To determine the source of inorganic elements collected at a crime scene, forensic scientists need dependable procedures. Physical evidence may be identified and compared using laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS), which can distinguish elemental and isotope variations down to the part per billion (ppb) level.
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Elemental Analysis of Forensic Samples
Elemental analysis is a material science and analytical chemistry process for determining the elemental composition of a material specimen, such as water, mineral deposits, or human fluids. Qualitative and quantitative elemental analysis are the two forms of elemental analysis. The kind of element is determined by qualitative examination, while the level/number of elements is determined by quantitative analysis.
Why Elemental Examination is Essential
An elemental examination of an item discovered during an inquiry might provide important information about its origin and history. However, since the item itself may need to be produced as evidence, any approach utilized for this examination should be non-destructive. A wide range of sample types and sizes are possible, ranging from a soil sample to a bullet to a minuscule particle of gunshot residue or a glass piece.
Elemental analysis may supply forensic scientists with a wealth of evidence. However, the samples that are provided for examination might be nearly anything, ranging from bulk material to minuscule fragments.
Preferred Techniques for Elemental Analysis
Non-destructive techniques are beneficial since it is preferred to maintain the sample for presenting as physical evidence. EDXRF is one of the few analytical methods with the sensitivity and elemental range to conduct many of these studies without requiring the material to be dissolved or otherwise tampered with.
The variants of EDXRF machines from SPECTRO Analytical Instruments and EDAX are intended to handle a broad range of sample types and can conduct the bulk of elemental analyses used in forensic investigations. SPECTRO Analytical Instruments' ICP-OES and ICP-MS spectrometers provide the very latest state-of-the-art methods when greater sensitivity is needed.
Laser ablation or photoablation is the process of removing material from a solid (or occasionally liquid) surface by bombarding it with a laser beam. The ingested laser energy heats the material, which dissolves or vaporizes at low laser flux. Using a high-intensity laser, the substance is often converted to plasma.
Which Lasers are Used for Ablation?
Laser ablation normally refers to peeling with a pulsed laser, however, it is viable to ablate material with a high-frequency laser beam provided the laser output is high enough. While reasonably long laser pulses can burn and destroy the treated material, ultrashort laser pulses cause only slight destruction during computation due to the ultrashort light-matter interaction, making them suitable for micromaterial processing as well.
Industrial Uses of Laser Ablation
The regulated removal of material from a solid surface is the most fundamental use of laser ablation. Laser milling and, in particular, laser drilling, are examples of how pulsed lasers can drill incredibly small, deep trenches through incredibly hard materials. Because very short laser pulses remove material so quickly so that the surrounding material receives very little heat, laser drilling may be used on delicate or heat-sensitive materials like tooth enamel (laser dentistry).
Metallic nanoparticles, inorganic materials, and metal carbides have been created by several researchers using laser ablation and gaseous condensation. Laser cleaning is also used to repair paintings, sculptures, and frescoes, as well as to eliminate rust from iron relics, clean oil or grease from various surfaces, and erase rust from ancient artifacts. Laser ablation is one of the preferred ways of cleaning rubber molds due to the mold's minimal surface damage.
LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry)
LA-ICP-MS is a powerful analytical method for performing very precise elemental and isotopic analyses on solid materials. By focusing a laser beam on the specimen surface, laser ablation is employed in LA-ICP-MS to generate tiny particles. The ablated particles are then transferred to the secondary excitation source of the ICP-MS apparatus, where the selected mass is processed and ionized. The excited ions from the plasma torch are then transferred to a mass spectrometer for elemental and isotopic evaluation.
Laser Ablation for Elemental Analysis in Forensic Applications
The forensic analyst's responsibility is to develop objective scientific evidence on trace elemental analysis. As a result, forensic samples provided for LA-ICP-MS analysis might range from tufts of hair to garment fibers. The glass used in automotive headlights and windows, for example, is often unique to a manufacturer, and the elemental profile may be used to determine the vehicle's trademark, brand, or even year of manufacture. The classic refractive index (RI) technique of discriminating is substantially inferior to trace element content.
Laser ablation (LA)-ICP-MS is employed directly on solids, and the original sample integrity is preserved. As a result, LA sampling is advantageous for forensic work due to its speed, ease, and minimized sample destruction. LA-ICP-MS ablation of samples is largely non-destructive and enables many observations of a sample, which may be necessary for forensic analysis.
LA-ICP-MS has been used to differentiate gold samples as well as to establish their origin. Other samples that have been identified based on trace element composition detected by LA-ICP-MS include jewels, soils, automobile paints, bones, dyes, and pigments.
Some examples of laser ablation for elemental analysis in forensic work are:
- Lipstick samples can be discriminated from each other based on trace element composition.
- Copper wire samples can also be analyzed using LA-ICP-MS to determine the diversity in trace element composition throughout the wire's length.
- The quantity of arsenic, mercury, lead, and copper inside tissue samples can be analyzed by LA-ICP-MS to indicate the existence of gunshot residue in tissue and larvae.
- Without the need for time-consuming sample preparation, LA-ICP-MS offers a quick and easy way to characterize glass shards encountered on clothes or at the scene of an accident.
Although the main and minor elemental compositions of glasses are highly similar, making standard techniques of characterization difficult or impossible to distinguish, these glasses may contain trace elemental signatures that allow reliable assessment of differences using LA-ICP-MS.
Why LA-ICP-MS is the Center of Attention
LA-ICP-MS is one of the most exciting analytical technologies in forensic analysis nowadays since it can do ultra-highly delicate chemical analysis down to ppb (parts per billion) levels without any sample pretreatment. The analysis may be carried out in the open air without the need for expensive vacuum apparatus, and the samples can be conductive or non-conductive.
LA-ICP-MS offers the fastest analysis speed of all analytical procedures, with a limit of detection near the ppb level, since the data are available in seconds. Sub-microscale sample mass values ranging from nanograms to femtograms are required for LA-ICP-MS analysis. Traditional liquid nebulization procedures for ICP-MS need the removal of milligrams of sample mass to be effective.
LA-ICP-MS provides elemental composition measurements at content levels that allow for robust sample discrimination among tiny, low-mass samples. These processes need little to no specimen preparation and often require less than 100 g of material for evaluation, ensuring that precious resources are not lost entirely. This allows you to run as many tests as you like.
Sensitivity down to the ppb level provides criminologists with the scientific information they need to determine how the evidence collected fits into each case. The findings are swift, requiring just a few minutes for LA-ICP-MS, which might help many labs manage sample backlogs faster.
Using ICP-MS or LA-ICP-MS, not all forensic samples can be easily separated from one another based on trace elemental analysis. The Federal Bureau of Research (FBI) conducted a rigorous investigation into the elemental analysis of bullet lead, which revealed several issues with the use of laser ablation for trace element analysis for the forensic evaluation of lead bullets.
It was estimated that a 40-grain bullet would need anything from 12,000 to 35 million lead. "Compositionally identical amounts of lead" might be used to make a 22 caliber long gun. Many bullet producers put leftover lead from bullet manufacturing into the melt at odd periods, causing the original melt's composition to occasionally shift.
Future Perspectives of Laser Ablation-ICP-MS
ICP-MS devices have been on the marketplace for about 40 years and have established themselves as standard in laboratories worldwide. With the present great interest in trace element evaluation in the ecosystem, the time has come for significant commercialization. The use of mobile LA-ICP-MS will also aid in the characterization of uranium waste generated during energy production.
In short, LA-ICP-MS is a powerful instrument for analyzing a broad range of forensic evidence. This approach is very useful for overcoming the restrictions of extremely tiny sample sizes or samples made of unreactive materials. LA-ICP-MS produces a definite "fingerprint" based on trace element ratio data that is used to validate or exclude the origin of forensic evidence. Using a proper plotting application, a direct visual form of data is often created, making it simpler to distinguish samples.
More from AZoM: A Current View of Scanning Electron Microscopy in Forensics
References and Further Reading
Neufeld, L. M. (2004). Introduction to laser ablation ICP-MS for the analysis of forensic samples. Agilent Technologies Publication, 1-6. https://www.agilent.com/cs/library/applications/5989-1565EN
Orellana, F. A., Gálvez, C. G., Roldán, M. T., & García-Ruiz, C. (2013). Applications of laser-ablation-inductively-coupled plasma-mass spectrometry in chemical analysis of forensic evidence. TrAC Trends in Analytical Chemistry, 42, 1-34. https://www.sciencedirect.com/science/article/pii/S0165993612002877?via%3Dihub
Jantzi, S. C., & Almirall, J. R. (2014). Elemental analysis of soils using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and laser-induced breakdown spectroscopy (LIBS) with multivariate discrimination: Tape mounting as an alternative to pellets for small forensic transfer specimens. Applied Spectroscopy, 68(9), 963-974. https://journals.sagepub.com/doi/10.1366/13-07351