Inductively Coupled Plasma Atomic Emission Spectroscopy
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Inductively coupled plasma atomic emission spectroscopy (ICP-AES), also referred to as inductively coupled plasma optical emission spectroscopy (ICP-OES), is an analytical technique used for the detection of chemical elements. It is a type of
emission spectroscopy The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to electrons making a transition from a high energy state to a lower energy state. The photon energy of th ...
that uses the inductively coupled plasma to produce excited atoms and ions that emit
electromagnetic radiation In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...
at wavelengths characteristic of a particular element. The plasma is a high temperature source of ionised source gas (often argon). The plasma is sustained and maintained by inductive coupling from electrical coils at megahertz frequencies. The source temperature is in the range from 6000 to 10,000 K. The intensity of the emissions from various wavelengths of light are proportional to the concentrations of the elements within the sample.


Mechanism

The ICP-AES is composed of two parts: the ICP and the optical
spectrometer A spectrometer () is a scientific instrument used to separate and measure Spectrum, spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomeno ...
. The ICP torch consists of 3 concentric
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
glass tubes. The output or "work" coil of the
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
(RF) generator surrounds part of this quartz torch.
Argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
gas is typically used to create the plasma. The ICPs have two operation modes, called capacitive (E) mode with low plasma density and inductive (H) mode with high plasma density, and E to H heating mode transition occurs with external inputs. The torch is operated in the H mode. When the torch is turned on, an intense
electromagnetic field An electromagnetic field (also EM field) is a physical field, varying in space and time, that represents the electric and magnetic influences generated by and acting upon electric charges. The field at any point in space and time can be regarde ...
is created within the coil by the high power
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
signal flowing in the coil. This RF signal is created by the RF generator which is, effectively, a high power radio transmitter driving the "work coil" the same way a typical radio transmitter drives a transmitting antenna. Typical instruments run at either 27 or 40 MHz. The argon gas flowing through the torch is ignited with a Tesla unit that creates a brief discharge arc through the argon flow to initiate the ionization process. Once the plasma is "ignited", the Tesla unit is turned off. The argon gas is ionized in the intense electromagnetic field and flows in a particular rotationally symmetrical pattern towards the magnetic field of the RF coil. A stable, high temperature plasma of about 7000 K is then generated as the result of the inelastic collisions created between the neutral argon atoms and the charged particles. A
peristaltic pump A peristaltic pump, also commonly known as a roller pump, is a type of positive displacement pump used for pumping a variety of fluids. The fluid is contained in a flexible tube fitted inside a circular pump casing. Most peristaltic pumps work th ...
delivers an aqueous or organic sample into an analytical nebulizer where it is changed into mist and introduced directly inside the plasma flame. The sample immediately collides with the electrons and charged ions in the plasma and is itself broken down into charged
ions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
. The various molecules break up into their respective atoms which then lose
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s and recombine repeatedly in the plasma, giving off radiation at the characteristic
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s of the elements involved. In some designs, a shear gas, typically
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
or dry compressed air is used to 'cut' the plasma at a specific spot. One or two transfer lenses are then used to focus the emitted light on a
diffraction grating In optics, a diffraction grating is an optical grating with a periodic structure that diffraction, diffracts light, or another type of electromagnetic radiation, into several beams traveling in different directions (i.e., different diffractio ...
where it is separated into its component wavelengths in the optical spectrometer. In other designs, the plasma impinges directly upon an optical interface which consists of an orifice from which a constant flow of argon emerges, deflecting the plasma and providing cooling while allowing the emitted light from the plasma to enter the optical chamber. Still other designs use optical fibers to convey some of the light to separate optical chambers. Within the optical chamber(s), after the light is separated into its different wavelengths (colours), the light intensity is measured with a
photomultiplier A photomultiplier is a device that converts incident photons into an electrical signal. Kinds of photomultiplier include: * Photomultiplier tube, a vacuum tube converting incident photons into an electric signal. Photomultiplier tubes (PMTs for sh ...
tube or tubes physically positioned to "view" the specific wavelength(s) for each element line involved, or, in more modern units, the separated colors fall upon an array of semiconductor photodetectors such as charge coupled devices (CCDs). In units using these detector arrays, the intensities of all wavelengths (within the system's range) can be measured simultaneously, allowing the instrument to analyze for every element to which the unit is sensitive all at once. Thus, samples can be analyzed very quickly. The intensity of each line is then compared to previously measured intensities of known
concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...
s of the elements, and their concentrations are then computed by interpolation along the calibration lines (use of a
calibration curve In analytical chemistry, a calibration curve, also known as a standard curve, is a general method for determining the concentration of a substance in an unknown sample by comparing the unknown to a set of standard samples of known concentration. ...
). In addition, special software generally corrects for interferences caused by the presence of different elements within a given sample matrix.


History

The first published attempt to use plasma emissions as a source for spectroscopic analysis were in 1956 by Eugen Bădărău. In 1964 Stanley Greenfield working at Albright & Wilson was the first to use ICP for non experimental analysis. The first commercial machine was produced by KONTRON in 1975.


Applications

Examples of the application of ICP-AES include the determination of metals in wine, arsenic in food, and trace elements bound to proteins. ICP-AES methods are used to test for metals contamination in drinking water and wastewater. ICP-AES is widely used in minerals processing to provide the data on grades of various streams, for the construction of mass balances. In 2008, the technique was used at Liverpool University to demonstrate that a ''
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amulet An amulet, also known as a good luck charm or phylactery, is an object believed to confer protection upon its possessor. The word "amulet" comes from the Latin word , which Pliny's ''Natural History'' describes as "an object that protects a perso ...
found in Shepton Mallet and previously believed to be among the earliest evidence of
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in
England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...
, only dated to the nineteenth century. ICP-AES is often used for analysis of trace elements in soil, and it is for that reason it is often used in forensics to ascertain the origin of soil samples found at crime scenes or on victims etc. Taking one sample from a control and determining the metal composition and taking the sample obtained from evidence and determine that metal composition allows a comparison to be made. While soil evidence may not stand alone in court it certainly strengthens other evidence. It is also fast becoming the analytical method of choice for the determination of nutrient levels in agricultural soils. This information is then used to calculate the amount of fertiliser required to maximise crop yield and quality. ICP-AES is used for
motor oil Motor oil, engine oil, or engine lubricant is any one of various substances used for the lubrication of internal combustion engines. They typically consist of base oils enhanced with various additives, particularly antiwear additives, deterge ...
analysis. Analyzing used motor oil reveals a great deal about how the engine is operating. Parts that wear in the engine will deposit traces in the oil which can be detected with ICP-AES. ICP-AES analysis can help to determine whether parts are failing. In addition, ICP-AES can determine what amount of certain oil additives remain and therefore indicate how much service life the oil has remaining. Oil analysis is often used by fleet manager or automotive enthusiasts who have an interest in finding out as much about their engine's operation as possible. ICP-AES is also used during the production of motor oils (and other lubricating oils) for quality control and compliance with production and industry specifications.


See also

* Ashing * Atomic absorption spectroscopy *
Atomic emission spectroscopy Atomic emission spectroscopy (AES) is a method of chemical analysis that uses the intensity of light emitted from a flame, plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample. The wavelength of ...
* Inductively coupled plasma mass spectrometry


References


External links


Inductively Coupled Plasma/Optical Emission Spectrometry in Encyclopedia of Analytical Chemistry
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