Evidence Processing

Senior Crime Scene Specialists assigned to the Evidence Processing Unit (EPU) work in the crime laboratory. Like all members of the Crime Scene Unit Senior Crime Scene Specialists assigned to the EPU are graduates of the Mesa Police Department Crime Scene Academy.  However, once assigned to the EPU, they receive specialized training in chemical processing for latent prints, latent print and biological evidence education, and biological evidence examination and processing.

Each item of evidence is carefully examined and processed according to the material the item is made of and the type of evidence being looked for.  Evidence made out of paper is processed differently than evidence made out of plastic.  Additionally, chemical processes must be conducted in a specific order to prevent latent prints and/or biological evidence from being destroyed.

Latent Print Processing

When processing evidence for latent prints, members of the EPU use various chemicals to develop--or make visible--the prints. Some of the chemicals used are:

  • Cyanoacrylate Esther ("Super Glue" and "Loctite" are brand name uses of this chemical)
  • Fluorescent Dye Stains (Ardrox, Basic Yellow, MBD, RAM, Rhodamine 6G, Red-drox, DFO, MRM-10)
  • Ninhydrin
  • Silver Nitrate
  • Physical Developer
  • Blood Reagents (Amido Black, Luminol, Leuco Crystal Violet)
  • Latent Print Powders

Once a latent print is developed, it is "collected" using comparative photography techniques.

Biological Evidence Processing

Members of the EPU examine and process evidence for biological fluids, such as blood or skin cells, through visual examination or the use of a Forensic Light Source (FLS).  Some bodily fluids, such as blood on an item of evidence, are easy to see without the aid of a FLS, and are collected by swabbing the fluid off the evidence.  More often, bodily fluids cannot be seen without assistance, and that is where the FLS comes in.  Using a FLS causes certain bodily fluids to luminesce, making them easier to locate and collect.  Once the biological evidence is collected, EPU members prepare any swabs and forward them to the Biology Unit for further analysis.

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