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Unit 21

Spectroscopic analysis

Analysts use light to weigh what they cannot see. This unit starts from the Beer–Lambert law, opens up the UV–visible spectrophotometer, watches atoms in flames and plasmas emit and absorb razor-sharp lines, “listens” to molecular vibrations with infrared spectroscopy, and ends by sorting molecules by mass — every band and peak computed from a model and checked against literature and measured data.

9 lessons ≈ 285 min 0 of 9 lessons complete

  1. 1 Light and absorbance: the Beer–Lambert law Double the concentration and the transmitted light does not halve. Transmittance falls exponentially; absorbance is what is proportional to concentration — and stray light, wide slits and chemical equilibria can bend the line. 25 min
  2. 2 The spectrophotometer Source, monochromator, cell, detector: a virtual instrument scans permanganate, copper ammine and dichromate. Learn to zero it, pick the analytical wavelength, and determine manganese and chromium in steel at once. 30 min
  3. 3 Fluorescence: how molecules give light back Tonic water glows blue under an ultraviolet lamp. The Jablonski diagram shows the routes an absorbed photon can take; a virtual time-resolved fluorometer measures quinine’s lifetime, and the Stern–Volmer equation shows how chloride puts it out. 30 min
  4. 4 Atomic spectroscopy: emission and absorption Only two sodium atoms in ten thousand glow in a flame. The Boltzmann distribution, lines a few picometres wide and the hollow-cathode lamp explain where flame photometry, atomic absorption and ICP each belong. 30 min
  5. 5 Infrared spectroscopy: molecular vibrations Bonds are springs and infrared spectra record their vibrations. Why does CO₂ absorb infrared while N₂ does not? Why are symmetric stretches invisible? And how do group frequencies reveal what a molecule contains? 30 min
  6. 6 Rotation and vibration: the infrared band of a gas The infrared absorption of gaseous HCl is not one peak but a comb of fine lines with a gap in the middle. Rotational levels, a selection rule and the Boltzmann distribution explain its shape; analyse a set of real lines with combination differences and you can measure the bond length to a thousandth of an ångström. 40 min
  7. 7 Counting vibrations with group theory: infrared and Raman Of a molecule’s 3N − 6 vibrations, which absorb infrared and which appear in the Raman spectrum? Turn symmetry operations into characters and reduce them: without solving a single equation of motion you can predict how many bands to expect — and tell cis from trans isomers. 40 min
  8. 8 NMR: listening to hydrogen nuclei Put a sample in a strong magnet and its hydrogen nuclei absorb radio waves of precise frequencies, like tiny compass needles. The chemical shift tells a proton’s environment, the integral how many there are, and the splitting who its neighbours are — enough clues to recognise a molecule. 35 min
  9. 9 Mass spectrometry: isotope clusters and fragments Ionise molecules and sort them by mass-to-charge: chlorine’s 3 : 1 and bromine’s 1 : 1 clusters follow from isotope convolution, the base peak is often a fragment, and high resolution tells CO⁺ from N₂⁺. 25 min