Imagine for a moment that you are a forensic scientist working for the FBI. You are attempting to determine if several seemingly unrelated crimes might be linked. In this instance, the FBI forensic chemistry laboratory needs to determine whether bullet lead fragments from different crime scenes can be linked to a single perpetrator. This is an analytical challenge that is ideal for elemental fingerprinting. This article will show how ICP's wavelength coverage and spectral data-handling capabilities make it possible to fingerprint bullet lead.

Experimental

Figure 1: Lead fragments mounted in a laser cell.

For this work, samples were introduced into the plasma using a laser ablation solid sampling device rather than dissolving them and using conventional solution nebulization. For some applications, the laser's ability to directly vaporize a small quantity of the sample in question (5–1000 mg) can simplify the sample-handling process and help to preserve samples for future investigative work. In this case, a New Wave Research Macro 266 laser ablation system (Fremont, California) was interfaced to the Teledyne Leeman Labs Prodigy ICP system (Hudson, New Hampshire). Figure 1 shows the lead fragments mounted in the laser cell. The white material in the photograph is a removable adhesive used to hold the fragments in position in the cell.

Table I: Instrument operating conditions

Operating conditions for the laser system and the ICP are listed in Table I.

Results and Discussion

Figure 2: Spectrum of unknown lead fragment 1.

The first step in this experimental work was to acquire the full ICP spectrum for the bullet fragments in question. An identical spectrum was then acquired for a high-purity lead blank. A spectral subtraction of the blank from each of the fragments was then performed using the ICP system's software. The net result is the elemental fingerprint for each fragment. A spectrum from an unknown lead sample and a high-purity lead blank are shown in Figures 2 and 3. The resulting elemental fingerprint is shown in Figure 4.

Figure 3: Spectrum of high purity lead blank.

Once the analyst has the resulting elemental fingerprint (spectrum) for each sample, there are multiple options for the next course of action. For example, one might choose to perform a qualitative analysis of the fingerprint spectrum or to do a fully quantitative analysis on a select number of elemental constituents.

Figure 4: Elemental fingerprint of unknown lead sample.

A qualitative analysis can be performed within the software, as shown in Figure 5.