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The Strengths and Limitations of ICP-OES Analysis
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What is ICP-OES?
Inductively coupled plasma-optical emission spectroscopy (ICP-OES) is an analytical technique that is used to identify the atomic composition of a particular sample. The technique makes use of the unique photophysical signals of each element to successfully detect the type and relative amount of each element within the complexity of a compound. ICP-OES has particular utility in the analysis of complex samples,1 and has been used in applications such as analyzing trace elements in the human brain,2 determining the chemical composition of electronic cigarettes,3 and assessing the purity of pharmaceutical compounds.4
How does ICP-OES work?
To perform ICP-OES, one needs the following key components:
Figure 1: Example of an ICP-OES setup.
(a) High energy plasma. This plasma is most commonly composed of argon,5 although nitrogen gas6 and mixed gas compositions7 have also been reported. It is generated through the use of a high-power radio frequency signal8 or through microwave irradiation,9 which causes the gas to ionize to form electrons and other charged species within the plasma matrix.
(b) A sample aerosolizer. Interactions between the plasma matrix and the sample are critical for successful analysis, and obtaining those interactions requires the sample to be aerosolized. Aerosolizing of the sample generally occurs through the use of a nebulizer,10 and also needs a mechanism for sample transport from the injection port to the point of aerosolization.11 Following successful aerosolization, interactions between the high energy plasma and the sample result in degradation of the sample to its individual elements, each of which has a characteristic optical signal that can be detected spectroscopically (see part d).
(c) A wavelength separation mechanism. Although each individual element absorbs and emits light at a characteristic wavelength, signals from multiple elements often overlap, leading to significant challenges in interpreting the results obtained. To address this issue, the wavelengths corresponding to each element are separated, generally via an optical grating device,12 so that each element can be individually detected. The configuration of the system into either an axial configuration13 (where the plasma is viewed head-on) or radial configuration (where the plasma is viewed from the side) has additional effects on the ability to observe the target signals: although generally radial configurations show improved detection capabilities,14 advances in axial configurations’ detection capabilities have recently been reported.15
(d) A detector and signal processor. This detector, after correlating the wavelengths of light to the identity of the elements, is used to determine the final sample composition. It generally uses either a photomultiplier tube-type mechanism or a charge coupled device (CCD).16 Moreover, the detector is calibrated with known quantities of the elements targeted for analysis, so that it can effectively match the signals obtained from the sample to its pre-calibrated signals to allow for effective quantitation.17 Finally, there is a need to remove potentially interfering signals that can compromise detection of the target analyte, although recent studies have used these non-analyte signals to understand the broader matrix effects and overall system composition.18
Analyzing a sample by ICP-OES first requires one to determine if and how it can be effectively aerosolized. While this is a relatively straightforward process for liquid samples (which can be accomplished with a nebulizer, vide supra),19 solid samples require additional effort, such as the use of electrothermal vaporization,20 electrothermal evaporation,21 laser ablation,22 or spark ablation.23 Finally, gas sensing via ICP-OES tends to be a straightforward process, as no aerosolization is needed. Rather, such systems require a mechanism for gas capture and for introduction of the gaseous sample into the detection system.24
In addition to figuring out how to successfully introduce a sample into the system, one has a number of choices regarding system configuration, many of which are outlined above. Selecting the gas composition for the plasma can have measurable effects on the ability to ionize the gas effectively and determine atomic composition of the sample, as can the viewpoint (radial, axial, or dual)25 of the sensor relative to the generated plasma. Many of these choices, however, are made by manufacturers of ICP-OES instruments, and therefore are not necessarily within the purview of the individual ICP-OES user to decide.
ICP-OES vs ICP-AES – Is there a difference?
Inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) are used interchangeably in many scientific publications,26, 27, 28 as both represent the emission of photons from an ionized sample that can be deconvoluted into signals from each of the constituent elements.
How do you analyze ICP-OES data and what does it tell you?
General guidelines for analysis of ICP-OES data are to look at the intensity of light emitted at particular wavelengths and compare that to calibration data to determine the concentration of atoms that are emitted at that particular wavelength. Most instruments currently in use allow for the selection of multiple wavelengths, and the user should select wavelengths that correspond to the emission signals from the atoms of interest.29 After correct wavelength selection, identifying the elements within the sample is generally an automated process, and in recent years has become more sophisticated to facilitate multivariate analysis and highly sensitive identification.30
Other concerns in analyzing ICP-OES data relate to potential interferents and their ability to compromise the system performance. To eliminate undesired interferences prior to analysis, users are advised to use an internal standard to correct for sample-to-sample variability and differences in sample processing conditions.31 Commonly used internal standards are scandium32 and yttrium,33 chosen because their wavelengths generally do not overlap with those of other atoms in the sample. After the successful implementation of the internal standards, calibration data allows for a direct comparison of the light intensity obtained from the sample to light intensities of known sample compositions, providing the types of elements found in the sample and their relative ratios within that sample as the key readout data from ICP-OES.
Figure 2: Typical line spectra produced by ICP-OES (left). The same spectra magnified in the y-axis demonstrates that, despite being spectral “lines”, they are still peaks and can thus suffer spectral interferences (right). This can be overcome by using software that enables selection of peaks that lack interference.
Figure 3: Example of a calibration curve.
Strengths and limitations of ICP-OES
Key strengths of ICP-OES include the ability to identify the types and ratios of elements in complex samples. For example, ICP-OES has been used effectively to analyze the composition of crude oil,34 contaminated soil,35 and heavy metal mixtures,36 all of which would have been challenging to analyze by other methods. Moreover, the ability to detect multiple elements simultaneously by ICP-OES presents another significant advantage,37, 38 with researchers reporting situations where ICP-OES has detected up to 19 elements in one analytical procedure.39 Advances in the ability to aerosolize a broader variety of samples has improved the general applicability of ICP-OES,40 as have advantages in spectral deconvolution41 and calibration procedures17 to facilitate effective detection. Even in the case of radioactive samples, ICP-OES can still be used to determine the elemental composition of the sample, with separate measurements used to determine the degree of radioactivity.42, 43 Finally, the ease of ICP-OES has allowed it to also be used in chemistry education contexts,44 with both analytical reagent grade and spectral pure grade solvents,45 and with relatively high throughput for sample preparation46 and analysis,47 highlighting the straightforward usability of the system.
Notable limitations of ICP-OES include the fact that samples must be aerosolized. Even though aerosolization procedures have undergone significant advances (vide supra), this means that solid and liquid samples cannot be analyzed while they are still in their solid and liquid forms. Moreover, ICP-OES is a destructive analytical procedure, meaning that the sample cannot be recovered after analysis. As a result, highly precious or rare samples cannot be analyzed via this method. Moreover, method development using ICP-OES can be a time-consuming process, as it necessarily involves multiple steps:28 (a) doing crude analysis to obtain a basic idea of the elements present in the sample; (b) wavelength selection based on that initial knowledge; (c) optimization of separation so that signals from the various wavelengths have limited overlap; (d) comparison with a internal standard to validate the method and system performance; and (e) analysis for spectral interferences and ways to eliminate those from the read-out without eliminating target signals. Finally, ICP-OES requires costly instrumentation for plasma generation, sample aerosolizing, and signal analysis, albeit at a relatively lower cost than other comparable methods such as ICP-MS,48 which means that access to this technique is necessarily limited.
Common problems with ICP-OES
Common problems with ICP-OES include poor precision,49 sample drift,50 non-ideal detection limits, and inaccurate identification.51 Each of these problems will be discussed in turn.
Poor precision is defined as a lack of reproducibility in results obtained for the same sample. Such challenges are likely to be due to issues in the sample introduction system, including mechanisms in which the sample is aerosolized, introduced into the system, and/or transported from the introduction site to the plasma matrix.
Sample drift refers to a situation in which the signal is not stable and changes in position over time. Such issues are usually due to instrument problems, including buildup of the parts of the sample that were not effectively aerosolized in the instrument tubing that slows flow rates, or degradation in the tubing due to highly acidic samples52 that cause system leakages.
Non-ideal detection limits means that in many cases, the detection limits obtained through the use of ICP-OES are higher than desired for the target application. While detection limits for ICP-OES can theoretically be as low as single digit parts-per-billion (ppb),53 they are more often reported in the parts-per-million (ppm) range.54, 55 Optimization of detection limits focuses on ensuring that sample preparation procedures limit dilution and/or sample degradation, as well as optimizing the view of the plasma-generated signal (axial, radial, or dual) to achieve the optimal signal capture.
Inaccurate identification refers to situations in which the ICP-OES signal identifies a signal as corresponding to one element when it in fact belongs to a different element. Such situations, while rare, can be minimized by selecting wavelengths for the desired elements that have limited overlap from competing elements. These situations have also been assisted by the recent application of multivariate spectral analysis to ICP-OES signal read-outs,56 which allows for the use of statistical analysis to deconvolute overlapping signals and facilitate accurate identification.
ICP-OES is often compared to ICP-MS (inductively coupled plasma – mass spectrometry).57 ICP-MS operates using many of the same principles as ICP-OES, except that the detection of elements from the aerosolized and ionized sample occurs via mass spectral analysis rather than being based on photon emission. Key advantages to the use of ICP-MS compared to ICP-OES are that sensitivities of mass spectral-based techniques are higher, with ICP-MS able to obtain parts-per-trillion (ppt) detection limits.58 Disadvantages to the use of ICP-MS focus on the limited tolerance for total dissolved solids (TDS),59 which is markedly higher in ICP-OES, allowing for greater sample tolerance.
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