Introduction

Instrumental spectrochemical methods are essential for trace element analysis across various sample types. Different calibration strategies, including external standard calibration (EC), matrix-matched calibration (MMC), internal standardization (IS), and standard additions (SA), are employed to mitigate matrix effects and enhance accuracy and precision. These traditional methods are closely associated with widely used spectroanalytical techniques.

Inductively coupled plasma mass spectrometry (ICP-MS) is highly sensitive to various interferences, necessitating the development of strategies to address them. Similarly, inductively coupled plasma optical emission spectrometry (ICP OES) faces non-spectral interferences due to sample transport and plasma effects. Interference processes, such as self-absorption in ICP OES, space charge effects in ICP-MS, and recombination processes in both techniques, can result in biased results. Spectral interferences are also prevalent, stemming from nearby emission lines, background signals, and polyatomic ions.

In quadrupole-based ICP-MS (ICP-QMS), spectral interferences can be corrected using collision/reaction cells (CRC) or tandem arrangements with two quadrupoles and a CRC. Alternatively, special calibration strategies can be employed to correct for interfering effects in both ICP OES and ICP-MS.

This blog explores the applications, advantages, and limitations of traditional and new calibration methods, focusing on ICP-MS applications. The discussion draws from landmark papers and recent works to provide insights into these strategies.

Traditional Calibration Methods

External Standard (EC) and Matrix-Matched Calibration (MMC)

In analytical chemistry, calibration involves determining a mathematical function that relates analyte concentration to instrument response. The EC method uses certified pure substances or standard solutions external to the sample. It assumes negligible matrix effects, making it straightforward and widely used. In EC, multiple calibration standards are analyzed to plot analyte concentration against instrument response, typically using least-squares regression. The AOAC International recommends using 6-8 standard concentrations close to the sample’s analyte concentration.

Linearity of the calibration model can be tested graphically or numerically using a correlation coefficient. Statistical tools, such as the F-test, can also verify linearity. Ordinary least-squares (OLS) is a linear model used when data are normally distributed, and the error in concentration values is minimal compared to the analytical signal error. Heteroscedastic data require alternative models, such as weighted least-squares (WLS), where lower-concentration standards have higher weights, enhancing accuracy.

Internal Standardization (IS)

Internal standardization corrects for interferences by adding a reference element (internal standard) to all samples, calibration standards, and the analytical blank. The analyte-to-IS signal ratio is used for calibration. Ideally, the IS species should behave similarly to the analyte during measurement, minimizing effects from fluctuations in nebulization, radiation source intensity, or sample position.

Standard Additions Method (SA)

The SA method involves adding increasing concentrations of the analyte to the sample, using the sample solution as solvent to minimize matrix effects. The analyte concentration added is plotted against instrument response, with the calibration function estimated by least squares fitting. This method is effective for reducing matrix effects in complex samples.

Innovative Calibration Methods for ICP-MS

Several non-instrumental strategies have been developed to minimize interferences in ICP-MS. Mathematical correction equations, based on the natural abundance of isotopes, can subtract interfering signals from the analyte’s m/z. However, these equations have limitations, especially in the presence of high concentrations of interfering species.

Interference Standard Method (IFS)

IFS involves using a separate standard that contains the interfering species at known concentrations. By measuring the interference in a controlled manner, corrections can be made to the analyte signal.

Standard Dilution Analysis (SDA)

SDA uses a dilution series of the sample, allowing for the calibration of both the sample and standards within the same matrix, effectively mitigating matrix effects.

Multi-Isotope Calibration (MICal)

MICal uses multiple isotopes of the analyte for calibration, improving accuracy by compensating for isotopic variations and interferences.

Multispecies Calibration (MSC)

MSC involves calibrating multiple species of the analyte, providing a more comprehensive correction for interferences and enhancing precision.

Selecting the appropriate calibration method is crucial for accurate and precise trace element analysis. Traditional methods, such as EC, MMC, IS, and SA, remain effective for many applications. However, innovative strategies like IFS, SDA, MICal, and MSC offer new solutions for overcoming interferences, particularly in ICP-MS.

Calibration is integral to analytical procedures, and careful consideration of calibration methods can significantly improve analytical performance. As analytical instrumentation advances, new calibration approaches will continue to emerge, supporting faster and more accurate analyses and contributing to a better understanding of trace elements in various fields.

By leveraging both traditional and innovative calibration methods, analysts can ensure the accuracy and precision necessary for reliable trace element analysis.