Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)
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INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY (ICP-MS) Hector Neff Department of Anthropology and IIRMES, California State University Long Beach, Long Beach, CA, USA
Synonyms Inductively coupled plasma mass spectrometry Definition A technique used for characterizing archaeological artifacts and sediments in terms of both elemental concentrations and isotope ratios. Introduction ICP-MS stands for inductively coupled plasma mass spectrometry. It is an analytical-chemistry technique that can measure even tiny (subpart per million) concentrations of most elements of the periodic table. In archaeology and geoarchaeology, elemental characterization by ICP-MS or other techniques is most commonly used for determining the sources of artifact raw materials. Other applications of elemental characterization include reconstructing technology and identifying and mapping activity areas on occupation floors. Some ICP-MS instruments can determine isotopic ratios with sufficient precision to be useful in human-population movement studies and uranium-thorium dating. ICP-MS instruments As indicated by its acronym, ICP-MS is a compound technique, consisting of an inductively coupled plasma connected to a mass spectrometer. Essentially, a sample is ionized initially within the plasma, and then the ions are separated and counted by the spectrometer.
Plasma is a state of matter having no defining shape like a gas but consisting of atoms together with positively charged ions and free electrons. Its level of ionization allows it to respond to magnetic fields and conduct electricity. In ICP, an argon plasma is produced by seeding argon gas flowing through a quartz tube with electrons, which are accelerated with a radio-frequency generator. Collisions of some of the electrons with argon atoms yield more electrons, lost from argon atoms, and these electrons are themselves accelerated and then may collide with additional argon atoms, thus sustaining the plasma as long as the RF field is applied. The loss of electrons and their cyclical rejoining of other argon atoms in need of an electron repeat many times per second and generate extraordinary heat. Inductively coupled plasmas were first applied to atomic emission spectroscopy (AES) (Greenfield et al., 1964), the plasma being used to excite atoms in an injected sample, which then gave off light of characteristic wavelengths upon de-excitation. Not long after the advent of ICP-AES, it was recognized that argon plasmas, which produce temperatures in the 8,000–12,000 K range at their cores, are also excellent sources of positively charged ions. Once ionized, an atom can be identified as mass-to-charge ratio (m/e) by mass-spectrometric techniques. In ICP-MS instruments, the ICP torch (which controls the flow of plasma) is oriented horizontally, and the stream of ions emerging from the torch is passed through an interface into a high vacuum environment conducive to measurement of mass-to-charge ratio. The interface usually consists of two nickel cones with small openings that allow passage of the
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