Scanning Electron Microscope (SEM) - GSR Cases

The use of Scanning Electron microscopy to positively analyze gun shot residue has become a topic of debate within the judicial, legal and scientific community. The controversy stems from various cases which exhibit the method as being unreliable. In particular, the events in Baltimore city exhibit some of the major issues associate with the use of scanning electron microscopy in gun shot residue analysis.

The world of biology bloomed when Knoll and Ruska invented the Electron microscope in 1933. The electron microscope used electrons instead of light, further enhancing the limit of resolution that limited light microscopes. The electron microscope ushered ion a new area where not only the general features of a cell can be observe, but its components, even reaching the point of distinguishing individual compounds. Aside from its impact on the field of biology, the electron microscope, in particular the scanning type has been employed to some degree of success in forensics. (Vermeij)

Forensics deals with the interpretation of evidence to solve a crime. It evolved from simple articles of personals belongings to todays use of technology to detect biological evidence, chemical traces and gunshot residues. (Saks  Faigman, 2008)

This paper aims to elucidate upon the use of the scanning electron microscope in forensics, particularly in solving cases by interpreting gunshot residue.

In contrast to light microscopy, electron microscopy employs a cathode filament as a source of radiation. Electromagnetic coils instead of glass lenses are used to direct the electron beams through a vacuum. Electrons are then directed to the specimen or substance by means of a condenser coil. The image produced by the electrons is then further magnified by the objective coils. Viewing of the image is done by conversion of the electrons to electronic signals that are translated into images through the use of either a cathode ray tube or a liquid crystal display monitor. (Vermeij)

Image formation however, differs in different types of electron microscopes. In transmission electron microscopy, image generation is based on differential electron absorbency. It is highly utilized in the biological sciences to obtain a cross section of cells and other molecules. It uses the degree of penetration of electrons upon the specimen to form an image of its inside structure. (Burnett, 2007)
The scanning electron microscope on the other hand, employs a different approach in using electrons to generate images. Scanning electron microscopy employs differential electron scattering to form an image of a specimens outside surface. (Vermeij) The release of secondary electrons upon striking of primary electrons on the surface of the specimen is passed unto a scintillator. The scintillator converts these electrons into light signals which are then amplified and projected unto a screen.   The resulting image would show bright regions representing elevated surfaces and dark regions representing low ones.

Scanning Electron Microscope in Forensics
The use of the scanning electron microscope in forensics can be traced back in 1968 in London. It was first used by the Metropolitan Police Forensic Laboratory to manually detect and analyze gun shot residue. Today, several hundred scanning electron microscopes are in use in various forensic laboratories around the world and most are still primarily used to analyze gun shot residue.  (Vermeij)

The use of the scanning electron microscope in forensics is derived from it being capable of performing imaging as well as elemental analysis digitally and automatically. Most forensic applications are found in gunshot radio and explosive residues, both of which possess monochromatic particles of heterogenic character. (Stone  Petty, 1974) Other forensic applications include fiber and biological residue. (Vermeij)

Gunshot residue is primarily composed of particles coming from the combustion of the bullet charge and primer as well as scrapings from the firearm barrel. Firing a gun would blow back some of these particles on to the shooters hand and other parts of the body. The particles can be obtained using adhesive material or by obtaining a part of the surface to be investigated.  (Romolo  Margot, 2001)

The detection of primer residues can be divided into analytical and qualitative Analytical methods usually rely on atomic absorption spectrophotometry as well as inductively coupled plasma-atomic emission spectrometry.

Qualitative analysis of gun shot residue usually involves both SEM- EDS( Energy Disruptive X-Ray Spectrometry) and atomic force microscopy. Todays GSR scanning usually uses automated SEM programs to test for heavy metals and other particles on the residue.  The detection of gunshot residue (GSR) using scanning electron microscopy with electron dispersive X-rays (SEM-EDX) technology was developed by J. E. Wessel, P. F. Jones, Q. Y. Kwan, R. S. Nesbitt and E. J Rattin in 1974. The scanning electron microscope is utilized to obtain a picture of the surface topography of the substance obtained, while X-ray spectrometry is used to obtain the chemical composition found in the particle.  (Vermeij)

Integrity of using SEM-EDS in analyzing gun shot residue
The standards of isolating and positively identifying gunshot residue on a subject have been a subject of debate since most of them have shifted through time. Prior to 2000, the standard of gunshot residue was having the presence of barium and antimony. However, it must be noted that firearms are not the only occurring source of these two elements in the environment. In 2002, the standard was set to a combination of barium, antimony and lead that were fused together, forming a single particle. Again, this was criticized since this combination of elements can also be found in car products. Also, lead is present in a large quantity of sources in the environment, thus contamination of residue is an inherent risk. (Romolo  Margot, 2001) The advent o new types of weapons utilizing different elements also complicate the issue of what to look for in making a positive identificatio for gunshot residue.
Being a qualitative study, the use of SEM-EDS requires highly skilled and proficient personnel to avoid lapses in interpreting results. The most common error is the assumption that residue containing lead, antimony and barium are automatically gunshot residue and the additional assumption that any residue found in the scene of the crime came from the weapon used.  (Nethercott  Thompson, 2005)

Perhaps one of the most prominent events involving the inherent difficulty in using gun shot residue as a basis of criminal persecution was the Baltimore GSR debacle. In2001, the Baltimore Sun published a story dealing with the occurrence of high amounts of gun shot residue contamination in police station where suspects and witnesses were led to be tested for gun shot residue. It was extrapolated that most of these reside was from gun ranges near the stations. Internal testing was done by the Baltimore police department and one station was remanded to not house testing facilities for gun shot residue. It was highly suspected that the internal decision was prompted by the occurrence of false positives by suspects and witnesses tested within the area.  (Bykowicz, 2005) The problem was also compounded when it was also found out the that the police officers themselves as well as their equipment contained high amounts of gun residue contamination, even while processing both witnesses and suspects.

But the worst problem within the area was the misinterpreting of the SEM-EDS results by the analyst. Evidence as to the presence of gun shot residue is usually introduced into a court room by a sworn testimony of the analyst, or a signed account. The analyst in this case identified the presence of barium and antimony as a positive for gunshot residue in 93 cases between 2003 and 2004, several years after the standard to include lead as a standard for GSR identification was used.

The Baltimore series of events highlighted the key disadvantages of using GSR testing by SEM-EDS. First, the standards of positive testing are still arbitrary and debated.  The three key elements, lead, antimony and barium can be found in other sources, with lead being found in large quantities in some environments. To really identify a sample as being unique, careful analysis must be performed not only on the types of substances present but also on their quantity and morphology. This brings us to the second problem, competence of analysts. Analysis of samples must be done in careful condition, starting from collection, to transport up until processing unto the SEM-EDS.

Perhaps the most relevant action to be done to address the use of SEM in gun residue testing is its weight as evidence in the courtroom. Since gun shot residue affects a certain area around the fire arm, it could not be a reliable basis in separating victim, shooter and witness. The use of GSR in criminal proceedings must still be corroborated by other forensic evidence.


Post a Comment