The Entrance Lens, Repeller, and Ion Focus all work together to affect high mass abundance; adjust them iteratively. The repeller is particularly effective at changing the high mass abundance relative to that of the lower masses.
If the repeller voltage is set too low, too few ions will leave the source, resulting in poor sensitivity and poor high mass response. If the repeller voltage is set too high, too many ions at too high a velocity will leave the ion source. This may result in precursors (poor mass filtering), peak splitting, and poor low mass resolution.