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Inductively coupled plasma mass spectroscopy (ICP-MS) is a highly sensitive mass spectroscopy technique that is often used in elemental analysis. It is capable of multi-elemental analysis at the sub-parts per trillion level, depending on the element to be analyzed, which is most commonly a metal.1) This instrumental technique couples together an inductively coupled plasma as a source of ionization with a typical mass spectrometer as a means of separating and detecting the generated ions.
A plasma, widely considered to be the fourth state of matter, refers to a gas in which a certain percent of the molecules are ionized; plasmas are electrically conductive.2) An inductively coupled plasma (ICP) is a plasma in which the energy is supplied by electrical currents produced by electromagnetic induction. In the typical ICP-MS setup, the ICP is formed in a stream of argon gas (typically 8-20 L/min) flowing through a chain of three concentric quartz tubes known as the plasma torch.3)
Figure 1: Detailed diagram of the ICP-MS torch
A copper coil surrounds the top portion of the torch and is connected to a radio frequency (RF) generator. As RF power is applied to the coil, an alternating current begins to oscillate (generally at a frequency of either 27 or 40 MHZ) within the coil.4) This oscillating current generates a strong electromagnetic field in the top of the torch. As argon gas flows through the torch, a high-voltage spark is applied to the gas, which strips some argon atoms of their electrons.5) These electrons are accelerated by the magnetic field and collide with other argon atoms, stripping off their electrons in turn. This process continues in a chain reaction, resulting in an ICP discharge consisting of argon atoms, argon ions, and electrons.6) This discharge is continually sustained within the torch by influx of RF power into the load coil and provides the temperature and energy required to ionize the majority of the elements in the periodic table.
Ultimately, the ions produced from the ICP are filtered through a mass spectrometer, which is an instrument that can separate and quantify the amount of each element in the sample according to the mass-to-charge ratios of their respective ions. The concentration of an element from the original sample can be elucidated via calibration with a reference standard or through isotope dilution analysis.7)
The typical ICP-MS setup consists of several components, namely a nebulizer, a plasma torch, a spray chamber, and a detector.
Figure 2: Schematic diagram of a typical ICP-MS setup
Components that vary between different ICP-MS interfaces include the ion focusing system, mass separation device (generally a mass spectrometer), and vacuum chamber.8) The initial sample to be analyzed (usually a liquid) is pumped into a nebulizer, where it is converted into an aerosol, a suspension of liquid droplets in a gas, with argon gas. The smaller, finer droplets (about 1-2% of the sample) are separated from larger ones through usage of a spray chamber. The fine aerosol then exits from the spray chamber and is transported into the plasma torch by means of a sample injector.9)
Once the sample enters the high-temperature inductively coupled plasma discharge (around 10,000 K), it is vaporized and broken down into atoms, the majority of which become ionized. This vaporization-atomization-excitation-ionization pathway is common to all ICP-MS systems.10) Ions produced in the plasma are then directed towards a vacuum chamber / interface region, which is kept at around 1-2 Torrs of pressure with a mechanical pump. This region contains two metallic cones (a sampler cone and a skimmer cone) with tiny orifices to allow ions to pass through to the next region of the ICP-MS system, ion optics. The ion optics system (held at 10-3 Torr) is a series of electrostatic lenses meant to preferentially gather ions and focus this ion beam towards the mass spectrometer while excluding photons, solid particulate matter, and neutral species.11) The ion beam then enters the mass spectrometer, typically a quadrupole, which is kept at 10-6 Torr via a turbomolecular pump. The ions are then detected by either an ion counting detector or an analogue detector.12) Most detection systems used can analyze samples from parts per trillion (ppt) levels up to a few hundred parts per million (ppm).13)
Due to the ability to detect a large number of elements at ultra-trace levels (as low as ppt amounts), ICP-MS has become invaluable in the field of environmental analysis. In this field, it is frequently used to assay for trace and ultra-trace concentrations of hazardous toxic elements, mainly heavy metals, metalloids, and various radionuclides.14) For example, ICP-MS is often used to determine dissolved and total concentrations of various toxic elements in surface water, drinking water, industrial waters, and waste water samples.15) Many publications utilizing ICP-MS deal with speciation of arsenic and selenium, but speciation and protein and DNA binding of other metals has been investigated as well.16) ICP-MS is often paired with effective separation techniques, such as high performance liquid chromatography (HPLC) and size exclusion chromatography (SEC). For example, a significant study paired ICP-MS detection with SEC to separate proteins from DNA fragments and to study the binding of Cr, Se, Cd, Th, and U to the various fractions.17) Many applications are found in physiology and medicine, where ICP-MS can be used to detect toxic metals in biological systems.18) A deterrent to the prevalence of ICP-MS systems in more laboratories is the price (upwards of 400,000 USD) of the instrument.
ICP-MS also lends itself to the possibility of isotope ratio measurements, such as isotope dilution (ID) analysis. Further application within isotope studies includes tracer and migration studies based on natural or anthropogenic differences in the isotopic composition of certain elements, such as Sr or Pb.19) Further, ICP-MS can be used to collect data on the environmental occurrence of long-living radionuclides such as U, Pu, and Tc.20) The study of isotope ratios, furthermore, is extremely important in both geochemistry and environmental science. To this end, isotope studies using ICP-MS have been carried out in applications such as approximating the age of rock formations and pinpointing the source of metallic pollutants in different environments and ecosystems.21)