How does ICR measure the mass of a peptide?
Electrospray produces protonated peptides that are present within and on the surface of microdroplets. As the droplets pass through a heated metal capillary, the solvent evaporates, and the droplets become over-charged and fission into ever-smaller droplets until only single, usually multiply charged peptide ions (e.g., 2+, 3+, 4+ for tryptic peptides) are left. During an FTICR experiment ions are accumulated continuously in an octopole ion trap which is external to the magnet. Then the ions are suddenly pulsed out of that trap (by simultaneously raising the dc voltage at the far end and dropping the front end potential), and then guided through a long octopole, into the ICR ion trap inside the middle of the magnet. After pulsing the ions into a high-radius cyclotron orbit, we measure the time-domain signal and FT (as noted above) to generate a spectrum of ICR frequencies. We then convert from ICR frequency to ion mass-to-charge ratio. Finally, we determine ion charge as the reciprocal
