The Forgotten Evidence

By McGinn, Terrence | Law & Order, November 2002 | Go to article overview

The Forgotten Evidence


McGinn, Terrence, Law & Order


The analysis of gunshot residue (GSR) utilizing Scanning Electron Microscopy interfaced with energy dispersive x-- ray (SEM/EDX) has undergone extensive research and development since its inception into the forensic world. At present it appears to be the most reliable technique for detection and identification of GSR. This is due to its high degree of sensitivity and specificity.

The American Society of Testing Materials defines GSR as that residue formed by the condensation of the vaporized bullet and primer materials that segregate into metallic and compound particles. In order to qualify as GSR, the elemental content of these particles must be: lead, antimony and barium; or antimony and barium. The morphology of each particle must also be considered. These metallic particles must be spheroidal in appearance and range in diameter from 0.5 microns to 50 microns-a micron is a millionth of a meter.

When a weapon is discharged, the components of a primer undergo an intense exothermic reaction: there is a rapid increase in temperature followed by a sharp decrease. This super-- cooling phenomenon leads to a unique spheroidal formation that is generally not observed in the natural environment. In other words, these metals could be detected on the hands of an individual who did not recently discharge a firearm. However, these particles would not be spheroidal and the concentration of these particles would be extremely small compared to that found in GSR.

The Scanning Electron Microscope (SEM) allows observation of each particle for these morphological characteristics. These conditions must be met to satisfy the elements of the definition and to conclusively report the presence of GSR.

The SEM is a complex but straightforward analytical instrument. The instrument utilizes a focused beam of electrons upon the specimen. Simultaneously, several electron probe-- induced signals are produced from the specimen. These signals generate images of the specimen surface. Material composition is also determined by an x-ray. There are many parameters within the instrument's controls that can be regulated. They are varied based upon the sample specimen and the data being sought in the analysis.

The major primer components in a center-fire cartridge are antimony sulfide, barium nitrate and lead styphnate. These compounds, as well as their relative amounts, will vary based upon brand and caliber. Thus, the analyses of each will yield different results, both qualitatively and quantitatively. In addition, numerous supporting metallic particles- zinc, copper or nickel- can and will most likely be observed. The interpretation of all the data is significant when rendering the results. With regard to law enforcement, the qualitative analysis (whether GSR is present) is of greatest importance.

There are several field or color tests available to law enforcement for the detection of possible GSR. Color tests are presumptive tests that are indicative of a certain component or components of the residue produced from the discharge of a firearm. The first is the paraffin test for nitrates, a common byproduct of the discharge of a firearm. However, there are numerous everyday chemical compounds that contain nitrates. Some of these nitrate-containing compounds are fertilizer, urine, certain pharmaceuticals, matches and explosives.

A second test is the modified Griess test, which reacts with nitrites, another byproduct of a firearm discharge. Nitrites are used extensively in the manufacture of dyes, photographic chemistry and as preservatives in meats. Another procedure is the sodium rhodizonate test, which is presumptive for lead and barium, both of which can be found frequently in the environment. Lead is found in numerous construction materials such as piping, solder, aerosols, batteries and pigments. Barium is found in road flares, paints and ceramics.

The problem with these color tests is that they cannot fulfill the criteria to meet the GSR definition. …

The rest of this article is only available to active members of Questia

Already a member? Log in now.

Notes for this article

Add a new note
If you are trying to select text to create highlights or citations, remember that you must now click or tap on the first word, and then click or tap on the last word.
One moment ...
Default project is now your active project.
Project items

Items saved from this article

This article has been saved
Highlights (0)
Some of your highlights are legacy items.

Highlights saved before July 30, 2012 will not be displayed on their respective source pages.

You can easily re-create the highlights by opening the book page or article, selecting the text, and clicking “Highlight.”

Citations (0)
Some of your citations are legacy items.

Any citation created before July 30, 2012 will labeled as a “Cited page.” New citations will be saved as cited passages, pages or articles.

We also added the ability to view new citations from your projects or the book or article where you created them.

Notes (0)
Bookmarks (0)

You have no saved items from this article

Project items include:
  • Saved book/article
  • Highlights
  • Quotes/citations
  • Notes
  • Bookmarks
Notes
Cite this article

Cited article

Style
Citations are available only to our active members.
Buy instant access to cite pages or passages in MLA, APA and Chicago citation styles.

(Einhorn, 1992, p. 25)

(Einhorn 25)

1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

Cited article

The Forgotten Evidence
Settings

Settings

Typeface
Text size Smaller Larger Reset View mode
Search within

Search within this article

Look up

Look up a word

  • Dictionary
  • Thesaurus
Please submit a word or phrase above.
Print this page

Print this page

Why can't I print more than one page at a time?

Help
Full screen

matching results for page

    Questia reader help

    How to highlight and cite specific passages

    1. Click or tap the first word you want to select.
    2. Click or tap the last word you want to select, and you’ll see everything in between get selected.
    3. You’ll then get a menu of options like creating a highlight or a citation from that passage of text.

    OK, got it!

    Cited passage

    Style
    Citations are available only to our active members.
    Buy instant access to cite pages or passages in MLA, APA and Chicago citation styles.

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn, 1992, p. 25).

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn 25)

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences."1

    1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25, http://www.questia.com/read/27419298.

    Cited passage

    Thanks for trying Questia!

    Please continue trying out our research tools, but please note, full functionality is available only to our active members.

    Your work will be lost once you leave this Web page.

    Buy instant access to save your work.

    Already a member? Log in now.

    Oops!

    An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.