Poor losses are generally of low probability. However, poor losses can't be rejected on chemical grounds. For example, a loss of 82 from the molecular ion is unlikely. However, H81Br has a mass of 82 Da. This mass is in the unlikely list however, because 82 Da occurs as a loss from the M+2 isotope peak and not the molecular ion.
1. Robert A. W. Johnstone, Malcolm E. Rose, Mass spectrometry for chemists and biochemists, 2nd. Ed., Cambridge University Press, Cambridge, Great Britain, 1996.
2. R.G. Dromey, B.G. Buchanan, D.H. Smith, J. Lederberg, and Carl Djerassi, "Applications of Artificial Intelligence for Chemical Inference. XIV. A General Method for Predicting Molecular Ions in Mass Spectra," J. Org. Chem., 40(6), 1975.
Colby College Chemistry, 1999