Lecture 5 trace evidence and microscopy glass and soil

forensic analysis of glass

composition of glass, float glass(windows/bottles) borosilicates (pyrex) and tempered glass

comparing glass fragments

finding and measuring properties that can be used to associate one glass fragment with another, while also eliminating other possible sources.

comparing glass density

glass chip of known density is immersed in a liquid, the glass chip is made to be suspended within the liquid mixture by adjusting the ratio of the solvents. The unkown sample of rougly the same size is added and compared to where the known glass sample is floating. the solvents are typically bromoform and bromobenzene

comparing refractive index

glass is immersed in liquid who’s refractive index is adjusted until it is the same as the glass (via temperature manipulation) the match point occurs when the becke line disappears.

The becke line

a bright halo that is observed near the border of a particle that is immersed in a liquid of a different refractive index.

classification of glass samples

what if two glass samples have comparable densitiies and refractive indices? In such cases, forensic scientists may utilize additional methods such as elemental analysis or isotopic composition to aid in distinguishing between the samples. This data is added to an index used by the FBI to find which glasses are more frequently used.rt

glass trace element analysis

trace element profile of glass is obtained by aiming a high-energy laser beam at a glass particle using a laser induced breakdown spectrometer.

Glass fractures

2 types of fractures: radial and concentric a high velocity projectile like a bullet leaves a round, crater shaped hole surrounded by a nearly symmetrical pattern of cracks and the hole is wider on the exit side.

stress marks in glass fractures

shaped like arches that are pependicular to one glass surface and curved nearly parallel to the opposite surface, the perpendicular edge always faces the surface on which the crack originated.

the 3R rule

radial cracks form a right angle on the reverse side of the force.

Successive penetration in glass fractures

a fracture always terminates at an exisiting line of another fracture

collecting glass evidence

the process of gathering small fragments of glass from a crime scene to analyze their physical and chemical properties for forensic investigations. Pieces of glass evidence are individually packaged.

what is soil?

soil is defined as any disintegrated material, natural and/or artificial, that lies on or near the earth’s surface

Why is soil evidence significant?

soil evidence can be used to link a suspect to a crime scene.

visual comparison of soil

• side by side visual comparison of color and texture

• microscopic analysis can show plant and animal material

• high power microscopes can be used to characterize minerals and rock.

what happens with soil evidence

• soil found adhering to an object, the object is brought into the lab

• otherwise brought in in plastic baggies

forensic microscopy basics

earliest and simplest micrscope was a magnifying glass, ordinary magnifying glasses can only achieve magnification of about of about 5 to 10 times.

virtual images

images seen through a lens are virtual images

real images

images seen directly by eye

the compound microscope

constructed of two lenses mounted at each end of a hollow tube, the object to be viewed is placed under the lower lense, called the objective Lens, the magnified image is viewed through the eyepiece lens.

the comparison microscope

• often used in forensic microscopy

• is essentially two compound microscopes combined into one

the stereoscopic microscope

• lower magnifying powrs than compound microscopes (10X to 125X)

• gives a three dimensional image of an object

• is most frequrntly used microscope in a crime lab.

The polarizing microscope

A compound or stereoscopic microscope fitted with a polarizer and an analyzer to detect polarized light. produes vivid colors and high contrast applications in forensic science include the anaylysis of refractive indices of glass, crystalline materials, minerals, and synthetic fibers.

the microspectrophotometer

Links a microscope to a computerized spectrophotometer, gives ability to view the absorption spectrum of a particle, can use infrared or visible light. Used in forensics for trace evidence that is generally too small to be analyzed by a normal spectrophotometer, used for fibers, paint, and ink.

The scanning electron microscope

image is formed by aiming a beam of electrons on a specimen and analyzing the emitted electrons. High magnification (10X to 100,000X), high resolution, great depth focus.