Part 2 of this series discussed the use of latent print development powders. This article will cover some of the most popular chemical development methods.
Latent print powders are of little use on porous surfaces such as paper, cardboard and raw wood unless prints are known to be fresh; the reason being that a latent print is over 90% water, which is absorbed and dispersed in the surface. However, if prints are known to be fresh, regular black powder and magnetic powders may produce good results, and use of powders will not interfere with chemical processing. Crime scene investigators (CSIs) are able to recover latent prints from porous surfaces without a great deal of difficulty using chemical methods.
In Part 1 of this series you learned that a latent print is a combination of substances that make up human perspiration (sweat). The sweat pores exude substances like water, amino acid, carbohydrates, choline, proteins and uric acid. The fingertips also deposit oils from contact with areas of the body that harbor sebaceous glands-like on the face, arms and chest.
When examining porous objects like those listed above, the CSI or laboratory technician will follow a specific regimen of tests. Usually the first test is iodine fuming, which we will discuss in a subsequent lesson. Iodine fumes are used first as they are non-destructive to the testing that will follow. But iodine fumes are highly toxic and special care must be exercised when using iodine fuming techniques.
Special Note! All items that are transportable should be taken to the crime lab for processing.
Several chemical reagents are used in many crime labs as they are very sensitive and often produce spectacular results. Let’s take a look at the processing sequence recommended by the FBI and the British Home office. The best order of processing is: Iodine Fuming, DFO, Ninhydrin, and then either Silver Nitrate or Physical Developer. If Ninhydrin is used prior to DFO, the DFO prints will not fluoresce.
Note that DFO and Ninhydrin are a biological stains, and when they react with an amino acid, they exhibit a visible color. Amino acids are a component of sweat and oddly enough they do not disperse into the surface. In fact amino acids form a permanent bond with the cellulose structure of paper, cardboard and raw wood. And this bond conforms to the friction ridge structure of the finger depositing the print.
Special Note: Iodine Fuming will be covered in another article covering fuming methods like iodine and superglue.
DFO (1,8-Diazafluoren-9-One) is a Ninhydrin analog that reacts to amino acids, but it also has fluorescent properties. Here are the steps to treating a document with DFO: (Author’s Note! It is preferable to run these treatments in the crime lab since it is more efficient and much, much safer when a chemical fume hood [exhaust fan] is available!).
1. Place the object to be treated in a fume hood. In the case of a document, suspend it using evidence clips. Cardboard objects and pieces of wood may be placed on the floor of the fume hood. Be certain to treat all surfaces of each object, since you probably will not know just where latent prints may be found.
2. Spray a heavy coating of the reagent of both sides of the document. Be certain to be wearing latex or nitrile gloves to keep your fingerprints off of the document and to prevent your fingertips from turning color.
3. The document or object should be allowed to air-dry.
4. If tray development is preferred, allow the document to soak for about 5 seconds, just dip and then hang it up to dry.
5. If no other methods are used to accelerate development, it could take several hours or more for latents to develop.
6. Acceleration may be used if available. Expose the document to heat at about 200-degrees. Prints will then pop out in a matter of minutes. Visible DFO prints are often a light pink color, but they usually produce bright fluorescence when exposed to an alternate light source.
7. Once the prints are visible, they must be photographed (with a scale).
After treatment and drying, the document is placed in a heat chamber (at about 200-degrees). The document should remain in the chamber for 5-10 minutes.
After removal from the heat chamber, examine the document under subdued room light using an alternate light source that operates in the range of 455, 470, 505 and 520nm. These light frequencies are available from Sirchie’s BLUEMAXX and megaMAXX alternate light sources. DFO is said to produce 2.5 times the number of latent prints than Ninhydrin.
Ninhydrin is mixed with several different solvents like Xylene or Acetone. Less hazardous formulas are also available since these two are highly flammable. This mixture may be supplied in an aerosol can or a manual pump spray bottle. It is also mixed and used in an open developing tray in the crime lab.
Ninhydrin processing should follow this order:
1. Place the object to be treated in a fume hood. In the case of a document, suspend it using evidence clips. Cardboard objects and pieces of wood may be placed on the floor of the fume hood.
2. Spray a heavy coating of the reagent of both sides of the document object. In the case of objects-be certain to coat all surfaces since you may not know just where latent prints were placed. Be certain to be wearing latex or nitrile gloves to keep your fingerprints off of the document and to prevent your fingertips from turning a purple color.
3. The document should be allowed to air-dry.
4. If tray development is preferred, allow the document to soak for 5 seconds, just dip and then hang it up to dry.
5. If no other methods are used to accelerate development, it could take 24 hours or more for latents to develop.
6. Acceleration may be used if available. Expose the document to moist heat at about 200-degrees. Prints will then pop out in a matter of minutes. Visible Ninhydrin prints are purple in color. An effective method of acceleration is to apply moist heat from a steam iron. Cover the document with a towel before applying the steam iron to prevent scorching of the document or object.
Even if prints are enhanced using the above methods, re-evaluate the results after waiting about 24 hours.
Ninhydrin prints may fade over a period of time so it may be advisable to use a Ninhydrin fixative (available from most Ninhydrin suppliers).
Should both of the above steps fail to produce useable latent prints, two alternative methods are available: Silver Nitrate and Physical Developer. Both of these reagents contain a silver salt, but they perform in a different manner. Silver Nitrate reacts to salt deposits in sweat while Physical Developer reacts to the presence of sebaceous secretions (oil).
- Silver Nitrate is a light sensitive material so it is usually sprayed onto a document from an amber, light-tight bottle. If tray development is desired, available light must be shut off. Development under red darkroom lighting is the best method.
- Allow the document to air-dry and then expose it to room light. Prints may become visible in several minutes, but development may be accelerated by briefly exposing the document to a shortwave ultra violet light source.
Physical Developer, while still using a silver compound, is not as light sensitive as Silver Nitrate. It produces a gray latent print, which is also somewhat fluorescent, so a normal longwave UV light may be used. The best method for Physical Developer processing is tray development.
When processing certain kinds of paper it is a good practice to neutralize them first with a short bath in Maleic Acid. This is especially important for bleached (typically any white) paper as these will turn completely brown-gray without the neutralizing step.
- Physical Developer is packaged in two containers: the small bottle contains the dry reagents, which are emptied into the large bottle of liquid. Once mixed the shelf life is limited to a few days.
- Tray-development is the most practical method of development. Latent prints, if present, will appear in a matter of minutes.
- Either Silver Nitrate or Physical Developer is the final step since either process is destructive to subsequent testing with other reagents. Be certain to obtain the Material Safety Data Sheets (MSDS) covering the use of the reagents discussed in this article.