A sample matrix consists of the components of an analytical sample other than the analyte being measured. The definition is relative to the analytical question: a constituent can be the analyte in one measurement and part of the matrix in another.[ref] The sample matrix is material, whereas an analytical matrix effect is its influence on the measurement.
For an invented composition, a sample contains of water, of glucose and of sodium chloride. If glucose is the analyte, the remaining supplies the sample matrix. If water is the analyte instead, glucose becomes a matrix constituent. Calling something a matrix component does not imply that it is chemically unimportant or present only in traces.
Sample preparation can change the relevant matrix. Extraction, dilution or addition of a reagent changes which constituents reach the detector and in what proportions. An extraction recovery measurement asks how much analyte survives a specified preparation stage; a mass spectrometry matrix effect asks how accompanying constituents change response. Matching only the starting sample type does not establish that final extracts have identical measurement behavior.
A MALDI matrix is a deliberately introduced material with a specific role in laser-assisted ion production. It can contribute to the final sample environment, but its name is not a general synonym for all non-analyte constituents. Nor does the word matrix here mean a mathematical array.
An analytical blank and a matched sample matrix answer different questions. A blank can estimate a background contribution, yet fail to reproduce a sample-dependent change in sensitivity. Matrix-matched calibration consequently tries to reproduce relevant measurement conditions as well as the absence or presence of the analyte; the label alone does not prove a match.
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