Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry that ionizes samples with an inductively coupled plasma to produce atomic ions that are detected. It is generally used for detecting metals and non-metals in liquid samples with low detection limits for quantification. It can also detect different isotopes of the same elements and therefore has applications in isotopic labeling. ICP-MS is fast, precise, and sensitive and has several applications in medical and forensic fields.
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An inductively coupled plasma is a plasma that is energized by heating the gas with an electromagnetic coil containing a concentration of ions and electrons to make the gas electrically conductive. Only a small percentage of the gas is required to be ionized to produce the characteristic plasma. The plasmas are usually electrically neutral where each positive charge on an ion is balanced by a free electron.
ICP-MS is operated in inductive mode. However, it can also be operated in capacitive mode with low plasma density. The sample is introduced to the ICP-MS instrument through the use of a nebulizer that converts the liquid into an aerosol, which is transmitted to the plasma to be ionized. The unique application of using inductively coupled plasma mass spectrometry is that it can be used to measure the analyte continuously. Hence, a high throughput analysis can be achieved.
The sample is analyzed at atmospheric pressure with ICP-MS. Ions created in the argon plasma are transmitted through the mass analyzer to the detector. This allows for high throughput sample measurements as well as time-resolved acquisition. When coupled to other chromatographic techniques such as liquid chromatography or flow injection, it allows for further analysis of components. The benefit of high throughput measurements of up to hundreds of samples a day has provided several applications in environmental analysis, which has reduced costs.
An inductively coupled plasma (ICP) for spectrometry is sustained in a torch that consists of three concentric tubes, usually composed of quartz. The end of the torch is placed inside the induction coil, which is supplied with an electric current.
Argon gas is introduced between the tubes and an electric spark is applied to generate free electrons into the gas stream. These electrons interact with the magnetic field of the induction coil and accelerate back and forth at high frequency, which collides with argon atoms causing them to be ionized. The free electrons are also accelerated by the changing magnetic field and the process continues until there is a balance in the rate of new electrons in collisions with the rate of recombination of electrons with argon ions.
Detection limits of ICP-MS
Detection limits in ICP-MS are much lower than traditional techniques such as flame atomic absorption and can be superior to graphite furnace atomic absorption. Typical limits of detection in ICP-MS are in the nmol/L range for most elements but are dependent on the element, sample type, and matrix, the dilution factor, introducing the sample to the system, instrument operating conditions, and background signals.
Detectors that are typically used operate in both a digital and analog mode. These dual detectors automatically switch from digital to analog mode when the signal intensity becomes out of range. This ensures the dynamic range of the detector remains linear and can be extended by several orders of magnitude.
Interferences in ICP-MS
There are different interferences in ICP-MS, which can be classified as either spectroscopic or non-spectroscopic. Spectroscopic interference is when non-analyte ions have the same mass-to-charge ratio as the analyte. In contrast, non-spectroscopic interference is often observed from sample matrix and instrument drift. However, these particular effects can often be overcome with dilution to reduce the severity of these effects and minimize the deposition of solids in the nebulizer.
Inductively coupled plasma-mass spectrometry
Applications
There are several applications of ICP-MS. One such application is found in profiling bodily fluids such as blood, urine, plasma, and serum for detecting heavy metal poisoning. Metal toxicity of factory workers working in certain fields is a real concern, so mandatory testing has become common practice. Additionally, the technique is found in the environmental testing of water and soil.
Analysis of soil particles as well as other suspensions of colloids. Furthermore, analysis has been achieved on thorium dioxide nanoparticles, zirconium dioxide, and gold nanoparticles, which have been used as substrates in nanopharmacy. ICP-MS has the capability to scan for all elements concurrently.
Furthermore, trace element analysis in biological samples is useful in a large number of clinical settings and is often monitored for nutritional purposes, which include essential elements such as iodine, manganese, copper, selenium, and zinc. Electron transport, oxygen transport, hormone synthesis, and catalysis of biological reactions are some of the key functions of the above elements listed.
On the other hand, other elements such as cadmium, arsenic, mercury, and lead are toxic and therefore ICP-MS is used to assess exposure in the body.
Sources:
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- C. Degueldre, P.-Y. Favarger, Colloid analysis by single particle inductively coupled plasma-mass spectroscopy: a feasibility study, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 217, Issues 1–3, 28 2003, Pages 137-142.
- C. Degueldre, P.-Y. Favarger, Thorium colloid analysis by single particle inductively coupled plasma-mass spectrometry, Talanta, Volume 62, Issue 5, 2004, Pages 1051-1054
- C. Degueldre, P.-Y. Favarger, C. Bitea, Zirconia colloid analysis by single particle inductively coupled plasma–mass spectrometry, Analytica Chimica Acta, Volume 518, Issues 1–2, 2004, Pages 137-142.
- C. Degueldre, P. -Y. Favarger, S. Wold, Gold colloid analysis by inductively coupled plasma-mass spectrometry in a single particle mode, Analytica Chimica Acta, Volume 555, Issue 2, 2006, Pages 263-268.
- Geldmacher-von Mallinckrodt M, Meissner D. General aspects of the role of metals in clinical chemistry. In: Seiler HG, Sigel A, Sigel H, editors. Handbook on metals in clinical and analytical chemistry. New York: Marcel Dekker, Inc; 1994. pp. 13–29.
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Further Reading