The figure shows the partial sequence of the β_A1 globin and the postulated sites and products of cleavages generated by CID in the gas phase. Below, a portion of the electrospray ionization mass spectrum of the β_A1 globin acquired under the conditions that induce partial protein fragmentation in the gas phase is shown. The spectrum shows multiply charged precursor molecular ions and product ions. The precursor ions represent intact protein and are marked with a number of positive charges that they carry (ie, 16⁺, 15⁺, 14⁺). Product ions are derived from protein fragments produced in the course of CID. They are annotated with letters (b and y for ions encompassing the N- and C-termini of the molecule, respectively), followed by numbers representing positions within the sequence where the cleavages occur (numbering starts from the N- and C-terminus for the b and y ions, respectively). The number of positive charges that each product ion carries is given in superscript. Note that intensities of the ions within the m/z range of 550 to 875 are magnified 3 times.