Mass spectrometry - How can you find the mass of the compound?

[Horseb0x]

The obvious way would be to look for the parent peak (the peak caused by the unfragmented molecular ion) at the end of the spectrum but how can you tell the parent peak from one caused by heavier isotopes? Also how do you know if there's a parent peak at all and that all the peaks aren't fragments? Are there any tricks I should know about for finding the molecular weight from the mass spectrum?

 

[Ygggdrasil]

There are ways of performing mass spectrometry that involve "soft" ionization methods that do not result in fragmentation. These methods can be used to determine the molecular weight of the species as only the parent ion will be present. Mass spectra obtained under fragmenting conditions can then be used to deduce structural features of the compound.

 

[Leakyrex1]

Using H1NMR, C13NMR and IR will help a great deal than using Mass spec by itself.