Recently I have seen discussions and questions about the analytical method of standard addition. Standard addition can apply to any analytical method which has a linear signal or response vs. concentration of the analyte; for example absorbance in spectroscopy or peak area in chromatography. Standard addition is used when the matrix effects of a sample changes the response in a linear manner by changing the slope of the standard curve y= mx+b; m would change as a result of the sample matrix. Standard addition will not work where matrix effect contributes to baseline response – b. If b is affected a sample matrix blank would have to be subtracted from all values to use the standard addition method.
In the graph above is a response of analyte concentration to absorbance. The lower regression line is the response of the analyte as a standard curve. The upper regression line is the standard addition in a sample matrix. The slope of the two lines is different due to matrix effects. If the sample analyte within the matrix with signal of 0.09 is compared to the standard curve a concentration of 8.9 ppm would be the result. Using the standard addition line the concentration of the sample is the absolute value as the regression line crosses the X axis: (-)7.1 ppm.
Standard addition can be used in a quality control setting using only one standard if the method has been verified to be linear in the sample matrix and it has been verified there is no enhancement of the baseline (blank) signal based on matrix effects. The level of standard to be added should be at least 2x the level of analyte expected in the sample* – as long as the resulting signal is within the linear range. The standard must be added to each sample for reliable results. The volume must either be kept constant or the change in volume must be considered in calculating the concentration of the standard.
*Thompson, Michael, ed. “Standard Additions: Myth and Reality.” Amc Technical Briefs 1 Mar. 2009. Print.