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Trace Evidence

The Trace Unit of the Physical Evidence Section is responsible for the examination of small (trace) particles of evidence left behind during a crime such as hair, fiber, paint, glass, fire debris, explosives, gunshot residue, and other miscellaneous particles.
This kind of evidence is often submitted in response to violent crimes including rape, homicide, robbery, arson, and hit-and-run incidents.
In the analysis of these trace materials, several different instruments and methods are used by the Forensic Scientists, as described below.

Infrared Spectroscopy 
The Fourier Transform Infrared Spectrophotometer (FTIR) instruments are equipped with microscopes and Attenuated Total Reflectance (ATR) accessories, and are primarily used to analyze fibers, paints, polymers, and explosives.

Inductively Coupled Plasma / Mass Spectroscopy
An Inductively Coupled Plasma / Mass Spectrometer (ICP/MS) analyzes for the presence of metal components associated with gunshot residue (GSR).  

Gas Chromatography -Flame Ionization Detection/Mass Spectroscopy
The Gas Chromatograph-Flame Ionization Detector (GC-FID) and the Gas Chromatograph / Mass Spectrometer (GC/MS) are used to analyze fire debris samples, volatile and semi-volatile compounds, and light products such as alcohols, acetone, and other solvents that may be submitted as evidence in connection to a crime.

Pyrolysis - Gas Chromatography / Mass Spectroscopy
A manual injection Pyrolysis unit is used with a Gas Chromatograph with Mass Selective Detector (Py-GC/MS)to analyze evidence such as paints, fibers, and polymers.

Glass Refractive Index Measurement System
The Glass Refractive Index Measurement System (GRIM) measures the refractive index of glass fragments using the oil-immersion method, a process that exploits the variation of refractive index with temperature. 

Scanning Electron Microscopy
A Scanning Electron Microscope/Energy Dispersive X-ray system (SEM/EDX) is used for elemental analysis and high resolution and magnification imaging with enhanced depth of field.  The process results in a non-destructive analysis of gunshot residue particles, paints, metals, powders, and other trace materials.

X-Ray Fluorescence
The Energy Dispersive X-ray Fluorescence (XRF) analyzer is used for the non-destructive elemental analysis of trace evidence such as glass, larger metal fragments, powders and certain liquids.

The Microspectrophotometer is employed to measure spectral (color) differences in polymeric samples such as paints and fibers.

Microscopy is used to examine evidence in nearly every analysis and in each sub-discipline in the Trace Unit.  Stereomicroscopes are utilized extensively when searching through debris and tapings for hairs, fibers, paint, and glass; Polarizing light microscopes (PLMs) are necessary when analyzing certain properties of fibers; and Comparison microscopes are essential when comparing the microscopic properties of questioned and known samples of hairs and fibers.

Physical  Evidence Section - Trace Unit:   Forensic Scientist Manager Dave Freehling