The LIT mass analyzer cavity contains helium gas that is used as a damping gas and a collision activation partner. The collisions of the ions entering the LIT with the helium gas slow the ions so that the RF field in the LIT can more efficiently trap them.
The presence of helium gas in the LIT cavity significantly enhances sensitivity and mass spectral resolution. Before their ejection from the mass analyzer cavity, sample ions collide with helium gas atoms. These collisions reduce the kinetic energy of the ions, which dampens the amplitude of their oscillations. The mass analyzer then focuses the ions to the axis of the cavity rather than allow them to spread throughout the cavity, which would spread their ejection times and degrade the mass spectral resolution.