Forensics Last Test

Fiber, glass, DNA, blood

Fiber characteristics

Type, Width, Condition, Color, Shape, Delustering Agents

Natural Fiber

straight from plant or animal

Regenerated fiber

digested and reformed cell walls (shaped by extrusion)

Synthetic fiber

made from oil (shaped by extrusion)

MSP profile

identifies color of fiber (Microspectrophotometer Test)

Shape of fiber

Longitudinal (along length) or cross-sectional (3D shape at end)

Delustering agents

reduces shine (only in synthetics)

Plate glass

breaks into large, sharp pieces

Wire glass

wire mesh embedded in plate glass
- holds onto shards \= not sharp, hard to break through

Tempered glass

Heat-treated
- very difficult to break
- explodes into tiny nuggets (not sharp)

Laminated glass

plastic embedded in plate glass
- benefits of wire glass, but looks better

Compare glass

chemical composition, Refractive Index (RI)

Refractive Index Test

Put glass in oil of known RI and look for becke lines (halos/edges)

Concentric cracks

show location of contact

Radial cracks

show order of strikes
- look for intersections: cracks can't cross existing ones

Stress lines

come out of one side and curve to parallel
- come out of face opposite to the face struck

Presumptive Blood Tests

Is it blood?
- Luminol
- Phenolphthalien

Luminol Test

spray - glows blue if positive

Phenolphthalien Test

drops - bright pink if positive

Low velocity blood

round, smooth edges (most likely dripping off something)

Medium velocity blood

Tails and secondary spatter (generally from strike)

High velocity blood

mist (generally from gunshot)

Origin of blood

- tails indicate direction of travel (always downstream)
- angle of impact

Area of Convergence

indicates number of bleeding events
- all spatter converges to one area
- trace line through long axis to determine

Angle of Impact (AOI)

inverse sine of (width/length) in degrees

Point of Hemorrhage (POH)

distance in Z axis off the surface that is the origin of blood
- POH \= tan (AOI) times distance from area of convergence

Precipitin Test

Test for species of blood
- known antibodies and blood in agar
- let sit and diffuse
- if match, antibodies will stick to blood cells and form clump in middle

Blood Typing

tests for proteins on outside of red blood cells (13 different tests)

ABO Test

mix blood with antibodies that stick to each protein and see if it clumps (with A and B proteins)
- if both clump \= AB
- if neither clump \= O

Frequency of A

43%

Frequency of B

12%

Frequency of AB

3%

Frequency of O

42%

Rh Test (D protein)

presence or absence of D protein
- have D (+)
- don't have D (-)

Frequency of D+

85%

Frequency of D-

15%

Blood Enzyme Types

PGM-1, PGM-2, PGM-2-1

RFLP Test (DNA Fingerprinting)

compare DNA - use restriction enzymes, compare size and number of cut pieces

Electrophoresis

pieces separate out by size (light ones travel farthest)
- Standard lane (control) \= known sizes

PCR

DNA copying process (billion copies in one day)

STR Testing (Short Tandem Repeats)

compare number of STR repeats
- copy (PCR) just STRs
- electrophoresis to separate pieces
- compare STR sizes
**if all 13 STRs match - positive identification**