Collisional Damping Interface for an Electrospray Ionization Time-of-Flight Mass Spectrometer

A new interface between atmosphere and high vacuum has been developed for orthogonal injection of electrosprayed ions into a time-of-flight mass spectrometer. A small rf quadrupole operating at 100 mtorr (1.33 × 10 −4 bar) is its key element. Ions enter the quadrupole with velocities acquired in the free expansion/declustering process. As they pass through the quadrupole their motion is constrained by the rf field. Meanwhile, they lose energy by collisions with the gas molecules. The time delays of ions passing through the quadrupole have been measured in order to determine the average velocities of the ions and the factors determining this value. In addition, a simple computational model based on a Monte Carlo approach has been developed to simulate the ion motion; it shows a considerable decrease in both transverse and axial ion velocity components. As the result of collisional damping the interface provides a dramatic improvement in the overall quality of the ion beam transported into the mass spectrometer. Both resolution and sensitivity of the time-of-flight instrument are improved and mass-to-charge ratio discrimination is greatly reduced.