Fundamental Principles, Method Development Optimization and Operational Requirements of ICP-Optical Emission

Since its introduction over 40years ago, inductively coupled plasma optical emission spectroscopy (ICP-OES) has significantly changed the capabilities of elemental analysis. This technique combined the energy of an argon-based plasma with an optical spectrometer and detection system capable of measuring low-level emission signals, which allowed laboratories to perform rapid, automated, multielement analyses at trace concentrations.1 This was approximately lOyears before the introduction of the first commercial inductively coupled plasma mass spectrometer, so ICP-OES became the workhorse instruments in many laboratories required to perform elemental analysis at trace level concentrations. This chapter, written by Maura Rury from the Applied Testing Reference Materials Division of LGC, gives a detailed description of the fundamental principles together with method development optimization procedures and operational requirements of this technique.

The advantage in using an atmospheric pressure ICP source for making optical emission measurements was first published in 1964,' and the sensitivity, speed of analysis, ease of use, and tolerance to high levels of dissolved solids are advantages that laboratories continue to rely on more than half a century later.2>3 The success of the technique itself can be measured by the fact that thousands of ICP-OES instruments have been installed between 1983 and 2020, which have resulted in approximately 59,000 publications, with over 28,000 published since 2012 (results courtesy of Google Scholar search). That published literature features elemental determinations in a variety of sample matrices in industries including: environmental, nuclear, mining and geochemistry, materials testing, semiconductor, industrial, petrochemical, clinical and toxicological, food safety, and pharmaceutical.

Basic Definitions

A full glossary exists at the end of this book for purposes of defining terms used throughout the text; however, several terms are defined here to ensure clarity while reading this chapter. Several optical emission techniques exist, based on atmospheric discharges, which include: inductively coupled plasmas (ICPs), direct coupled plasmas (DCPs), microwave-induced plasmas (MIPs), DC arcs, and

AC sparks. Each discharge is generated via a different mechanism and has its own inherent advantages and disadvantages; however, a comparative discussion of these techniques is outside the scope of this text. The remainder of this chapter will focus solely on ICP-OES.

It should be noted that ICP-AES and ICP-OES are terms that are sometimes used interchangeably; however, the former term can be a source of error and confusion. The term ICP-AES refers to “atomic emission spectroscopy” which nominally excludes emission contributions from other species such as ions and molecules. The latter term refers to “optical emission spectroscopy” and is more commonly used as it includes emission from multiple contributors. Only the term ICP-OES will be used in this text.

Principles of Emission

For most ICP-OES applications, a sample is delivered to the instrument’s plasma in the form of an aerosol. As the aerosol travels from the base of the plasma to its tail, it travels through a variety of heated zones where it gets desolvated (unless delivered as a dry aerosol), vaporized, atomized, and ionized. Further time spent in the plasma allows the atoms and ions to absorb additional energy which excites an outer electron and produces excited state species. Relaxation back to a ground-state atom produces energy in the form of a photon. This production of photons from excited atoms and ions forms the basis for atomic emission measurements. There are many species in a sample that may absorb energy from the plasma and produce emission spectra. These species include atoms, ions, and molecules. For the purpose of this section, the contribution from molecular emission will be excluded. All references to emission will include the contribution from atoms and ions only.

Atomic and Ionic Emission

Elemental analysis by ICP-OES relies on the emission from excited atoms and ions within a sample. Argon plasmas contain ~15.8eV of energy, which is sufficient to remove one or two electrons from the outer orbital of most atoms. This results in the presence of both atoms and ions in the plasma, all of which are in their ground (lowest level) energy state. Excitation, and subsequent emission, occurs when a species’ absorbed energy from the plasma is released in the form of wavelength-specific photons.

A simplified schematic of atomic absorption (AA) and emission is illustrated in Figure 25.1. The horizontal lines represent energy levels in an atom. The lowest horizontal line and the four remaining horizontal lines represent the ground state and excited states, respectively. If included

Diagram depicting energy transitions involved in an atom’s absorption and emission of energy

FIGURE 25.1 Diagram depicting energy transitions involved in an atom’s absorption and emission of energy.

in the schematic, additional horizontal lines to represent ionic ground and excited states would be illustrated above the atomic excited states. The vertical arrows represent an energy transition for an electron, following the absorption or emission of a photon. The length of each vertical arrow correlates to the amount of energy involved in the transition.

As the schematic indicates, absorbed energy can shift electrons to different excited states, both atomic and ionic. Relaxation of these excited electrons produces energy in the form of photons. Photons vary in energy and can be correlated to their associated emission wavelength using Einstein’s equation4 which relates the energy of light and its frequency according to:

where E represents the energy of light, h represents Planck’s constant, and v represents the frequency of light. In OES, it is more practical to speak in terms of wavelength, so the term c/X can be substituted to yield:

where E represents energy in Joules, h represents Planck’s constant in units of Joule seconds, c represents the speed of light in meters per second, and X represents the wavelength in units of meters. From this equation, it becomes clear that each emitted photon is wavelength specific and represents the inverse relationship between energy and wavelength. These emission wavelengths represent the energy levels that are characteristic to each element, thus making OES a useful technique for identifying and quantifying elements in unknown samples.

 
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