0.2 Forensics lab  (Page 13/4)

Forensic lab

Objective

• To appreciate the variety of tests available to the Forensic Scientists
• To observe latent fingerprinting development
• To studyink identification
• To do a breathalyzer analysis

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.

Most fingerprint development techniques are based on specific chemical reactions between the oily residues of a latent print and an applied chemical. The product of the reaction will have a new composition that is more visible and can be photographed to compare to other prints. A simple chemical method for fingerprint development is cyanoacrylate fuming.