Mass spectrometry — FAQ

During electron impact ionisation, it requires a certain amount of energy to remove one electron. Removing a second electron from an already positive ion requires significantly more energy. While it is possible to form +2 or +3 ions, it is far less likely. Therefore, the vast majority of ions detected have a +1 charge.

Yes, often it can. Structural isomers have the same molecular formula and therefore the same relative molecular mass, so they will have the same molecular ion peak ($M^+$). However, they have different structures and bond arrangements, so they tend to break apart (fragment) in different ways. This leads to different fragmentation patterns, with different peaks and/or different relative abundances, allowing them to be distinguished.

Electrospray ionisation is a 'softer' ionisation technique. The sample is dissolved in a volatile solvent and sprayed through a fine, high-voltage needle. This creates charged droplets, and as the solvent evaporates, ions are formed (typically $[M+H]^+$). ESI is much less energetic than EI and causes little to no fragmentation. It is useful for finding the $M_r$ of large, fragile molecules like proteins that would be destroyed by electron impact.