Mass spectrometry is a technique that measures the mass-to-charge ratio of ions. It works by ionizing analyte molecules, then separating the ions based on their m/z ratios using electric and magnetic fields. The data produced are mass spectra, which provide information about molecular structure. Key developments included techniques like electron ionization, electrospray ionization, and matrix-assisted laser desorption/ionization, which allowed analysis of nonvolatile and thermally labile compounds. Mass spectrometry determines isotopic composition and is used to study elemental, molecular, and isotopic properties of substances.