Spectroscopy: How Scientists Identify Substances Using Light

In 1814, Joseph von Fraunhofer observed that sunlight passed through a prism produced not a smooth rainbow but a rainbow interrupted by hundreds of dark lines at precise wavelengths. He catalogued over 570 of these lines, labeled them with letters, and could not explain them. Forty-five years later, Gustav Kirchhoff and Robert Bunsen showed that each line corresponded to an element absorbing its characteristic wavelengths of light. Fraunhofer's lines were the absorption spectrum of the solar atmosphere — and the technique of reading chemical identity from patterns of light and dark had been born. Spectroscopy now identifies molecules in meteorites, determines protein structures, diagnoses diseases from breath, and monitors industrial processes continuously in real time.

The Electromagnetic Spectrum as an Analytical Tool

Every region of the electromagnetic spectrum interacts with matter in a different way. The photon energy E = hν determines which physical process it drives. X-rays eject core electrons. Ultraviolet light promotes valence electrons to excited states. Visible light also excites electronic transitions in colored molecules. Infrared radiation drives molecular vibrations. Microwaves drive molecular rotations. Radio waves flip nuclear spins. Each region probes a different structural feature of the molecule.

Spectroscopic Region	Wavelength Range	Molecular Process	Information Obtained
X-ray	0.01–10 nm	Core electron ejection; diffraction by crystal lattices	Elemental composition (XPS); crystal structure (XRD)
Ultraviolet-Visible (UV-Vis)	200–800 nm	Valence electron transitions (π→π*, n→π*)	Conjugated systems, aromatic rings, transition metal complexes; concentration (Beer-Lambert law)
Infrared (IR)	2.5–25 μm (4000–400 cm−1)	Molecular bond vibrations (stretching, bending)	Functional groups (–OH, C=O, N–H, C–H); molecular fingerprinting
Raman	Variable (shift from laser)	Inelastic light scattering changing vibrational states	Symmetric vibrations not seen in IR; materials characterization
Microwave	1 mm–1 m	Molecular rotation	Bond lengths, bond angles (rotational spectroscopy); interstellar molecules
NMR (radiowave)	0.1–10 m (at 1–14 T magnets)	Nuclear spin transitions in magnetic field	Complete molecular structure; H and C connectivity; 3D conformation in solution

Infrared Spectroscopy: Reading Functional Groups

Every bond in a molecule vibrates at a characteristic frequency determined by the masses of the atoms and the bond strength (analogous to a spring: ν ∝ √(k/μ), where k is the force constant and μ is the reduced mass). When infrared light of matching frequency strikes a molecule, the bond absorbs it and vibrates more strongly — provided the vibration changes the molecule's electric dipole moment.

An IR spectrum plots absorbance against wavenumber (cm⁻¹). Functional groups absorb at characteristic positions, allowing rapid identification:

• O–H stretch: 2,500–3,300 cm⁻¹ (carboxylic acid, broad) or 3,200–3,550 cm⁻¹ (alcohol, broad)
• N–H stretch: 3,300–3,500 cm⁻¹ (amines and amides)
• C–H stretch: 2,850–3,000 cm⁻¹ (alkane sp³), 3,000–3,100 cm⁻¹ (alkene sp²), ~3,300 cm⁻¹ (alkyne sp)
• C≡N stretch: ~2,200 cm⁻¹ (nitrile, sharp)
• C=O stretch: 1,650–1,850 cm⁻¹ (varies: acid ~1,710, ester ~1,735, aldehyde ~1,720, amide ~1,680)
• C=C stretch: 1,620–1,680 cm⁻¹ (alkene)
• Fingerprint region: 400–1,500 cm⁻¹ — complex combination of bends; unique to each molecule, used for identification by library matching

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR is the most powerful routine method for determining the complete structure of an organic molecule. When nuclei with nonzero spin (most commonly ¹H and ¹³C) are placed in a strong magnetic field, they align parallel or antiparallel to the field. Radiofrequency pulses knock them out of alignment; they precess back at the Larmor frequency, emitting a signal. The precise frequency (chemical shift, measured in ppm) depends on the electronic environment of each nucleus.

Electrons shield nuclei from the external field. More shielded nuclei (surrounded by electron density) resonate at lower frequency (upfield, lower ppm). Deshielded nuclei (near electronegative groups, aromatic rings, or double bonds) resonate at higher ppm. The chemical shift reveals what functional groups are nearby. Coupling constants (J values) reveal how many neighboring hydrogen atoms are present. Integration reveals the number of each type of hydrogen.

Proton Environment	Typical 1H Chemical Shift (ppm)
Alkyl (–CH3, –CH2–)	0.9–1.8
Next to halogen (–CH–X)	2.5–4.5
Alkene (=CH–)	4.5–6.5
Aromatic (Ar–H)	6.5–8.5
Aldehyde (–CHO)	9.4–10.0
Carboxylic acid (–COOH)	10–13

Mass Spectrometry: Weighing Molecules

Mass spectrometry (MS) measures molecular masses with extraordinary precision. In electrospray ionization (ESI-MS), a solution of the analyte is sprayed through a charged needle; solvent evaporates and the molecule acquires protons (in positive mode), producing [M+H]⁺ or [M+nH]ⁿ⁺ ions. These ions are accelerated into a mass analyzer — quadrupole, time-of-flight (TOF), or Orbitrap — that separates them by mass-to-charge ratio (m/z).

High-resolution mass spectrometry measures masses to 4–5 decimal places, allowing exact molecular formula determination. Carbon-12 and nitrogen-14 have integer masses by definition, but ¹H = 1.00783, ¹³C = 13.00335, ¹⁴N = 14.00307, ¹⁶O = 15.99491. A molecule with m/z = 194.0793 could be C₁₀H₁₀N₂O₂ (194.0793 calculated) but not C₉H₁₀O₅ (198.0528 calculated). High-resolution mass alone often uniquely identifies the molecular formula.

Astronomical Spectroscopy: Reading the Universe

Spectroscopy has identified over 300 molecules in interstellar space, from simple H₂ and CO to complex polycyclic aromatic hydrocarbons and even glycine precursors. Rotational spectroscopy at microwave frequencies gives molecular fingerprints that match laboratory standards to parts-per-billion precision.

Stellar spectroscopy measures the surface temperatures and chemical compositions of stars across the observable universe. The Doppler shift of spectral lines reveals whether a star is moving toward or away from Earth, enabling the measurement of galaxy rotation curves (which revealed dark matter) and the cosmological redshift (which confirmed the expansion of the universe). The techniques Fraunhofer applied to sunlight in 1814 now extend to galaxies 13 billion light-years away.