Micro-Electrospray (uESI) with flow rates around 25 nL/min and 30 minutes of stable ion signal from approx. 1 µL of sample was developed my M. Mann et. al., and adapted to an external-source rf-only ion trap. A non-linear trap with ion ejection at superimposed multipole resonances provides very fast scan rates, and resolution of up to 0.lu on ESI multilet.
The ion trap m/z range has recently been extended to < 6,000 for high m/z multiplycharged uESI generated protein ions. Moreover, examples of protein identification by partial sequencing using nozzle-skimmer dissociation followed by MS3 in the ion trap are shown for peptide mapping and for obtaining characteristic sequence tags directly in the mass spectrometer.