Experiment Essay Example: Forensics and Fiber Evidence

Introduction 

As people are in constant contact with both natural and synthetic fibers in almost any environment they may be in, the transfer of these fibers can occur so frequently that they are submitted as evidence in nearly a quarter of all forensic cases involving trace evidence.1 Because of their relative persistence and their numerous characterizing traits that can differentiate one type of fiber from another, forensic fiber analysis can provide comparative and associative evidence by examining and comparing them to references fibers involved in a crime. For example, often a fiber from a victim is collected via a lint roller or tape and then compared to various other known samples that have been collected from a suspect location or article of clothing.2 Analysis can then be performed by a series of microscopic comparison tests that look for various defining features on each fiber to conclude whether they originate from the same source.

Fibers can be broadly categorized as either synthetic or natural. Synthetic fibers are man-made and involve the repurposing of natural fibers to create an original, completely manufactured product. As for natural fibers, they can be further classified as belonging to an animal (silk or hair), they can be from a mineral, or they can be botanical, originating from the seed, bast, or leaf of a plant.3

To analyze botanical fibers a Herzog test can be performed to determine their characteristic fibrillary twists, resulting in either an S twist or a Z twist. An S twist is described by a yellow color produced when the fiber is oriented perpendicular to the analyzer (the N-S position), and a blue color produced when the fiber is oriented parallel to the analyzer (the E-W position). A Z twist is the opposite, meaning the perpendicular would be blue, and the parallel would be yellow. It should be noted that because a red compensator must be inserted to view the result of a Herzog test, the colors will have a pink tint to them, and thus will not be a clear, vibrant yellow or blue. Additionally, cotton is an outlier because while it is a botanical fiber, it is classified as having neither an S twist nor a Z twist, rather it is in its own category because of its unique behavior. When cotton undergoes a Herzog test it will show irregular alternations of yellow and blue colorations throughout the length of the fiber. This banding of yellow and blue is characteristic of cotton so, while it may not be defined as S or Z, it can still be identified using the Herzog test.

While Herzog is useful for botanical fibers, oil immersion can be used on both natural and synthetic fibers. Oil immersion is used to determine the two characteristic refractive indices (RI) of a given fiber, one for the parallel axis, one for the perpendicular. To find one of the RI values the contributions of the other RI must be eliminated so that it doesn’t obscure the readings. To do this the analyzer must be used and the fiber must be put into total extinction, which will effectively eradiate the impact of the second RI. Once this is done, oil immersion can proceed by following the movement of the Becke line. After the first RI has been found when the fiber appears invisible, the sample stage is simply rotated ninety degrees and the process is repeated.   

Another tool useful in the identification of both synthetic and natural fibers is the Michel-Levy chart. When a fiber is viewed in cross polars at 45° under a microscope, the colors observed can be found on a Michel-Levy chart and will yield a relative retardation (RR) value. The fiber can then have its diameter measured and between the RR and the diameter, the chart will give a birefringence number that correlates to those values. Further, the sign of elongation can be found by inserting a compensator and seeing how the RR is affective. If the colors move approximately one order to the right on the Michel-Levy chart, then the sign of elongation is positive. If the compensator is inserted and the order moves to the left, then the sign is negative. The birefringence value and the sign of elongation are characteristic features of a fiber and therefore can aid in the identification process.

Materials and Methods 

To analyze the botanical fibers kapok, linen, sisal, cotton, mercerized cotton, and nonmercerized cotton, the Herzog test was performed to determine each of their characteristic twist patterns. To do this, the fibers were cut with scissors from the source material, and then individually mounted on a sample slide. Then, a coverslip was laid over the fiber and a Cargille oil with an RI under 1.500. Once mounted the slide was placed under a Lecia DM EP compound microscope, the analyzer was flipped on, and a red compensator was inserted. The sample stage was then rotated so that the fiber was oriented in either the N-S direction (perpendicular to the analyzer), or the E-W direction (parallel to the analyzer) and the colors of each position was recorded and analyzed to determine the twist’s direction.

After the Herzog test was performed on the aforementioned botanical fibers, oil immersion was then performed on a given unidentified fiber in order to find its two RI values. To do this, the compensator was removed from the microscope, a D-line filter was inserted, the analyzer was flipped on, and the polarizer was set to 0 (or whichever nearby value provided the deepest black color). Because there are two RI values that belong to any given fiber, the sample stage must be rotated to orient the fiber either in an E-W or N-S position to find total extinction, achieved once the fiber appeared to be completely blackened. Then, with an RI range of 1.4-1.8 a mid-value Cargille Oil was selected as a starting point and was used to mount the fiber. Once mounted and placed under the microscope, the sample stage was slowly lowered to follow the movement of the Becke line so the next oil could be properly selected. After a couple of trials, the RI match between the fiber and oil was found, indicated by the disappearance of the fiber., the sample stage was then rotated ninety degrees and the process of oil immersion was repeated to find the second value. Because the oil gives its RI values as an n_D^(25°C) value, both found values were then temperature corrected to the temperature of the lab during the analysis, using the relationship seen in the equation below.

n_D^T=n_D^(T_0 )+(T-T_0 )   (1)

With Herzog performed on the botanical fibers and the RI values found for an unspecified fiber, a Michel-Levy chart was used to ascertain the characteristic birefringence value and sign of elongation for samples of kapok, linen, sisal, nonmercerized cotton, mercerized cotton, rayon, silk, wool, orlon, nylon 6, nylon 6.6, acetate, cresson, veral and dacron. To do this each fiber was mounted in a midrange oil (1.580 was used in this analysis) and placed at 45° under the microscope. The microscope was the set up in cross polars and the color seen on the fiber was then compared to the Michel-Levy chart. Often there are multiple colors produced from a fiber in this setting so the color that had the highest RR value (the furthest on the right on the chart) was noted as the RR for the fiber. If the color was difficult to note because of the repetitions found throughout the chart, the spectrum of colors on the fiber were able to help narrow it down. After an RR was selected, the diameter of the fiber was measures so as to use in conjunction with the RR value to find the birefringence value. To find the sign of elongation, a compensator was then inserted into the fiber and the new color of the fiber was recorded and found on the Michel-Levy chart.