Laser ablation-inductively coupled plasma-mass spectrometry (ICP-MS)
Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is an advanced analytical technique that utilizes a laser to vaporize a small amount of solid material for elemental analysis. This method is significant in forensic science, allowing for the detection of trace elements in various materials, such as glass, bullets, and paper, with minimal sample preparation. The process begins with the laser vaporizing a portion of the sample, which is then carried into a plasma where it is atomized and ionized. The resulting ions are analyzed in a mass spectrometer, producing a mass spectrum that reveals the relative abundances of different elements.
One of the key advantages of LA-ICP-MS is its exceptional sensitivity, with detection limits in the parts per billion range, enabling the differentiation of nearly identical samples. However, it is important to note that LA-ICP-MS focuses solely on elemental analysis, making it unsuitable for identifying organic compounds, as any molecules in the sample are destroyed during the process. Developed in the mid-1980s, this technique has gained prominence in forensic investigations over the years, although it requires expensive equipment and skilled operators. Forensic scientists must carefully consider both intrasample and intersample variations when interpreting results to establish meaningful comparisons between samples.
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Laser ablation-inductively coupled plasma-mass spectrometry (ICP-MS)
DEFINITION: Technique in which a laser is used to vaporize a small amount of a solid sample for subsequent elemental analysis in a mass spectrometer.
SIGNIFICANCE: Laser ablation-inductively coupled plasma-mass spectrometry enables forensic scientists to determine trace elements in solid samples with almost no sample preparation. This technique plays an important role in the forensic analysis of samples of materials such as glass, bullets, and paper.
Analysis by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) begins with the vaporization of a small portion of a solid sample by a pulsed laser. The vaporized sample is entrained in a flowing gas and carried into a plasma, where it is atomized and ionized. The ions produced in the plasma are drawn into the mass spectrometer, where they are filtered according to their mass-to-charge ratio. The form in which the resulting data are produced is referred to as a mass spectrum. It indicates the relative amounts of the different elements present in the vaporized portion of the sample. The detection limits for most elements by LA-ICP-MS are parts per billion or better. Because of the low detection limits, the technique has the ability to distinguish between samples that are nearly identical in elemental composition. The equipment required to perform the technique is relatively expensive and requires a skilled user.
The technique was first developed in the mid-1980s and later used as a forensics tool in criminal investigations. In the early 2010s, LA-ICP-MS was only used in a limited number of forensic laboratories, but the technology has become far more common in the 2020s.
In contrast to many other mass spectrometric techniques, LA-ICP-MS is purely an elemental technique. Any molecules present in the original sample are destroyed by the laser pulse or the plasma; only atoms remain. LA-ICP-MS is thus not a suitable technique for identification of organic compounds. In forensic applications the most common samples analyzed are glass, bullets, and paper.
In using LA-ICP-MS to establish a match between two samples, the forensic scientist must remember that samples are not always homogeneous in their composition. Because the laser vaporizes less than a microgram of a sample, samples that appear to be homogeneous can be shown to vary in composition from one location to the next. The scientist must compare the variation in composition within a single sample (intrasample variation) with the variation in composition among different samples (intersample variation) in order to establish a match or a difference.
Bibliography
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"Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)." National Aeronautics and Space Administration, ares.jsc.nasa.gov/research/laboratories/la-icp-ms.html. Accessed 16 Aug. 2024.
Orellana, Francisco Alamilla, César González Gálvez, Mercedes Torre Roldán, and Carmen García-Ruiz . "Applications of Laser-Ablation-Inductively-Coupled Plasma-Mass Spectrometry in Chemical Analysis of Forensic Evidence." TrAC Trends in Analytical Chemistry, vol. 41, January 2013, pp. 1–34, doi.org/10.1016/j.trac.2012.09.015. Accessed 16 Aug. 2024.
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