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Monday, July 16, 2012

Forensics Q&A: Fingerprinting

By Kristy Lahoda | @KristyLahoda

Disclaimer: The information provided in this post should not be used for malicious intent unless it is in the form of crime writing. The author is an explosives expert, not an expert in fingerprint analysis. 

QUESTION:  In my novel the protagonist is at a crime scene where fingerprints are found. What are some ways the fingerprints would be readied for analysis and what are some technical bits I can use to make it sound more authentic?

ANSWER: Fingerprints are analyzed by physical or chemical methods.

Types of Fingerprint Evidence

There are three types of prints that can be found at a crime scene: patent, impressions, and latent. Patent prints are those visible prints left as a result of contact with items such as dirt, blood, oil, and grease. Impressions are three-dimensional prints left in soft surfaces such as silly putty, tar, and butter. Latent prints require processing in order to be visible and will be our focus.

Latent Print Residue Composition

Our fingertips have skin called friction ridges that secrete sweat through pores. The composition of this sweat actually forms the latent print as well as any variety of materials that are on the friction ridges.

Latent Print Development

There are two types of methods for print development: physical and chemical methods. The methods used are primarily dependent on the surface the print is found on. The primary physical method is powder dusting. In this method, the powder adheres to the residue allowing for print visualization. Afterward, a piece of transparent lifting tape is applied to the powered print and then mounted onto a background that contrasts with the color of the print powder. Speaking from the limited experience I had during my training in fingerprints, dusting and lifting are art forms. Both procedures require patience and a great deal of skill. The prints can easily be damaged and you are out of luck if only one print is found and it is damaged during processing. Another physical method involves magnetic powder. It has magnetic properties and sticks to a wand. Since there are no brush bristles, the risk of damage to the print is reduced. This method is really cool, but again, I found that it requires technique.

There are several chemical methods. The chemical method that we use often in the crime lab is cyanoacrylate fuming, otherwise known as superglue fuming. The chemical name of superglue is alkyl-2-cyanoacrylate ester. The fumes interact with the latent print residue by polymerizing (i.e., the process of many small molecules combining). We place the item containing the print or prints into a chamber. We place a small dish of superglue in the chamber. The chamber is kept at a certain relative humidity and heat is used as a catalyst (i.e., to speed up the process of the reaction). There are several nice things about superglue fuming. It is an easy procedure, but it also allows for further processing if necessary. The print can be treated with a dye stain that induces fluorescence or luminescence when an alternate light source (ALS) or a laser illuminates the print at a specific wavelength. Examples of two dye stains that the crime lab uses are ardrox and Rhodamine-6G. Another common chemical process is called ninhydrin and is useful for porous surfaces. Ninhydrin reacts with components from the ridge lines containing amino acids. It is commonly applied by spraying, painting, or dipping. The process is slow, but like the superglue process, it can be sped up by adding heat in humid atmosphere. The ninhydrin develops bluish-purple prints. An analog of ninhydrin is 1,8-diazafluorenone (DFO). Ninhydrin and DFO react with components in the latent print residue that are water soluble. A follow-up treatment to ninhydrin and DFO is called physical developer (PD). PD is often used as a follow-up method because it reacts with components in the residue that have not already reacted—water-insoluble components such as lipids. It involves a process based on silver deposition onto the latent print residue.

There are many other chemical methods that can be used. I highlight a few of the more common methods here. The important takeaway is that even though it may seem to be an easy job, it takes a skilled crime scene technician or lab analyst to nondestructively lift the prints.

Kristy Lahoda, Ph.D., is an explosives analyst contractor in a crime lab as well as a technical editor for a scientific journals publishing company.  She writes Christian forensic suspense and discusses forensics on her blog called Explosive Faith.  You can follow her on Facebook and Twitter.

If you have a forensics question for Dr. Lahoda that you'd like to see answered on the QueryTracker Blog, send your question via Carolyn Kaufman using the email link under Contact Us in the right-hand column of the main QTB page.

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