Chemical Advances in Helping Police Detect Latent Fingerprints, Part 3


Latent print development powders were the topic of Part 2 of this series. Some of the most well-known approaches to chemical innovation are discussed here.

Because a latent print is more than 90% water, it is quickly absorbed and disseminated through porous surfaces like paper, cardboard, and raw wood unless the images are known to be recent. The use of powders will not interfere with chemical processing. However, ordinary black powder and magnetic powders may yield good results if prints are known to be fresh. Using chemical procedures, forensic scientists may easily remove latent fingerprints from porous surfaces and return them to the police.

You learned in the first installment of this series that a latent print is the chemical makeup of sweat. Various chemicals, including water, amino acids, carbohydrates, choline, proteins, and uric acid, are secreted through the sweat pores. Oils from the skin of the face, arms, and chest, among other places, are transferred to the fingertips.

The CSI or lab technician inspecting porous objects like those mentioned above will use a specialized battery of tests. In most cases, the first test performed is the iodine fuming test, which we shall cover in the following unit. Since the subsequent testing won’t be affected, iodine fumes are utilized first. However, iodine fuming procedures require extreme caution due to the severe toxicity of iodine fumes.

Necessary: Please bring everything that can be transported to the crime lab.

Many crime laboratories use several chemical reagents since their results are so sensitive and eye-catching. Let’s analyze the order in which the FBI and the British Home Office suggest processing information. The recommended processing sequence is iodine fuming, DFO, ninhydrin, and finally, silver nitrate or physical developer. DFO prints will not glow if Ninhydrin is applied beforehand.

DFO and Ninhydrin are biological stains that change color in response to an amino acid. Sweat contains amino acids, which, interestingly enough, do not evaporate. Amino acids react with the cellulose in paper, cardboard, and raw wood to form a robust and irreversible connection. And the structure of the friction ridges on the finger that left the print is reflected in the bond that develops.

An upcoming post will discuss several fuming techniques, including using iodine and super glue.

DFO Therapy

The amino acid-reactive fluorescent compound DFO (1,8-Diazafluoren-9-One) is a Ninhydrin analog. Here are the procedures for handling a document in DFO format: Note from the Author! When a chemical fume hood [exhaust fan] is available, performing these procedures in a forensics lab is preferable for efficiency and safety.

Step one: put the thing in the fume hood. Use evidence clips to hold up the document in question. The fume hood floor is suitable for cardboard and wood objects. Since you likely won’t know where latent prints may be located, it’s essential to treat all surfaces of each object.

Second, spray the reagent generously on both sides of the paper. Ensure you use latex or nitrile gloves to avoid leaving fingerprints on the form and staining the page.

The third step is to let the paper or other items dry naturally.

If you want to use tray development, you need only soak the document for a few seconds—just a few seconds—before hanging it up to dry.

5. It may take several hours or more for patients to develop if no other measures are utilized to speed up development.

When available, acceleration can be used. 6. Put the paper in a 200-degree oven for a while. After that, you’ll only need a few minutes to get your prints. The visible hue of DFO prints is often a pale pink, but when exposed to a different light source, they typically flash brightly.

Once the imprints can be seen, photographs of them (with a scale) must be taken.

The document goes into a heat chamber (at around 200 degrees) following treatment and drying. The room should be left for 5-10 minutes with the paper.

After taking the document out of the heating chamber, you should review it under normal lighting conditions, with a light emitting in the 455, 470, 505, and 520 nm range. Sirchie’s BLUEMAXX and megaMAXX additional lighting options provide access to these wavelengths. It has been claimed that the number of latent prints produced by DFO is 2.5 times that of Ninhydrin.

Injection of Ninhydrin

Solvents like Xylene and Acetone are combined with ninhydrin to create various solutions. Since both of these are highly combustible, safer alternatives exist. You may get this concoction in an aerosol can or a bottle with a pump. It is also utilized in a developing tray with an open top in the forensics lab.

This is the recommended sequence for processing ninhydrin:

Step one: put the thing in the fume hood. Use evidence clips to hold up the document in question. The fume hood floor is suitable for cardboard and wood objects.

Second, spray the reagent generously on both sides of the paper/object. To avoid missing any latent fingerprints on an object, coat every surface. Wearing latex or nitrile gloves can help you avoid leaving fingerprints on the paper and will stop your fingers from turning purple.

Third, you need to let the paper air dry.

If you want to use tray development, you should only soak the document for 5 seconds, dip it, and then let it dry in the air.

5. Patients may take up to 24 hours to form if no extra measures are taken to speed up development.

When available, acceleration can be used. 6. Use moist heat of roughly 200 degrees on the document. After that, you’ll only need a few minutes to get your prints. Ninhydrin leaves purple traces when it is visible. Moist heat, such as that produced by a steam iron, can be used to great success for speeding up. Cover the paper or object with a cloth before using the steam iron to avoid scorching it.

After 24 hours, take another look at the prints, even if they were improved by utilizing the above techniques.

If you want your Ninhydrin prints to last as long as possible, you might consider using a Ninhydrin fixative, which can be purchased from any company selling Ninhydrin.

Other Approaches

Silver nitrate and physical developer are backup options if neither of the above techniques results in usable latent prints. These reagents have silver salt, but their functions are quite different. Silver nitrate develops a reaction to sweat’s salt deposits, while the physical developer develops a response to the oil.

Because of its sensitivity to light, silver nitrate is typically applied to paper by spraying it from an amber bottle. The light must be removed from the area if growth on trays is to occur. The best way to develop a film is in a darkroom lit by a red bulb.
Dry the document by exposing it to room light and letting it air dry. If you want to speed up the development time, you can tell the document to a source of shortwave ultraviolet light for a few seconds.
While both Physical Developer and Silver Nitrate use silver compounds, Physical Developer is less sensitive to light. The resulting latent print is gray and somewhat luminous under standard longwave UV illumination. Tray development is the recommended processing method when using Physical Developer.

It is recommended to soak certain types of paper in Maleic Acid to neutralize them before further processing. Bleached (usually any white) paper requires this extra process because it will turn brown-gray entirely without it.

The dry reagents for Physical Developer are in a smaller bottle, then dumped into the giant bottle of liquid. There is just a temporary storage window after mixing has occurred.
The most efficient way of development is tray-based. If latent prints exist, they will become visible within a few minutes.
Due to the damaging nature of Silver Nitrate and Physical Developer, they are both reserved for final processing. Please consult the MSDSs for the reagents mentioned in this article before using them.
Check out my blog for further details on the equipment actual CSIs use in the field. You may get more information about CSI by checking out this blog []. There is also much more information about latent fingerprints, arson investigation, and blood spatter analysis. Please go to [] to read the CSI Tech Blog.

The city of Raleigh is home to Don Penven, a freelance writer. He has worked in law enforcement and forensics in some capacity for over 30 years.

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