Elemental analysis by energy dispersive x-ray: a significant factor in the forensic analysis of glass.

Eighty-one glass samples were analyzed by EDX; two were indistinguishable. However, when the physical properties of the glass samples were considered, all glass samples were distinguishable from each other. The need to examine glass with respect not only to physical properties but also to elemental composition has been demonstrated. The authors believe that if glass is examined only by density and refractive indexes it is quite possible to conclude incorrectly that glass samples with the same physical qualities could have originated from the same source. To confirm that the glass specimens may have in fact originated from the same source their elemental compositions in addition to density and refractive indexes must be determined. The analysis by EDX of glass samples can selectively eliminate most specimens on the basis of elemental differences. If this step is performed prior to refractive index or density measurements considerable time savings can be realized in casework situations.     

The classification and discrimination of glass fragments using non destructive energy dispersive X-ray microfluorescence

Frequency of analytical characteristics is best estimated on glass recovered at random. However, as such data were not available to us, we decided to use control windows for this estimation. In order to use such a database, one has to establish that the recovered fragment comes from a window. Therefore, elemental analysis was used both for classification and discrimination of glass fragments. Several articles have been published on the subject, but most methods alter the glass sample. The use of non destructive energy dispersive X-ray microfluorescence (microXRF) for the analysis of small glass fragments has been evaluated in this context. The refractive index (RI) has also been measured in order to evaluate the complementarity of techniques. Classification of fragments has been achieved using Fisher's linear discriminant analysis (LDA) and neural networks (NN). Discrimination was based on Hotelling's T2 test. Only pairs that were not differentiated by RI followed by the Welch test were studied. The results show that neural network and linear discriminant analysis using qualitative and semi-quantitative data establishes a classification of glass specimens with a high degree of reliability. For discrimination, 119 windows collected from crime scene were compared: using RI it was possible to distinguish 6892 pairs. Out of 129 remaining pairs, 112 were distinguished by microXRF.     

Analysis of glass fragments by laser ablation-inductively coupled plasma-mass spectrometry and principal component analysis

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is used to differentiate glass samples with similar optical and physical properties based on trace elemental composition. Laser ablation increases the number of elements that can be used for differentiation by eliminating problems commonly associated with dissolution and contamination. In this study, standard residential window and tempered glass samples that could not be differentiated by refractive index or density were successfully differentiated by LA-ICP-MS. The primary analysis approach used is Principal Component Analysis (PCA) of the complete mass spectrum. PCA, a multivariate analysis technique, provides rapid analysis of samples without time-consuming pair-wise comparison of calibrated analyses or prior knowledge of the elements present in the samples. Probabilities for positive association of the individual samples are derived from PCA. Utilization of the Q-statistic with PCA allowed us to distinguish all samples within the set to a certainty greater than the 99% confidence interval.     

Mixed effects modelling for glass category estimation from glass refractive indices

520 Glass fragments were taken from 105 glass items. Each item was either a container, a window, or glass from an automobile. Each of these three classes of use are defined as glass categories. Refractive indexes were measured both before, and after a programme of re-annealing. Because the refractive index of each fragment could not in itself be observed before and after re-annealing, a model based approach was used to estimate the change in refractive index for each glass category. It was found that less complex estimation methods would be equivalent to the full model, and were subsequently used. The change in refractive index was then used to calculate a measure of the evidential value for each item belonging to each glass category. The distributions of refractive index change were considered for each glass category, and it was found that, possibly due to small samples, members of the normal family would not adequately model the refractive index changes within two of the use types considered here. Two alternative approaches to modelling the change in refractive index were used, one employed more established kernel density estimates, the other a newer approach called log-concave estimation. Either method when applied to the change in refractive index was found to give good estimates of glass category, however, on all performance metrics kernel density estimates were found to be slightly better than log-concave estimates, although the estimates from log-concave estimation prossessed properties which had some qualitative appeal not encapsulated in the selected measures of performance. These results and implications of these two methods of estimating probability densities for glass refractive indexes are discussed.     

An investigation into the spatial elemental distribution within a pane of glass by time of flight secondary ion mass spectrometry

Advances in the technology employed for the manufacture of glass have resulted in a final glass product with little variability in terms of its physical and optical properties. For example, the refractive index of Australian float glass tends to lie between 1.5189 and 1.5194. It has therefore become necessary to complement physical and optical methods for forensic glass comparison with instrumental elemental analyses. In a previous study, time-of-flight secondary ion mass spectrometry has been shown to offer potential for the analysis of glass particles as small as a few tens of microns across. In this study, the three-dimensional homogeneity of a sheet of float glass is described, and consequences for forensic elemental analysis of glass particles of such size are explored. Variation in Si, Ca, Mg, and Na levels immediately under the nonfloat surface was observed, with the variance accompanied by a decrease in refractive index.     

Elemental analysis of glass fragments by ICP-MS as evidence of association: analysis of a case

Inductively coupled plasma mass spectrometry (ICP-MS) was used to analyze glass evidence from a case in which a person broke a window in each of 15 vehicles in a parking lot in order to gain entry into the vehicles. The results of the analysis by traditional methods, which measure the properties of color, thickness, density, and refractive index, are also reported. A total of 15 known samples representing the windows on the cars and 42 questioned glass fragments recovered from the suspect and the police vehicle where the suspect was sitting were submitted for analysis. Density comparisons separated one of the known samples into three samples, increasing the number of known samples from 15 to 17. The concentrations of 16 elements were measured for all but three of the samples using an external calibration ICP-MS method with internal standardization. Color assessment (non-instrumental) separated the 17 known samples into two groups, and refractive index measurements resulted in six groups when the Emmons double variation method was used and ten groups when the Glass Refractive Index Measurement 2 (GRIM2) system was used. Elemental analysis, by itself, differentiated all of the known samples from each other and associated four of the known sample fragments with several of the questioned sample fragments. The informing power of RI, density, and elemental analysis comparisons is evaluated and a summary of the case results is reported.     

A survey of glass fragments recovered from clothing of persons suspected of involvement in crime

The refractive indices (RI) of over 2000 glass fragments recovered from items of clothing submitted to the Birmingham and Wetherby Home Office forensic science laboratories in connection with crimes involving broken glass have been measured. The glass fragments have been classified according to their location on the clothing, and a grouping algorithm has been applied to the data.The refractive index distribution was found to be dependent on the location of the glass on the clothing. The significance of the results in the interpretation of glass evidence is discussed.     

The consequences of potentially differing distributions of the refractive indices of glass fragments from control and recovered sources

In forensic cases involving glass evidence, the variance of the recovered glass refractive index (RI) has been observed to be larger than the variance of the control glass RI. This has consequences for subsequent interpretation. To investigate this phenomenon, a study was made of the probable distributions of refractive indices of the recovered glass given a range of casework type scenarios involving breakage and backscatter. An investigation of the consequences of any distribution differences with regard to casework was also made. It was discovered that the inclusion of surface fragments from the float surface of the glass can have a profound effect on the comparison of recovered and control glass samples. The effect was largest when a breaker was striking the float surface of a window. In particular, it was found that the inclusion of a few surface fragments even post grouping is sufficient by itself to explain the observed differences in variance of RI between control and recovered groups. Surface fragments, if present, are very likely to be treated as outliers and to be deemed as non-matching. The findings of this paper challenge routine glass examination procedure and suggest that knowledge of which surface is facing the striker is valuable information in interpretation.     

The effect of re-annealing on the distribution of refractive index in a windscreen and a windowpane classification of glass samples

Tiny glass fragments of two typical objects being of interest in forensic investigations: car windscreen and windowpane were examined from the point of view of their importance as crime evidence. Both examined objects were made from float glass and were toughened. The present paper concerns examination of refractive index distribution across the objects under investigation before and after the glass fragments were annealed according to previously chosen procedure. The annealing procedure was carried out in order to increase discrimination power of refractive index (RI). The following conclusions can be drawn from the results obtained. For both examined objects the mean RI was significantly higher after annealing and, at the same time, standard deviations in RI were smaller. The distributions of RI for both examined objects appeared not to be normal; the deviations from normality were observed at both sides of RI distributions. It was found that the difference in the values DeltaRI (difference between mean values of RI after annealing and before annealing) for both examined object was not significant and thus it would be not a good parameter to differentiate between two heat strengthen objects. The attempt to classify 181 glass samples on the basis of theirs RI and DeltaRI was performed. Increased discrimination of glass samples was observed.     

汽车风挡玻璃的鉴别方法研究

目的研究油浸法和X射线荧光光谱法对汽车风挡玻璃的鉴别能力。方法应用油浸法对30个风挡玻璃样品进行折射率测试，并将所得数据进行t检验分析。对不能区分的样品再用X射线荧光光谱进行元素成分分析。结果30个样品共组成的435对中，折射率t检验在置信度959／5时区分427对，其余8对通过元素成分实现区分。结论折射率测试和元素分析相结合，可对常见汽车风挡玻璃进行有效区分。     

Defining event reconstruction of digital crime scenes

Event reconstruction plays a critical role in solving physical crimes by explaining why a piece of physical evidence has certain characteristics. With digital crimes, the current focus has been on the recognition and identification of digital evidence using an object's characteristics, but not on the identification of the events that caused the characteristics. This paper examines digital event reconstruction and proposes a process model and procedure that can be used for a digital crime scene. The model has been designed so that it can apply to physical crime scenes, can support the unique aspects of a digital crime scene, and can be implemented in software to automate part of the process. We also examine the differences between physical event reconstruction and digital event reconstruction.     

The effect of annealing on the variation of glass refractive index

The variation of refractive index (RI) over a non-toughened, float pane of glass and a toughened, float pane of glass was investigated. The two panes of colourless, float glass were cut into 150 5 cm × 5 cm squares. The pre- and post-annealing RI values from three random areas from each square were measured. Bayesian statistical hierarchical modelling of the results showed that for the non-toughened, float glass pane annealing increased the variability in RI by a factor of 1.29-1.58, with a mean of 1.43 (with 95% probability); and for the toughened, float pane of glass annealing decreased the variability in RI by a factor of 0.63-0.76, with a mean of 0.69 (with 95% probability). In addition it was found that although there were no systematic differences in ΔRI across either pane of glass, there were observable differences across both panes of glass. These results provide information regarding the expected RI variation over entire panes of both non-toughened and toughened float window glass for both pre- and post-annealing RI measurements.     

The role of isotope ratio mass spectrometry as a tool for the comparison of physical evidence

This paper considers how likelihood ratios can be derived for a combination of physical, chemical and isotopic measurements. Likelihood ratios were formulated based on the characteristics of a small convenience sample of 20 duct tapes. The propositions considered were:

• H_ptwo samples are from the same batch
• H_dtwo samples are from different batches.

The physical and isotopic characteristics of ten rolls of duct tape were shown to be consistent throughout each roll.The width and thickness of the tapes and the density of the scrim fibres provided equivalent information and the combined physical characteristics provided a basis upon which to discriminate between many of the samples.Scatter-plots and confidence ellipses provided a convenient method to group the isotopic composition of the tape backing material and provided a basis to discriminate between samples which were physically indistinguishable. Considering both the physical and isotopic characteristics it was possible, at best, to ascertain that the evidence provided moderately strong support for the proposition that two samples of tape were derived from the same batch (LR = 400).Kernel density estimates were used to model the distribution of isotopic compositions of the backing material. Using this technique it was possible to estimate objectively the probability that a sample with given characteristics could be drawn, at random, from the background population and to calculate a likelihood ratio based on the propositions above.The strength of evidence which could be presented by either model was ultimately limited by the size of the background sample.     

油浸法鉴别汽车后视镜玻璃的研究

目的研究油浸法对汽车后视镜玻璃的鉴别能力。方法应用油浸法对20种汽车后视镜玻璃进行折射率值测定并用t检验法对数据进行处理。结果在置信度为95％时，20个后视镜玻璃样品的区分率为97．9％。结论油浸法对汽车后视镜玻璃有很好的区分能力，而且使用样品量少，可以达到微克级，是一种鉴别后视镜玻璃的有效方法。     

5.6. Hochauflösende ultradünnschicht-isoelektrofokussierung zur differenzierung der PGM1-subtypen in der forensischen blutgruppenkunde

The analytical data (five elemental concentrations plus refractive index) for 349 glass samples have been assessed for the purpose of glass classification by the application of multivariate statistical procedures. The requirements of a classification scheme for glass samples are discussed, and such a scheme for unknown samples in presented. The use of this scheme to classify a glass sample involves a comparison of the glass analysis with a reference collection of analyses. By identifying glasses of similar composition to the unknown it is possible in many cases to classify the sample as sheet, container, tableware, headlamp, etc. The results are expressed as definite classifications, suggested classifications and as unclassifiable. The classification scheme was assessed by means of a blind trial set up within the Metropolitan Police Forensic Science Laboratory. For the type of sample occurring most often in casework the scheme produced a rate of successful definite classifications of 91%. No wrong classifications were obtained in the trial.     

平板玻璃受热后折射率的变化规律初探

目的探索平板玻璃受热后折射率的变化规律。方法首先采用差示扫描量热仪测定玻璃样品的转变温度Tg,将样品加热至不同的温度(温度范围从室温到Tg以上),自然冷却至室温后再用GRIM3测量折射率。结果加热温度在Tg以下时,玻璃的折射率基本不变,加热温度在Tg以上时,折射率有明显变化。结论对于从火灾和爆炸现场提取的玻璃,在进行折射率比对时应当考虑现场高温对测量值的影响,并进行必要的修正。     

Spatial variation of refractive index in a pane of float glass.

In the statistical interpretation of forensic glass evidence it is standard practice to make the assumption of homogeneity of the refractive index (RI) of the source glass, or of localized homogeneity. However, the work of Locke and Hayes showed that, for toughened windscreen glass, this assumption might not be true. This work is well cited, but there appears to have been little follow-on published research. Furthermore, the toughening process is something known to affect the refractive index, and is a process that float glass does not undergo. Float glass is a major component of casework in New Zealand and for that reason it would be interesting to know whether the findings of Locke and Hayes apply when dealing with float glass. In this paper we describe an experiment similar to that of Locke and Hayes, systematically examining the variation of RI in a pane of float window glass. It was found that, although there were no systematic differences in refractive index, there were observable differences across the pane.     

Classification of Glass Fragments Based on Elemental Composition and Refractive Index*

Abstract: The aim of this study was to assess the efficiency of likelihood ratio (LR)‐based measures when they are applied to solving various classification problems for glass objects which are described by elemental composition, and refractive index (RI) values, and compare LR‐based methods to other classification methods such as support vector machines (SVM) and naïve Bayes classifiers (NBC). One hundred and fifty‐three glass objects (23 building windows, 25 bulbs, 32 car windows, 57 containers, and 16 headlamps) were analyzed by scanning electron microscopy coupled with an energy dispersive X‐ray spectrometer. Refractive indices for building and car windows were measured before (RI_b), and after (RI_a) an annealing process. The proposed scheme for glass fragment(s) classification demonstrates some efficiency, although the classification of car windows (c) and building windows (w) must be treated carefully. This is because of their very similar elemental content. However, a combination of elemental content and information on the change in RI during annealing (ΔRI = RI_a?RI_b) gave very promising results. A LR model for the classification of glass fragments into use‐type categories for forensic purposes gives slightly higher misclassification rates than SVM and NBC. However, the observed differences between results obtained by all three approaches were very similar, especially when applied to the car window and building window classification problem. Therefore, the LR model can be recommended because of the ease of interpretation of LR‐based measures of certainty.     

The interpretation of refractive index measurements. III

The question of whether or not to group recovered glass fragments before comparison with a control sample is fundamental to the interpretation of refractive index measurements. A computer program has been written to perform the grouping and explore the consequences. Grouping the fragments is shown to increase the chances of finding all recovered fragments similar when they have in fact come from the same source as the control, and to give enhanced discrimination when the recovered glass has come from a different source. When the recovered fragments have come from two different sources the consequences of the grouping procedure are preferable to non-grouping.     

DO NEIGHBORHOODS GENERATE FEAR OF CRIME? AN EMPIRICAL TEST USING THE BRITISH CRIME SURVEY*

For a long time, criminologists have contended that neighborhoods are important determinants of how individuals perceive their risk of criminal victimization. Yet, despite the theoretical importance and policy relevance of these claims, the empirical evidence base is surprisingly thin and inconsistent. Drawing on data from a national probability sample of individuals, linked to independent measures of neighborhood demographic characteristics, visual signs of physical disorder, and reported crime, we test four hypotheses about the mechanisms through which neighborhoods influence fear of crime. Our large sample size, analytical approach, and the independence of our empirical measures enable us to overcome some of the limitations that have hampered much previous research into this question. We find that neighborhood structural characteristics, visual signs of disorder, and recorded crime all have direct and independent effects on individual‐level fear of crime. Additionally, we demonstrate that individual differences in fear of crime are strongly moderated by neighborhood socioeconomic characteristics; between‐group differences in expressed fear of crime are both exacerbated and ameliorated by the characteristics of the areas in which people live.