Charles A. Peters

Comparison of refractive index, energy dispersive X-ray fluorescence and inductively coupled plasma atomic emission spectrometry for forensic characterization of sheet glass fragments

Fragments (in the milligram size range) from 81 tempered sheet glasses were used in order to evaluate the source discrimination capabilities of refractive indices (RI) and elemental composition, by using energy dispersive X-ray fluorescence (EDXRF) and inductively coupled plasma atomic emission spectrometry (ICP-AES). The X-ray intensities of five elements were determined by EDXRF with precisions of between 1 an 25%. The concentrations of nine elements were determined using ICP-AES and precisions of from less than 1 to about 10% were obtained. Both methods offer improved discrimination capability over RI measurements alone. The technique of EDXRF provides rapid, non-destructive testing and is widely available in forensic laboratories. The ICP-AES method offers the advantages of providing quantitative data on the concentration of elements, applicability to a greater number of elements and improved discrimination.

Determination of Barium in Gunshot Residue Collection Swabs Using Inductively Coupled Plasma-Atomic Emission Spectrometry

Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was compared with atomic absorption spectrophotometry (AAS) for barium determination in gunshot residue (GSR) collection swabs. Lack of interferences by common swab extract constituents, a wide linear dynamic range, and good precision and accuracy of ICP-AES make it superior to AAS for barium determination in GSR swab extracts.

Analysis of Gunshot Primer Residue Collection Swabs Using Flameless Atomic Absorption Spectrophotometry: A Reexamination of Extraction and Instrument Procedures

Methods of extraction of gunshot residue (GSR) swabs for determination of antimony (Sb), barium (Ba), and lead (Pb) by flameless atomic absorption spectrophotometry (AAS) have been studied. Optimum extraction parameters were determined using recovery studies of Sb, Ba, and Pb added to swabs at amounts representative of actual GSR and handblank levels. Two difficulties with most extraction-AAS procedures are incomplete extraction of Sb and incorrect Ba results arising from improper matching of sample and standard matrices before AAS determinations. Utilization of standards made by spiking analyte elements on swabs and extraction along with samples by an efficient procedure minimizes errors in Sb, Ba, and Pb determination.

Validation of Tool Mark Analysis of Cut Costal Cartilage

Abstract:  This study was designed to establish the potential error rate associated with the generally accepted method of tool mark analysis of cut marks in costal cartilage. Three knives with different blade types were used to make experimental cut marks in costal cartilage of pigs. Each cut surface was cast, and each cast was examined by three analysts working independently. The presence of striations, regularity of striations, and presence of a primary and secondary striation pattern were recorded for each cast. The distance between each striation was measured. The results showed that striations were not consistently impressed on the cut surface by the blade’s cutting edge. Also, blade type classification by the presence or absence of striations led to a 65% misclassification rate. Use of the classification tree and cross‐validation methods and inclusion of the mean interstriation distance decreased the error rate to c. 50%.

Barium and antimony distributions on the hands of nonshooters

Barium and antimony levels from selected areas of the left and right hands of 269 nonshooters provide a database for interpretation of gunshot residue swab analysis results. The database represents a variety of activities of individuals sampled by collectors throughout the United States. Nonshooting exposure to barium and antimony can generally be distinguished from firearms-associated exposure by considering the relative levels of the elements, location on the hands, and condition of the swabs. Consistent definition of sampling procedures and accurate analytical results make this database applicable for interpretation of data generated by most gunshot residue swab examiners.

Analysis of Gunshot Primer Residue Collection Swabs Using Flameless Atomic Absorption Spectrophotometry and Inductively Coupled Plasma-Atomic Emission Spectrometry: Effects of a Modified Extraction Procedure and Storage of Standards

Two modifications to a previously reported extraction procedure for determination of antimony, barium, and lead in gunshot primer residue swab extract solutions have been studied. Removal of cotton tips from swabs by cutting their shafts results in extraction efficiencies for all elements that compare favorably with those obtained by complete removal of the cotton from the swab shaft. Swab-mounted standards can be made and stored in a dry condition for periods of at least four months before use without adverse effects on analytical results. Long-term relative precision estimates for the combined extraction and analysis are approximately 5% for all three elements.

Microscopic Saw Mark Analysis: An Empirical Approach

Microscopic saw mark analysis is a well published and generally accepted qualitative analytical method. However, little research has focused on identifying and mitigating potential sources of error associated with the method. The presented study proposes the use of classification trees and random forest classifiers as an optimal, statistically sound approach to mitigate the potential for error of variability and outcome error in microscopic saw mark analysis. The statistical model was applied to 58 experimental saw marks created with four types of saws. The saw marks were made in fresh human femurs obtained through anatomical gift and were analyzed using a Keyence digital microscope. The statistical approach weighed the variables based on discriminatory value and produced decision trees with an associated outcome error rate of 8.62–17.82%.

Forensic Comparison of Household Aluminum Foils Using Elemental Composition by Inductively Coupled Plasma—Atomic Emission Spectrometry

Inductively coupled plasma-atomic emission spectrometry (ICP-AES) provides the precision, sensitivity, and specificity for element concentration determinations required for good discrimination among sources of household aluminum foil. The concentrations of copper (Cu), gallium (Ga), iron (Fe), magnesium (Mg), manganese (Mn), titanium (Ti), vanadium (V), and zinc (Zn) were determined in aluminum foil by ICP-AES with analytical precisions (rsd) in the 1 to 2% range and nickel (Ni) and silicon (Si) with analytical precisions of about 5%. For these ten elements, the combined analytical uncertainties and variations along the length of a single roll of foil are much smaller than the variations from roll to roll, both within a single brand and across brands. The aluminum foils in this study exhibit measurable roll-to-roll compositional variation across the different brand names as well as across those rolls produced by the same manufacturer in different production runs. In some instances, rolls of foil produced in a single production run have analytically distinguishable compositions. Thus, a high degree of discrimination capability exists among sources of household aluminum foil.

Feature selection for best mean square approximation of class densities

Abstract A criterion for feature selection is proposed which is based on mean square approximation of class density functions. It is shown that for the widest possible class of approximants, the criterion reduces to Devijers Bayesian distance. For linear approximants the criterion is equivalent to well known generalized Fisher criteria.

Should Charity Air Medical Organizations Require Commercial Certification of Their Pilots

OBJECTIVE Fixed wing medical transportation crashes operating under 14 CFR Part 91 show higher fatal outcomes than nonmedical Part 91 flights. Advanced certification may translate into increased safety, yet we know of no charity air medical transportation requiring such certification. Herein, in a retrospective study, we determined whether commercial certification is associated with a reduced fatality rate compared with the less stringent private pilot certificate and accident causes. METHODS The National Transportation Safety Board accident database was queried for fatal accidents in single-engine aircraft occurring between 2002 and 2012. Poisson and proportion tests were used in statistical analyses. RESULTS For the period spanning 2002-2012, commercial pilots showed a lower fatality rate. Under visual meteorologic conditions, aerodynamic stall was a frequent cause for fatal accidents affecting both airman cohorts equally. For operations in instrument meteorologic conditions, fatal accidents were most commonly attributed to instrument approach deficiency and spatial disorientation. At night, failure to maintain obstacle/terrain clearance was the most prevalent cause of fatal crashes. CONCLUSION Our data suggest that charity air medical transportation organizations should encourage their pilots to acquire commercial certification. Furthermore, our study indicates areas in which general aviation training/currency should be directed to reduce fatal accidents.