Chemical Examination of Glass Fragments

Glass is one of the most frequently submitted and important materials for forensic trace evidence analysis.

Glass is commonly found at crime scenes, especially those involving car accidents, car theft, and burglaries. Glass fragments that are likely to remain on clothing for an extended period of time are extremely stable. They don’t degrade or change over time like biological evidence.

Glass has different physical and chemical properties due to different manufacturing methods and variations in raw material composition. Even two seemingly identical glass sources can be distinguished using sensitive and highly discriminating techniques.

Also Read: Glass (Physical Evidence)

Glass fragments chemical composition can be determined by a variety of Instruments:

▪️Scanning Electron Microscopy–Energy Dispersive X-ray Spectrometry (SEM-EDX)

▪️Laser-Ablation Sampling (LA-ICP-MS).

▪️Inductively Coupled Plasma–Optical Emission Spectrophotometry (ICP-OES)

▪️Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), and,

▪️X-ray Fluorescence Spectrometry (XRF)

Also Read: Glass Fracture Patterns

▪️Scanning Electron Microscopy:

A focused electron beam is scanned over the surface of a sample in scanning electron microscopy, causing, among other things, the emission of X-rays. The intensity of the X-ray peaks in the measured spectrum correlates with the quantities of each element present in the sample area exposed to the electron beam, and the wavelengths or energies of the detected X-rays are used to identify the elements.

▪️Laser Ablation ICP-MS:

Laser ablation– inductively coupled plasma–mass spectrometry (LA-ICP-MS) is a relatively new technique that offers significant advantages over all previous methods. It can determine the elemental composition of a 0.5 mm fragment. A laser beam vaporises a small spot on the sample, which is then carried to an ICP furnace by a stream of inert gas, where the elements in the plasma sample are identified using MS.

The crater created in the LA step is about 75 mm in diameter, or less than the width of a human hair. The technique preserves the majority of the sample, can process very small fragments, is extremely effective at distinguishing between samples of the same type of glass, and can distinguish between different types of glass. However, the cost of the equipment, as well as the limited number of cases in which it can be used, do not justify its widespread use at this time.

▪️Inductively Coupled Spectrophotometry:

In most ICPs, an electrical discharge is started in a flowing stream of inert gas, usually argon, and then sustained by a radio frequency field around it. With temperatures in the range of 7000–10,000 K, the resulting stable discharge, or plasma, resembles a small, continuously glowing flame. When a sample is introduced into plasma, it undergoes extensive atomization, ionisation, and excitation. The ions and atoms in the sample emit light at specific wavelengths as they enter cooler parts of the plasma and drop to lower excited states. This emission is dispersed with a spectrophotometer in an ICP-OES, and its intensity is measured.

▪️ICP Mass Spectrometry:

An awesome ionisation device is the inductively coupled plasma torch. Instruments that combine inductively coupled plasma as an ion isolator and detector with mass spectrometry as an ion detector have shown improved analytical capabilities suitable for glass fragment analysis. With single or multiple electron multiplier detectors, mass spectrometry instruments can be quadrupole, time-of-flight, or magnetic-sector designs.

▪️X-Ray Fluorescence Spectrometry:

X-ray fluorescence spectrometry is an elemental analysis technique that measures the characteristic X-rays emitted from a sample after it has been excited by an X-ray source. The detected X-ray energies or wavelengths are used to identify the elements, and the intensities of the X-ray peaks in the measured spectrum correlate with the amounts of each element present in the sample area exposed to the X-ray beam.

Sources & Differences
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  • Houck, M. M. (2004). Trace evidence analysis: more cases in mute witnesses. San Diego, CA: Academic Press.
  • Koons, R. D., Buscaglis, J., Bottrell, M., & Miller, E. T. (2002). Forensic glass comparisons. In R. Saferstein (Ed.), Forensic science handbook (2nd ed., Vol. 1). Upper Saddle River, NJ:Prentice-Hall.
  • Petraco, N., & Kubic, T. (2004). Color atlas and manual of microscopy for criminalists, chemists, and conservators. Boca Raton, FL: CRC Press.
  • Scientific Working Group for Materials Analysis (SWGMAT). (2005). Elemental analysis of glass. Forensic Science Communications, 7(1). http://www.i.gov/about-us/lab/forensic-science-communications/fsc/jan2005/ standards/2005standards10.htm