matrix matching
Matrix matching means making your standards live in the same surroundings as your sample, so the comparison is fair. The matrix is everything in the sample besides the analyte — the salt, the protein, the soil, the blood. If your standards sit in clean water while your sample swims in seawater, they are not really comparable; matrix matching closes that gap.
Concretely, you prepare calibration standards in a medium that mimics the sample's matrix as closely as possible — adding the same salts, acids, or background substances at similar levels. Then a standard and a sample of equal analyte concentration experience the same conditions and give the same signal, so the calibration truly applies to the sample.
It matters because the matrix can quietly boost or suppress the signal — a matrix effect — and standards made in pure solvent would not feel it, biasing the result. Matrix matching is a common cure when many samples share a similar, known matrix. Its limit: if the matrix varies from sample to sample or is poorly known, matching becomes impractical and standard addition is often the better choice.
To measure sodium in seawater, the calibration standards are prepared in artificial seawater of matching salinity rather than pure water, so the heavy salt background affects standards and samples alike.
Standards built in a sample-like matrix make the calibration genuinely comparable.
Matrix matching works only when the matrix is known and reasonably constant. When the matrix is variable or unknown, standard addition — calibrating inside each sample — sidesteps the need to reproduce the matrix in separate standards.