Mass spectrometry (Forensic Science)
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.
(The entire section is 112 words.)
Ion Source (Forensic Science)
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 reagent 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.
(The entire section is 113 words.)
Mass Analyzer (Forensic Science)
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...
(The entire section is 319 words.)
Detector (Forensic Science)
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.
(The entire section is 165 words.)
Mass Spectrometry in Forensic Science (Forensic Science)
In GC-MS and LC-MS, samples are injected into the chromatography 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.
(The entire section is 210 words.)
Further Reading (Forensic Science)
Bell, Suzanne. Forensic Chemistry. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006. Provides informative description of the forensic applications of mass spectrometry.
Houck, Max M., and Jay A. Siegel. Fundamentals of Forensic Science. Burlington, Mass.: Elsevier Academic Press, 2006. Good general textbook provides a brief description of the instrument components of mass spectrometers.
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. Presents discussion of mass spectrometry techniques in relation to forensic applications, including forensic toxicology, identification of illicit substances, and DNA analysis.
Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007. General introductory text provides a brief overview of mass spectrometry.
Siegel, Jay A. Forensic Science: The Basics. Boca Raton, Fla.: CRC Press, 2007. Text includes a general discussion of mass spectrometry illustrated with the example of cocaine.
(The entire section is 151 words.)
Mass Spectrometry (Encyclopedia of Science)
Mass spectrometry is a method for finding out the mass of particles contained in a sample and, thereby, for identifying what those particles are. A typical application of mass spectrometry is the identification of small amounts of materials found at a crime scene. Forensic (crime) scientists can use this method to identify amounts of a material too small to be identified by other means.
The basic principle on which mass spectrometry operates is that a stream of charged particles is deflected by a magnetic field. The amount of the deflection depends on the mass and the charge on the particles in the stream.
Structure of the mass spectrometer
A mass spectrometer (or mass spectrograph) consists of three essential parts: the ionization chamber, the deflection chamber, and the detector. The ionization chamber is a region in which atoms of the unknown material are excited so as to make them lose electrons. Sometimes the energy needed for exciting the atoms is obtained simply by heating the sample. When atoms are excited, they lose electrons and become positively charged particles known as ions.
Ions produced in the ionization chamber leave that chamber and pass into the deflection chamber. Their movement is controlled by an electric field whose positive charge repels the ions from the ionization chamber and whose negative...
(The entire section is 570 words.)