Instructor Side JCE Classroom Activity: #16 Chemical Methods for Developing Latent Fingerprints Gurvinder S. Sodhi, Department of Chemistry, SGTB Khalsa College, University of Delhi, Delhi–110007, India Jasjeet Kaur, College of Applied Sciences for Women, Delhi University, Jhilmil Colony, Vivek Vihar, Delhi–110095, India
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About the Activity Sweat secreted by the eccrine glands on the fingertips is deposited on surfaces the fingers touch. The deposition is in the form of contours which are the mirror images of the ridge patterns on the fingers. The patterns formed by fingerprints fall into three well-defined categories: arches, loops, and whorls (Figure 1, Student Side). The constituents of sweat may be selectively fixed by different chemical reagents so as to make latent (invisible) fingerprints visible. The simplest and most commonly used procedure for revealing the ridge pattern is powder dusting, which relies on the mechanical adherence of fingerprint formulation to the moisture and oily components of the skin deposit. Regular fingerprint powders consist of a colorant and a resinous polymer or inorganic salt. The resinous polymer or inorganic salt is adsorbed by the moisture or oily constituents of sweat, and the colorant is adsorbed on the resin or salt. Colorants are charcoal, manganese dioxide, and ferric oxide. The commonly used resinous polymers are silica gel, starch, and rosin and typical insoluble inorganic salts are zinc carbonate, barium carbonate, and magnesium carbonate. The moisture or oil secreted along with sweat tends to dry up within two or three days. Hence old prints cannot be developed by this method. Ninhydrin reagent may be used to develop very old prints. Ninhydrin reacts with amino acid components of sweat, giving a red-purple coloration that makes the finger ridges visible. Yet another reagent used for developing latent fingerprints is silver nitrate. It reacts with the sodium chloride in sweat in a simple ionic reaction, and silver chloride is formed. On exposure to sunlight, silver chloride decomposes into finely divided black silver, depositing along the ridges and making the fingerprints visible. This activity should be performed in the chemistry laboratory with supervision by the instructor. Students must wear appropriate eye protection, such as splash-proof goggles. Gloves are recommended when working with both solutions, as they can cause long-lasting stains on the skin. If students have difficulty getting good prints by rolling a finger, have them make a plain print by simply pressing the finger into the paper. This method will give a clear ridge pattern, but the number of characteristics (Figure 2, Student Side) will be reduced.
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Formulations It is recommended that the instructor prepare the formulations described below for student use. Eye protection and gloves should be used when preparing the formulations. A hood is recommended for preparation of the ninhydrin solution. The directions below should produce enough of each formulation for a class of approximately 20 students. 1. Weigh out 3 g of one of the following: charcoal powder, manganese dioxide, ferric oxide (A). Weigh out 9 g of one of the following: starch powder, silica gel, rosin, zinc carbonate, barium carbonate, magnesium carbonate (B). Mix A and B. To the mixture, add 100 mg of talcum powder. Grind, using a mortar and pestle. Store in a tightly sealed, labeled bottle. 2. Weigh out 0.5 g of ninhydrin. Add 30 ml of ethanol. Transfer the solution to a labeled spray bottle. (CAUTION: ninhydrin stains skin.) Freshly made solution works best. 3. Weigh out 2.5 g of silver nitrate. Add 45 ml of distilled water. Transfer the solution to a labeled spray bottle. (CAUTION: silver nitrate stains clothes and counter tops indelibly and skin for a long time.) If you do not have reagent spray bottles (such as No. Z 12630-6, Aldrich Chemical Co. catalog), plastic spray bottles intended for household or cosmetic use can be substituted. You may need to prepare a larger volume of solution in order to use such bottles. Gloves are recommended when handling and spraying solutions, especially if household spray bottles are used, because they may leak or splatter a small amount of solution backward.
Integrating the Activity into Your Curriculum Human sweat is 98% water, 1% organic constituents, and 1% inorganic ions. The major organic and inorganic components of sweat can be discussed in a biochemistry course. The observation that the fingerprint powders adhere to moisture or oily constituents of sweat may be cited as an example of adsorption in the surface chemistry class. The chemical reactions involved in fixing the amino acid and sodium chloride components of sweat can be incorporated into a qualitative analysis course. This topic may also be related to the solubility of polar and nonpolar molecules, precipitation reactions, and oxidation–reduction reactions.
More Information 1. Advances in Fingerprint Technology; Lee, H. C.; Gaensslen, R. E., Eds.; Elsevier: New York, 1991; p 59. 2. Wilshire, B. Endeavour 1996, 20, 12. 3. Clark, S.; Quigley, M.; Tezak, J. J. Chem. Educ. 1993, 70, 593. Kimbrough, D. R.; DeLorenzo, R. J. Chem. Educ. 1998, 75, 1300. 4. FBI Fingerprint Identification. http://www.fbi.gov/kids/finger/finger.htm (accessed Feb 1999). 5. Integrated Automated Fingerprint Identification System (IAFIS). http://www.fbi.gov/iafis/iafis.htm (accessed Feb 1999).
This Activity Sheet may be reproduced for use in the subscriber’s classroom. JChemEd.chem.wisc.edu • Vol. 76 No. 4 April 1999 • Journal of Chemical Education
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JCE Classroom Activity: #16 Student Side Chemical Methods for Developing Latent Fingerprints Gurvinder S. Sodhi, Department of Chemistry, SGTB Khalsa College, University of Delhi, Delhi–110007, India Jasjeet Kaur, College of Applied Sciences for Women, Delhi University, Jhilmil Colony, Vivek Vihar, Delhi–110095, India
The tiny ridges on the fingertips of each human being are unique. Because a copy of these ridges is left on just about everything we touch, fingerprints at a crime scene are important evidence that can be used to identify the criminal. Fingerprints may be visible, but are more often invisible or latent, consisting mostly of sweat. There are several methods that can be used to make latent fingerprints visible. Your instructor will provide you with three different formulations you can use to develop latent fingerprints.
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CAUTION: You must wear eye protection when working with the chemicals in this activity. Take special care that the solutions do not contact your skin or clothing, as both cause long-lasting stains. Gloves are recommended for working with the formulations.
__1. Collect Fingerprints on Paper: Cut 20–30 threeinch squares of white, unlined office paper. Select one thumb to use for making all prints. Make a thumbprint on a paper square by first rubbing the thumb against your forehead to pick up sweat, and then placing the left edge of the thumb on the sheet, with the right edge pointing up off the paper. Slowly roll it to the right until its right edge touches the paper, and the left edge is pointing up off the paper. Develop the print using one of the three formulations described in steps 2, 3, and 4. If the fingerprint ridges are not clear, try variations in making prints until you get a good result. You may need to practice rolling the thumb to get a good print. In addition, if you are wearing makeup, or if your skin is very oily, try rubbing your thumb against your palm rather than your forehead. Repeat until you have obtained at least one clear print using each of the formulations. __2. Powder: Your instructor will tell you what the powder contains. Sprinkle fingerprint powder over a latent fingerprint. Spread it with a small artist’s paint brush. Dust off excess powder. Cover the fingerprint with a piece of transparent tape to protect it. __3. Ninhydrin Solution: Ninhydrin is an organic compound that reacts with amino acids to form a deep purple complex. Spray a latent fingerprint with ninhydrin solution from a distance of about 6 inches. Wait a few moments until most of the solvent evaporates, then spray again. Allow it to dry completely. The print will appear only after the surface is completely dry—this may take a hour or more. (You can dry it more quickly in an oven at 50–70 °C.) __4. Silver Nitrate Solution: Place the paper containing the latent print on a protected surface. Spray it with silver nitrate (AgNO3) solution. Expose the sprayed surface to sunlight. The print will become visible in 10–15 minutes. __5. Prints To Develop Later: Using your best technique, make thumbprints on each of six paper squares. Label them on the back side, and save them in an envelope. Save the paper squares with the best developed prints for each formulation in a second envelope, making sure each is labeled with the method used to develop it. One Week Later: Remove three of the paper squares from the first envelope. Develop one with each formulation. Compare results with those from the previous week. Label and save the developed prints. Two Weeks Later: Remove the remaining paper squares from the first envelope. Develop one with each formulation. Compare with results from the last two weeks. Which formulation gives the best results with the older fingerprints? __6. Collect Fingerprints on Other Surfaces: Locate objects with smooth surfaces made of as many as possible of the following: glass, porcelain, china, polished wood, stainless steel, plastic, and ceramic. Wipe each surface with a moist paper towel. Make thumbprints on each surface and develop one with each of the three formulations. Figure 1. Ridge patterns: arches (left), loops (center), and whorls (right). Use a horizontal surface for the solutions so the liquid does not roll off as quickly. Which formua b c d e f g lation gives the best results on each surface? When you are finished, wash the surfaces with soap and warm water to remove Figure 2. Basic characteristics in fingerprints: a. ridge termination; b. fork; c. lake; powder and marks. d. island; e. short independent ridge; f. hook; g. crossover. __7. Identification: Use ten paper squares to make a set of all your fingerprints. Be sure to label the squares on the back side with the finger used. Develop all ten prints using any one of the three formulations. Examine each fingerprint with a magnifying glass. Identify the ridge pattern for each finger (Figure 1). Parallel ridges may be interrupted by one of seven types of irregularities, known as characteristics (Figure 2). It is the combination of these irregularities that makes each fingerprint unique. Try to identify some of the characteristics of your fingerprints. This Activity Sheet may be reproduced for use in the subscriber’s classroom. 488B
Journal of Chemical Education • Vol. 76 No. 4 April 1999 • JChemEd.chem.wisc.edu
