Forensic Lab

May 26, 2006 /MM

Objective

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.
Fingerprints have been the reason for the solving of 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:

‘Powder and brush’ technique: The surface is dusted with a very fine powder that sticks to the oils and perspiration that are left behind from the friction ridges of the skin. Some surfaces, however, absorb this powder and the fingerprints are not identifiable.

Laser luminescence: Involves illumination of fingerprints which fluoresce due to particles picked up during everyday life such as paints, inks and oil. It can be used on metals, plastic, cloth and wood.

Ninhydrin test: Indantrione hydrate is sprayed onto the fingerprint where it reacts with the amino acids, giving a dark purple deposit. It can be used to develop very old prints (made over 30 years ago).

Iodine vapor: Can be used to develop fingerprints on fabrics and rough surfaces. Iodine vapor alone is useful only for prints up to 24 hours old, however a mixture of the vapor with steam allows this method to be effective for up to two months. Prints developed by this method disappear rapidly, so it works well in situations where you want to conceal your work.

Silver nitrate: Silver nitrate reacts with chlorides in the fingerprints, to give the insoluble salt, silver chloride, which rapidly turns black on exposure to light. This method is not suitable for fabrics or rough surfaces.

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

In document examination, the examination of inks often plays an important part. 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 examination 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) Click here for more information on 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), 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

Cr2O72- + H++ C2H5OH ---> Cr3+ + CH3CO2H + H2O

 

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.