Sample Notes: Analytical Techniques

A2 Level Chemistry – Detailed Notes

Chapter 37: Analytical Techniques

37.1 Thin-Layer Chromatography (TLC)

Key Terms and Concepts

• Stationary Phase:
  • A solid material that does not move.
  • Example: Aluminium oxide (Al₂O₃) or silica gel coated on a glass/plastic/metal plate.
  • It adsorbs compounds based on polarity and forms the basis for separation.
• Mobile Phase:
  • A liquid solvent (polar or non-polar) that moves up the TLC plate via capillary action.
  • The choice of solvent affects how far each compound travels.
• Rf Value (Retention factor):
  • A ratio used to compare how far a substance travels on the TLC plate relative to the solvent front.
  • Formula:
    Rf = distance moved by substance / distance moved by solvent front
  • Always between 0 and 1.
• Solvent Front:
  • The farthest point reached by the mobile phase during the run.
• Baseline:
  • The initial line where the sample is spotted on the TLC plate.

How TLC Works

• A small spot of the sample mixture is placed on the baseline.
• The plate is placed vertically in a beaker containing a shallow layer of solvent.
• As the solvent rises:
  • More soluble substances in the mobile phase move faster.
  • Greater adsorption to the stationary phase causes substances to move slower.
• Different components are separated based on their relative interactions with both phases.

Interpreting Rf Values

• Smaller Rf → stronger interaction with stationary phase (more polar compound).
• Larger Rf → weaker interaction with stationary phase, more soluble in solvent.

Applications

• Identifying substances by comparing Rf values with known standards.
• Used in forensics, drug purity testing, and biochemical analysis.

37.2 Gas/Liquid Chromatography (GLC or GC)

Key Terms and Concepts

• Stationary Phase:
  • A non-volatile, high-boiling, non-polar liquid (e.g., long-chain alkane) coated on a solid support inside the column.
• Mobile Phase:
  • An inert/unreactive gas such as helium or nitrogen.
• Retention Time:
  • Time taken for a component to pass through the column to the detector.
  • Different compounds have different retention times based on:
    • Boiling point
    • Solubility in stationary phase
    • Size and polarity

How GC Works

• Sample is vaporised and carried by the gas into the column.
• Each compound interacts with the stationary liquid phase.
• Compounds with:
  • Low boiling points or weak interaction → pass quickly (short retention time).
  • High boiling points or strong interaction → retained longer.

Interpreting a GC Chromatogram

• Each peak = a different component.
• Retention time = identifies the substance.
• Area under each peak = relative amount (percentage composition).

Applications

• Drug testing, blood alcohol levels, air pollution studies.
• Often used in conjunction with mass spectrometry (GC-MS).

37.3 Carbon-13 NMR Spectroscopy (¹³C NMR)

Key Principles

• Carbon atoms in different chemical environments produce different signals.
• Each unique carbon environment shows one peak in the ¹³C NMR spectrum.
• Only carbon-13 isotopes (1% abundance) are NMR-active.

Reading a ¹³C NMR Spectrum

• X-axis = chemical shift in ppm (parts per million).
• Number of peaks = number of different carbon environments.
• Peaks do not split – no coupling in ¹³C NMR (no spin-spin splitting).

Interpretation Tips

Deducing Structure

• Use:
  • Number of peaks
  • Chemical shifts
• Combine with molecular formula to propose structure.

37.4 Proton NMR Spectroscopy (¹H NMR)

Key Concepts

• Hydrogen atoms (protons) in different environments absorb different frequencies.
• ¹H NMR provides more detailed information than ¹³C NMR:
  • Chemical shift
  • Relative peak area
  • Splitting pattern (spin-spin coupling)

Reading a ¹H NMR Spectrum

• Chemical Shift (ppm):
  Indicates type of proton environment.

• Peak Area:
  • Proportional to number of equivalent protons.
• Splitting Pattern:
  • Determined by the number of adjacent protons (n).
  • n + 1 rule:
    • 0 neighbours → singlet
    • 1 neighbour → doublet
    • 2 neighbours → triplet
    • 3 neighbours → quartet

    • 3 → multiplet

Use of Tetramethylsilane (TMS)

• Chemical shift standard (set to 0 ppm).
• Inert, volatile, non-toxic.
• Produces one sharp peak away from others.

Use of Deuterated Solvents

• Solvent like CDCl₃ used so proton peaks from the solvent do not appear in the spectrum.
• Deuterium (²H) does not give signal in ¹H NMR.

Proton Exchange with D₂O

• Used to identify –OH and –NH groups.
• Add D₂O → proton exchanges with deuterium → signal disappears.