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.
Gentian violet stain has been used for many years to develop latent fingerprints on nonporous surfaces, particularly the adhesive side of sticky tapes. The specimen to be treated is simply immersed in or floated on a 0.1% w/v aqueous solution of gentian violet for one to two minutes and then rinsed with water. Repeated stain/rinse cycles may be used to intensify the color of the stains. With some types of tape a clearing solution of 1 M hydrochloric acid is used as the final rinse, to remove background staining without affecting the developed prints. When the print is fully developed, it can be viewed and photographed under ordinary light. Gentian violet is non-destructive, so it is often used first on sticky tape specimens. If gentian violet fails to develop the print, other powder-based methods can be attempted. Obviously, the aqueous solution of gentian violet should not be used on tapes that use water-soluble adhesives.
In this lab session, we’ll use gentian violet stain to develop latent fingerprints on various types of tape and other sticky surfaces.
Required Equipment and Supplies
- goggles, gloves, and protective clothing
- magnifying glass, loupe, or stereo microscope to examine specimens (optional)
- camera (optional)
- forceps, hemostat or tweezers
- evaporating dish
- microscope slides (optional; see Substitutions and Modifications)
- transparent stiff plastic sheet (optional; see Substitutions and Modifications)
- gentian violet solution (see Substitutions and Modifications)
- hydrochloric acid, 1 M (optional)
- tape specimens (see Substitutions and Modifications)
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.
Hydrochloric acid is corrosive. Gentian violet solution stains anything it contacts. Wear splash goggles, gloves, and protective clothing.
Substitutions and Modifications
- For ease of handling, we used microscope slides to secure our specimens and prevent them from curling. We placed each of them sticky side up on a slide and then used additional tape to secure the two ends.
- Preserving developed prints on sticky tape can be problematic because of the nature of the surface. Transparent tape can be pressed into contact with a white sheet of paper, but that reverses the prints to a mirror image and is not usable for opaque tapes. Because the adhesive often remains sticky after treatment, you can’t simply leave the surface exposed. One method is to mount the developed prints by placing the sticky side of the tape in contact with a thin sheet of stiff transparent plastic. (We used a sheet of plastic that was used to protect the screen of a new LCD display we’d purchased. Acetate theater gels or similar sheets also work.) The drawback to mounting the developed prints is that it may make it difficult to see the prints on some surfaces. Overall, the best method of preserving gentian violet developed prints is to photograph them.
- You can make up a 0.1% w/v solution of gentian violet by dissolving about 0.1 g of gentian violet crystals in about 100 mL of water. Gentian violet is available from lab supply vendors and some drugstores in solid form, and often as a 1.0% aqueous solution, which can be diluted one part solution to nine parts water to make up a 0.1% solution. Gentian violet is sold under many names, including crystal violet, methyl violet 10B, basic violet 3, brilliant violet 58, Gram stain, and many others. (Gentian violet is the hexamethyl form of methyl violet, which is also available in tetramethyl (methyl violet 2B) and pentamethyl (methyl violet 6B) forms. Make sure you know what you’re getting.)
- * If you don’t have a bench solution of 1 M hydrochloric acid, you can make it up by adding 8.3 mL of concentrated hydrochloric acid (or 9.7 mL of hardware store muriatic acid) to about 80 mL of water and then making it up to 100 mL. Using this final hydrochloric acid rinse is optional, but it may improve the contrast of gentian violet prints on some surfaces.
- Obtain as many different types of sticky tape and other adhesive materials as possible. In addition to ordinary cellophane tape, you can try masking tape, packaging tape, electrician’s tape, medical adhesive tape, sticky computer labels, and so on.
- If you have not already done so, put on your splash goggles, gloves, and protective clothing.
- Pour sufficient gentian violet solution into the evaporating dish to cover the bottom to a depth of 5 mm or so.
For small specimens, including microscope slides, you can conserve gentian violet solution by placing the specimen flat in a dish and applying a few drops of the solution with a disposable pipette until the surface is covered. Discard the solution after treating each specimen.
- Put a latent fingerprint on your first tape specimen by pressing the tape against your fingertip and then peeling it away.
- Place the specimen sticky side up on a microscope slide and carefully secure both ends of the tape to the slide using additional tape.
- Repeat steps 3 and 4 with each of your other tape specimens.
- Using the forceps, place the first specimen face up in the gentian violet solution, and allow it to soak for one to two minutes.
- Using the forceps, remove the specimen from the gentian violet solution and rinse it under cold running water for several seconds, as shown in Figure 8-18. (You can begin staining the next specimen while you rinse the current specimen.)
Figure 8-18. Rinsing a specimen after treating it with gentian violet solution
- Examine the rinsed specimen for visible fingerprints. If the fingerprints are distinct, set the specimen aside to drain and dry. If the prints are stained only lightly, repeat the stain/rinse cycle several times, until no further improvement is evident. If the contrast between the developed prints and the background is poor, try using the dilute hydrochloric acid final rinse to improve it.
- Place the specimen on a clean, flat surface and examine it carefully under strong oblique light with the magnifier or loupe. You should see fingerprints revealed in considerable detail as violet stains. 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.
- Repeat steps 3 through 9 for your other specimens.
Figure 8-19 shows a latent fingerprint revealed by gentian violet staining.
Figure 8-19. Latent fingerprint revealed by gentian violet staining
Q1: For which specific types of specimens is gentian violet solution best suited?
Q2: You are presented with a cellophane tape specimen that may contain latent fingerprints on either or both surfaces. How would you proceed, and why?