Mass spectrometry identifies a molecule by fragmenting it and weighing the pieces — a destructive technique that consumes the sample in the process. Spectroscopy takes an entirely different, non-destructive approach: it shines electromagnetic radiation — visible light, infrared, or radio waves, depending on the specific technique — at a sample and reads structural information from exactly which wavelengths the molecule absorbs, leaving the sample itself completely physically unchanged.
Different wavelengths, different structural information
UV-Vis spectroscopy measures which wavelengths of ultraviolet and visible light a molecule absorbs, which is particularly sensitive to certain electron arrangements within the molecule, especially useful for compounds with specific conjugated bonding patterns. Infrared (IR) spectroscopy measures absorption in the infrared range, which corresponds to the specific frequencies at which chemical bonds within the molecule naturally vibrate — different types of bonds (a carbon-hydrogen bond versus a carbon-oxygen double bond, for instance) vibrate at reliably different, characteristic frequencies, so an IR spectrum effectively reveals which types of bonds are present in a molecule, functioning almost like a fingerprint of its functional groups. Nuclear magnetic resonance (NMR) spectroscopy, arguably the most structurally informative of the three, exploits the fact that certain atomic nuclei behave like tiny magnets and respond to a strong external magnetic field in ways that depend sensitively on their specific chemical environment within the molecule — revealing not just what types of atoms are present, but how they're connected to their immediate neighbours.
Reading structure from an untouched sample
Because none of these techniques breaks the molecule apart, they can often be run on the same physical sample multiple times, or combined with other analytical methods afterward, in ways mass spectrometry's destructive approach doesn't allow. Chemists routinely combine several spectroscopic techniques together — using IR to identify functional groups present, NMR to map out the detailed connectivity between atoms, and UV-Vis for specific electronic features — building up a full structural picture of an unknown compound from several independent, complementary, non-destructive readings rather than relying on any single measurement alone.
What we're still unsure about
The physical principles underlying UV-Vis, IR, and NMR spectroscopy are well established quantum mechanics and molecular physics, thoroughly validated across decades of use — this isn't in dispute. The genuinely difficult, still partly manual part of the process is interpretation: correctly reading a complex spectrum from a large, unfamiliar molecule and translating it into a confident structural conclusion still requires significant chemist expertise and judgment, and while computational tools increasingly assist with this interpretation, fully automating structure determination from spectroscopic data for genuinely novel, complex molecules remains an active area of research in computational and analytical chemistry rather than a fully solved problem.
This sits inside Spectroscopy (UV-Vis, IR, NMR), one of seven topics in Analytical Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.