Modus Operandi
Written by David A. Thornton   

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Modus Operandi: The way something operates or works
How ninhydrin helps reveal latent prints

WORKING IN HIS LAB in 1910, English chemist Siegfried Ruhemann had no way of knowing that he was actually becoming a crime fighter. But the ability of today’s forensic scientist to develop certain latent prints is owed to Ruhemann’s use of ninhydrin to stain amino acids. Later, in the mid-1950s, Swedish scientists Oden and von Hofsten first used ninhydrin to visualize latent prints on porous surfaces—a process that has evolved over time to become the workhorse of the crime lab.

Techniques that develop latent fingerprints focus on creating a reaction between a chemical and one or more components in fingerprint residues. Sensitivity is an important aspect of a fingerprint-development technique. Fingerprint powders are the least sensitive technique, requiring 500 to 1,000 ng (1 ng = 0.000000001 g) of residue material to develop a print, while ninhydrin needs 100 to 200 ng. Ninhydrin’s analog, 1,8-diazafluoren-9-one, or DFO, requires only 1 to 10 ng.

Reacting with amines, ninhydrin forms a violet compound called Ruhemann’s purple. In the case of latent-fingerprint development, the compound reacts with the proteins in fingerprint residue deposited along the edges of friction ridges. This forms a dark, purplish hue along the ridge lines.

Fingerprint residue originates from glands in the subcutaneous skin. Apocrine and eccrine glands are commonly referred to as sweat glands. Apocrine glands are associated with hair follicles, while eccrine glands are primarily located on the papillary ridges of the hands and feet. Eccrine glands are positioned so their secretions produce a mirror image of the print when deposited on a surface. Composed of about 99 percent water, eccrine secretions also contain about 1 percent organic and inorganic materials such as chlorides, metal ions, lactic acid, ammonia, creatinine, and amino acids.

The oily sebaceous glands, associated with hair follicles, are also often involved with the deposition of fingerprints. They produce an oily, waxy substance called sebum that protects the hair and skin from water. Unlike eccrine glands, sebaceous glands secrete directly into the follicle rather than onto the skin. Sebum largely consists of lipids (fats, oils, and waxes). Sebaceous secretions may be unwittingly deposited on the fingertips through common actions such as running fingers through hair or scratching.

By understanding the residues that make up the material left behind on a surface as a fingerprint, a criminalist can target specific materials with the most effective technique for the circumstance to visualize the most prints. Black powder, for example, adheres to water and oils. Silver nitrate reacts with chlorides. And ninhydrin reacts with amino acids.

Amino acids in the fingerprint residue have an affinity for the cellulose in paper and do not disperse over time. Because of this property, decades-old latent prints have been developed with ninhydrin.

Progressive processing, or the use of different techniques in succession on a single print, is possible because techniques have been identified that target different components without interfering with other methods. It is important to follow the proper order of different development techniques. For example, iodine processing must be done before ninhydrin processing, because the solvents in ninhydrin, such as acetone, will often dissolve the oils and waxes that react with iodine.

Proteins, amino acids, and amines are distinct but related, and are often referred to interchangeably. About 20 different types of amino acids combine to form proteins. Amines are nitrogen-based molecules that form amino acids. Consider amines the foundation that comprises amino acids, which in turn create the proteins. An amine molecule consists of hydrogen, carbon, and nitrogen atoms, plus a side chain of protein-specific molecules.

Controlling the complex reaction is important to ensure the best results. The concentration of the ninhydrin in the carrier fluid or solvent is important. Too rich of a mixture may cause crystals to form on the treated surface, inhibiting subsequent treatments or creating excessive background staining.

Selecting the correct solvent for ninhydrin is a critical consideration. Many solvents, such as acetone or fluorocarbons, are hazardous to people and the environment. Acetone can destroy evidence such as handwriting by causing ink to run. Eco-friendly carriers have been developed recently that are safer and also increase ninhydrin’s effectiveness.

When examined under light using green wavelengths of 550nm, ninhydrin will absorb the energy and darken. The ninhydrin re-emits this light at a longer wavelength, called Stoke’s shift.

Barrier filters act similarly to dust masks, except they block light rather than dust particles. The barrier filter absorbs all visible and invisible light below its designated wavelength, while allowing longer wavelengths to pass through. This creates contrast, making the print developed with ninhydrin more readily visible.

It should be noted that ninhydrin is not the only amino-sensitive reagent. DFO reacts in much the same manner as ninhydrin, although DFO creates a paler pink-tinged fingerprint. Using an alternate light source set to 352 to 591nm and orange or red goggles, the developed prints brightly fluoresce in orange or yellow.

Although a standard of chemical development techniques, ninhydrin is just one of several available options for criminalists. Understanding how techniques work allows the correct technique to be selected and more prints developed.

About the Author

This e-mail address is being protected from spam bots, you need JavaScript enabled to view it is a crime-scene investigation training consultant with Thornton Consulting & Investigation in Thornton, Colorado.

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