Forensic Lab

Objectives

To appreciate the variety of tests available to the Forensic Scientists

Introduction

Part 1: Latent Fingerprint Development

The earliest recognition of the uniqueness of fingerprints and their suitability for personal identification came from the ancient Chinese, who employed a thumbprint in lieu of a signature on legal conveyances and even criminal confessions. Since literacy was uncommon, this proved a practical measure. The first scientific recognition of fingerprints in the West came in the 17th century, when the first studies on fingerprints were published in England and Italy. Two hundred years later Sir Francis Galton published a book, Finger Prints, were he proposed that no two fingers have identical ridge characteristics and fingerprints remain unchanged during the individual’s lifetime. Today the practice of utilizing fingerprints as means of identification is an indispensable aid to modern law enforcement.Fingerprint identifications have solvied a vast amount of cases. Crime scene fingerprints fall into three types:

1. Patent or visible impressions occur as the result of transferring a foreign material (paint, grease, blood or ink) coating the skin of the fingers to the object.2. Plastic or molded impressions are deposited when the hands, fingers or feet are pressed into a soft rubbery type material (wax, putty, clay or tar) that will retain the impression of the ridge pattern in this material.3. Latent or hidden impressions are left on polished surfaces such as wood, metal or glass by the sweat-moist ridges of the fingertips. Since latent fingerprints are not visible to the naked eye, they need to be developed using one of the following techniques:

After developing the latent impression it is photographed and lifted with a clear tape to be placed on a backing card with a contrasting background. It can then be entered into a computer, which allows it to be quickly and easily recalled and compared to the fingerprint of a suspect. Identification depends on showing a minimum of twelve matching characteristics in the ridge pattern. When these points of comparison are shown, it is considered that the proof of identity has been established. In this lab you will be developing your fingerprints using the iodine vapor and silver nitrate methods.

Part 2: Identification of Inks

The identification of inks often plays an important part in document examination. As a rule, the examination centers on the question as to whether the ink of certain passages or of alternations in the text is identical with the ink found in the possession of the suspect. For this reason the examination of questioned documents seldom consists of a complete determination of the inks in question but is usually restricted to a comparative analysis of certain properties of these inks.

Many different nondestructive techniques of the examination of inks are available: reflected infrared radiation, reflectance microspectrophotometry, lasers and scanning electron microscopy. Unfortunately, the reflectance methods are often subject to interference effects from “bronzing” or “sheering” of the ink.

Semi-destructive methods involve high-performance liquid chromatography (HPLC) and thin layer chromatography (TLC). Most chromatographic techniques are based on the minute sampling of a single written character representative of the questioned text. Small samples of ink bearing paper are removed from the document, they are then extracted with a suitable solvent, and the components of the solution are separated using HPLC and TLC. If the inks being compared show different composition, they did not come from the same pen.

In this lab you will be separating dyestuffs of several ballpoint pens using thin layer chromatography. Comparison of the dye composition will allow you to find out which pen was used by your TA to spot the TLC plate.

Part 3: Invisible Ink

Invisible ink has been used to conceal secret messages for a long time. Many different liquids can be used as invisible inks such as lemon juice, milk, vinegar or a solution of phenolphthalein.

Part 4: Breathalyzer

To determine whether a driver is driving under the influence of alcohol, law enforcement officers perform a Breathalyzer test to measure the blood alcohol content of the bloodstream. In the breath analyzer test, a breath sample is passed through a solution containing acidified potassium dichromate (K2Cr2O7)(K2Cr2O7) size 12{ \( K rSub { size 8{2} } ital "Cr" rSub { size 8{2} } O rSub { size 8{7} } \) } {}, which is bright yellow. Potassium dichromate, a strong oxidizing agent, oxidizes ethyl alcohol to acetic acid (vinegar). The chromium is consequently reduced from the VI to the III oxidation state, which is green. The unbalanced equation for this reaction is

The amount of alcohol in a breath analyzer sample is therefore proportional to the amount of potassium dichromate that is used up and also therefore to the loss of yellow color.

The Blood Alcohol Concentration (BAC) may then be calculated from the equation

BAC = 0.8 A/WR

Where W is a body weight of the individual being tested, A is the amount of alcohol in the body (in mL) and R is a “Widmark R Factor”, approximately 0.68 for men and 0.55 for women. In most states, a BAC of 0.1 percent is sufficient to be convicted for driving under the influence of alcohol; in some states the threshold BAC is even lower.

Part 5: Blood Stain Analysis Using Chemiluminescence

Investigators often find bloodstains during their examination of a crime scene. They also find stains that could be similar substance, something other then blood, such as red paint. How would you test a stain to see if it is blood? Human blood contains a pigment called hemoglobin, which is used to transport oxygen through our body. This pigment is used by number of tests to identify the presence of blood. One most common test used by investigators that reveals the presence of blood is the Luminol Test. In this test the bloodstain can be made to glow with a blue light due to chemoluminescent reaction of the luminol reagent with the iron (Fe) in the hemoglobin. Chemoluminescence is the reversed case of photoreaction: by a chemical reaction, an excited particle is formed, which looses its energy by producing a light quantum of light. The most important characteristic is that the light is emitted in cold. In other words, chemoluminescence happens when a molecule capable of fluorescing is raised to an excited level during a chemical reaction. Upon its return to the ground state, energy in the form of light is emitted. Luminol is one of the most outstanding molecules that emit appreciable amounts of light.

Experimental Procedure

Part 1: Latent Fingerprint Development

Your TA will fingerprint each person in the group on a glass microscope slide. Each student should use the ink pad to fingerprint themselves on a piece of paper.

1. After your TA develops your slides, use your ink prints to identify each group members’ print on the glass slides.

Part 2: Ink Identification

1. Obtain a precut TLC plate. Do not touch the white surface and handle carefully only by the edges.

2. Using a pencil, draw a light line across the shorter dimension 1 cm from the bottom. Using a ruler as a guide on the line, mark off five equally spaced intervals on the line as shown in Figure 1.

Figure 1

Figure 1. TLC plate

3. Cut a few letters out of the ransom note written in ink. Dissolve the pieces in about 1 mL of acetone. Use this to spot your TLC plate as the unknown.

4. In a similar manner, spot your TLC plate with three ‘standards’ diluted in test tubes: pen #1 (Pilot Easy Touch), pen #2 (Bic) and pen #3 (Papermate Eraser). The applied spots should be no bigger than 1-2 mm.

5. Use a 400 mL beaker for the development chamber. Add the solvent mixture (ethyl acetate/ethanol/water) to the beaker to a depth of about 0.5 cm. Remember: the level of the solvent must be below the spots on the plate. Using tweezers or forceps, place the spotted plate in the development chamber so that it rests in the solvent and against the beaker wall. Cover the beaker (with a paper towel) and allow the plate to develop. It might take more than 10 min, so you can proceed with Part 3 and Part 4.

6. When the solvent has risen on the plate to within 1 cm from the top, remove the plate from the beaker with tweezers or forceps. Using a pencil, mark the position of the solvent front.

7. Allow the plate to air dry and observe the colored separation. Note how many dye components make up the individual pens, along with their color.

8. Using a millimeter ruler, measure the distance that each spot (use the center of each spot for consistency) has traveled relative to the solvent front. Calculate the RfRf size 12{R rSub { size 8{f} } } {} values for each spot.

Part 3: Invisible Ink

Method 1

Caution! Iodine vapors are poisonous and should not be inhaled. Keep the jar with iodine vapor in the fume hood at all times.

1. Mix 1 g of cornstarch with 10 mL of water in a beaker and stir until smooth.

2. Heat the mixture for several minutes. Soften the point of a toothpick in the mixture and write a letter or a message with it. Let the paper dry.

3. To observe the message, place it in a jar with iodine vapor in the hood. Recap the jar and let it stand for more than 1 minute.

Method 2

1. Pour about 5 mL of phenolphthalein solution in a beaker. Soften the point of a toothpick in the solution and write a letter or a message with it. Allow the paper to dry.

2. To read the invisible writing, dip a small piece of cotton wool into the 1 M NaOH solution and carefully wipe the paper (do not rub!).

Part 4: Breathalyzer

Caution! Sodium dichromate is a strong oxidizing agent. Avoid contact with the skin.

You are given four unknown clear, colorless liquids. Use the sodium dichromate to determine which one is ethyl alcohol.

1. Pour 2 mL of each unknown into the test tube with the same number label and add 2 mL of acidic sodium dichromate solution to each test tube. Mix gently and record your observations.

Part 5:Luminol Analysis

The Luminol solution and fake blood are prepared as follows, but it will be made for you ahead of time for this lab.

Solution 1: Prepare by dissolving 0.1g of Luminol in 20 ml of 10% NaOH in a 50 ml beaker and diluting this solution to 200ml in your spray bottle and set aside.

Solution 2 (“blood”): Prepare by dissolving 0.5g of K3[Fe(CN)6]K3[Fe(CN)6] size 12{K rSub { size 8{3} } \[ ital "Fe" \( ital "CN" \) rSub { size 8{6} } \] } {} in 20 ml of 5% hydrogen peroxide solution. This solution is only stable for a short period of time so look ahead and know what you need to do next before you make the solution.

1. Go to a hood which is blacked out by construction paper and smear some “blood” (solution 2) onto the bench inside the hood with a paper towel. (make sure you are wearing gloves).

2. Quickly spray the wet “blood” stain with the luminol solution (solution 1) and close the hood to the point that you can barely see at the bottom and record your observations.