Glass Analysis

 

Glass is often encountered in many types of forensic cases. It is important when collecting glass samples that all the known glass from a window or container be collected in order to enhance the probability of a “physical match” between known and questioned glass. A physical match describes a physical fit between glass from two pieces, which is the only scenario whereby a scientist can conclusively determine that two pieces of glass originated from the same source.

Measurement and observation of thickness, color, fluorescence and refractive index are methods traditionally employed in the forensic examination of glass. The refractive index is the ratio of the speed of light in a vacuum over the speed of light through the glass and is measured by immersing the glass in oil of known refractive index and altering the temperature of the oil until the refractive indices of both the oil and the glass are the same. This point is determined microscopically by noting the temperature where the “becke line”, a halo of light surrounding the glass, disappears.

Because of more stringent quality control guidelines in the glass manufacturing industry, the refractive indices of glass are becoming less variable and therefore a less probative factor for comparison. For this reason, some forensic scientists are now utilizing methods of comparing trace elements present in glass. These methods include inductively coupled plasma mass spectrometry (ICP-MS) and scanning electron microscopy-energy dispersive x-ray spectrometry (SEM-EDS). It has been determined that comparison of both optical properties and elemental profiles results in a much more probative examination than comparison of optical properties alone. Our scientists have been active in development of forensic methods for the elemental analysis of glass samples.

In glass comparisons, and well as many other types of forensic analyses, the scientist attempts to exclude questioned and known evidence as having originated from the same source. If all measured properties are consistent between known and questioned samples, and absent a physical match, it is determined that the questioned and known samples could have originated from a common source.

We utilize ICP-MS, considered by most glass examiners as the most discriminating method for glass analysis and comparison. Measurement and comparison of refractive index and other physical properties is also performed in our analytical scheme.



 



 
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