Outside the field of polarography, Harveys book Spectrochemical Procedures was the next earliest reference book to mention standard addition. Harveys approach, which involves the successive addition of standards, closely resembles the most commonly used method of standard addition today.
To apply this method, analysts prepare multiple solutions containing equal amounts of unknown and spike them with varying concentrations of the analyte. The amount of unknown and the total volume are the same across the standards and the only difference between the standards is the amount of analyte spiked. This leads to a linear relationship between the analyte signal and the amount of analyte added, allowing for the determination of the unknown's concentration by extrapolating the zero analyte signal. One disadvantage of this approach is that it requires sufficient amount of the unknown. When working with limiting amount of sample, an analyst might need to make a single addition, but it is generally considered a best practice to make at least two additions whenever possible.
Note that this is not limited to liquid samples. In atomic absorption spectroscopy, for example, standard additions are often used with solid as the sample.
In atomic emission spectroscopy, background signal cannot be resolved by standard addition. Thus, background signal must be subtracted from the unknown and standard intensities prior to extrapolating for the zero signal.
As this approach involves varying amount of standards added, it is often referred in the plural form as standard additions.