Matrix
Everything in a sample other than the analyte itself.
Unit
Dimensionless
In depth
The matrix can enhance or suppress the analytical signal, an interference known as the matrix effect. It is countered by matrix-matched calibration standards, by the method of standard additions, or by separating the analyte from the matrix before measurement.
Examples in the real world
Blood proteins and salts interfering with a drug assay; soil organic matter affecting a metal determination.