Controlling the expansion into vacuum-the enabling technology for trapping atmosphere-sampled particulate ions

A new inlet has been designed to control the kinetic energy distributions of ions into a large-radius, frequency-adjusted, linear quadrupole ion trap. The work presented here demonstrates trapping singly-charged, intact proteins in the 10 to 200 kDa range injected from the atmosphere. The trapped ions were held while collisions with a buffer gas removed the remaining amounts of expansion-induced kinetic energy. The ions were then ejected from the trap on-demand into an awaiting detector. There is no low mass limit for ion injection and trapping. The upper limit presented in this study was defined by the limit of the conversion dynode-based detector at approximately 1.5 MDa. Trapping larger masses should be achievable. The transmission and capture efficiency across the entire mass range should be very high because the entire flow from the inlet empties directly into the trap. The kinetic energy distribution of massive ions is the primary reason for the working range limitation of mass spectrometers. Trapping ions with collisional cooling before mass analysis permits the motion of the ions to be completely defined by the applied fields. For this reason, this new inlet and trapping system represents a large step toward sensitive, high-resolution mass spectrometry into the megadalton range and beyond.