The method of standard addition is used in instrumental analysis to determine concentration of a substance (analyte) in an unknown sample by comparison to a set of samples of known concentration, similar to using a calibration curve. Standard addition can be applied to most analytical techniques and is used instead of a calibration curve to solve the matrix effect problem.
Standard addition is frequently used in atomic absorption spectroscopy and gas chromatography.
The matrix effect problem occurs when the unknown sample contains many impurities. If impurities present in the unknown interact with the analyte to change the instrumental response or themselves produce an instrumental response, then a calibration curve based on pure analyte samples will give an incorrect determination.
One way to solve this problem is to use standard addition. The standard solution (solution of known concentration of analyte) is added to the unknown solution so any impurities in the unknown are accounted for in the calibration. The operator does not know how much was in the solution initially but does know how much standard solution was added, and knows how the readings changed before and after adding the standard solution. Thus, the operator can extrapolate and determine the concentration initially in the unknown solution. In practice, the volume of standard solution added is kept small to avoid dilution of the unknown impurities.
A typical procedure involves preparing several solutions containing the same amount of unknown, but different amounts of standard. For example, five 25 mL volumetric flasks are each filled with 10 mL of the unknown. Then the standard is added in differing amounts, such as 0, 1, 2, 3, and 4 mL. The flasks are then diluted to the mark and mixed well.
The idea of this procedure is that the total concentration of the analyte is the combination of the unknown and the standard, and that the total concentration varies linearly. If the signal response is linear in this concentration range, then a plot similar to what is shown above is generated.