Latent prints
The Latent Print Section employs a variety of techniques to develop latent prints on items of evidence submitted to the laboratory by law enforcement agencies throughout Minnesota. Latent Print Examiners attempt to identify the latent prints to individuals that may be connected to a crime scene. If they cannot be identified, or if there are no known individuals of whom to compare, then latent prints can be searched in the Midwest Automated Fingerprint Identification Network, a regional database of fingerprint and palm print arrest records from the states of Minnesota and North Dakota.
The Latent Print Section also works in cooperation with Medical Examiner Offices throughout the state to assist in identifying unknown decedents. The presence of latent prints is important to a criminal investigation because it can link a suspect to a crime scene, provide proof of contact, and identify people that may not previously have been known.
Latent print evidence
Non-porous surfaces
These are mainly smooth surfaces of which the latent print resides on the surface. Examples of non-porous surfaces include glass, plastics, metals, and varnished wood. Latent prints on non-porous surfaces tend to be fragile, so they must be preserved as soon as possible.
The primary technique is to expose the non-porous surface to superglue fumes (cyanoacrylate ester). Superglue reacts with the moisture present in the latent print, converting the latent print to a hard plastic, and bonds it to the surface.
The surface can then be treated with a fluorescent dye stain and examined under a laser. The dye stain adheres favorably to the superglue residue, so any latent prints preserved by superglue fuming process will fluoresce under the laser.
Porous surfaces
These are surfaces of which the latent print is absorbed into the material. Examples include paper, cardboard, and untreated woods. Various chemical treatments are used to develop latent prints in porous materials. The chemicals target certain components present in the latent print residue, reacting as a color change, making the latent print visible.
Indanedione targets particular groups of amino acids that are present in latent print residue. The surface is treated with the chemical and examined under a laser. If latent prints are present, they will fluoresce upon exposure to the laser.
Ninhydrin also reacts with amino acids in latent print residue. An advantage of ninhydrin is that a laser is not needed. If latent prints are present, the ninhydrin will turn them purple.
Physical Developer reacts with fatty acids that are sometimes present in latent print residue, turning them black. Because fatty acids are insoluble in water, Physical Developer is useful on porous items that have been wet.
Other surfaces
The laboratory occasionally receives items with surfaces that do not necessarily fall into the category of either of non-porous or porous. Some examples of other surfaces and the preferred processing technique include:
| Adhesive Tape | Sticky-side powder |
| Waxy Surface | Sudan Black |
| Glossy Cardboard | Superglue fuming and fingerprint powder |
| Surfaces with Blood | Amido Black or Acid Yellow 7 |