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Mass spectrometry is an analytical technique used to determine the molecular weight, molecular formula, and chemical structure of a compound. The technique generates ions from the analyte molecule and measures their abundances against their mass-to-charge ratio. This information allows scientists to reconstruct the native molecule's properties and identify unknown compounds.

In electron ionization, a high-energy electron beam bombards analyte molecules in the gas phase, displacing an electron from the molecule. This creates a charged species called a molecular ion, which is a radical cation containing an unpaired electron and a positive charge. Since electron mass is negligible, the molecular ion's mass-to-charge ratio equals the analyte molecule's molecular mass.

After formation, the molecular ion becomes unstable due to its positive charge and fragments into smaller cations, radicals, neutral molecules, and other radical cations. Cleavage occurs at weakened bonds to produce relatively stable fragments. The mass-to-charge ratio of charged species among these fragments is measured, providing structural information about the original molecule.

Beyond electron impact ionization, alternative ionization methods include chemical ionization and electrospray ionization. These methods provide different ways to generate ions from analyte molecules, each suited to different sample types and analytical requirements. The choice of ionization method affects the fragmentation pattern and information obtained about the molecule.

The electron mass is negligible because it is extremely small compared to the mass of the molecular ion and parent molecule. Therefore, the molecular ion and its parent molecule have comparable molecular weights. This allows the mass-to-charge ratio measured in mass spectrometry to accurately represent the molecular mass of the analyte molecule.

Mass spectrometry separates and weighs individual fragments produced from molecular ion fragmentation. By analyzing the mass-to-charge ratios and abundances of these fragments, scientists can reconstruct the molecular weight, structure, and molecular formula of the native molecule. Fragment patterns reveal how bonds break, providing clues about the molecule's composition and connectivity.

The mass-to-charge ratio is the fundamental measurement in mass spectrometry. For a molecular ion with a single positive charge, the mass-to-charge ratio directly equals the molecular mass of the analyte. This relationship allows mass spectrometers to determine molecular mass by measuring the mass-to-charge ratios of ions generated from the sample.