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 analysis of a range of metals and several non-metals at 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 identifying and detecting the ions.
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- ICP (Inductively Coupled Plasma) - this is a high temperature plasma (partly ionised argon) sustained with a radiofrequency electric current, which acts to produce ions. The electric current is transferred to the plasma by an induction coil, wrapped around a configuration of concentric quartz tubes (the plasma torch). The common operating frequencies are 27.12 and 40.68 MHz and operating power is in the range between 800 and 1500 W. The plasma is sustained within a constant flow of argon gas, open to atmosphere and reaches temperatures as high as 10,000 K in the hottest part. To prevent melting of the torch a high flow rate of argon is used in the outermost tubing. The total gas consumption of a typical analytical ICP is in the range of 14 - 18 L/min. In order to reduce capacititive coupling between the coil and plasma, some designs employ an additional metal sheet between the torch body and the induction coil.
- 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] Instrument operation
A sample is injected into the instrument, normally by an auto sampler. The Sample is atomised 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 plasma passes through a ~1mm hole and then a ~0.4mm hole. The purpose of which is to allow a vacuum that is required by the mass spectrophotometer. The sample then passes through charged lenses through the quadrapole 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 gasses. 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 a 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 analysis of elements with 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 ppt to 10 or 100 ppm or around 8 orders of magnitude.
Unlike Atomic Absorption Spectroscopy which can only scan for 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 analyse for metals in urine in the case of mining activities. The instrument is very sensitive to particulate matter and high concentrations of organics will cause the instrument to cease function, requiring cleaning.cs:ICP-MS de:ICP-MS
