Suppose an analyst is determining the concentration of silver in samples of waste solution in photographic film by atomic absorption spectroscopy. Using the calibration curve method, the analyst can calibrate the spectrometer with a pure silver aqueous solutions, and use the calibration graph to determine the amount of silver present in the waste samples. This method, however, assumes the pure aqueous solution of silver and a photographic waste sample have the same matrix and therefore the waste samples are free of matrix effect.
Matrix effects occur even with methods such as plasma spectrometry, which have a reputation for being relatively free from interferences. As such, analyst would use standard additions in this case.
For standard additions, equal volumes of the sample solutions are taken, and all are separately spiked with varying amounts of the analyte – 0, 1, 2, 3, 4, 5 mL, where 0 mL addition is a pure test sample solution. All solutions are then diluted to the same volume of 25 mL, by using the same solvent as the one used to prepare the spiking solutions. Each prepared solution is then analyzed using an atomic absorption spectrometer. The resulting signals and corresponding spiked silver concentrations are plotted, with concentration on the x-axis and the signal on the y-axis. A regression line is calculated through least squares analysis and the x-intercept of the line is determined by the ratio of the y-intercept and the slope of the regression line. This x-intercept represents the silver concentration of the test sample where there is no standard solution added.