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Background, Theory, Use, Benefits, Drawbacks
Introduction
The technology behind inductively coupled plasma (ICP) optical emission spectroscopy (OES) is credited as being introduced and developed by Fassel, et. al. at Iowa State University and by Greenfield, et. al. in the United Kingdom in the mid-1960s. The popularity of using ICP optical analysis rose during the 1980s, as the elemental analysis technique became more accessible to the analytical laboratory, leading to decreased instrument and method development costs. ICP-OES is regarded as one of the most popular techniques for the analysis of trace elements in a variety of sample types, most notably for its simple method development and capacity for high throughput.
- How does plasma play a role in elemental analysis?
- What is the difference between ICP-OES and ICP-AES?
- How does ICP-OES or ICP-AES differ from ICP-MS?
- How do you choose a method?
We will explore that in this special technology “face-off.”
ICP-AES vs. ICP-OES Background: It Starts with Plasma
Plasma is a gaseous mixture containing cations and electrons that retains electrical conductance. The key to using plasma for elemental analysis is found in the interaction of molecules with electromagnetic radiation. Acid digestion and hydride generation are often used to prepare the sample solution for effective elemental analysis. The solution enters a spray chamber and is carried by argon gas into a torch heated to approximately 10,000 °C, which ionizes the argon discharge. In ICP-OES and ICP-AES, the ionized gaseous mixture emits photons that are collected by a lens or concave mirror. The emission of atoms, in this case, photons, lends this technology its hyphenated sub-names: -OES and -AES. “OES-optical emission spectroscopy” and “AES-atomic emission spectroscopy” effectively represent the same method and technology used for elemental analysis. The names have been used interchangeably over the decades. From here on in this manuscript, and, for the sake of our technology discussion, ICP-OES and ICP-AES will represent the same thing.
System Design
High detection limits are made possible by the extremely high temperatures found in plasma-based systems (up to 10,000 K). A photodetector converts the wavelength exiting the monochromator to an electrical signal. From that signal, element-identifying data are processed by a computer. The high temperature of the ICP system design contributes to the excellent atomization and excitation of many elements. In ICP-OES, the amount of element detected in a sample is proportional to the intensity of energy emitted at a particular wavelength. Because of its atomization and excitation potential, plasma allows for the simultaneous analysis of about 60 elements, far more than the single element that can be analyzed by a flame-based spectrometry technique (such as FAAS). The linear dynamic range of ICP-AES/ICP-OES is up to six orders of magnitude (106). ICP-MS provides the added bonus of being able to detect the ions themselves rather than just the photons (-OES and -AES), as ICP-MS mates the plasma coupling technology with mass spectrometry. The markedly higher sensitivity of ICP-MS allows for detection limits in the parts per trillion (ppt) range. ICP-MS boasts a greater linear dynamic range than ICP-OES, all the way up to eight orders of magnitude (108) in current instruments.
Sample Preparation
A typical “sample” fit for testing by ICP-OES analysis is a solid sample consisting of a metal, trace minerals, food substances, or other dissolved compound for which metals analysis is required. For example, a food testing laboratory might want to use ICP-OES to detect for the presence of arsenic in a sample of fish flesh. As mentioned in the preceding text, the sample might require additional steps of acid digestion or hydride generation for the most effective analysis. The sample is solubilized in the appropriate medium, dissolved, and injected into the instrument. A nebulizer within the instrument injects a gaseous mixture of an inert gas and the solubilized, sometimes acidified, sample into the test chamber.
What about ICP-MS?
ICP-MS differs from ICP-OES in that it builds upon the principles of ICP-OES and utilizes ICP to dissociate atoms from a sample and then sends those atoms into a mass spectrometer (MS) system to separate the atoms or ions based on their mass-to-charge ratios. Therefore, it provides an extra set of data: valuable isotopic information.
ICP-OES vs. ICP-MS: Practical for your laboratory?
ICP-OES: Uses, Benefits, Drawbacks
ICP-OES is regarded as suitable for the detection of most elements (73), with the exception of radioactive elements requiring analysis by gamma-ray spectroscopy, the halogen group, and trace contaminants found in the argon gas mixture that is used in the ICP-OES testing procedure. Common areas of application include food and beverage, environmental, toxicology, photonics, agricultural testing, petrochemicals, and other areas of application where rapid elemental analysis is of great interest. ICP-OES efficiently measures 1 to 60 elements per minute. Method development is relatively simple, and “analytical grade” solvents and reagents may be used for testing. Running ICP-OES samples does not require the attention of a specialist: a method can be calibrated by a specialist and run by average laboratory personnel. Medium sample volumes are best for ICP-OES. The biggest drawback is the high potential for spectral interference. Also, an ICP-OES system requires a high-volume gas installation in the laboratory.
ICP-MS: Uses, Benefits, Drawbacks
ICP-MS boasts excellent detection limits. By passing the positive ICP sample ions through a quadrupole mass filter and then the mass detector, the ICP-MS provides isotopic information and mass data. Of comparative elemental analysis techniques, including ICP-OES, FAAS, and GFAAS, ICP-MS is able to detect the largest number of elements (82). Very small volume samples can be used in ICP-MS. Run times for both ICP-OES and ICP-MS are relatively short, but ICP-MS is able to detect most elements in less than one minute, and even older systems can detect all the elements in less than five minutes. Some spectral and isotopic interference may be seen in ICP-MS, as can matrix effects and ionization effects. Some elements (S, K, Ca, Se, B, Br, etc.) have high detection limits via ICP-MS and appropriate configurations must be made to enable efficient testing.
An excellent ICP-MS set up may cost 2 to 3 times the cost of an ICP-OES system. The coating on ICP-MS detectors is light sensitive and wears off as ions hit the detector surface, so it must be replaced as needed. Operation can be left unattended, but should be monitored by a highly knowledgeable specialist, as method development is significantly more difficult than for ICP-OES. The operating costs for an ICP-MS system typically are and can be considerably greater than for an ICP-OES system, especially since cleanroom conditions must often be established to properly test in the parts per trillion (ppt) level. High purity grade reagents must be used with ICP-MS.
How to Choose ICP-OES vs. ICP-MS
The specificity of elemental analysis required and the laboratory budget are the biggest considerations in choosing an ICP-OES system over an ICP-MS system. The amount of samples to be tested per day, as well as regulatory testing standards and frequency of changing analytical requirements, may factor into selecting a system. For the broadest range of information, an ICP-MS is ideal as it provides isotopic information. However, the cost of installing and maintaining the proper conditions for an ICP-MS system may be over two to three times the cost of a highly suited ICP-OES system, making long-term budget an important factor in the purchasing consideration. Method development for an ICP-OES system is simpler than for ICP-MS systems. A laboratory should also factor in reagent costs, replacement parts, calibration, and training of personnel and cleanroom requirements into their operating budget for an ICP instrument.
References
- Meyers, R.A. Encyclopedia of Analytical Chemistry pp. 9468-9485. UK: John Wiley & Sons, Ltd., 2000.
- AAS, GFAAS, ICP or ICP-MS? Which technique should I use? An elementary overview of elemental analysis. MA: Thermo Elemental, 2001. http://www.thermo.com/eThermo/CMA/PDFs/Articles/articlesFile_18407.pdf
- Tyler, Geoffrey. ICP-MS, or ICP-AES and AAS?—a comparison. Australia: Varian, 1994. http://concours.univ-lyon1.fr/servlet/com.univ.collaboratif.utils.LectureFichiergw?ID_FICHIER=1320397709943
- What is ICP-MS?... and more importantly, what can it do?. U.S. Geological Survey: Crustal Geophysics and Geochemistry Science Center. http://crustal.usgs.gov/laboratories/icpms/intro.html
Emilia Raszkiewicz is Managing Editor, American Laboratory/Labcompare; e-mail:[email protected].