​​​Glossary of ICP Terms

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​Term
​​Definition
​Array Detectors​
​An array detector is a silicon chip with a one or two-dimensional array of photosensitive material on its surface.  The photons strike the photosensitive regions called pixels and are converted into electrons which are collected and read out as a charge.  Examples of array detectors include Photodiode Arrays (PDAs), Charge-Coupled Devices (CCDs), Charge-Injection Devices (CIDs), and Segmented-array Charge-Coupled Devices (SCDs).
​Autosampler
​A device that automatically sequences the delivery of sample and/or standard solutions to the ICP. Autosamplers vary considerably.  The system should accommodate large sample vials for standards, and optional different sizes for samples based on the analytical requirement.  The autosampler should be “random-access”, allowing the operator to program functions such as logic (i.e., what to do when a QC standard fails) and sample/standard positioning.
​Axial View
​Axial viewing is the positioning of an ICP torch such that the spectrometer views the plasma end-on. Usually, the torch is positioned in a horizontal orientation for axial viewing. When configured properly, axial viewing provides an increase in sensitivity, typically 5-20x greater than radial viewing.
​Background Correction
​This is a spectral correction technique that compensates for an elevated background signal at the analyte wavelength. Instruments measure background at one or more points positioned slightly off-peak from the emission wavelength, and then subtract the background from the total intensity measured at the analytical wavelength.
​Background Equivalent Concentration (BEC)
​Defined as the apparent concentration of the background signal based on the sensitivity of the element at a specified wavelength. The lower the BEC value, the more easily a signal generated by an element can be discerned from the background. Many analysts believe BEC is a more accurate indicator of the performance of an ICP-OES system than detection limit, especially when making comparisons of background correction methods. Refer to “Background Correction”.
​Background Noise
The background noise is the precision of the measurement of the background signal at the analyte wavelength, and is normally determined by taking the square root of the spectral background of the instrument (in emission intensity). Refer to “Detection Limit”.
​Background Shift
​This is a change in the emission intensity at the analyte wavelength due to some other spectral feature, other than the analyte.
​Blaze
​Echelle and mechanically-ruled gratings are optimized (“blazed”) to be more efficient near certain wavelengths, which is dictated by the blaze angle of the groove etched/cut into the surface. Since most lines used in ICP are at relatively short wavelengths, gratings are often blazed in the neighborhood of 200-250 nm, the “blue” end of the spectrum. Holographic gratings are not blazed in the traditional sense because the process is based on photolithography. Echelle gratings are blazed by design for all wavelengths
​Blurring
​ The effect known as blurring occurs when the image quality on the edge of a CTD detector is worse than the image quality in the center where the optimum (blaze) wavelength falls. Blurring affects the practical resolution at the detector and will vary from the theoretical resolution, if blurring is severe. Blurring is measured by comparing theoretical resolution to practical resolution. The ratio should be as close to 1:1, as possible.
​Claibration Curve
​A graph or equation generated using calibration standards and a blank, which defines the relationship between the concentration of an element and the signal intensity produced at the wavelength of interest. This relationship is used to determine the analyte concentration in unknown samples.
​Charge-Coupled Device (CCD)
​A charge-coupled device is a two-dimensional array detector that reads the charge accumulated on the photosensitive surface by moving it sequentially down to the next row of pixels before being read out. The disadvantage of CCD technology is that the charge is lost when the accumulated charge on the detector is read out. A variation on the CCD is the SCD (Segmented Array Charge-Coupled Device).
​Charge-Injection Device (CID)
​A charge-injection device is a two-dimensional array detector that reads the charge accumulated on the photosensitive surface and then stores it by injecting it into the silicon substrate. The benefit of CID technology is that the charge is not destroyed after being read-out, allowing for interrogation of the data at any time.
​Charge-Transfer Device (CTD)
​A charge-transfer device is a generic name given to a solid-state detector which may be used for spectrochemical applications in order to convert photons into electrical energy.
​Cooled Spray Chamber
​A spray chamber that is cooled in order to keep the sample aerosol at a constant temperature. For hassle-free aspiration of organic samples, the temperature is lowered using a Peltier cooling device, to reduce the amount of solvent reaching the plasma.
​Crystal-Controlled (RF) Generator
​This is a class of radio frequency (RF) generator that uses the principle of an oscillating crystal source to control the frequency of operation. Refer to “Free-Running (RF) Generator”.
​Czerny-Turner Spectrometer
​This is a design of scanning monochromator that uses two collimating mirrors and a stepper motor driven, ruled or holographic, diffraction grating. As the incident light from the plasma falls upon the grating, it is separated into its various component wavelengths of light corresponding to the elements present in the sample. The element of interest is selected by moving the grating in small steps to focus the chosen spectral emission line onto an exit slit with a PhotoMultiplier Tube (PMT) detector behind it. 
​Demountable Torch

​A demountable torch is an ICP torch where the three concentric tubes (injector tube, quartz inner and quartz outer tubes) are not permanently connected together. Refer to “Plasma Torch”.Detection Limits: Refer to “Instrument Detection Limit”.
​Detectors
​Detectors in ICP-OES are devices which convert light energy (photons) from the plasma into electrical energy. Examples are PhotoMultiplier Tubes (PMT) and Charge-Transfer Devices (CTD).
​Diffraction Grating
​A diffraction grating is an optical component with many finely spaced, etched or ruled, grooves on its reflecting surface. Gratings are used to disperse light according to wavelength and/or spectral order.
​Direct Current Plasma (DCP)

​This is a high temperature plasma discharge that is created by a direct current flowing through a conductive gas, between a set of two or three electrodes. DCP technology had many limitations, based on its low temperature compared to ICP, and is not commercially-available today.
​Dispersion
​This is a measure of how well wavelengths from the ICP spectrometer are separated in space along the focal plane. Usually expressed as “Linear Dispersion”, in nm/mm, the lower this value, the better the dispersion. An important relationship exists between resolution, dispersion, and the amount of energy that reaches a detector: Resolution (in nm) is directly proportional to the dispersion and primary slit image size. Energy (of light at the detector) is directly proportional to the size of the image focused on the detector, which in turn depends on entrance slit width. A smaller entrance slit width gives smaller (better) resolution, but decreases the amount of light reaching the detector. Optics with higher dispersion will be able to achieve better resolution with wider entrance slits. Refer to “Entrance Slit” and “Exit Slit”.
​Double-Pass Spray Chamber
​A spray chamber that is comprised of an inner (central) tube inside the main body of the spray chamber. The smaller droplets are selected by directing the aerosol from the nebulizer into the central tube. The aerosol emerges from the tube, where the larger droplets fall out (because of gravity) through a drain tube at the rear of the spray chamber. The smaller droplets then travel back between the outer wall and the central tube into the sample injector of the plasma torch. The most common type of double-pass spray chamber is the Scott design. Dynamic Range: The concentration’s range over which a calibration curve is linear. It is usually expressed as orders of magnitude and is a measurement of the maximum concentration that can be measured at a particular analyte wavelength. An ICP is capable of up to 6 orders of linearity (low ppb to high ppm levels).
​Echelle Grating
​Echelle gratings are precisely-ruled diffraction gratings used in ICP spectrometry that utilize very high angles of incidence, very low ruling density (grooves/mm), and provide the use of high orders of diffraction, typically in the range of 20-120th order, using a prism as the 2nd dispersion optic. The main benefits of Echelle gratings for ICP-OES is that they provide an optimum combination of resolution, dispersion, and high light throughput for multi-element detection on a small CTD detector.
​Emission Intensity
​This is the measurement of the amount of light (photons) at a given wavelength (emission line) emitted from a spectral source such as an ICP. 
​Entrance Slit
​A narrow slit through which light enters a spectrometer. The size of the entrance slit will determine the resolution of the spectrometer.
​Excitation
​This is a process in which an electron is promoted to a higher energy level, resulting in an atom or ion said to be in an excited state.
​Excitation Potential
​This is a measure of the amount of energy required to promote an electron in an atom to a higher energy level. This excited state is necessary for the atom to emit light. Different elements have different excitation potentials, based on their electronic configuration.
​Exit Slit
​A narrow slit through which light exits the spectrometer before it is converted into an electric current by the detection system. The size of the exit slit will dictate the resolution of the spectrometer.
​Fixed Optics
​Fixed optics do not move, such as those found in all direct-reading echelle sequential and simultaneous spectrometers. Moving optics, such as those found in all peak search (i.e. Czerny-Turner) spectrometers, are inherently unstable and require integration at several points to acquire correct peak wavelength data. This leads to extended analysis times and incorrect peak wavelength identification. Fixed optics integrate directly on-peak, and therefore are not subject to the pitfalls associated with moving optics.
​Free-Running (RF) Generator
​This is a class of radio frequency (RF) generator that uses the electronic constants of their circuit components to determine and control the frequency of operation, without the use of external triggers such as an oscillating crystal unit.
​Focal Length
​An optical element (e.g. mirror, prism, lens, or curved grating) that focuses light on exit slits. The focal length is the distance from the optical element to the point of focus. In the early days of spectrometer development, increasing focal length (and consequently the length and size of the instrument) was the only way to improve resolution. Improved grating production methods have eliminated this requirement, allowing for the compact instruments we enjoy today.
​Grating
​The grating is the optical element that disperses light. Echelle gratings provide exceptionally high resolution, dispersion, and light throughput. Conventional gratings are manufactured by the mechanical ruling of concave blanks or via holographic reproduction techniques. Concave ruled and holographic gratings typically do not compare in optical performance with Echelle gratings.
​Grating Density
​ The number of lines or grooves per unit length on a diffraction grating, usually expressed as lines/mm or grooves/mm. The higher the grating density, the greater the dispersion of the light diffracted by the grating. 
​Grating Drive
​The grating drive is the mechanism used to move the grating by sequential spectrometers without fixed optics. Common grating drives include stepper motor-driven worm gears, sine-bar drives, as well as galvanometer systems.
​Holographic Grating
​A holographic grating is a type of diffraction grating formed by an interference-fringe field of two laser beams whose standing-wave pattern is exposed to a polished substrate coated with a photoresist material. Photolithography processing of the exposed medium results in a pattern of straight lines with a smooth sinusoidal cross-section, as opposed to sharp cut edges with a traditional ruled grating.
​Hydride Generation (HG)
​A very sensitive analytical technique for determining trace levels of volatile elements such as As, Bi, Sb, Se, and Te. Generation of the elemental hydride is carried out in a closed vessel by the addition of a reducing agent, such as sodium borohydride, to the acidic sample. The resulting gaseous hydride is swept into the plasma where atomization occurs. Multi-element quantitation is then carried out in the conventional way, by comparing the emission intensity of unknown samples against known calibration or reference standards. Hydride generation can also be used with AA using a special heated quartz cell in place of the traditional flame burner head.
​Inductively Coupled Plasma (ICP)
​ The high-temperature source used to generate ions in ICP-OES. It is formed when a tangential (spiral) flow of argon gas is directed between the outer and middle tube of a quartz torch. A load coil (usually copper) surrounds the top end of the torch and is connected to an RF generator. When RF power (typically, 750–1500 W) is applied to the load coil, an alternating current oscillates within the coil at a rate corresponding to the frequency of the generator. The RF oscillation of the current in the coil creates an intense electromagnetic field in the area at the top of the torch. With argon gas flowing through the torch, a high-voltage spark is applied to the gas, causing some electrons to be stripped from their argon atoms. These electrons, which are caught up and accelerated in the magnetic field, then collide with other argon atoms, stripping off still more electrons. This collision-induced ionization of the argon continues in a chain reaction, breaking down the gas into argon atoms, argon ions, and electrons, forming what is known as an “inductively coupled plasma (ICP) discharge” at the open end of the plasma torch.
​Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)
​A multi-element technique that uses an inductively coupled plasma to excite ground-state atoms to the point where they emit wavelength-specific photons of light, characteristic of a particular element. The number of photons produced at an element-specific wavelength is measured using a spectrometer to separate the analyte wavelengths and a photon-sensitive detection system (PMT or CTD) to measure the intensity of the emission signal produced. This emission signal is directly related to the concentration of that element in the sample. Commercial instrumentation comes in two configurations: a traditional radial view, where the plasma is vertical and is viewed from the side (side-on viewing), and an axial view, where the plasma is positioned horizontally and is viewed from the end (end-on viewing).
​Instrument Detection Limit (IDL)

​Also called Detection Limit (DL) or Limit of Detection (LOD)​
​The concentration equivalent to the analyte signal which is equal to three times the standard deviation of a series of 10 replicate measurements of the calibration blank signal at the same wavelength.
​Integration Time:
​This is the length of time the detector is allowed to acquire an intensity measurement. The most precise measurements are taken at the wavelength peak intensity. Some instruments that do a peak-search do not integrate the actual peak signal, but instead calculate an intensity measurement by fitting a curve to many measurements made at wavelength intervals across the width of the peak.
Inter-Element Interference Correction (IEC): 
When emission lines from two different elements overlap at the exit slit, light measured by the detector becomes a measurement of the combination of both signals. The undesirable element interferes with the correct analyte at the wavelength in question, and is subsequently deemed an “inter-element interference”. If wavelengths which are free of inter-element interferences for the desired element cannot be found (at the desired levels of sensitivity), the spectrometer approximates a correction (an “inter-element correction”) to remove the interfering element signal. This is performed by calibrating a solution containing only the interfering element at a separate wavelength, and comparing the ratio of concentration at this separate interfering element wavelength to that observed at the original desired element’s wavelength. In this way, any time the interfering element is detected at its separate wavelength, a corresponding proportional deduction is made at the desired analyte wavelength. Some spectrometers, such as Echelle systems, suffer less from inter-element interferences, because of their high resolving power.
​Internal Standardization (IS)
​A quantitation technique used to correct for changes in analyte sensitivity caused by variations in the concentration and type of matrix components found in the sample. An internal standard is a non-analyte element that is added to the blank solution, standards, and samples before analysis. It is typical to add two or three internal standard elements to the samples to cover all the analyte elements of interest across the wavelength range. To work properly, it is critical that the internal standards are of a similar excitation potential to the analytes and that they are monitored at exactly the same time (simultaneously). The software adjusts the analyte concentration in the unknown samples by comparing the intensity values of the internal standard elements in the unknown sample to those in the calibration standards. Because ICP-OES is prone to many matrix and sample-transport-based interferences, internal standardization is considered necessary to analyze most sample types.
​Laser Ablation
​A sample preparation technique that uses a high-powered laser beam to vaporize the surface of a solid sample and sweep it directly into the ICP-OES system for analysis. It is mainly used for samples that are extremely difficult to get into solution or for samples that require the analysis of small spots or surface inclusions. Depth profiling and surface rastering studies are also possible with a laser ablation system.
Linear Dynamic Range
Linear dynamic range is the concentration range that an element may be calibrated for at a particular wavelength. Linear dynamic range is dependent on source conditions and detector, the latter of which is often the limiting factor. For example, PMTs routinely accommodate a linear dynamic range of 106 or better, but solid-state detectors, such as SCCD’s and CID’s can be limited to 104. Lower linear dynamic range forces the use of smaller integration times, sample dilutions, or use of alternative wavelengths when exceeding the limits of the range.
​Load Coil
​ Another name for the RF coil used to generate a plasma discharge. Refer to “Matching Network (RF)”. 
​Limit of Quantification (LOQ)
​The limit of quantification is the lowest level at which reliable measurements can be made during an analysis. Typically, this is approximately three times the detection limit. See also Detection Limits.
​Matrix Interferences
​There are basically two types of matrix-induced interferences in ICP-OES. The first, and simplest to overcome, is often called a “sample transport effect” and is a physical suppression of the analyte signal brought on by the level of dissolved solids or acid concentration in the sample. The second type of matrix suppression is caused when the sample matrix affects the excitation conditions of the plasma discharge, which results in varying amounts of signal suppression depending on the concentration of the matrix components. The classical way to compensate for matrix interferences in ICP-OES is to use internal standardization. Refer to “Internal Standardization”.
​Method Detection Limit (MDL)
The minimum concentration of an analyte that can be identified, measured, and reported with 99% confidence that the analyte concentration is greater than zero.

The MDL is measured by fortifying a reagent blank with analyte at a concentration 2 to 3 times the IDL.  This solution is then analyzed 7 times and the standard deviation of the analyte concentration is multiplied by 3.14 (the students t-statistic for a 99% confidence interval).Microwave Digestion: A method of digesting difficult to dissolve solid samples using microwave technology. Typically, a dissolution reagent such as a concentrated mineral acid is added to the sample in a closed acid-resistant vessel contained in a specially designed microwave oven. By optimizing the current, temperature, and pressure settings, difficult samples can be dissolved in a relatively short time compared to traditional hot plate sample digestion techniques.

​Monochromator
​A monochromator is a spectrometer design used in sequential ICP that presents one wavelength at a time to the detector.
​Monochromatic Light
​Generally referred to as the separation of white light into its different spectral components or wavelengths.
​Nebulizer

​A nebulizer is a sample introduction device that creates a fine aerosol of sample solution for delivery to the plasma source. Various types of nebulizers include:

  • ​​​​​Babington-Type (or High Solids) Nebulizer: Sample solution is pumped through an orifice located at the top of a flat, circular face at one end of the nebulizer, from where it proceeds (by gravity) down a groove (often described as a “V-groove”). At the bottom of the groove is another orifice, through which argon carrier gas projects, causing the sample to form an aerosol. This nebulizer is commonly used for the delivery of wear-metals in oil solutions, or for the introduction of samples containing high amounts of dissolved solids. 
  • Concentric Nebulizer: This nebulizer is composed of two concentric tubes which meet very closely (but do not touch) at the delivery end. Sample is pumped or allowed to freely aspirate through the central tube, while argon carrier gas is introduced through the outer tube. A resulting aerosol is generated at the delivery end.
  • Cross Flow Nebulizer: Sample is pumped into a cross-flow nebulizer through an orifice, at which point it is directly sheared with the impact of ​​a fast-flowing stream of argon carrier gas presented at a right angle to the sample flow, causing it to burst into an aerosol.
  • Hildebrand Grid Nebulizer: Sample is pumped through an orifice onto a fine wire mesh grid, at which point the impact of a fast flowing stream of argon carrier gas converts it into an aerosol. This is a multi-purpose nebulizer, commonly associated with aqueous applications. It is also compatible with hydrofluoric acid (HF) solutions.
​Microconcentric Nebulizer

​The microconcentric nebulizer is based on the concentric nebulizer design, but operates at much lower flow rates. Conventional nebulizers have a sample uptake rate of about 1 mL/min with an argon gas pressure of 1 L/min, whereas microconcentric nebulizers typically run at less than 0.1 mL/min and typically operate at much higher gas pressure to accommodate the lower sample flow rates.
​Ultrasonic Nebulizer
​Ultrasonic nebulizers are configured with a transducer assembly containing a piezoelectric ceramic material bonded to a thin quartz blank (often referred to as a “crystal”). While sample solution is allowed to drip over the crystal, the ceramic material is caused to oscillate by a small RF generator (usually operating at about 1.4 MHz). The energy provided to the crystal results in the sample bursting into an ultra-fine cloud or mist. This mist is passed first through a heated tube, which is hot enough to effectively drive off much of the solvent from the analyte present. Subsequently, the mist is passed through a cooling tube, which condenses and removes the separated solvent via gravity drain. The end-product remaining is a dry aerosol, which is then passed directly to the torch. The ultrasonic nebulizer excels over conventional nebulizers in two ways: First, it is much more efficient at delivering sample to the torch - 10% efficiency, as opposed to 3% for typical pneumatic nebulizers. Second, by removing the solvent, less quenching of the plasma occurs, providing better effective excitation within the ICP itself. In this way, ultrasonic nebulizers typically provide a factor of 10 enhancement over standard pneumatic nebulizers.
​Normal Analytical Zone (NAZ)
​This is the region of the plasma where the most stable and useful analyte emission originates.
​Order (Spectral)

​Gratings disperse light spatially by wavelength. Light of individual wavelengths leaves the grating at specific angles. Most of the light leaves at a particular angle, slightly less at a slightly larger angle, and so on. Looking at white light dispersed by a grating, you will see a series of spectral “rainbows”. These rainbows are actually different orders of the light from the grating. When an echelle grating is employed, these orders are spaced quite close together (actually overlapping), allowing the use of very high orders (typically 20th-120th) for measurement. A sorting device (usually a prism) separates the orders after they come from the grating, providing an (ICP) echelle spectrometer with a two-dimensional (“X-Y”) array of resulting light on its focal plane. Conventional gratings typically use only 1st-4th orders, which are not sorted. The resulting spectral display from a conventional grating is a linear array from shortest to longest wavelength, in which light from order multiples overlap.
​Paschen-Rünge Spectrometer
​This is an optical design used in emission spectroscopy, where the diffraction grating mount and exit slits are stationary and fixed in position in relation to the detector. Traditional mounting for an ICP-OES is in the Rowland circle configuration. 
​Peltier Cooler

​A thermoelectric cooling device that uses a temperature gradient between two different materials to generate a cold environment. It uses electrical energy via a solid-state heat pump to transfer heat from a material on one side of the device to a different material on the other side, thus producing a temperature gradient across the device (similar to a household air conditioning system) In ICP-OES, they are typically used to cool the solid state detector or the sample introduction system.
​Peristaltic Pump
​A pump in which the fluid is pushed through a length of flexible tubing by waves of mechanical contractions, usually caused by a series of rollers that travel along the length of the tubing.

​PhotoMultiplier Tubes (PMT)
​PMT’s are one type of detector used in ICP instruments. When struck by incident light, a PMT generates a current proportional to the intensity of the input light. This current can be multiplied by several orders of magnitude (typically 6-8), and processed as an output signal by a readout device. Simultaneous ICPs use an array of fixed PMT’s optimized for particular wavelengths. Sequential ICPs use one or two broad-range PMT’s to cover the entire ICP spectrum. The signal from each PMT has an Analog/Digital (A/D) converter that changes current to a digital signal, registering intensity as counts integrated over a given time period.
​Pixels
​ These are the photosensitive regions on the surface of a Charge-Transfer Device (CTD) detector. As photons of light strike the surface of the detector, each pixel accumulates an electrical charge.

​Plasma Source
​Refers to the RF hardware components that create the plasma discharge, including the RF generator, matching network, plasma torch, and argon gas pneumatics.
​Plasma Torch
​Another name for the quartz torch that is used to generate the plasma discharge. The plasma torch consists of three concentric tubes: an outer tube, middle tube, and sample injector. The torch can either be one piece, where all three tubes are connected, or it can have a demountable design, in which the tubes and the sample injector are separate. The gas (usually argon) that is used to form the plasma (plasma gas) is passed between the outer and middle tubes at a flow rate of 12–17 L/min. A second gas flow (auxiliary gas) passes between the middle tube and the sample injector at 1 L/min, and is used to change the position of the base of the plasma relative to the tube and the injector. A third gas flow (nebulizer gas), also at 1 L/min delivers the sample, in the form of a fine-droplet aerosol, from the sample introduction system and physically punches a channel through the center of the plasma. The sample injector is often made from other materials besides quartz, such as alumina, platinum, and sapphire, if highly corrosive materials need to be analyzed.
​Pneumatic Nebulizer
​A type of nebulizer that uses the impact of a high-speed gas flow on a liquid to create an aerosol.
​Polychromator
​A polychromator is the spectrometer design of a simultaneous ICP; it measures multiple wavelengths at the same time.Polychromatic Light: This is commonly referred to as white light consisting of multiple wavelengths.
​Precision
​The reproducibility of a set of measurements. Typically expressed as standard deviation (SD) when measuring raw emission counts, or relative standard deviation (RSD) or coefficient of variation (CV) when measuring concentration units.
​Purged/Vacuum Spectrometer
​A purged spectrometer is continuously displaced inside with nitrogen or argon gas, or alternatively evacuated (in a vacuum system). This removes oxygen from the system, allowing use of emission wavelengths in the short-UV region of the ICP spectrum (below 190 nm). Sulfur determinations, with only two wavelengths (180 and 182 nm) available, require a purged or vacuum spectrometer for analysis via ICP.
​Rapid Sampling Systems
​These are automated delivery systems that minimize delivery time of the sample to the sample introduction system, as well as optimizing the washout time after the analytical measurement has been carried out. They offer quite a significant increase in sample throughput over traditional autosamplers.
​RF Generator
​The RF generator provides a radio frequency alternating current to sustain the ionized argon plasma at a temperature of approximately 8,000 °C. Two popular RF generator designs include the “tuned oscillator” (or “free-running”) design, and the older “crystal-controlled” design. The tuned oscillator is smaller and more tolerant of sample variations (organic and aqueous). The crystal-controlled design is much larger and requires a costly electro-mechanical impedance-matching device to control power levels. The inherent design of the tuned oscillator demands much less strain on the power tube, increasing its lifespan over those in a crystal-controlled system. Two frequencies are commonly used for RF power, 40.68 and 27.12 MHz. While both provide similar performance, there is conclusive evidence that 40.68 provides higher tolerance to variation in sample matrices.
​Radial View
​Radial positioning of an ICP torch allows the spectrometer to view the plasma from the side. This is the classic viewing position of the ICP, and the torch is normally positioned in a vertical orientation.
​Resolution
​Resolution or resolving power is one of the most important performance specifications in ICP spectrometry. It is defined as the wavelength difference in nanometers between adjacent distinguishable emission lines. Because the ICP source excites all the atoms in a sample to emit very complex spectra, the light coming from the ICP contains many discrete wavelengths requiring separation. The spectrometer separates white light into individual spectral lines so that each wavelength may be integrated independently. The higher the resolution of the spectrometer, the closer the two lines may be located and analyzed independently. Poor resolution requires the use of inexact inter-element corrections to account for spectral overlap, thereby degrading accuracy. It is a well-recognized fact that Echelle-based spectrometers provide the best optical resolution currently available.
​Ruled Gratings
​Ruled gratings are diffraction gratings where the grooves are ruled or scribed into the surface of the mirror with a diamond cutting tool.
​Sample Introduction System
​The portion of the instrument that delivers the liquid sample to the plasma torch as a fine-droplet aerosol. It is comprised of a nebulizer to generate the aerosol, a spray chamber to only allow smaller droplets to reach the plasma discharge (rejecting larger droplets) and a drain system to take the redundant sample to waste. Refer to “Nebulizer” and “Spray Chamber”.
​Sample Injector
​The central tube of the plasma torch that carries the sample aerosol and the nebulizer gas. It can be a fixed part of the quartz torch or it can be separate (demountable). Sample injectors can be made from a variety of materials such as alumina, platinum, and sapphire, for the analysis of highly corrosive materials.
​Sample Transport Interferences
​A term used to describe a physical suppression of the analyte signal caused by matrix components in the sample. Sample transport interferences are more exaggerated with samples having high levels of dissolved solids, because they are transported less efficiently through the sample introduction system than aqueous-type samples.
​Scan
​A scan is a plot of light intensity vs. wavelength (in an ICP), usually centered on a particular emission wavelength. Scans are most commonly used during method development for locating background correction points or checking for interferences. Scans can also be useful as a quantitative analysis tool. 
​Scanning Spectrometers
​These are sequential spectrometers which use the principle of mechanically or electrically moving a dispersion optic (typically a ruled or holographic grating) to select a particular wavelength of light corresponding to the element of interest.
​SCD Technology
​A refinement of the CCD technology is the Segmented-array Charged-Coupled device Detector (SCD), which was designed for a particular kind of Echelle optical system. Instead of using a traditional CCD with hundreds of thousands of pixels, the SCD was designed with over 200 small sub arrays, positioned to take advantage of 3-4 of the most common emission lines of the 70 elements that can be determined by ICP-OES. Because each sub array has its own photosensitive area, storage area and output electronics, it allows individual sub arrays to be addressed and read-out. This means that unlike conventional CCD technology, the information stored on the small SCD sub arrays may be rapidly interrogated in any sequence or order. However, like all CCD technology, the charge is destroyed in the process of being read-out. Additionally, the SCD has a limited (“segmented”) wavelength range, and can only analyze wavelengths within the regions of its segments. As a result, it is analytically blind in all other areas on the detector.
​Self-Absorption
​The absorption of intense analyte emission by atoms or ions of that same analyte element within the plasma. Self-absorption occurs at high analyte concentrations and is generally responsible for determining the maximum linear concentration for strong analyte emission lines.
​Sequential ICP
​A sequential ICP measures elements one wavelength at a time. This design allows selection of any useful ICP wavelength, which provides the benefit of a wavelength selection not prone to potential interferences or for the analysis of uncommonly analyzed elements. 
​Signal-to-Background Ratio (S/B)
​The ratio of the signal intensity of an analyte to its background level at a particular wavelength. When considering the noise of the background signal (standard deviation of the signal), the S/B ratio is typically used as an assessment of the detection limit for that element. Refer to “Detection Limit” and “Background Signal”.
​Simultaneous ICP
​A simultaneous ICP measures all desired elements in a sample at the same time. 
​Slits
​Slits are image-forming or masking devices. Light from an ICP source passes through an entrance slit, through the optics, and through an exit slit to a detector. The larger the slit, the more light is allowed through (resulting in better sensitivity at the detector), but at a cost of diminished resolution. Refer to “Resolution” and “Dispersion”.
​Spray Chamber
​A spray chamber is a device, placed between a nebulizer and an atomization/excitation source, whose function is to separate the aerosol droplets according to their size and to smooth fluctuations in the sample-carrying gas flow. Various types of spray chambers include:
  • ​Cyclonic Spray Chamber: A spray chamber that operates using the principle of centrifugal force. Droplets are discriminated according to their size by means of a vortex produced by the tangential flow of the sample aerosol and argon gas inside the spray chamber. Smaller droplets are carried with the gas stream into the sample injector of the plasma torch, while the larger droplets impinge on the walls and fall out through the drain.
  • Impact Bead Spray Chamber: A type of spray chamber more commonly used in atomic absorption spectrometers. The aerosol from the nebulizer is directed onto a spherical bead, where the impact breaks the sample into large and small droplets. The large droplets fall out due to gravitational force and the smaller droplets are directed by the nebulizer gas flow into the sample injector of the plasma torch.
  • Scott Spray Chamber: A sealed spray chamber with an inner tube inside a larger tube. The sample aerosol from the nebulizer is first directed into the inner tube. The aerosol then travels the length of the inner tube, where the larger droplets fall out by gravity into a drain tube and the smaller droplets return between the inner and outer tube, where they eventually exit into the sample injector of the plasma torch. 
Spectral Interferences
A generic name given to interferences that produce a spectral overlap or shift at or near the analyte wavelength of interest. It is usually produced by a component in the sample matrix.​
​Stability

​The ability of a measuring device to consistently replicate a measurement. In ICP-OES, it usually refers to the capability of the instrument to reproduce the signal intensity of the calibration standards over a fixed period of time without the use of internal standardization. Short-term stability is generally defined as the precision (as %RSD) of 10 replicates of a multi-element solution, measured over a few minutes. Whereas long-term stability is defined as the precision (as %RSD) of a multi-element solution over a 4–8 hour time period. It can also refer to wavelength calibration stability, which is the ability of the spectrometer to repeatedly replicate the correct wavelength position every time, when carrying out a multi-element analysis.
​Standard Reference Materials (SRM)
​ Well-established reference standards that come with certified values and associated statistical data which have been analyzed by other complementary techniques. Their purpose is to check the validity of an analytical method, including sample preparation, instrument methodology, and calibration routines, to achieve sample results that are as accurate and precise as possible and can be defended under intense scrutiny.
​Subarray
​A subarray is a group of pixels on an array detector which are strategically positioned to measure a small, specific wavelength range. 
​Torch
​A torch is a device constructed of concentric tubes (usually quartz) that contain the flows of argon (coolant or “outer”, auxiliary or “plasma”, and sample carrier gases) to the ICP. The two common types of torch are “demountable”, in which all tubes can be disassembled (common for hydrofluoric acid applications), and “one-piece”, for all other applications. Refer to “Sample Injector” and “Plasma Torch”.
​Transient Signal
​A transient signal is one that lasts a finite period of time. This type of analysis, sometimes called Time Resolved Analysis (TRA), presents unique challenges not seen in the more common continuous nebulization sample introduction approach. Some examples of transient signal include when an ICP-OES is used for the analysis of elemental speciation using chromatography separation techniques, spatially resolved laser ablation experiments such as depth profiling or surface ​rastering of solid samples and Flow Injection Analysis (FIA) involving the discrete introduction of samples into a continuous flow of a carrier solution.
​UV Region

​Generally refers to the spectral region of 200-350 nm. The low UV region is normally 165-200 nm and the deep UV region is considered any wavelength below 165 nm where many of the halogens have sensitive emission lines.
​Viewing Height
​The vertical position in the plasma where the analyte emission is being viewed by the spectrometer. With a radial ICP, it is quoted as the number of mm above the top of the load coil.
​Wavelength Calibration
​An instrument’s internal positioning mechanism for ensuring the correct wavelength is being measured every time
​Wavelength Range
​ This is the range of usable wavelengths for ICP analysis. Some instruments have spectrometers limited to this range, and concentrate primarily on shorter wavelengths. Alkali metals and/or alkaline earths, which are most sensitive in the longer wavelength region, may not be accessible with all ICP instruments. Auxiliary spectrometers, in addition to the main optics, are sometimes employed to measure these longer wavelengths. Not all instruments provide access to the full ICP spectrum.