Trace Metal Analysis by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry and X-Ray K-Edge Densitometry of Forensic Samples

Author: Jonna Elizabeth Berry

Abstract:

This report describes a variety of studies determining trace elements in samples with forensic importance using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and x-ray, K-edge densitometry. The elements studied were lipstick, copper wire, and gunshot and bullet residue.

Original sample integrity is retained when laser ablation (LA-ICP-MS) is used on solids. Therefore, the speed, convenience, and limited sample destruction of LA sampling is an advantage for forensic work. Ablation of samples by LA-ICP-MS is relatively non-destructive and allows for replicate measurements of a sample, which may be required for forensic analyses.

The report highlights studies on three types of forensically related samples that were analyzed for trace element composition that impact forensic investigations. LA-ICP-MS was used to determine the trace element composition of numerous lipstick samples. LA-ICP-MS was also used to analyze copper wire, which is used for binding or strangling victims, often is left at crime scenes, and can be recovered from electronic circuits or components after a bomb explosion.

LA-ICP-MS also assessed the presence of gunshot residue (GSR) on tissue within pig carcasses. Finally, pig decomposition studies were made to detect bullet fragments in bones using radiography and x-ray, K-edge densitometry. This method avoids even the small amount of sample destruction inflicted by LA-ICP-MS.

The lipstick study found samples of different brands or colors to be distinguishable based on their trace element compositions. This suggests LA-ICP-MS could assist in the identification of the brand and color of an unknown lipstick sample recovered at a crime scene.

The copper wire study found that variations in the trace elemental composition cannot be readily distinguished from one another, suggesting spatial heterogeneity should be evaluated in forensic applications, particularly in cases where only small samples are available (e.g., copper strand fragments after an explosion).

In the GSR study, larvae, temperature, and precipitation had substantial effects on the retention of GSR in the skin around gunshot wounds on decomposing carcasses. In general, these results indicate potential forensic value of measuring GSR elements in skin and larvae for a period of time throughout the decomposition process.

Finally, the pig decomposition studies resulted in bones that had very small bullet fragments in them. This result demonstrates the potential for GSR to be detected on skeletonized remains using radiography and x-ray, K-edge, densitometry. Identification of projectile trauma on skeletonized remains would be important in forensic investigations in order to assist in determining the cause of death.

Overall, the studies found that LA-ICP-MS is a relatively nondestructive technique, whereas x-ray, K-edge densitometry is completely nondestructive. Both techniques have low detection limits for the measurement of trace elements, and most, or all, of the original sample integrity remains intact, which is advantageous in forensic analyses.

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