Trace evidence part II: fibers
Fibers are often significant in solving crimes. Fiber evidence shows class characteristics, not always individual characteristics, but if fibers are found at a crime scene which are not widely distributed, then the appearance of these rare fibers can help in matching suspects to a crime (as in Wayne Williams case). The FBI reference on fibers is useful: ttp://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric3.htm
We will look at synthetic and natural fibers. Crime labs use other techniques besides microscopy to identify fibers: IR spectroscopy and chromatography. We don’t have the time or equipment for these analyses, but we will discuss the techniques (see pp. 217 –
219 and class material). We will concentrate on visible microscopy, including examination of refractive indexes of synthetic fibers.
Procedure:
Make slides of several fibers and draw sketches in your notebook. Note the differences between synthetic and natural fibers
To see if a synthetic fiber is isotropic, hold a polarizing filter over the lamp, with the axis of the polarized light parallel to the fiber axis. Then rotate the polarizer 90 degrees. If you see a change, the fiber is birefringent. Study the handout.
You can even estimate the degree of anisotropy using the Becke line method as shown in the handout. The Becke line is the bright line seen where the fiber and mounting medium meet. First focus the microscope as well as possible. Then move the focus up. The
Becke line moves toward the substance of higher refractive index. If the line moves toward the sample, the sample has a higher RI than the medium. If the Becke line moves away from the sample, the mounting medium has a higher refractive index. This takes a bit of practice but will be useful in the glass lab also.
Report: Make separate sections for hairs and fibers.
The purpose should be clear. The introduction for the hair sections should include relevant definitions. Most of the hair section should consist of copies of your sketches, and a summary of characteristics you can use to distinguish hair samples. Also include relevant size measurements.
The fiber section should be divided into natural and synthetic fibers. Include sketches of cotton and wool fibers. The synthetic section should include an introductory paragraph containing definitions of the relevant terms, including refractive index. Conclude the fiber section of the report with a discussion of characteristics which can be used to distinguish fibers.