Determination of Fracture Patterns in Glass and Glassy Polymers.

The study of fractures of glass, glassy-type materials, and plastic has long been of interest to the forensic community. The focus of this interest has been the use of glass and polymer fractures to associate items of evidence under the assumption that each fracture is different. Generally, it is well-accepted that deviations exist; however, the emphasis has been on classifying and predicting fracture rather than determining that each fracture is different. This study documented the controlled fracture patterns of 60 glass panes, 60 glass bottles, and 60 plastic tail light lens covers using both dynamic impact and static pressure methods under closely controlled conditions. Each pattern was intercompared, and based on the limited specimens tested in this study, the results illustrate that the fracture patterns are different. Further repetitive studies, under controlled conditions, will be needed to provide more statistical significance to the theory that each fracture forms a nonreproducible fracture pattern.

Quantitative bloodstain analysis: differentiation of contact transfer patterns versus spatter patterns on fabric via microscopic inspection.

In crime scene reconstruction, it is often necessary to differentiate "contact transfer" and "spatter" bloodstain patterns found on clothing. Current methodologies, however, are qualitative and prone to context bias. In this work, we demonstrate that microscopic inspection of the stain orientations provides a quantitative differentiation of bloodstains resulting from spatter versus contact transfer. Specifically, common knitted fabrics are comprised of parallel rows of left loop legs, in an upward diagonal orientation (/), and right loop legs in a downward diagonal orientation (\). Our microscopic examination of more than 65,000 individual stained loop legs shows that spatter stains are approximately evenly distributed between left and right loop legs, but contact transfer stains are unevenly distributed: depending on the type of surface contacted, as many as 82% of the stains were preferentially located on the left loop legs. We further show that in these fabrics the left loop legs protrude further out than the right loop legs by approximately 50 µm, indicating that the observation of left loop legs preferentially stained over right loop legs is associated with the topography of the fabric. These findings suggest that microscopic quantification of the relative loop leg stain distributions could provide an objective means of differentiating contact transfer versus spatter patterns in crime scene reconstruction.

A quantitative analysis of torn and cut duct tape physical end matching.

Forensic scientists are often asked to physically compare duct tape samples found in association with criminal activity. This study was designed to statistically evaluate the error and accuracy rates associated with duct tape physical end matching. The experimental design consisted of a blind study in which three researchers independently analyzed eight types of tape subjected to four methods of separation. The lowest mean accuracy observed was 98.15%, the highest mean false-positive rate observed was 3.33%, and the highest mean false-negative rate was 2.67%. Overall, high accuracy with low false-positive and false-negative error rates were observed. This study confirms the use of physical end matching in identifying duct tape samples as matching or nonmatching and that the differences between analysts, brands, tape grades, tape color, and methods of separation have varying contributions to misidentifications and inconclusive results. This study also demonstrates the importance of peer review in duct tape analysis.

Determination of mouth alcohol using the Dräger Evidential Portable Alcohol System.

Drivers suspected of alcohol intoxication are observed for a period of 15 min prior to quantitative breath alcohol testing. This is to preclude the interference of alcohol-based substances such as cough medicine, mouthwash, and breath spray just prior to actual evidential testing. To determine whether a 15 min observation period was necessary when performing evidential breath tests in the field, a mouth alcohol experiment was performed using the Dräger Evidential Portable Alcohol System (EPAS). Five types of alcohol beverages and the effects of expectorating versus swallowing were tested on twenty-five volunteer subjects. Serial measurements of breath and blood alcohol levels were performed at fixed time intervals. All alcohol beverage types gave two sequential measurements within 0.02 g/210 L of each other before 15 min had passed. Fifteen minutes was necessary to ensure there was no residual mouth alcohol. If the 15 min waiting period was not observed, the safety feature of the EPAS requiring two sequential measurements 2 min apart within 0.02 g/210 L would not ensure against mouth alcohol interference.

Morphological and spectroscopic measurements of plastic bags for the purpose of discrimination.

The discrimination of noncolored transparent polyethylene bags was studied by several nondestructive and semidestructive analytical methods. X-ray diffraction, infrared spectroscopy, and optical microscopy (differential interference contrast microscopy and phase contrast microscopy) were applied to polyethylene films. X-ray diffraction was used to distinguish variations in the crystalline phase, infrared spectroscopy was used to distinguish variations in the molecular components, and optical microscopy was used to distinguish the different surface morphologies. The results show that X-ray diffraction classifies the crystalline phase of the film depending on whether it is made from low-density polyethylene, linear low-density polyethylene, or high-density polyethylene; that infrared spectroscopy is useful to distinguish the molecular components and it is the most discriminating technique; and that optical microscopy discriminate films easily by their morphological differences.