Unit 13: Comprehensive Summary

Analytical Chemistry answers two questions: "What is it?" (Qualitative) and "How much is there?" (Quantitative).

Spectroscopy is the study of the interaction between matter and the electromagnetic spectrum. Molecules absorb specific frequencies of radiation, causing transitions like vibrations (IR), electronic excitations (UV-Vis), or nuclear spin flips (NMR). The resulting spectrum is a unique "fingerprint" used for identification, quantification, and structure elucidation.

• Infrared (IR) Spectroscopy: Measures the absorption of IR radiation, which causes molecular bonds to vibrate. It is primarily used to identify functional groups (e.g., broad O-H peak, sharp C=O peak). The fingerprint region (< 1500 cm⁻¹) is unique to each molecule.
• Nuclear Magnetic Resonance (NMR) Spectroscopy: Uses radio waves in a strong magnetic field to study nuclei with spin (¹H, ¹³C). It is the most powerful tool for structure elucidation.
  • ¹H NMR: Provides information on the number of proton environments (number of signals), the ratio of protons (integration), and the number of adjacent protons (splitting pattern via the n+1 rule).
  • ¹³C NMR: Provides information on the number of non-equivalent carbon environments.
• UV-Visible (UV-Vis) Spectroscopy: Measures the absorption of UV or visible light, which causes electronic transitions in molecules with chromophores (e.g., conjugated systems). Its main use is quantitative analysis via the Beer-Lambert Law (A = εbc), which relates absorbance directly to concentration.
• Mass Spectrometry (MS): Measures the mass-to-charge ratio (m/z) of ions. It provides the relative molecular mass from the molecular ion peak (M⁺) and structural clues from fragmentation patterns. Isotope patterns (e.g., M+2 peaks) can reveal the presence of Cl or Br.

Chromatography separates mixtures based on the differential distribution of components between a stationary phase and a mobile phase.

• Gas Chromatography (GC): Separates volatile compounds. Uses a gas mobile phase. Components are identified by their retention time.
• High-Performance Liquid Chromatography (HPLC): A high-resolution technique for separating non-volatile compounds using a liquid mobile phase under high pressure.
• Thin Layer Chromatography (TLC): A simple, rapid method used for monitoring reactions and checking purity. Results are quantified by the Rf value.

Other Techniques:

• Atomic Absorption Spectroscopy (AAS): A highly sensitive method for the quantitative analysis of individual elements (trace metals).
• X-ray Diffraction (XRD): Used to determine the precise 3D structure of crystalline solids.
• Titration: A classical quantitative method to determine the concentration of a solution.