Presentation on theme: "Issues in Analyzing Physical Evidence"— Presentation transcript:
1 Issues in Analyzing Physical Evidence Clear as GlassIssues in Analyzing Physical Evidence
2 A “Simple” Example: Glass What are some possible sources of glass evidence at a crime scene?Broken head light at hit and runBroken window at break inBroken bottle at bar fightBroken vase at struggle in homeBroken glasses at mugging
3 Some Obvious Properties to Match ThicknessColorUniformityCurvatureSurface ConditionSoilTintingTexture
4 Distinguishing Plate Glass Plate glass is made by floating the glass on liquid tin as it coolsSome tin will diffuse into the hot glassTin atoms will fluoresce under UV lightOnly the side next to the tin will fluoresce
5 How does glass break?Glass is elastic—it initially bends away in response to the forceGlass is weaker under tension than compressionOnce elastic limit is reached, radial cracks form first on the side opposite the forceContinued force places the front surface in tensionConcentric cracks form second on the side nearest the force
13 Where ever did that little piece of glass come from? Trace EvidenceWhere ever did that little piece of glass come from?
14 Classifying Properties Physical v. ChemicalThe property is physical if the composition of the substance does not change during the test (eg. density)Extensive v. IntensiveThe property is extensive if it depends on the amount of the substance (eg. mass)Class v. IndividualThe property is a class property if it is common to all samples in a category
16 Intensive Physical Properties DensityColorOdorLusterDuctilityMalleabilityHardnessThermal ConductivityElectrical ConductivityRefractive Index (function of wavelength)Optical absorption coefficient (function of wavelength)
17 Testing Do nondestructive tests first Destructive v. Nondestructive Nondestructive preserves the evidenceChemical is invariably destructiveDo nondestructive tests firstMust leave untouched sample for possible defense analysis
18 Characteristics of Glass Class characteristicsDensityRefractive IndexChemical compositionNot specific enough!Individual characteristicsReams and striations from manufacturingIrregular edges that can be pieced together
19 What is Glass? Mixture of silicon oxides and other metal oxides Hard, brittle, amorphousSand (SiO2)+ soda (Na2CO3) lowers melting point and viscosity, making mix easier to workLIme (CaO) is added to prevent it dissolving in water60-75% silica, 12-18% soda, and 5-12% limeTraces of magnesium and aluminumPyrex and auto headlights add boron oxide to form borosilicates
20 Amorphous Structure Silicon atoms are gold and have four bonds Oxygen atoms are red and have two bondsRandom network, no repeating structureBond distances are uniform
21 Other Types of Glass Tempered glass Laminated glass Induced stress by repeated heating and coolingDices when broken rather than splintersUsed in side and rear car windows, shower doors, sliding glass doorsLaminated glassLayer of plastic between two sheets of glassUsed in all US car windshields
22 Density: An initial analysis Density = Mass/VolumeIntensive physical propertyClass characteristicVaries with composition and thermal historyCan be measured nondestructively
23 How you measured density in chemistry Use a balance to find the weight of the sampleDetermine the volume of the sampleMeasure the volume directly (if liquid)Water displacement if irregular solidWhy doesn’t this work on forensic samples?
24 Why choose density?Can be used as a screening technique with large numbers of larger fragmentsUseful in identifying multiple sources present in the known and/or questioned samplesNondestructive
25 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?
26 Density of Glass Crown 2.500 g/cm3 Lead Crystal 3.100 g/cm3 Densest Flint g/cm3Fused Silica g/cm3For most samples the range will be about 10%Need to measure in parts per thousand or betterHow uniform is density across a pane?Weast, Robert C. Handbook of Chemistry and Physics. 61st Edition. Florida: CRC, 1981:
27 Using Archimedes Principle An object will float if its average density is less than or equal to that of the liquid it is suspended inThe bouyant force is equal to the weight of the liquid that is displacedTo float glass you need high density liquidsbromoform (2.85 g/mL), Bromobenzene (1.50 g/mL)“Tune” density by making mixtures of the two
29 Flotation: A whole new approach Use a column with varying density, less dense on top, more dense on bottomObject will sink to the point where its average density equals the density of the fluidUse calibrated reference samples as checkPure crystals with known densitySolves the problem of tiny, irregular samples
30 Density Gradient Column Layers of liquids of different compositionEach has a slightly different densityNeeds to be temperature stabilized to minimize convectionLittle mixing between layers even though they are miscible
31 Using Optical Properties to Identify Glass Clear as GlassUsing Optical Properties to Identify Glass
32 What is refractive index? n = c / vRatio of speed of light in vacuum to speed of light in the materialn always greater than 1Light can’t go faster than its speed in a vacuumDepends on wavelength of light (dispersion)Why you get a rainbow from a prism
33 Snell’s Law The higher the n, the more the light bends n=1.50 n=1.335
34 Snell’s Law Mathematically N1 x sin(θ1) = N2 x sin(θ2)Animation
35 Properties of Glass Crown 2.500 g/cm3 1.52 Lead Crystal 3.100 g/cm3 Densest Flint g/cmFused Silica g/cmFor most samples the range will be about 5%Need to measure in parts per thousand or betterHow uniform is refractive index across a pane?Weast, Robert C. Handbook of Chemistry and Physics. 61st Edition. Florida: CRC, 1981:
36 FBI Glass DatabaseA 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
37 ConsequencesNeed to measure refractive index to 5 significant figures (1 part in 10,000)Measured at sodium D line (single wavelength)Flat glass samples onlyWindow, autos, display cases, mirrors from actual criminal investigations
38 The Key IssuesWhere 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?
39 The Wave of the Future: The search for individual characteristics Laser Ablation Inductively Coupled Plasma Mass SpectrometryDetect 46 trace elements in glassFBI and Oak Ridge National LaboratoryDeveloping a national database to determine likelihood of a match