ICP-MS
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ICP-MS (Inductively Coupled Plasma Mass Spectrometry) is a type of mass spectrometry that is highly sensitive and capable of the determination of a range of metals and several non-metals at concentrations below one part in 1012. It is based on coupling together an inductively coupled plasma as a method of producing ions (ionization) with a mass spectrometer as a method of separating and detecting the ions.
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[edit] Categories
- ICP (Inductively Coupled Plasma) - A plasma is a gas that contains a sufficient concentration of ions and electrons to make the gas electrically conductive. The plasmas used in spectrochemical analyis are essentially electrically neutral, with each positive charge on an ion balanced by a free electron. In these plasmas the positive ions are almost all singly-charged, so there are equal numbers of ions and electrons in each unit volume of plasma
An inductively coupled plasma (ICP) for spectrometry is sustained in a torch that consists of three concentric tubes, usually made of quartz. The end of this torch is placed inside an induction coil supplied with a radio-frequency electric current. A flow of argon gas (usually 14 to 18 liters per minute) is introduced between the two outermost tubes of the torch and an electrical spark is applied for a short time to introduce free electrons into the gas stream. These electrons interact with the radio-frequency magnetic field of the induction coil and are accelerated first in one direction, then the other, as the field changes at high frequency (usually 27.12 million cycles per second). The accelerated electrons collide with argon atoms, and sometimes a collision causes an argon atom to part with one of its electrons. The released electron is in turn accelerated by the rapidly-changing magnetic field. The process continues until the rate of release of new electrons in collisions is balanced by the rate of recombination of electrons with argon ions (atoms that have lost an electron). This produces a ‘fireball’ that consists mostly of argon atoms with a rather small fraction of free electrons and argon ions. The temperature of the plasma is very high, of the order of 10,000 K.
The ICP can be retained in the quartz torch because the flow of gas between the two outermost tubes keeps the plasma away from the walls of the torch. A second flow of argon (around 1 liter per minute)is usually introduced between the central tube and the intermediate tube to keep the plasma away from the end of the central tube. A third flow (again usually around 1 liter per minute) of gas is introduced into the central tube of the torch. This gas flow passes through the centre of the plasma, where it forms a channel that is cooler than the surrounding plasma but still much hotter than a chemical flame. Samples to be analyzed are introduced into this central channel, usually as a mist of liquid formed by passing the liquid sample into a nebulizer.
As a droplet of nebulized sample enters the central channel of the ICP, it evaporates and any solids that were dissolved in the liquid vaporize and then break down into atoms. At the temperatures prevailing in the plasma a significant proportion of the atoms of many chemical elements are ionized, each atom losing its most loosely-bound electron to form a singly charged ion.
- MS (Mass Spectrometry) the ions from the plasma are extracted through a series of cones into a mass spectrometer, usually a quadrupole. The ions are separated on the basis of their mass-to-charge ratio and a detector receives an ion signal proportional to the concentration.
The concentration of a sample can be determined through calibration with elemental standards. ICP-MS also lends itself to quantitative determinations through isotope dilution, a single point method based on an isotopically enriched standard.
Other mass analyzers coupled to ICP systems include double focusing magnetic-electrostatic sector systems with both single and multiple collector, as well as time of flight systems (both with axial and orthogonal accelerators).
Another type of spectrometer using ICP is ICP-AES (Atomic Emission Spectrometer).
There is an increasing trend of using ICP-MS as a tool in Speciation Analysis, which normally involves a front end chromatograph separation and an elemental selective detector, such as AAS and ICP-MS. For example, ICP-MS may be combined with size exclusion chromatography and quantitative preparative native continuous polyacrylamide gel electrophoresis (QPNC-PAGE) for elucidating native metal cofactor containing proteins in biofluids.
[edit] Transfer of ions into vacuum
A sample is injected into the instrument, normally by an auto sampler. The sample is nebulized and delivered through a glass tube by an argon carrier gas. The sample is then exposed to radio frequency which converts the gas into a plasma. A fraction of the formed ions passes through a ~1mm hole (sampler cone) and then a ~0.4mm hole (skimmer cone). The purpose of which is to allow a vacuum that is required by the mass spectrometer.
The vacuum is created and maintained by a series of pumps. The first stage is usually based on a roughing pump, most commonly a standard rotary vane pump. This removes most of the gas and typically reaches a pressure of around 1 torr. Later stages have their vacuum generated by more powerful vacuum systems, most often turbomolecular pumps. Older instruments may have used oil diffusion pumps for high vacuum regions.
Before mass separation, the ion beam is shaped somehow. The method varies from instrument to instrument. Some use simple ion steering plates, while others utilize more complex methods like quadrupoles, hexapoles, or octopoles to guide the ions.
In the last few years, many instruments have begun to use the beam shaping region of the instrument as a collsion cell or dynamic reaction cell. In the process, the ion beam is allowed to collide with a relatively low pressure gas. The most common gases used are argon or helium, but more exotic gases have been used as well. This allows for colision reactions to occur which alter the nature of the beam, generally removing or altering interferences that would otherwise be problematic.
The sample then passes through a charg-to-mass analyzer and into the detector.
[edit] Plasma Generation
As stated above the mode of ionisation is via an argon plasma. Argon has the advantage of being abundant (in the atmosphere, as a result of the radioactive decay of potassium). It is therefore available more cheaply than the other inert gases. Argon also has the advantage of having a higher first ionisation potential than all other elements except He, F and Ne.
The radio frequency causes the following reaction: Ar → Ar+ + e-. Given the high ionisation potential as cited above reverse reaction will take electrons from any species. This recombination of Ar with an electron Ar+ + e- → Ar is likely to cause the loss of an electron from a metal M → M+ + e-. Group II metals may become doubly charged species due to their low second ionisation potential.
[edit] Elemental Analysis
The ICP-MS allows determination of elements with atomic mass ranges 7 to 250. This encompasses Li to U. Some masses are prohibited such as 40 due to the abundance of argon in the sample. A typical ICP-MS will be able to detect in the region of nanograms per liter to 10 or 100 milligrams per liter or around 8 orders of magnitude of concentration units.
Unlike atomic absorption spectroscopy, which can only measure a single element at a time ICP-MS has the capability to scan for all elements simultaneously. This allows rapid sample processing.
[edit] Usage
ICP-MS can be used for analysis of environmental samples such as water and various other non-particulate samples. The instrument can also determine metals in urine to check for exposure to toxic metals. The instrument is very sensitive to particulate matter and high concentrations of organics will cause the instrument to cease function, requiring cleaning.
