A Split-Field Drift Tube for Separation and Efficient Fragmentation of Biomolecular Ions

A new ion mobility instrument that incorporates a low-field region for ion separation and a high-field region for collisional activation is described. In this approach, mixtures of ions are separated based on differences in their mobilities in a ~20-cm-long low-field (~5 V cm^-1) region of a drift tube. As the ions approach the drift tube exit, they are exposed to a large focusing potential drop; at high fields, ions are efficiently collisionally activated and dissociate as they exit the drift tube. We have demonstrated this approach by examining the fragmentation pattern for electrosprayed bradykinin ions. These studies show that the activation process is highly tunable; it is possible to modulate the field such that precursor ion mass spectra as well as several high-field collision-induced fragmentation patterns can be obtained. The approach also appears to be a simple means of activating protein ions, as demonstrated by examining electrosprayed myoglobin ions.