Why is mass spectrometry useful?
Mass spectrometry analyzes the mass-to-charge ratios of molecules by ionizing a gaseous sample, sending it through a magnetic field to separate it, and detects it with some sort of particle counter that functions well in a vacuum, like a Faraday cup. The signal is then converted to a current that is proportional to the molecular abundance, and the spectrum is plotted as abundance vs. mass-to-charge ratio.
It is useful because by analyzing the fragmentation patterns of molecules, we can determine what they are by their fragmentation, even if they are constitutional isomers. It is fast, reliable, and typically accurate (especially Ion Cyclotron Resonance); it is typically useful in conjunction with infrared spectroscopy or gas chromatography for the full characterization of compounds.