Comparative Bullet Lead and Antimony Analysis using XRF and ICP-OES
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Transcript of Comparative Bullet Lead and Antimony Analysis using XRF and ICP-OES
Comparative Bullet Lead and Antimony Analysis using XRF
and ICP-OESRob Harvey
Nate Birth
When the physical markings of a fired bullet recovered from a crime scene are too mutilated for visual comparison or the firearm used in the crime is not recovered, the bullet can be compared with other bullets associated with a suspect by its elemental composition (Peters, 2002)
Background
During the manufacturing processes, thousands of lead specimens (bullets and bullet cores) are produced with analytically indistinguishable compositions. However, those lead specimens that share the same composition are generally packaged within the same box of cartridges, or in boxes of cartridges of the same caliber and type at the same manufacturing plant, on or about the same date (Peters, 2002)
Background
In one research effort, the group acquired and analyzed samples from bullet lead manufacturers. The results of these analyses confirmed that a cast billet poured from a pot of molten lead is relatively homogeneous, but that leads poured from separate molten batches are distinguishable. As a result, comparative bullet lead analysis has been adopted by laboratories and accepted by courts internationally (Peters, 2002)
Background
Compositional bullet lead comparisons are possible because each melt of lead has its own characteristic composition (Peters, 2002)
Also backed by a study in 2005 by Koons and Buscaqlia◦ Looked at 1837 samples, determined that 76% of
them can be distinguished from one another
Significance
"The bullet removed from the victim and 10 of the 15 analyzed cartridges from the suspect residence are analytically indistinguishable from one another. Therefore, they likely originated from the same manufacturer's source (melt) of lead.” (Peters, 2002)
This conclusion does not associate a bullet to a box but rather to a melt of lead that has bullet specimens within that box and perhaps other boxes.
Quote from FBI
Methods
Flowchart4 Bullet Samples
Run in XRF
Dissolve Bullets
Run in ICP-OES
Compare Bullets
Bullet 1◦ Fired, Smashed, Full Metal Jacket
Bullet 2◦ Unfired, Hollow Point
Bullet 3◦ Unfired, Standard
Bullet 4◦ Unfired, Standard
Bullet Samples
XRF works by exciting the sample
Reads the unique X-rays given off
Gives percentage amount of metals
Bullets ran, and the resulting table printed
XRF Runs
Concentrated Nitric Acid was used
Bullets were cut with a hack saw
Shavings were weighed as close to 1.00 g as possible
The bullets sat in the acid for a week
Bullet Dissolving Method
Standards were made for both lead and antimony for 10, 20, 30, 40, 50, 75, 100, and 500 ppm
Samples spiked with 2 ppm of both metals, before dilution, after dilution, value is 0.20 ppm
The bullet/acid solution was diluted to 50 ml with a volumetric flask, and then later to 500 ml
Standards were ran to make a calibration curve Samples were ran to plot on the curve
ICP-OES
To get total ppm of the dissolve bullet: To make standard solutions: Experimental values were calculated using
the linear regression equation
Percent Composition from ICP:
Calculations
Results
XRF Data
ICP Calibration Curve Table
Bullet 1 – 2136 ppm
Bullet 2 – 2004 ppm
Bullet 3 – 2000 ppm
Bullet 4 – 2014 ppm
Bullet ppm Levels
Bullet 1 – 150.8 ppm
Bullet 2 – 1391.0 ppm
Bullet 3 – 1077.3 ppm
Bullet 4 – 385.5 ppm
Experimental Lead Levels
Bullet 1 – 0 ppm
Bullet 2 – 1.26 ppm
Bullet 3 – 0.016 ppm
Bullet 4 – 6.83 ppm
Experimental Antimony Levels
Summary Table
Discussion
Used the XRF and ICP-OES to look at bullet composition
XRF provided a fast accurate analysis ICP-OES was able to detect lower levels of
analytes◦ Detected antimony in all samples but 1, XRF only
found 2 bullets with antimony Bullet Dissolving process could be changed
Evaluation of Method
Dissolving process◦ Not everything dissolved, longer sonication could
result in more material dissolving◦ Place watch glass on top of beaker
ICP Runs◦ Concentration of the bullets was higher then
expected, not enough dilution was done Calculating ppm and percent composition
◦ Account for the fact that not all the mass measured was dissolved, lead to percents from ICP to be low
Issues to be Addressed
Concept that different bullets have different compositions was shown◦ XRF data and ICP-OES data both shows this
Lead varied among the bullets, but was always the most prevalent
Antimony was found in all bullets but one, so it could be a good analyte to look for
Comparison of Bullets
Increase Sample Size
Compare different brands
Compare different types
Look at different metals
Future Studies
Project showed that the levels of lead and antimony vary between bullets
Method is fairly straightforward and solid◦ Has kinks that need to be worked out
Issue of using a watch glass to minimize evaporation, diluting the sample enough, and accounting for undissolved mass
With the few issues resolved this method can be used to help possibly identify bullets at crime scenes that have been too badly damaged for visual study
Overall
Forensic significance of bullet lead compositions RD Koons and J. Buscaqlia Journal of Forensic Science 2005 50(2), 341-351
The Basis for Compositional Bullet Lead Comparisons Charles A. Peters Forensic Science Communications 2002 4(3)
Literature Cited