In tandem mass spectrometry, fragmentation is used to break up the ions of interest. This means that you can first filter for specific ions, break up these into fragments specific for the ions, and then filter for the fragments. Though this results in a lower amount of ions reaching the detector, as the fragmentation is never 1:1 from precursor to product ion, it increases both sensitivity and selectivity as the noise is reduced considerably and the so-called transition between precursor and product can be designed to be very specific.