Practical Forensic Microscopy

A Laboratory Manual

Barbara P. Wheeler

• Editore: Wiley
• Anno: 2021
• ISBN:
  9781119154518
• Edizione: 2
• Argomento: Law
• Categoria: Forensic Science

An applied approach to teaching forensic microscopy in educational settings, featuring new experiments and an up-to-date overview of the field

Practical Forensic Microscopy: A Laboratory Manual, 2nd Edition, is a unique resource that brings the microscopic procedures used by real-world forensic investigators to the college laboratory, providing hands-on knowledge of the microscopes and microscopic techniques used in the field. Presenting a balanced, skills-based approach to the subject, this student-friendly lab manual contains dozens of experiments designed to cover the various microscopic evidence disciplines, including examinations of fingerprints, firearm, toolmark, shoeprint and tire impressions, gunshots, fibers, soil, glass breakage, drugs, semen, and human hair.

The second edition includes revised and updated experiments that reflect current technologies and techniques used in forensic science, including new experiments examining plastic film, food condiments, feathers, building materials, explosive residue, cigarette butts and more. Each chapter includes a list of simple objectives for the experiment, a general overview of the topic, further readings, and selected references. The manual contains worksheets and templates for students to use when compiling analytical results. The concluding chapter features an innovative case scenario that requires students to analyze items of evidence, complete a laboratory report, reach a conclusion, and present their findings. This popular lab manual:

• Teaches practical forensic microscopy skills through hands-on experiments and engaging practical activities
• Covers a wide range of microscopes and forensic tools, including stereomicroscopes, ocular micrometers, and fluorescence, polarized light, and phase contrast microscopes
• Explains simple stereomicroscopic techniques for analyzing various types of common forensic evidence
• Includes more complex procedures for examining biological, drug, and trace evidence Discusses laboratory safety, microscope maintenance, and the Micro Kit

Written by an author with years of academic and professional experience, Practical Forensic Microscopy: A Laboratory Manual, 2nd Edition, is a must-have companion for any college-level forensic science course with a laboratory component, and is a useful supplement for related courses that cover microscopy and the principles of forensic lab procedures.

Application Experiments

Chapter 6
Experiment 6: Physical Match Examinations

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Recommended pre‐lab reading assignments:

Thornton JI. Fractural Surfaces as Models of Physical Matches. Journal of Forensic Sciences. 1986; 31(4): 1435–1438.

Adolf FP. Physical Fits Between Textiles. In: Proceedings of the 3rd Meeting of the European Fibres Group, Linkoping, Sweden. 1995; 36–41.

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Objective

Upon completion of this practical exercise, the student will have developed a basic understanding of:

1. physical match examinations

Introduction

A comparative examination is the process of ascertaining whether two or more objects have a common origin. One way that this is accomplished is by a physical match examination. Once a comparative examination has been completed, the forensic scientist must be prepared to render a conclusion with respect to the origins of the samples. Do they or do they not originate from the same source? Evidence is said to possess class characteristics when it can be associated with a group and not with a single source. Evidence that can be associated with a common source with an extremely high degree of probability is said to possess individual characteristics. When a physical match is obtained, it is considered unique, or an individual characteristic, and an analyst can determine that two portions were one at an earlier time.

Physical matching of broken, cut, or torn objects can be encountered in many types of forensic investigations. These types of examinations are performed to associate an item with the material from which it is thought to have originated. Physical match determinations may be performed in situations in which an item has been cut, torn, broken, or has otherwise undergone a separation into two or more pieces. If a separation is the result of a random process, the particular separation would not be expected to be repeated and should be considered unique. It is therefore possible that broken pieces could be fit together in a unique fashion to demonstrate a common origin.

The process of determining a physical match depends on the actual items themselves. For most items that have been cut or torn, a forensic scientist is able to determine whether or not the items were one piece at an earlier time by using edge shapes and surface features, as shown in Figure 6-1. These items are typically examined as a whole with low magnification on a stereomicroscope. Edge shapes are examined for a physical “fit,” similar to what is done when working on a jigsaw puzzle. Cut edges will appear sharp and crisp, allowing edges to be easily examined and aligned. Torn edges may be frayed or rough, so in some instances, a closer examination with slightly higher magnification may be necessary. Surface features can also be used when performing a physical match. Surface features such as colored or molded patterns may also be useful in determining the actual placement of a section. Unique surface markings such as manufacturing striations or polishing scratches can also be helpful. In some cases, it may be necessary to use surface features only, since items that have been separated by “sawing” have actually had small portions of the newly formed edge removed.

Figure 6-1 This section of fabric displays a physical match that can be determined by both cut edges and surface features.

Items that have been broken may also be examined using edge shapes and surface features. However, at times additional examinations are also necessary. Prior to breaking, some items tend to bend or stretch. This can cause unique stress markings perpendicular to the break edge. These markings, which will appear as mirror images, can also be examined to determine whether or not two items were one piece at an earlier time.

Several conclusions can be reached for physical match examinations. First, if a physical match is obtained, it can be determined that the items were one at an earlier time. Sometimes a physical match is not possible. This may be a result of differences or the lack of characteristics to make a determination. In these situations, no indication should be given as to whether or not the items were one at an earlier time. However, when appropriate, further testing may be suggested to either prove or disprove similarities between the items.

Equipment and Supplies

Stereomicroscope

Previously prepared sets of plastic pieces, cut up

Previously prepared sets of playing cards, cut up

Previously prepared sets of matchbooks and matches

Previously prepared sets of fabric, cut up

Previously prepared sets of broken microscope slides

Safety

Use standard laboratory safety procedures as described in gui...