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Calibration Curves
Understanding Calibration Curves
Both the accuracy and precision of ICP measurements are dependent, in part, upon the calibration technique used.
This section will focus upon errors (both fixed and random) that can be introduced through the use of:
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Different calibration techniques using accurate calibration standards.
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Samples that have been prepared accurately to within defined error limits.
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Instruments that have been 'set-up' correctly using a procedure programmed to account for spectral/mass interferences that include background correction.
You may believe that if the above errors have been confined to within acceptable and known limits that there is nothing else to worry about. Unfortunately, this is not the case.
Common Calibration Technique Options
The most common calibration technique options for ICP measurements are calibration curves and standard additions. The calibration curve method involves plotting signal intensity against known concentrations, while the standard additions method helps to correct for matrix effects by adding known quantities of analyte to the sample.
Internal standardization can also be used with the calibration curve technique to improve accuracy by compensating for variations in sample introduction or instrument performance. Additionally, matrix matching may be employed to minimize matrix effects.
In ICP-MS, isotope dilution is an advanced method that uses an enhanced isotope of the analyte as an internal standard to improve accuracy, especially in complex samples. This discussion will focus on these approaches, along with the introduction of the analyte as a nebulized solution and the use of argon (Ar) as the plasma gas.
Basic Considerations that Affect ICP Accuracy
The calibration of Inductively Coupled Plasma (ICP) spectrometry relies on a set of key assumptions to ensure the accuracy and reliability of measurements. These assumptions guide the comparison between known chemical standards and unknown samples, addressing the linearity of the calibration, the precision of standard preparation, and the stability of the standards.
Additionally, assumptions are made regarding the absence of spectral interferences, sample contamination, and sampling errors. While these conditions may not always hold true in practice, they provide a framework for focusing on the potential errors specific to the calibration process itself.
Key Assumptions for ICP Calibration
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ICP is a 'Comparative Method' where the measurement of an unknown sample is based upon chemical standards i.e. the measurement is a comparison process.
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It is not assumed that the calibration standards and samples have identical matrices.
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It is assumed that the calibration is linear. This means that the standard and sample elemental concentrations give an instrumental response that is described by the equation for a straight line.
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It is assumed that the analyst has prepared the chemical standards accurately to within defined error limits (i.e., the uncertainty of the prepared standard solution is known and has been calculated).
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It is assumed that the stability of the standards, however and by whomever prepared, is known and the standards are only used within these defined limits of time, matrix, concentration, temperature/humidity, and container material(s).
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The uncertainty of the measurement of an unknown can only be worse (greater) than the uncertainty of the calibration.
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It is assumed that there are no spectral/mass interferences. This of course is an area of great concern and effort on the part of the analyst. This assumption is made to allow us to focus completely upon the potential errors involved with the calibration process.
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It is assumed that the sample prepared for analysis involves no positive or negative contamination errors and no sampling errors. It is therefore assumed that the uncertainty in preparation can be described by the random and known sampling, weighing and volume dilution errors. Again, this is an assumption that is often inaccurate but is made to allow us to focus completely upon the potential errors involved with the calibration process.
Calibrations Standards
ICP is a matrix-dependant technique. Based upon the above assumptions and the fact that ICP is a comparative method, the primary concern is the availability and use of appropriate calibration standards. The problem analysts face is that ICP (ICP-OES and ICP-MS) is extremely matrix-dependent. Therefore, the ideal situation is that the matrices of the standards and samples be identical.
The Role of Matrix Matching in ICP Calibration
Matrix matching is crucial for accurate results in ICP analyses because the technique is highly matrix-dependent, meaning that the composition of the sample matrix can significantly affect the measured analyte signal. Elements or compounds in the sample matrix may cause ionization suppression or enhancement, leading to inaccurate results if not properly accounted for. By matching the calibration standards to the sample matrix as closely as possible, matrix effects are minimized, ensuring that the calibration curve or standard additions accurately represent the sample’s true composition, thereby improving measurement reliability.
Matrix components, including salts, organic compounds, and acids, can significantly affect nebulization efficiency. High concentrations of salts or heavy metals may cause ionization suppression or clogging, while organic compounds can alter the formation of aerosol droplets, leading to inconsistent sample delivery to the plasma. Similarly, the viscosity and concentration of common matrix component acids like nitric or hydrochloric acid can impact the ability to form fine droplets, further affecting nebulization efficiency. These variations can result in inaccurate or unreliable measurements, underscoring the importance of matrix matching to ensure that the calibration reflects the sample’s true composition.
Recommendations
This section lists several recommendations. Discussions relating to these recommendations are provided in the next section for the reader who would like more detail.
Recommendation (a) - Match the acid content of your calibration standards and samples in both the type of acid used and the concentration of the acid.
Recommendation (b) - Match the elemental matrix components of your calibration standards and samples to the greatest extent possible. In this situation, the analyst must know the composition of the sample.
Recommendation (c) - With unknown sample matrices, matching is not possible and is most accurately dealt with using the technique of standard additions. However, this approach is slow compared to the calibration curve technique with the use of internal standardization.
Recommendation (d) - The use of internal standardization is very effective in many cases but may introduce--or fail to correct for--all errors. This statement does not apply to isotope dilution ICP-MS which is considered to be a primary analytical technique.
Recommendation (e) - "Chemical calibration is an approximation at best. The analytical chemist must be constantly aware of the possibility of bias introduced by the nature of the standards used, which may be the major source of bias in the analytical data. Appropriate reference materials should be used to evaluate this and other aspects of the measurement process."
Strategies for Matching Complex Matrices
When dealing with complex sample matrices in ICP analyses, matching the acid type, concentration, and elemental composition between the sample and the calibration standards is critical to achieving accurate results. Here are some practical strategies for handling this:
For environmental samples like soil or sediment, which may require digestion in a combination of acids like nitric and hydrofluoric acid (HF), ensure that both the sample and calibration standards use the same acid mixture at comparable concentrations. This will help to minimize discrepancies that might arise from differences in acid strength and ionic behavior. Be aware that a portion of the acid(s) may be consumed during the digestion process, so the amount of acid needed to matrix-match in the standards may be less than the amount originally added to the sample digestion.
When analyzing solutions like mining brines, which may contain high concentrations of certain elements and salts (like sodium, potassium, and calcium) you might also be interested in trace metals such as copper, lead, or arsenic. To ensure accurate results, your calibration standards should reflect the same levels of these high-concentration elements while still maintaining the low-level trace metal concentrations you're analyzing. This helps to account for any interference or ionization effects caused by the high concentration matrix, allowing for better differentiation of the trace metals.
Use internal standards that are resistant to matrix effects and exhibit similar behavior to the target analytes. This helps compensate for potential variations in the sample matrix and enhances accuracy. In the analysis of industrial wastewater, where the matrix may vary significantly from sample to sample, adding an internal standard such as indium (In) or rhodium (Rh), which behaves similarly to many transition metals, can help correct for changes in signal intensity due to matrix effects. It may be necessary to utilize more than one internal standard for a multi-analyte analysis to ensure availability of an appropriate internal standard for all analytes of interest.
Consider the analysis of trace metals, such as copper or iron, in a fruit juice sample. The juice matrix, which contains high concentrations of sugars and organic acids, may cause interferences during the measurement of the target metals. Instead of relying on matrix-matched standards, known amounts of copper or iron are added to aliquots of the sample, and the signal response is measured at each spiked level. By plotting the change in response against the concentration of added analyte, the concentration of the target metal in the original, unspiked sample can be accurately determined, effectively accounting for matrix effects that could otherwise distort the results.
Nebulization Efficiency and Matrix Effects
Nebulization efficiency is the percent of solution that reaches the plasma. Therefore, if the nebulization efficiency is 1%, then 99% of the solution is going to waste and 1% is making it to the plasma. Typically, nebulized solution 'mist particles' that are greater in diameter than 8 microns will go to waste. If a matrix component changes the efficiency from 1.0% to 0.8%, then a relative drop of ~ 20% would be expected from this effect alone.
Plasma Temperature Considerations
The droplet size distribution of a pneumatic nebulizer is governed by the physical properties of the solution as well as the volume flow rates of liquid (influenced by peristaltic pump speed and tubing diameter) and gas (sample Ar flow rate). The physical properties claimed to influence the droplet size distribution are the surface tension, viscosity, and density. See Inductively Coupled Plasmas in Analytical Atomic Spectrometry; Montaser, A., Golighty, D. W., Eds.; VCH Publishers: New York, 1992 - page 703 for more detail and additional references on this topic.
For the ICP analyst, the most common matrix component that will alter the physical properties of a solution is the acid content. This is not to say that other differences such as the presence of trace organics (added intentionally or not) should not be considered. However, the identity and concentration(s) of one or more acids is an issue that virtually all ICP analysts have to determine. The ICP analyst is most commonly involved in the preparation of samples where one or more inorganic mineral acids are required to bring about dissolution of the sample and/or to maintain solution stability of the analyte(s) of interest. The acids most commonly used are HNO3, HCl, HF, HClO4, H2SO4, and H3PO4 and are listed in the order of best to worst.
The effect of acid matrix upon nebulization efficiency is such that a change in acid content from 5 to 10% v/v will cause a decrease in efficiency of 10 to 35% depending upon the acid used, nebulizer design and liquid and gas flow rates. Matching the matrix to within 1% relative is necessary for the most accurate (we use the term "assay") work (i.e., a 5% HNO3 acid solution would be made to 5.00 ± 0.05%.
The matrix will influence the plasma temperature, which is related to the signal intensity for ICP-OES. The other effect matrix components have on the ICP cannot be explained by a change in nebulization efficiency. The effect is one where the matrix components give the appearance of taking power away from the plasma (lowering the temperature of the plasma). It has been reported that this effect is related to the excitation potential of the line and that the effect increases as the excitation potential increases. A similar effect would be seen by decreasing the applied RF power or by increasing the sample (nebulizer) Ar flow rate since both result in a reduction of the plasma temperature.
Therefore different lines of the same element would be affected differently according to their excitation potentials. In addition, when choosing an internal standard element it follows that the excitation potentials of the internal standard and analyte lines should be as close as possible, unless the calibration standards and samples are matrix matched.
For more information and additional references, see: Inductively Coupled Plasmas in Analytical Atomic Spectrometry; Montaser, A., Golighty, D. W., Eds.; VCH Publishers: New York, 1992 - pages 279-281
Quenching in ICP-MS
The effect most commonly encountered is referred to as 'quenching' and is thought to be due to defocusing of the ion optics by space charge effects. Generally, as the concentration of the 'matrix element(s)' increases, the analyte signal will be suppressed. Quenching increases in effect as the matrix element absolute concentration increases, the matrix element mass increases and the analyte mass decreases. This effect is absolute in nature and not a function of the relative concentrations of the matrix elements and analyte elements. Therefore, when sensitivity allows, it can be diluted out. It is also greater in effect as the RF power is lowered. The effect is such that an element matrix concentration of 100 ppm can severely suppress a 'cool plasma' (RF power ~ 800 W) and has little effect at normal power (an RF power of ~ 1300 W).
For more information and additional references, see: Inductively Coupled Plasma Mass Spectrometry; Mantaser, A., Ed.; Wiley-VCH: New York, 1998 - page 543
Considering Isotope Dilution for ICP-MS
Isotope dilution mass spectrometry (IDMS) is an analytical technique that modifies the natural isotope composition of elements or compounds by adding an enriched isotope or an isotopically labeled form of the analyte to the sample. The isotope composition in the mixture is then measured using Mass Spectrometry, and the concentration of the analyte in the sample is determined through calculations. The technique is highly regarded as a primary method, providing results that are both accurate and precise, and directly traceable to the International System of Units (SI).
Advantages of Isotope Dilution
Isotope dilution mass spectrometry offers several key advantages, particularly in minimizing calibration errors that can arise in traditional analytical methods. By adding a known amount of isotopically enriched or labeled analyte to the sample, IDMS effectively compensates for variations in sample preparation, instrument performance, and matrix effects. This process reduces the potential for systematic errors, ensuring greater accuracy and reliability in the measurement. Additionally, since the technique directly measures the ratio of isotopes, it bypasses the need for complex external calibration standards, which can introduce additional uncertainties. Moreover, IDMS is highly resistant to matrix interferences, providing precise results even in complex sample matrices. These advantages make IDMS an invaluable tool for trace analysis and quantification in a wide range of scientific and industrial applications.
Practical Limitations
Isotope dilution mass spectrometry has several practical limitations. One challenge is the availability of isotopically enriched standards, which can be difficult to obtain and cost prohibitive, especially for less common elements. IDMS is also not suitable for monoisotopic elements, which have only one naturally occurring isotope and lack alternatives for dilution.
The method requires precise sample preparation, as the isotopically enriched standard must be added accurately and thoroughly mixed within the sample. Any errors in this process can affect the results. Additionally, IDMS requires careful calibration and optimization, adding complexity to the method. These factors make IDMS less ideal for high-throughput analysis or labs with limited resources or expertise.
Additional things to note:
Documentation & Traceability
In ICP calibration, the accuracy of measurements is directly linked to the careful preparation of calibration standards and the quality of the reference materials used. It is crucial to record all steps taken during the preparation of standards, such as the weighing of reagents, dilution procedures, and any adjustments made to ensure the correct concentrations. Proper documentation provides transparency and allows for the process to be replicated or audited if needed, ensuring consistency and traceability.
Verifying the certificates of reference materials
Additionally, verifying the certificates of reference materials is essential, as these certificates confirm the material's composition, certified concentration, and associated uncertainties. By ensuring that reference materials are appropriate and within the defined limits of accuracy, analysts can reduce potential errors in ICP calibration. This attention to detail in both standard preparation and reference material verification helps to maintain the reliability and validity of your calibration, ensuring precise and accurate results.
The Role of Instrument Maintenance in Ensuring Consistent ICP Performance
Consistent performance in ICP analysis is not only dependent on accurate calibration and high-quality reference materials, but also on the regular maintenance of the instrument itself. Routine upkeep is essential to ensure the reliability and precision of results over time. Key components, such as the nebulizer, spray chamber, and torch, can experience wear and tear that may impact the efficiency of sample introduction or plasma stability. For instance, worn nebulizer parts can lead to inefficient aerosol formation, causing fluctuations in signal intensity and inaccurate readings.
The Importance of Cleaning
A dirty or worn spray chamber can lead to inconsistent particle size of the sample that is able to reach the plasma, as well as increased washout or carryover issues. Regular inspection of the plasma torch is also crucial, as any damage or contamination can affect the plasma's stability and, in turn, the consistency of the analysis. Replacing parts, cleaning components, and performing routine diagnostics helps to maintain optimal performance, ensuring that measurements remain precise and reproducible. By implementing a proactive maintenance schedule, analysts can minimize instrument-related errors, reduce downtime, and maintain consistent, high-quality results in every analysis.
In Summary
Accurate instrument calibration is essential for accurate ICP measurements, and the precision and accuracy of these measurements depend heavily on the chosen calibration technique. The most common methods include calibration curves, standard additions, and internal standardization.
Calibration curves involve plotting the signal intensity against known concentrations of an analyte, while standard additions help to correct for matrix effects. Proper matrix matching is crucial to minimize interferences and ensure accurate results. The calibration process is also built on key assumptions, such as linearity and the stability of standards, though practical challenges like spectral interferences and sample contamination may still arise.
Matrix effects play a significant role in ICP accuracy, as variations in sample composition can lead to ionization suppression or enhancement. Therefore, matching the matrix of calibration standards to the sample matrix is recommended to minimize these effects. When matrix matching is not possible, techniques like standard additions or internal standardization can help. Additionally, maintaining proper documentation of calibration procedures and performing regular instrument maintenance, including replacing nebulizer and spray chamber parts and inspecting the plasma torch, is essential to ensure consistent and reliable performance over time.
In complex matrices, it’s critical to match both the acid content and elemental composition of standards with samples. For samples with unknown matrices, using standard additions or isotope dilution mass spectrometry (IDMS) offers highly accurate results. The use of internal standards can also help compensate for matrix effects, though it may not completely correct for all errors.
By carefully considering these factors—accurate calibration techniques, matrix matching, and consistent instrument upkeep—analysts can achieve precise, reliable ICP measurements.