This article incorporates, in modified form, material from the not-yet-published Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement.
Since it was first used for developing latent fingerprints in 1954, ninhydrin has become the most common method used to reveal prints on porous surfaces. Nearly all forensics labs use ninhydrin for this purpose, and some seldom use anything other than ninhydrin. Ninhydrin is cheap, sensitive, and commercially available in disposable spray cans. The developed prints are a high-contrast purple that’s readily visible on most paper backgrounds. If iodine fuming or DFO is to be used, either or both must be used before ninhydrin, in that order. If it is to be used, ninhydrin must be used before silver nitrate or PD.
Because the solvent does not take part in the reaction that forms Ruhemann’s purple, nearly any organic solvent can be used successfully. We’ve used ordinary rubbing alcohol (ethanol or isopropanol), acetone, petroleum ether, and the mixed alcohols recommended by the FBI, and all appear to work identically. Ninhydrin development occurs slowly at room temperature and humidity. Although some stains may appear within seconds to minutes of applying the ninhydrin solution, complete development may take 24 to 48 hours. After the ninhydrin solution dries, the development process can be accelerated by increasing the temperature and humidity. We used an ordinary steam iron with the specimen sandwiched between paper towels, and found that full development occurs within a few minutes under those conditions.
Various after-treatments are sometimes used to enhance prints developed by ninhydrin, including treating the developed prints with nickel nitrate solution, zinc chloride reagent, or other metal-based reagents, which may intensify the prints and/or make them fluorescent under an ALS. One of the more effective after-treatments is simply to saturate the developed prints with blank solvent (e.g., if the specimen was treated with ninhydrin in acetone, saturating the developed prints with plain acetone) and then exposing the developed prints to heat and humidity. The exact mechanism of this enhancement is uncertain, but it seems likely that the use of blank solvent allows unreacted ninhydrin still present on the specimen to combine with unreacted amino acids from the latent prints.
In this session, we’ll use ninhydrin to reveal latent prints on paper specimens that have not previously been treated, as well as on specimens that have undergone iodine fuming. We’ll then treat developed prints with blank solvent to see if we can improve the visibility of the prints.
Required Equipment and Supplies
- goggles, gloves, and protective clothing
- magnifying glass, loupe, or stereo microscope to examine specimens (optional)
- camera (optional)
- small sprayer bottle (2)
- steam iron (see Substitutions and Modifications)
- paper towels
- ninhydrin solution (see Substitutions and Modifications)
- acetone (optional)
- specimens of paper with untreated latent fingerprints
- specimens of paper with iodine-developed fingerprints
The MAKE Forensic Fingerprinting Kit includes all of the specialty items needed for this lab session. Alternatively, you can purchase these items individually from Maker Shed or other laboratory supplies vendors.
Acetone is flammable. Ninhydrin is irritating. Wear splash goggles, gloves, and protective clothing, and use the ninhydrin spray outdoors or under an exhaust fan. Note that ninydrin reacts with any amino acids, including those on your skin, to form Ruhemann’s purple. If you get ninhydrin solution on your skin, the purple stains, although harmless, will persist for several days.
Substitutions and Modifications
- You may substitute discarded decongestant nasal sprayers or similar spray bottles for laboratory spray bottles. You can also buy small fingertip sprayer bottles at the drugstore for about $0.50 each.
- If you don’t have a steam iron, you can cover each specimen with a damp paper towel and heat it in an oven set to 175 °F to 200 °F (~ 80 °C to 95 °C).
- Ninhydrin solution is sold by forensic supplies vendors, or you can make up own by dissolving about 0.15 g to 0.20 g of ninhydrin in about 25 mL of acetone. Acetone sold in the painting supplies section of hardware stores is fine.
In this lab session, we’ll use ninhydrin solution to develop both untreated latent prints and prints that have already been processed by iodine fuming. For the former, create some fresh fingerprint specimens using the procedure described in the preceding lab session. Ninhydrin is used to develop prints on various nonporous surfaces, but is used primarily to develop latent prints on paper. Select various types of paper from around the house to use for your specimens. Use only paper items you are willing to discard after the experiment; ninhydrin stains are persistent. For the latter, use specimens that you processed in the preceding lab session.
- If you have not already done so, put on your splash goggles, gloves, and protective clothing.
- Working outdoors or under an exhaust fan, place the specimen print-side up on paper towels or old newspaper to protect the surface against overspray.
- Spray the specimen with ninhydrin solution sufficient to wet the surface, as shown in Figure 8-11. Don’t drench it, but make sure the entire surface is dampened with ninhydrin solution.
Figure 8-11. Spraying a specimen with ninhydrin solution
- Allow the specimen to air dry for a few minutes. Some bluish or purplish ninhydrin stains may be faintly visible at this point, but do not be concerned if no stains are evident.
- Make a sandwich with two thicknesses of paper towels, followed by the specimen (print side up), and then two more layers of paper towels.
- Set the steam iron to low heat and iron the sandwich for several minutes, using steam occasionally, as shown in Figure 8-12. You can check development progress periodically by peeling back the top layer of paper towels.
Figure 8-12. Applying moist heat to develop the fingerprints treated with ninhydrin solution
- When development appears to be complete, place the specimen on a clean, flat surface and examine it carefully under strong light with the magnifier or loupe. You should see fingerprints revealed in considerable detail. If you have a camera, shoot an image of the developed fingerprints for your records. Record the pertinent details for the specimen in your lab notebook.
- Spray the specimen with plain acetone until the surface is damp, and then repeat steps 4 through 7 to see if you can develop additional detail.
- Record your observations in your lab notebook and tape the developed specimen into your lab notebook.
- Repeat steps 2 through 9 for your other specimen, including those that have already been iodine fumed. Retain at least one ninhydrin-developed specimen for use in the following lab session, as well as at least one specimen that has been iodine fumed but not treated with ninhydrin.
Figure 8-13 shows the latent fingerprints revealed by ninhydrin development.
Figure 8-13. Latent fingerprints revealed by ninhydrin development
Q1: With which component of fingerprint residues does ninhydrin react to form Ruhemann’s purple?
Q2: Which two common fingerprint development methods must be used before ninhydrin if they are to be used at all?
Q3: What three after-treatment methods are commonly used to enhance fingerprints developed with ninhydrin?