The illustration below shows one method for making a standard addition. The volumetric flask on the left contains a portion of the sample, Vo, and the volumetric flask on the right contains an identical portion of the sample and a spike, Vstd, of a standard solution of the analyte. Both flasks are diluted to the same final volume, Vf. The concentration of analyte in each flask is shown at the bottom of the figure where CA is the analyte’s concentration in the original sample and Cstd is the concentration of analyte in the external standard.
A second approach to making a standard addition is shown here. In this case we add a spike of the external standard directly to the sample without any further adjust in the volume. Symbols are the same as in the previous illustration.