precursor ion scan

In an MS/MS experiment,mass-selected precursor ions are induced to dissociate into product ions, which are then mass analyzed by a second analyzer. The three common scans in the single-step MS/MS experiment are: product ion scan, precursor ion scan, and then the constant neutral loss scan. In  the precursor ion scan, the second mass analyzer is set at the mass of the selected product ion, and then the first mass analyzer is scanned from that mass upwards. The result is a mass spectrum that contains signals for all the precursor ions that dissociate to that selected product ion.