Forensic Glass Analysis

What is Glass? -Considered trace evidence -Embedded in hair clothing, shoes, and other objects -Hard, amorphous material, usually transparent, composed primarily of silica (SiO2) and various amounts of element oxides.

Classification of Glass -Can be classified into families (taxonomy) -Families differ in chemical composition and physical properties -Many of the properties of glass cannot be measured from small fragments and are of little concern to the forensic scientists.

Breaking of Glass

-Its brittle -Can occur during burglary, breaking and entering, assault, hit and run, and car-pedestrian accident. -Investigators need to be able to distinguish between various types of glass such as automobile, window, household glass.

Fracture Patterns -Tell investigators whether the breakage was due to a high velocity, a low velocity bullet, or another type of projectile or object. Ex.) A bullet at high velocity produces a hole in the glass where the exit side is wider then the entry side. Low velocity bullet hole will exhibit more shattering around the hole and the hole may be irregular in shape.

Concoidal Fractures -Shell-like fracture patterns in the glass are the result of stress on the glass and breakage due to application of force.

2 Types

Radial Fractures -Extend outward in a line from the point on the glass where the force originated

Concentric Fractures: -Circular patterns around the point of origin of the force.

-Knowing the type of fracture allows the investigator to determine the direction of the impact on the glass.

*Label the radial and concentric fractures.

-For a piece of glass that has been fractured radically, the lines will intersect the surface of the glass at right angles on the side of the glass OPPOSITE the side that received the force. -For a piece of glass that has been fractured concentrically, the force of impact is on the SAME SIDE of the glass that has lines intersecting the surface at right angles.

-In the case of multiple fracture patterns in a single piece of glass, the sequence of events can be determined by looking closely at the radial fracture pattern. A fracture pattern will stop when it intersects with an existing fracture line.

*Which pattern occurred first? Second? Third? A,B,C

Flotation Method -Observation that a solid particle will float in a liquid medium of greater density, sink in a liquid of lower density, or remain suspended in a liquid of equal density.

Refractive Index -A physical property of any material that will transmit light. -When light strikes the surface of a transparent material such as glass, it can be refracted, absorbed, and transmitted. -When light passes from one medium into another, there is a change in its direction and velocity-----Refraction. -Light traveling from a less dense substance to a more dense substance (as from air to glass) will slow down.

-The refractive index (n) is a comparison of the speed of light in a vacuum to the speed of light in another substance.

(air)

-For example, the speed of light in water is 225,000,000 m/sec -Its refractive index can be calculated as: 3.00 x 108----speed of light in air

2.25 x 108----speed of light in water Equals--1.33

-Light rays can be used to find refractive index of an unknown substance. By projecting a laser light through a liquid and then out into the air, the amount of refraction can be measured.

-If a clear material, such as glass, is immersed in a liquid that has the same refractive index, both the glass and the liquid will bend light at the same angle, and the glass will appear to disappear!!

Snell’s Law-A ratio of the angle of incidence to the angle of refraction as light travels from one medium to another.

Snell’s Law: (n1)(Sing Angle 1)=(n2)(Sin Angle 2)

Where: n1= the refractive index of first medium n2= the refractive index of second medium Angle 1= the angle of incidence between the incoming ray and the normal line Angle 2= the angle of refraction between the outgoing ray and the normal line

How can glass be used as evidence? -Glass fragments found at a crime scene can be used to place a suspect at the scene. -May be found in a suspect’s clothing or embedded in shoes as a result of breaking a window to gain entry to a house, or they may be found on a victim of a hit-&-run. -Glass shatters into so many fragments that piecing them together is impossible; thus it must be considered as class evidence. -Forensic scientists must use as many physical properties of glass as possible to characterize the fragments and associate the fragments from the crime scene to a suspect.

Blood

Karl Landsteiner-1901; discovered the typing of blood. -Won noble prize in 1930 -Recognized that human blood is not the same -From his work came the classification system--ABO System

Blood Components

DNA -Molecules carrying the body’s genetic information -Double stranded in the shape of a double helix

Erythrocytes -Red blood cells

Antigen -A substance, usually a protein, that stimulates the body to produce antibodies against it.

Antiserum -Blood serum in which there are specific antibodies.

Plasma -The fluid portion of unclotted blood.

Serum -The liquid that separates from the blood when a clot is formed.

Antibody -A protein that destroys or inactivates a specific antigen. Antibodies are found in the blood serum.

Hemoglobin -A red blood cell protein responsible for transporting oxygen in the bloodstream and the red coloring of blood

Agglutination -The clumping together of red blood cells by the action of an antibody.

A A Anti-B

B B Anti-A

AB AB Neither Anti-A or Anti-B

O Neither A nor B Both anti-A and Anti-B

For example, the transfusion of type A blood into type B patient will cause the natural anti-A in the blood of the type B patient to react promptly with the incoming A antigens, resulting in agglutination.

Forensic Characterization of Blood Stains

The criminalist must be prepared to answer the following questions when examining dried blood:

1. Is it blood? 2. From what species did the blood originate? 3. If the blood is of human origin, how closely can it be associated to a particular individual?

Determination of Blood

Preliminary Test:

Benzidine Color Test -Not used anymore; benzidine is a known carcinogen -Phenolphthalein is usually substituted in its place (Kastle-Meyer Color Test) -Both tests are based on the observation that blood hemoglobin possesses peroxidase-like activity. -When blood, phenolphthalein, and hydrogen peroxide are mixed together, the blood’s hemoglobin will turn a deep pink color. Hemastix Strips -Used by Field Investigators -Moisten with distilled water and placed in contact with a blood stain. -An appearance of a green color is positive for blood.

Determination of Blood Cont.

Luminol Test -The reaction of luminol with blood results in the production of light rather than color. -Also known as chemiluminescence--same as in fireflies and light sticks -Luminol (C8H7O3N3) plus hydrogen peroxide reacts with the iron in hemoglobin, a bluish-green light appears.

Stain Patterns of Blood

The crime scene investigator must not overlook the fact that the location, distribution, and appearance of blood stains and spatters may be useful for interpreting and reconstructing the events that must have occurred to have produced the bleeding.

Herbert L. MacDonell --In-depth study of this subject. -Many of his observations and conclusions have important implications for investigators who Want to trace the direction, dropping distance, and angle of impact of a blood stain.

1-Surface Texture- The harder and less porous the surface, the less spatter results.

2-The direction of travel of blood striking an object. The pointed end of a bloodstain always faces its direction of travel.

3-It is possible to determine the impact angle of blood on a flat surface by measuring the degree of circular distortion of the stain. A drop of blood striking a surface at right angles gives rise to a nearly circular stain; as the angle decreases, the stain becomes elongated in shape.

4-The origin of a blood spatter in a two-dimensional configuration can be established by drawing straight lines through the long axis of several individual bloodstains. The intersection or point of convergence of the lines, represents the point from which the blood emanated.