A method of mass spectrometry comprising: providing an attenuation device, an ion accumulation device upstream of said attenuation device, wherein said ion accumulation device comprises a first upstream ion accumulation region and a second downstream ion accumulation region and an ion trap arranged downstream of said attenuation device; accumulating a first population of ions in said second downstream ion accumulation region and a second population of ions in said first upstream ion accumulation region; determining a first ion current I 1 using said first population of ions; controlling said attenuation device based upon said determined first ion current I 1 so as to set the intensity of ions in said second population of ions transmitted by said attenuation device and passed to said ion trap at a first level; allowing said second population of ions to accumulate within said ion trap for a first fixed period of time T 1 which is substantially independent of said determined first ion current I 1; performing an analytical scan of said second population of ions; accumulating a third population of ions in said second downstream ion accumulation region and a fourth population of ions in said first upstream ion accumulation region whilst the analytical scan of said second population of ions is being performed; determining a second ion current I 2 using said third population of ions; controlling said attenuation device based upon said determined second ion current I 2 so as to set the intensity of ions in said fourth population of ions transmitted by said attenuation device and passed to said ion trap at a second different level; allowing said fourth population of ions to accumulate within said ion trap for a second fixed period of time T 2 which is substantially independent of said determined second ion current I 2, and wherein T 1 equals or substantially equals T 2. providing an attenuation device, an ion accumulation device upstream of said attenuation device, wherein said ion accumulation device comprises a first upstream ion accumulation region and a second downstream ion accumulation region and an ion trap arranged downstream of said attenuation device; accumulating a first population of ions in said second downstream ion accumulation region and a second population of ions in said first upstream ion accumulation region; determining a first ion current I 1 using said first population of ions; controlling said attenuation device based upon said determined first ion current I 1 so as to set the intensity of ions in said second population of ions transmitted by said attenuation device and passed to said ion trap at a first level; allowing said second population of ions to accumulate within said ion trap for a first fixed period of time T 1 which is substantially independent of said determined first ion current I 1; performing an analytical scan of said second population of ions; accumulating a third population of ions in said second downstream ion accumulation region and a fourth population of ions in said first upstream ion accumulation region whilst the analytical scan of said second population of ions is being performed; determining a second ion current I 2 using said third population of ions; controlling said attenuation device based upon said determined second ion current I 2 so as to set the intensity of ions in said fourth population of ions transmitted by said attenuation device and passed to said ion trap at a second different level; allowing said fourth population of ions to accumulate within said ion trap for a second fixed period of time T 2 which is substantially independent of said determined second ion current I 2, and wherein T 1 equals or substantially equals T 2.