If an ion, whether molecular ion or fragment ion (m1+) is accelerated before it breaks down, then it decomposes in the acceleration region and gives m2+ and m3 ions. The part of the kinetic energy of m1+ is lost to the neutral fragment m3.
m1+ = m2+ + m3
The m2+ ion continues to accelerate and then it is collected and detected. The m2+ ion produced in this way is not recorded as the mass m2 but recorded as them* which is called metastable ion and recorded as the week broad peak with less intensity.
Metastable ion (m*) = m22 / m1 or (m2+)2 /m
The metastable ion peaks are observed generally when the lifetime of the original species m1 is in the range 10-4 – 10-6 sec which is enough to reach the accelerating chamber but cannot be fully accelerated. The metastable peaks help in deducing the fragmentation mechanism as it is used to determine the both original mass and size of ejected fragments.