An ion produced in a source has no distribution in mass. However,a peak profile of mass has a width after the mass is separated by the mass analyzer.The left figure shows how this spreading of mass occurs.The blue area is a stability region of mass,which moves to the right hand by scan operation. When a mass enters the region between two dotted lines,an ion having that mass can go through the quadrupole.As long as the mass stays inside this region, the ions will be detected. This is a reason why the peak profile has a width.

How Dose a Mass Peak Define Width