[Reader Insight] Understanding the Standard Addition Method in Quantitative Analysis

This article is written by an expert chromatographer under the pen name of Chromatography Mound. Welch Materials, Inc. is authorized to translate this article to English and publish it on behalf of the author.

The external standard method and the standard addition method are two methods in quantitative analysis. While their principles and requirements appear straightforward, it is still worth discussing whether these methods are suitable for every compound in practice, and which method should be chosen for each compound to be analyzed.

To achieve accurate quantification, it is crucial to first understand the principles and detailed considerations of each quantitative method, to ensure the scientific rigor of the entire analytical process.

The external standard method involves preparing standard solutions of varying concentrations, fitting a linear equation curve based on mass concentration and response values, and calculating the mass concentration of the sample using its response value. The external standard method generally includes single-point quantification and curve-based quantification. In mass spectrometry, curve-based quantification is often relied on to ensure accuracy.

Due to its simplicity and efficiency, the external standard method is the most widely used quantitative method. However, it should be noted that, in mass spectrometry, where matrix effects can significantly influence results, matrix solutions should be used to prepare calibration curves.

More importantly, it is essential to monitor the slopes of the curves. Even with a high correlation coefficient, an overly steep slope can lead to significant deviations in the quantitative results, in particular in pollutant detections (e.g., for phthalates, perchlorates, bisphenol A, or nonylphenol). In such cases, either conduct blank solvent experiments to subtract blank values, or include the zero point in the curve fitting calculation. Additionally, multiple parallel experiments and recovery rate calibration are required.

The standard addition method fundamentally resembles the external standard method. But the key difference is that it does not use pure solvent or matrix solution to prepare the calibration curve; instead, it employs spiked samples with varying concentrations (levels) of standard solutions. Essentially, it uses multi-level spiked recovery experiments for curve fitting.

This distinction means that the added standard is processed alongside the sample from the preparation stage to instrumental measurement. Therefore, the added standard undergoes the same loss during sample preparation and is affected by matrix effects within the instrument. Factors such as container or operator variability also influence both the sample and the added standard to the same extent.

As a result, this quantitative method not only corrects for matrix effects but also for recovery rates. It is even argued that the recovery rate of the standard addition method is 100%, i.e., the measured value represents the true value, to which the author agrees.

However, this method requires spiked experiments for every concentration point, making it labor-intensive. Furthermore, spiked curves must be recreated for different samples and detectors, with strict procedural demands. Additionally, the curve must exhibit a good linear relationship, otherwise the quantitative results are meaningless. These limitations explain why the standard addition method is rarely applied.

Despite its complexity, the standard addition method has other advantages. Its calibration curve not only ensures accurate quantification but also allows for determining sample values even when the sample itself is positive, as shown in the diagram below.

In the diagram, the baseline value of the sample raises the response of each concentration point by A₀. The intercept at zero concentration corresponds to the response caused by the baseline value. Thus, when the correlation coefficient of the curve is high enough, the absolute value of the intercept |Cₓ| on the X-axis represents the baseline value of the sample. If the correlation coefficient equals 1, |Cₓ| can be interpreted as the true value of the sample.

However, this also highlights a limitation of the standard addition method. For samples with high baseline concentrations, A₀ becomes excessively large, flattening the curve slope and introducing significant deviations. Besides, spiking becomes cumbersome in such cases.

To accurately quantify the baseline value of a sample, the standard addition method is recommended. When isotopic internal standards are unavailable, participation in proficiency testing using the standard addition method significantly improves passing rates.

For batch measurements, however, the external standard method with recovery rate calibrations is more practical.