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