Issues in Analyzing Physical Evidence

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Issues in Analyzing Physical Evidence
Clear as Glass Issues in Analyzing Physical Evidence

A “Simple” Example: Glass
What are some possible sources of glass evidence at a crime scene? Broken head light at hit and run Broken window at break in Broken bottle at bar fight Broken vase at struggle in home Broken glasses at mugging

Some Obvious Properties to Match
Thickness Color Uniformity Curvature Surface Condition Soil Tinting Texture

Distinguishing Plate Glass
Plate glass is made by floating the glass on liquid tin as it cools Some tin will diffuse into the hot glass Tin atoms will fluoresce under UV light Only the side next to the tin will fluoresce

How does glass break? Glass is elastic—it initially bends away in response to the force Glass is weaker under tension than compression Once elastic limit is reached, radial cracks form first on the side opposite the force Continued force places the front surface in tension Concentric cracks form second on the side nearest the force

How does glass break? 2 1 2

Radial and Concentric Fracture Lines

Stress Marks Radial cracks form right angles on the reverse
Conchoidal fracture lines Almost parallel Almost perpendicular

Use Marks and Scratches
Diagonal marks where wiper blades do not overlap Vertical scratches on side windows from grit Cross hatching where wiper blades overlap

Which bullet was fired first?
Crack propagation is stopped by earlier cracks

Which side was the bullet fired from?
Exit side is wider than entry side Stress lines for radial cracks form a right angle on the reverse side of the force Exit Entry

Were the lights on?

Where ever did that little piece of glass come from?
Trace Evidence Where ever did that little piece of glass come from?

Classifying Properties
Physical v. Chemical The property is physical if the composition of the substance does not change during the test (eg. density) Extensive v. Intensive The property is extensive if it depends on the amount of the substance (eg. mass) Class v. Individual The property is a class property if it is common to all samples in a category

Extensive Physical Properties
Mass Weight Length Volume Absorption Resistance

Intensive Physical Properties
Density Color Odor Luster Ductility Malleability Hardness Thermal Conductivity Electrical Conductivity Refractive Index (function of wavelength) Optical absorption coefficient (function of wavelength)

Testing Do nondestructive tests first Destructive v. Nondestructive
Nondestructive preserves the evidence Chemical is invariably destructive Do nondestructive tests first Must leave untouched sample for possible defense analysis

Characteristics of Glass
Class characteristics Density Refractive Index Chemical composition Not specific enough! Individual characteristics Reams and striations from manufacturing Irregular edges that can be pieced together

What is Glass? Mixture of silicon oxides and other metal oxides
Hard, brittle, amorphous Sand (SiO2)+ soda (Na2CO3) lowers melting point and viscosity, making mix easier to work LIme (CaO) is added to prevent it dissolving in water 60-75% silica, 12-18% soda, and 5-12% lime Traces of magnesium and aluminum Pyrex and auto headlights add boron oxide to form borosilicates

Amorphous Structure Silicon atoms are gold and have four bonds
Oxygen atoms are red and have two bonds Random network, no repeating structure Bond distances are uniform

Other Types of Glass Tempered glass Laminated glass
Induced stress by repeated heating and cooling Dices when broken rather than splinters Used in side and rear car windows, shower doors, sliding glass doors Laminated glass Layer of plastic between two sheets of glass Used in all US car windshields

Density: An initial analysis
Density = Mass/Volume Intensive physical property Class characteristic Varies with composition and thermal history Can be measured nondestructively

How you measured density in chemistry
Use a balance to find the weight of the sample Determine the volume of the sample Measure the volume directly (if liquid) Water displacement if irregular solid Why doesn’t this work on forensic samples?

Why choose density? Can be used as a screening technique with large numbers of larger fragments Useful in identifying multiple sources present in the known and/or questioned samples Nondestructive

The BIG Questions for Any Analytic Technique
How much variation is there in what you are measuring? Is the range of potential values large enough that you can exclude lots of samples? How precisely can you measure it? OR Gee, I knew significant figures would come in useful sometime! What are the limitations of your instrumentation? What are the limitations of your sample?

Density of Glass Crown 2.500 g/cm3 Lead Crystal 3.100 g/cm3
Densest Flint  g/cm3 Fused Silica  g/cm3 For most samples the range will be about 10% Need to measure in parts per thousand or better How uniform is density across a pane? Weast, Robert C. Handbook of Chemistry and Physics. 61st Edition. Florida: CRC, 1981:

Using Archimedes Principle
An object will float if its average density is less than or equal to that of the liquid it is suspended in The bouyant force is equal to the weight of the liquid that is displaced To float glass you need high density liquids bromoform (2.85 g/mL), Bromobenzene (1.50 g/mL) “Tune” density by making mixtures of the two

Fun with Flotation

Flotation: A whole new approach
Use a column with varying density, less dense on top, more dense on bottom Object will sink to the point where its average density equals the density of the fluid Use calibrated reference samples as check Pure crystals with known density Solves the problem of tiny, irregular samples

Layers of liquids of different composition Each has a slightly different density Needs to be temperature stabilized to minimize convection Little mixing between layers even though they are miscible

Using Optical Properties to Identify Glass
Clear as Glass Using Optical Properties to Identify Glass

What is refractive index?
n = c / v Ratio of speed of light in vacuum to speed of light in the material n always greater than 1 Light can’t go faster than its speed in a vacuum Depends on wavelength of light (dispersion) Why you get a rainbow from a prism

Snell’s Law The higher the n, the more the light bends n=1.50 n=1.335

Snell’s Law Mathematically
N1 x sin(θ1) = N2 x sin(θ2) Animation

Properties of Glass Crown 2.500 g/cm3 1.52 Lead Crystal 3.100 g/cm3
Densest Flint  g/cm Fused Silica  g/cm For most samples the range will be about 5% Need to measure in parts per thousand or better How uniform is refractive index across a pane? Weast, Robert C. Handbook of Chemistry and Physics. 61st Edition. Florida: CRC, 1981:

FBI Glass Database A rough statistical estimate of the likelihood of finding glass of that refractive index (2337 samples) Manufacturing changed in late 1970’s making glass more uniform

Consequences Need to measure refractive index to 5 significant figures (1 part in 10,000) Measured at sodium D line (single wavelength) Flat glass samples only Window, autos, display cases, mirrors from actual criminal investigations

The Key Issues Where would you want to be on the distribution if you wanted to convince the jury using a class characteristic? Is it easier or harder to use refractive index to match glass now than it was 20 years ago? Does it help to measure both density and refractive index?

The Wave of the Future: The search for individual characteristics
Laser Ablation Inductively Coupled Plasma Mass Spectrometry Detect 46 trace elements in glass FBI and Oak Ridge National Laboratory Developing a national database to determine likelihood of a match

Focused UV Laser Vaporizes the Glass Sample

Inductively Coupled Plasma Mass Spectrometer
Make Ions in plasma by knocking off electrons Sort ions by mass in Mass Spec by pushing them around with electric fields

Comparing Trace Elements in Different Samples at ppm
A jury friendly data display—Is it easy to tell the samples apart?

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