Mass spectrometry in Forensics (MS)
Mass spectrometry (MS) is an analytical technique utilized in forensic science to identify substances by measuring the mass-to-charge ratio of ions derived from a sample. Its significance lies in the unique mass spectrum generated by different molecules, allowing forensic scientists to accurately detect components in various sample types, including drugs, explosives, and residues from arson. The mass spectrometer, the instrument central to this process, comprises three main components: the ion source, mass analyzer, and detector. Various ionization methods, like electron impact and chemical ionization, create ions from the samples, which are then separated based on their mass using different analyzer types, such as quadrupole or time-of-flight analyzers.
Mass spectrometry is often combined with other techniques, such as gas chromatography (GC) or liquid chromatography (LC), enhancing its analytical capabilities by enabling the separation of complex mixtures before identification. The resulting data is presented as a mass spectrum, a visual representation that helps forensic experts conclusively identify substances based on their unique ion fragmentation patterns. This versatile technique plays a crucial role in forensic investigations, offering insights into trace evidence analysis, substance identification, and even the detection of metal impurities in biological samples like hair.
Subject Terms
Mass spectrometry in Forensics (MS)
DEFINITION: Technique in which ions of a sample are formed and subsequently separated according to their mass-to-charge ratio.
SIGNIFICANCE: Because each element’s molecules yield a unique mass spectrum, forensic scientists use the sensitive and versatile technique of mass spectrometry to identify conclusively the components of a variety of sample types, including drugs, explosives, and ignitable liquid residues in arson debris.
Almost any type of sample can be analyzed by mass spectrometry, either through direct introduction of the sample or through use of the mass spectrometer as a detector for another analytical technique. In forensic science, gas chromatography-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS), and inductively coupled plasma-mass spectrometry (ICP-MS) are among the more common techniques in which mas spectrometry is used as a detector for the preceding technique.
The instrument used in mass spectrometry, the mass spectrometer, consists of three major parts: the ion source, the mass analyzer, and the detector. Because ions are being formed, the instrument operates under vacuum to prevent collisions of the sample ions with other atmospheric gases.
Ion Source
Ions of the sample are produced in the ion source. Electron impact (EI) ionization and chemical ionization (CI) are perhaps the most commonly used ionization methods in forensic science. In EI, the sample is bombarded with high-energy electrons from an electron beam, causing fragmentation of the sample into ions of varying masses. The fragment ions formed are characteristic of the original sample.
In CI, a gas (commonly methane) is first ionized through interaction with the electron beam. The resulting reagent gas ions then react with sample molecules to form sample ions. CI results in significantly less fragmentation than does EI ionization and is useful to determine the molecular weight of the sample.
Mass Analyzer
The sample ions formed are then directed to the mass analyzer, which separates the ions according to their mass-to-charge ratio (m/z). In most ionization processes, only singly charged ions are produced, so the m/z indicates the mass of the ion. Different types of mass analyzers are available, including magnetic sector, quadrupole, and time-of-flight (TOF) mass analyzers.
The magnetic sector analyzer consists of a magnet to which a magnetic field is applied. Under the influence of the magnetic field, ions travel in curved paths toward the detector. For a given value of the applied magnetic field, only ions of a given m/z will reach the detector; ions of other m/z collide with the surface of the spectrometer and are not detected. The magnetic field values are thus scanned in order to detect ions of all m/z.
The quadrupole mass analyzer is commonly used in such so-called hyphenated techniques as GC-MS and ICP-MS. This analyzer consists of four parallel rods arranged in two pairs; one pair is positioned above the other, essentially defining the corners of a square. A combined electric and radio frequency field is applied to opposite pairs of rods. Ions travel through the space defined by the rods and reach the detector. For a given electric/radio frequency field combination, only ions within a very narrow m/z range can reach the detector; all other ions hit the rods and are not detected. The electric/radio frequency field is thus varied during the analysis to allow ions of all m/z to reach the detector.
In the TOF analyzer, sample ions are separated based on the time they take to drift down a flight tube and reach the detector. Smaller ions drift more quickly and reach the detector ahead of larger ions. The time taken for the ion to reach the detector is converted into an m/z value by the data system.
Detector
Electron multipliers are commonly used as detectors in mass spectrometry because of their ability to amplify the ion signal. The detector consists of a curved glass tube that is coated with a substance that readily emits electrons, such as lead oxide. As ions from the mass analyzer hit the surface of the multiplier, electrons are emitted and travel farther along the multiplier tube, striking the surface and causing the emission of even more electrons. By the time the end of the multiplier is reached, a cascade of electrons is produced, which is measured. The more ions there are of a given m/z, the more intense the measured signal is, because more electrons are emitted from the surface.
Data are displayed in the form of a mass spectrum, which is a plot of ion intensity versus m/z. The forensic scientist can use the mass spectrum to identify a molecule conclusively based on the pattern of fragment ions observed, which is unique to that molecule.
Mass Spectrometry in Forensic Science
In GC-MS and LC-MS, samples are injected into the system and separated in the normal manner. Separated sample components then pass into the mass spectrometer for subsequent detection. Because narrow chromatography columns are used in GC, the carrier gas flow rates are compatible with the mass spectrometer; hence the GC column passes through a short transfer line directly into the mass spectrometer. The transfer line is heated in order to prevent loss of the separated analytes (the substances being analyzed) during transfer. Coupling LC with MS is more difficult, as the mobile phase solvent must be removed and the separated analytes must be transformed from solution into the gaseous state.
ICP-MS is slightly different from the other hyphenated techniques; in this case, the ICP is the ion source. Sample solutions are introduced into the ICP at atmospheric pressure, the solvent is evaporated, and the sample is ionized. The interface between the ICP and the MS consists of two metal cones, each with a pinhole aperture. The sample ions pass through the first aperture into a chamber with lower pressure and then through the second aperture into the mass analyzer, which is maintained at even lower pressure. Once in the mass analyzer, ions are separated and detected as described previously.
Mass spectrometry has many applications in forensics and medicine. The test is commonly used to trace metal impurities in hair and identify substances including drugs and explosives.
Bibliography
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Houck, Max M., and Jay A. Siegel. Fundamentals of Forensic Science. Burlington, Mass.: Elsevier Academic Press, 2006.
Jackson, Glen P., and Mark A. Barkett. "Forensic Mass Spectrometry: Scientific and Legal Precedents." Journal of the American Society of Mass Spectrometry, vol. 34, no. 7, 2023, pp. 1210-1224, doi: 10.1021/jasms.3c00124. Accessed 16 Aug. 2024.
James, Stuart H., and Jon J. Nordby, eds. Forensic Science: An Introduction to Scientific and Investigative Techniques. 2d ed. Boca Raton, Fla.: CRC Press, 2005.
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