The precursor scan type also uses two stages of analysis as shown in the following figure.
In the first stage, the ion source forms ions that are introduced into the precursor mass analyzer, which is scanned to transmit precursor ions sequentially into the collision cell.
In the second stage of analysis, in the collision cell, precursor ions fragment to produce product ions by unimolecular decomposition of metastable ions or by CID. The collision cell forms ions that enter the product mass analyzer, which transmits a selected product ion. (The product set mass is the m/z of ions transmitted by the product mass analyzer.)
The resulting spectrum shows all the precursor ions that fragment to produce the selected product ion. For a mass spectrum obtained in the precursor scan type (precursor mass spectrum), note that data for the m/z axis is obtained from Q1 (the precursor ions), whereas data for the ion intensity axis is obtained from Q3 (from monitoring the product ion).