Enhancement of Ion Activation and Collision-Induced Dissociation by Simultaneous Dipolar Excitation of Ions in x- and y-Directions in a Linear Ion Trap.

Collision-induced dissociation (CID) in linear ion traps is usually performed by applying a dipolar alternating current (AC) signal to one pair of electrodes, which results in ion excitation mainly in one direction. In this paper, we report simulation and experimental studies of the ion excitation in two coordinate directions by applying identical dipolar AC signals to two pairs of electrodes simultaneously. Theoretical analysis and simulation results demonstrate that the ion kinetic energy is higher than that using the conventional CID method. Experimental results show that more activation energy (as determined by the intensity ratio of the a4/b4 fragments from the CID of protonated leucine enkephalin) can be deposited into parent ions in this method. The dissociation rate constant in this method was about 3.8 times higher than that in the conventional method under the same experimental condition, at the Mathieu parameter qu (where u = x, y) value of 0.25. The ion fragmentation efficiency is also significantly improved. Compared with the conventional method, the smaller qu value can be used in this method to obtain the same internal energy deposited into ions. Consequently, the "low mass cut-off" is redeemed and more fragment ions can be detected. This excitation method can be implemented easily without changing any experimental parameters.