Amino Acid Fingerprints

Introduction

Detectives use ninhydrin to reveal fingerprints left at crime scenes. Ninhydrin reacts with amino acids found in the natural oils on our skin to produce a purple product. The intensity of the color may also be used as quantitative test for the amount of amino acids.

Concepts

• Amino acids
• Ninhydrin
• Forensic chemistry

Materials

Safety Precautions

Work in a fume hood or well-ventilated area. Ninhydrin is a skin and eye irritant and is dissolved in butyl alcohol, a flammable solvent. Keep away from flames or other sources of ignition. Ninhydrin will stain skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Blotting paper may be disposed of in the trash following Flinn Suggested Disposal Method #26a. The ninhydrin solution may be stored for future use.

Prelab Preparation

Cut each sheet of blotting paper (9" x 12") into thirds lengthwise to obtain three 9" x 4" pieces. Each piece of paper can be used to make two sets of prints, one set with ninhydrin, another set with ink to serve as a reference.

Procedure

1. Ask a student volunteer to make a set of fingerprints along the top half of a 9" x 4" sheet of blotting paper. Note: Oily fingerprints work best. Have student run their fingers through their hair before making fingerprints.
2. Place the blotting paper on paper towels and lightly spray the paper with ninhydrin solution. Note: Allow the paper to dry for a few minutes until the solution will not “run.” This will prevent the fingerprints from smudging.
3. Pick up the blotting paper with a gloved hand and use a fanning motion to “speed dry” the paper.
4. When dry, hold the blotting paper above a heat source such as a hot plate or hair dryer. Hold the paper about 10 cm above the heat source to prevent scorching. Note: It takes about 2–3 minutes of heating over the low setting of a hot plate for fingerprints to appear.
5. A set of purple fingerprints or spots will soon appear.
6. Have the student volunteer produce a corresponding set of fingerprints along the bottom edge of the blotting paper using a water-soluble inkpad.
7. Compare the two sets of fingerprints.

Teacher Tips

• Enough blotting paper (10 sheets) and ninhydrin solution (150 mL) are provided to make 30 sets of “amino acid fingerprints.”

• Some people have very pronounced or sharp fingerprints whereas others may leave only smudges. To get better fingerprints, have student volunteers run their fingers through their hair several times to pick up more oil.
• All amino acids and most proteins give positive test results with ninhydrin. The ninhydrin test is commonly used to detect amino acid “spots” in the separation of amino acids by paper chromatography. The purple color does not develop until the amino acid–ninhydrin mixture is heated.
• In forensic chemistry, ninhydrin is used to detect latent fingerprints left behind on porous surfaces, such as cloth, paper and cardboard. Prints on hard, nonabsorbent surfaces, such as mirrors, tile or glass, are detected using fingerprint powders (graphite or aluminum dust).
• A series of reference tests may be performed to demonstrate the positive reaction of ninhydrin with amino acids. Prepare a set of 1% reference solutions containing amino acids, such as phenylalanine and tyrosine. Add 2 mL of ninhydrin solution to 1 mL of each reference solution in a test tube, and heat the solutions at 75–80 °C for 3–5 minutes. If desired, these positive reference tests may be compared against a blank (distilled water) or negative controls. Water, salt and sugar solutions should all test negative (no purple color) with ninhydrin.

Discussion

Ninhydrin is a pale yellow solid. It reacts with amino groups in amino acids and proteins to produce a purple product (Equation 1). The reaction is very slow in the absence of heat—heat is therefore used to speed up the “fingerprinting” reaction. All amino acids will test positive with ninhydrin.

Latent fingerprints are composed of several chemicals that are naturally present in skin oils or released through the pores of the skin via perspiration. Some of the chemicals in fingerprints include sodium chloride, amino acids, glucose, lactic acid and ammonia.

References

This activity was adapted from Biochemistry—The Molecules of Life, Flinn ChemTopic™ Labs, Volume 20; Cesa, I., Editor; Flinn Scientific: Batavia, IL (2002).