standard addition
Standard addition is a way to measure an unknown by feeding it known boosts and watching how its signal grows. Picture not knowing how loud a radio is set: you turn the volume up by exactly three notches, hear how much louder it got, and from that step you work backward to where it started. You add known amounts of the analyte to the sample itself and see how the signal climbs.
Concretely, you split the sample into portions and spike each with a different known amount of the analyte, then measure them all. Plotting signal against amount added gives a line; extending that line backward to where the signal would be zero reveals how much analyte the sample held before any was added.
It matters because the sample's own matrix — everything in it besides the analyte — can quietly distort the signal, so an outside calibration curve would mislead. By doing the calibration inside the real sample, standard addition automatically accounts for those matrix effects. The trade-off is more work per sample and the need for a signal that grows in proportion to concentration.
To measure lead in river water by atomic absorption, four portions are spiked with 0, 5, 10, and 15 µg/L of added lead. The plot of signal versus added lead extrapolated back to zero signal gives the lead originally present.
Calibrating inside the sample itself cancels matrix effects an outside curve would miss.
Standard addition relies on extrapolating the line backward past the lowest point, which can magnify uncertainty. It corrects for proportional matrix effects but cannot fix a constant background — that still needs a proper blank.