The electron multipler converts incident ions into an amplified electric current to measure the ion signal. As voltage increases, amplification factor (gain) increases.
The electron multiplier for the 4500 ICP-MS should be replaced when the total ion count reaches more than 3.2 x 1011 (this value is displayed in the software run time window) and the sensitivity is less than 4000 counts with 10 ppb Y (89 amu) even when applying detector voltage above 2700V. In general, the number of counts an EM can measure is 1.6 x 1010.