Energy and Elements, Part II: A Closer Look at ICP

In this issue, we will continue the discussion of energy and elements used in atomic spectroscopy by looking at one of the most commonly used analytical techniques in the elemental analytical laboratory—inductively coupled plasma-atomic emission spectroscopy (ICP-AES). This technique is used to achieve high levels of accuracy while being able to discern increasingly low levels of detection in comparison with some of the other techniques we have reviewed. We will examine the structure and function of modern ICP systems to understand how to optimize these techniques for different types of samples and analytical targets.

The discussion of atomic spectroscopy started with the basic principles of changes in energy states that result in the absorption, emission, and other changes in energy measured by atomic absorption spectroscopy (AAS) or atomic emission spectroscopy (AES). In the first elemental spectroscopy column (September 2022), we looked at the form and function of many absorption techniques and started to examine the fundamentals of emission spectroscopy leaving off with the fundamental skeleton of inductively coupled plasma (ICP) (1). The basic components of emission spectroscopy techniques are similar, starting with sample induction to a nebulizer then the passage of the sample to an excitation source leading to optics that directs emission to a detector (see Figure 1).

Investigating ICP

Most modern laboratories rely heavily on atomic emission spectrometers with a plasma source either ICP-AES or ICP-mass spectrometry (MS). The sample introduction and nebulization systems for ICP systems are similar or identical to the nebulizers described previously for other emission spectrometers. The primary difference between ICP and other emission spectrometers is the excitation source and the detector. In ICP, the excitation source is plasma—a homogenous mixture of ionized gases (usually containing argon) composed of electrons, ions, and neutral species. ICP systems contain an ICP torch where argon moves an aerosolized sample through an applied Rf power supply. The plasma is ignited by a spark from a Tesla coil (see Figure 2).

The early generations of ICP systems used a side-on or radial view of the plasma. The radial view takes a plasma cross-section from the side (Figure 3a). Later, alternative configurations developed end-on or axial observation of the plasma along the entire length of the plasma (Figure 3b).

Modern systems can have either type of viewing or be a dual configuration (combination of both axial and radial viewing). In dual view systems, the torch is oriented most often in a horizontal configuration and optics are used to create the axial and radial views. This horizontal configuration allows the optics to stay cleaner as the heat and exhaust of the torch vents away from the path of the optical components.

Radial plasma tends to have less sensitivity than the axial configuration but is preferred with samples with complex or difficult matrices. Samples with large amounts of organic compounds or dissolved solids perform better with a radial view opposed to the axial view. One of the reasons is that an axial view boosts sensitivity of all the elements in the plasma including the background and can be susceptible to interference.

ICP Interferences

An interference is any effect that can alter the measurement of the target elements. In ICP spectroscopy there are three important types of interference: physical, chemical, and spectral.

Physical interferences are the differences between the samples being tested and any standards, calibration curves or blanks that can alter the sample introduction and nebulization. Most of the difference stems from the makeup of the matrices of the sample and standards. Changes in density, viscosity, and dissolved organic matter from either the matrix or, as a result of any digestion or preservation techniques, can create these physical interferences. Some of these difficulties can be addressed by diluting samples, using internal standards, standard addition, and matrix matching.

Chemical interferences are differences in the way in which a sample, standard, or element reacts in the plasma. These interferences can result in ionization effects, formation of molecules, or changes to the plasma. Some elements, such as alkali metals, often are seen to have ionization interactions resulting in interferences. Chemical interferences can be addressed by adding ionization buffers.

Spectral interferences are changes to the optical or detector spectral response from background shifts, interference from other elements in the sample, or spectral overlaps. In background shifts, the baseline or background of the sample is different between the sample injection and injections of standards, calibrants, or blanks. These shifts can occur because of the presence of an element in one of the injections that is not present or in the same concentration as the other injections.

Simple background interference is the difference in the baseline, either higher or lower than the target sample’s baseline in the immediate area of a targeted spectra (see Figure 4a). A sloping background shift is also often due to the presence or concentration of another element that causes the baseline or background to slope either upward or downward in the area of a target spectrum (see Figure 4b). Complex background shifts are interferences from multiple sources or elements creating increased noise in the baseline interfering with the detection of the target spectrum.

Another type of spectral interference are partial or full spectral overlaps (see Figures 4c and 4d). Direct overlaps occur when the wavelengths between two elements are too close to fully resolve. Despite the fact that no two elements have exactly the same spectral emission, there are many situations where the width of the spectral lines or resolution of the optical system can create overlap. When these overlaps occur, a correction called an inter-element correction (IEC) may be applied. The correction applies a ratio based on the concentration of the interfering element. To properly use the IEC, the concentration or contribution of the interfering element must be measured at another wavelength and a correction factor applied. Each suspected interfering element must be measured and used in the correction. Complexity of the sample can cause the use of an IEC to be cumbersome and may be limited by the optics of the system.

Spectral interferences can be mitigated by understanding the composition of your samples and running single element standards for would be targets and interferences. Many analytical methods include standard wavelength suggestions that can be adjusted for specific types of matrices. For unknown samples, it may be necessary to run some preliminary semi-quantitative runs to determine the overall composition before fine tuning analysis for the targets of primary interest. Once the probable composition is known, wavelengths can be selected to minimize interferences. Many references can be found for the most common wavelengths for ICP amenable elements including an online tool kit from myperiodictable.us (see Tables I and II).

Examining Energy, Wavelengths, and Optics

At the base of all ICP-optical emission spectrometry (OES) detection is the ability of the system to measure the change in energy resulting in specific wavelengths detected by an optical detector. If you remember previous discussions of the electromagnetic spectrum, the larger the wavelength the lower the energy state. In elements, the more transitions of energy state the more energy is released and detectable as lower wavelengths. So, a transition of an electron through two or more orbitals has more energy and a shorter wavelength than a transition of an electron through a single orbital (see Figure 5).

The ability of an ICP-OES system to detect and isolate elements and their wavelengths depends on the system’s optics and detector. A typical ICP-OES system must be able to produce wavelengths over a wide range of energies, typically from approximately 150–850 nm, and have resolution typically better than 0.03 nm over several wavelengths. The emitted energy from a single element must be first differentiated and then isolated to programmed wavelengths. This task is accomplished by a series of slits, mirrors, diffusion gratings, filters, and prisms. There are two basic types of optical configurations: monochromator and polychromator. A monochromator is usually a sealed system pumped with a gas such as argon where there is one entrance and one exit to the optical system allowing for the isolation of one element and wavelength at a given point in time to pass through to a detector. The angle of the grating can be altered quickly, allowing for scanning of more than one wavelength during sampling but it is limited by the sample and sampling rate. There are two common mount configurations for a monochromator: Czerny-Turner and Ebert. The Czerny-Turner possesses two concave mirrors while the Ebert mount has one large concave mirror that spans between the entrance and exit slits. Monochromators with their ability to examine one wavelength after another are known as sequential configurations.

A polychromator is a system that has an entrance slit and multiple exit slits leading to more than one detector and allowing for the detection of more elements and more than one wavelength in a set time period. This allows for more information to be collected across multiple elements, but also will reduce resolution of any one element or wavelength since the signal is split up into dozens of wavelengths limited by the number of detectors. This multi-element capacity makes polychromators known as a simultaneous configuration. The polychromators are less flexible than monochromators and are set to fixed wavelengths where the monochromators can be change quickly and frequently (see Figure 6).

Both types of systems use a diffraction grating to split the signal into selected wavelengths. The basic principle is that incident light hits the highly polished and etched mirror finish of the diffraction grating and splits into component wavelengths. The surface of the diffraction grating mirror is finely etched with lines closely spaced together resulting in several hundred to thousands of lines per millimeter. As the light strikes the surface of the grating, it is diffracted at specific angles depending upon the wavelength of light and the density of lines.

Another type of grating common in ICP-OES analysis is an echelle grating. This grating has a course set of lines compared to the other diffraction grating and separates light of a variety of wavelengths and produces overlapping spectra. A second device (either another grating or prism) further cross disperses the light into a two-dimensional array called an echellogram.This type of grating diffracts lights and focuses it on the slits. This type of grating is often used in polychromator systems.

Deciding on Detectors

As with all the other analytical systems we have examined the last component of the analytical system is the detector. In ICP systems, since the resulting energy are wavelengths of light the detector will use some combination of a photomultiplier, photo diode array, or solid-state device (similarly to the same components discussed in the previous columns regarding chromatography and spectrophotometry). In the past, analytes were measured by photomultipliers. Modern systems rely on technology called solid-state charge transfer devices (CTDs). These devices have almost completely replaced photomultipliers. There are several types of CTDs used to process signals including charge injection (CID) and charge coupled (CCD) devices. The CID and CCD technology is based on the light sensitivity properties of silica and have the benefit of being two-dimensional detectors compared to the older photodiode arrays and photomultipliers. The silicon atom structure forms crystalline lattice with each silicon atom bonding to the adjacent atoms to form a three-dimensional structure. When photons break one of the bonds it results in a release of an electron and leaves a hole in the lattice called the electron-hole pair (see Figure 7). The released electron moves in the opposite direction while the hole moves with the energy direction causing a charge.

A two-dimensional silicon wafer composed of elements called pixels stores and detects photonic charge. CIDs allow activation of random and individual pixels with an electrode at each pixel site. CIDs have a very light sensitive surface divided into thousands of pixels with rows and columns of electrodes that allow for detection of wavelengths from about 160–900 nm. The integration of CID pixels means that the charge is not destroyed. CIDs have higher noise and require extreme cool as with liquid nitrogen to reduce noise.

CCDs have a more systematic charge output measurement from the pixels. The charge is transferred sequentially from each pixel to a buffer then to a recorder. Pixels are processed in rows or columns that are read by the CCD. As the pixels are read, the charge is destroyed. These devices are very fast, but require cooling with a cooling system such as a Peltier unit.

Some devices combine features of both the CID and CCD. Systems such as a segmented array charge coupled device (SCD) uses an echelle array and is designed with thousands of pixels arranged in small subarrays (about 200) positioned on the silicon wafer producing an echellogram. These subarrays mirror the most important ICP spectral lines of about 70 common elements. The subarrays each have their own electrode and can be read in any order. These systems have higher sensitivity than the other types of CCD systems.

Final Thoughts

Inductively coupled plasma spectroscopy is a widely used instrumentation in inorganic analysis. The technique has existed for many years with great improvements on range of analytes and limits of detection that benefit the analyst. There are many choices in the individual components of the system that can be made to further refine and target analysis—from the sample induction to the torch, optics, and detector—the choice of configuration is important to improving your methodology. By understanding the physical configurations and their use, the analyst has another tool to improve their workflow.

Further Reading

1. Atkins, P. Energy and Elements, Part I: Understanding Atomic Spectroscopy, Cannabis Science and Technology 2022, 5 (7), 16-22.
2. Boss, C. B.; Fredeen, Instrumentation and Techniques in Inductively Coupled Plasma Optical Emission Spectrometry, K. J. Concepts, 120.
3. Neubauer, K., How to Choose the Correct Wavelength in ICP-OES, 2019, 34 (8), 24–26.
4. MyPeriodicTable.US | AA/ICP Wavelenths, http://myperiodictable.us/?page_id=3506 (accessed Dec 11, 2022).

About the Columnist

Patricia Atkins is a Senior Applications Scientist with Spex, an Antylia Scientific company and has been a member of many cannabis advisory committees and working groups for cannabis including NACRW, AOAC and ASTM.

How to Cite this Article:

Atkins, P., Energy and Elements, Part II: A Closer Look at ICP, Cannabis Science and Technology®, 2023,6(1), 20-25.