| Popis: |
Blood is a biological fluid frequently encountered at crime scenes and can be an important source of information regarding the events that occurred and individuals involved. Blood evidence can be the subject of bloodstain pattern analysis (BPA) to ascertain the details of a blood shedding event as well as deoxyribonucleic acid (DNA) analysis to identify potential individuals that were present at the event. BPA becomes more complicated when bloodstains are found on clothing items due to the wide range of fabrics and their inherent properties, along with a range of factors, including but not limited to, treatments, effects of laundering, and wear. The interaction between fabrics and bloodstains has not been wholly explored in the forensic literature. Additionally, most forensic research on bloodstain and fabric interactions have been conducted on laundered, absorbent fabrics. Waterproof fabrics are designed to repel water and protect underlying surfaces from the effects of weather such as rain and snow, often having very different properties from other fabrics. In this research, the characteristics of bloodstains deposited on waterproof and absorbent fabrics were assessed, as well as the ability to chemically detect the presence of blood after physical alteration of the stains. Six unlaundered, waterproof fabrics with different waterproof coatings, fiber content, and other properties in addition to three different types of absorbent fabrics were cut into swatches and utilized for this study. Whole human blood was used to create a variety of bloodstain types that are often observed at a crime scene: drip stains at two different impact angles, spatter stains, and transfer stains. Ten replicates were completed for all nine fabrics and each bloodstain type, for a total of 360 bloodstains. The dried bloodstains were photographed with a handheld digital camera and a digital camera mounted on a stereo microscope to better visualize the interactions between the blood and fibers. The bloodstained fabric swatches were then physically manipulated to assess the persistence of the stains and the ability of a presumptive blood assay to detect the remaining traces. Four mechanisms of blood deposition created bloodstains with different characteristics. Differences were observed among the variety of waterproof fabrics as well as the different absorbent fabrics. Bloodstains on the waterproof fabrics appeared to rest on the surface with very little wicking into the fabric. The largest drip stain diameters were observed on the cotton t-shirt, implying that the cotton t-shirt had the greatest wicking ability. Pearling of the blood droplets was observed on the waterproof fabrics used in this experiment, but appeared to occur irrespective of the type of waterproof finish applied. The drip stains on the nylon fabrics appeared flatter and frequently caved in at the center, even though the waterproof finishes were the same as two of the polyester fabrics, which produced more spherical drip stains. The contrasting appearances of the drip stains on cotton jersey and the cotton t-shirt is likely due to the presence of 5% spandex in the cotton jersey fabric. Bloodstain characteristics thus appeared to be influenced by fiber content, fabric structure and thickness, and the presence of waterproofing surface treatments, as well as the blood deposition mechanism. The bloodstains on absorbent fabrics did not separate greatly from the fabric after alteration and appeared largely unchanged. The bloodstains were dislocated from the waterproof fabrics to varying extents, but all greater than the absorbent fabrics. Traces of the bloodstains were frequently observed under the stereo microscope when the pre-existing stains were not observed macroscopically, indicating that a microscopic examination may be a useful tool in the forensic laboratory. The altered drip stains at both impact angles were overwhelmingly detected by the presumptive Kastle-Meyer blood test, with only a single instance of a negative result. In contrast, test results were more evenly split among the spatter stains on waterproof fabrics and were less likely to produce positive test results. Positive test results were obtained for the majority of altered stains, including stains that were not visible to the naked eye, indicating that a presumptive blood test may be useful for detecting latent bloodstains on clothing items such as the ones analyzed in this research. |
