[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

Terry et al 706 pioneered use of the semedx method on

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Terry et al. [706] pioneered use of the SEM/EDX method on its own for the determination of elemental composition of different groups of glass used in Australia. Currently, SEM/EDX and SEM/WDX are applied in the rou- tine examination and evaluation of glass evidence in the majority of forensic laboratories. For qualitative determination, a glass fragment is placed on a stub and, after sputtering with carbon, assuring its conductivity, the elemen- tal composition is determined. In the case of quantitative analysis, sample preparation is required, involving embedding the glass fragment in a plastic resin and then polishing the surface until it is flat, using grinding methods.
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Methodological Developments and Applications 723 The surface is usually coated with a carbon layer and the fragment is sampled at different locations. Glass samples for examination by the XRF method are prepared in a similar manner. The technique of X-ray fluorescence is widely used for the qualitative and quantitative analysis of elements having atomic numbers greater than oxy- gen. Instrumentation to detect the emission can be divided into two types, wavelength dispersive and energy dispersive. Quantitative elemental analysis is possible thanks to the linear correlation between the intensity of the char- acteristic X-radiation of the element generated in the sample by electrons and the concentration of this element. This is a relative method involving com- parison of the measured intensity of the characteristic radiation of a given element in the sample with the intensity of radiation measured for a reference (standard) sample. In practice, quantitative analysis of forensic glass samples is best achieved by an evaluation of the ratios of elements rather than by measurement of ab- solute concentrations. Very small and irregularly shaped samples, which are commonly found in forensic casework, are not suitable for this type of analy- sis. Samples with flat surfaces and known working angles are necessary for quantitative determinations. Quantitative analysis with or without standards can be used for the analysis of elements such as Na, Mg, Al, Si, K and Ca, assuming the rest of the sample matrix is oxygen. The measurement of major, minor and trace elemental composition of glass is very important (valuable) for its discrimination and classification into glass types. It is usually helpful to be able to classify the questioned glass into one of a number of possible categories, such as sheet, container, vehicle window, vehicle headlamp or tableware. It is necessary to apply statistical methods in the characterization of glass evidence by its elemental composition. Traditional treatment of the data involves determining the mean concen- tration and the standard deviation for each element and then comparing the means using a “3 sigma rule” or testing the match criteria with a strict range overlap for each of the elements. Advanced methods of comparison use statis- tical tests, cluster analysis or Bayesian approach [694–696, 707].
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  • Spring '14
  • MichaelDudley

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