusting in the lab.

Key takeaways

• A latent print contains mostly water, plus amino acids, salts, urea and oils.
• Powders stick physically to fresh residue on non-porous surfaces.
• Ninhydrin reacts with amino acids to give Ruhemann’s purple, ideal for paper.
• Superglue vapour polymerises on print residue to give a hard white copy.
• Iodine dissolves in oils; silver nitrate reacts with chloride; physical developer targets fats on wet paper.
• The choice and order of methods depend on the surface and which components survive. See also forensic chemistry.