Mass spectrometry is an analytical technique used to determine the molecular mass and molecular formula of a compound. The basic principle of mass spectrometry is to generate ions from the analyte molecule and measure these ion abundances against their molecular mass. One common type of ionization, known as electrospray ionization or EI, bombards the analyte molecules in the gas phase with high-energy electron beams. The electron beams displace an electron from the molecule and leave behind a charged species called a molecular ion. A molecular ion is a radical cation, meaning it contains an unpaired electron and has a positive charge. The molecular ion has effectively the same mass as the analyte molecule, considering the electron mass is negligible. Therefore, given a single charge on the molecular ion and a similar mass as the analyte molecule, the mass-to-charge ratio measured in the mass spectrometry is the same as the molecular mass of the analyte molecule.
The instability that arises due to the charge leads to molecular ion fragmentation into cations, radicals, neutral molecules, and other radical cations with smaller molecular mass. The molecular ion fragments at its weakened bonds to produce stable fragments. The mass-to-charge ratio of the charged species among the fragments is also measured. The information about the molecular ion and the charged species fragmented from the molecular ion can provide details about the molecular mass, chemical structure, and molecular formula of the analyte molecule.
