Nicholas Petraco

The Morphology and Evidential Significance of Human Hair Roots

A discussion of the morphology of human hair roots is presented. In addition to descriptions of variants of the root appearance for hairs removed from follicles in the three classical growth phases, several other commonly occurring root configurations are described and illustrated with photomicrographs. The possible evidential significance of each in certain case situations is discussed.

Forensic surface metrology: tool mark evidence

Over the last several decades, forensic examiners of impression evidence have come under scrutiny in the courtroom due to analysis methods that rely heavily on subjective morphological comparisons. Currently, there is no universally accepted system that generates numerical data to independently corroborate visual comparisons. Our research attempts to develop such a system for tool mark evidence, proposing a methodology that objectively evaluates the association of striated tool marks with the tools that generated them. In our study, 58 primer shear marks on 9u2009mm cartridge cases, fired from four Glock model 19 pistols, were collected using high-resolution white light confocal microscopy. The resulting three-dimensional surface topographies were filtered to extract all waviness surfaces-the essential line information that firearm and tool mark examiners view under a microscope. Extracted waviness profiles were processed with principal component analysis (PCA) for dimension reduction. Support vector machines (SVM) were used to make the profile-gun associations, and conformal prediction theory (CPT) for establishing confidence levels. At the 95% confidence level, CPT coupled with PCA-SVM yielded an empirical error rate of 3.5%. Complementary, bootstrap-based computations for estimated error rates were 0%, indicating that the error rate for the algorithmic procedure is likely to remain low on larger data sets. Finally, suggestions are made for practical courtroom application of CPT for assigning levels of confidence to SVM identifications of tool marks recorded with confocal microscopy.

Case studies in forensic soil examinations.

The examination and comparison of forensic soil samples is discussed. The origin of a simple and easy to learn procedure used and modified by the authors is reviewed. The process begins with a preliminary observation, removal of artifacts, and sieving of each specimen. A specific size fraction is split into three fractions for color matching, polarized light microscopy (PLM) examination (particle counting) and optional gradient comparison. Next, several cases are reviewed in which the modified method was used to evaluate the likelihood of common origin for questioned and known specimens.

A Density Gradient Technique for Use in Forensic Soil Analysis

A high-density aqueous salt solution for the preparation of density gradients is presented. It has been used successfully by the authors in forensic soil analysis. It has a density range that allows for the separation of a soil specimens heavy mineral components. It has no odor or toxic fumes, which eliminates the need to use a hood during preparation, and is far superior to the organic liquids normally used to prepare density gradients. This liquid should cause many forensic scientists to reexamine their attitudes towards using density gradients in forensic soil casework.

A Technique for Comparing Soil Colors in the Forensic Laboratory

The implementation of a simple aid that facilitates the designation and comparison of color in forensic soil cases is presented. The aid, a mount for forensic soil samples, when used in conjunction with the Munsell® Soil Color Charts allows the color of forensic soil samples to be accurately described and compared.

Trace Evidence—The Invisible Witness

This paper reviews the use of microscopic trace evidence in actual casework. Three cases are discussed in which the microscopic analysis of trace evidence was used to: associate the people, places, and things involved in the incident; reconstruct the event; and describe the occupation(s) of the participants. Each case is discussed in detail from the initial stages of the investigation through the use of the trace evidence during the investigation and at the trial.

The Occurrence of Trace Evidence in One Examiner's Casework

This study reviews the occurrence of trace evidence in routine casework from 1977 through 1983. Four primary categories of trace evidence are described. Each item of trace evidence was placed into one of the four primary categories. The occurrence of each category was evaluated for six different crime classifications over a period of seven years. The diversity of the trace evidence encountered, and the locations from which it was obtained, was also examined and evaluated.

A New Approach to the Microscopical Examination and Comparison of Synthetic Fibers Encountered in Forensic Science Cases

This paper describes a refined approach to the problems of identification and comparison of synthetic fibers encountered in forensic science case work. The necessary apparatus consists of a polarizing microscope equipped with a Mettler hot stage. Observations concerning the variation of a fibers birefringence with temperature have been found to be very useful, both for determining the generic type of an unknown fiber and for detecting subtle differences among fibers of the same generic type from different sources. Data, including semilogarithmic plots of birefringence as a function of temperature, are given for nine different generic types of synthetic fibers.

A comparison study of hair examination methodologies

A study was conducted to investigate the accuracy between two methods of hair analysis: PCR-STR DNA analysis and microscopic comparison analysis. Standard sets of pubic hairs were collected from volunteers, and unknown sets were generated from these samples. Three out of five (60%) of the hairs analyzed produced full DNA profiles that were correctly matched to the standard sets. DNA analysis was inconclusive (partial or no DNA profile) for two out of five (40%) of the samples. In contrast, the microscopic comparison analysis correctly matched four out of five (80%) of the samples to the standard sets but mis-identified one out of five (20%) of the samples. These results reinforce the practice of preliminary microscopic hair examination in narrowing down a set of hairs for DNA analysis. Microscopic comparison analysis is sufficiently reliable to remain a rapid and inexpensive method for forensic hair analysis.

Trajectory Reconstructions I: Trace Evidence in Flight

This paper reviews the use of trace evidence recovered from spent bullets in helping to establish trajectories. The use of information derived from such trace evidence combined with that from geometrical techniques of trajectory reconstruction and other data is discussed. Five cases are reviewed in which the analysis of trace evidential materials adhering to bullets was used to help reconstruct the event.