combined techniques

• Sequence for identification:

  1. Elemental analysis
  2. Mass spectra
  3. Infrared spectra
  4. NMR spectra
• Elemental analysis:

  Use of percentage composition by mass to determine the empirical formula of a compound
• Mass spectra:

  • Use of the molecular ion peak to determine the molecular mass and fragment ions to determine parts of the molecule
  • The molecular formula can then be determined from the empirical formula and the molecular mass
• Infrared spectra:

  Use of absorption peaks to identify bonds present and functional groups
• NMR spectra:

  To determine the number and types of carbon and hydrogen atoms from the chemical shifts of peaks and the order of atoms within molecules from splitting patterns.
• Analysis of an unknown compound

  STEP 1: Determine the empirical formula from data by converting percentage by mass to moles (by dividing by Mr) and then finding the smallest whole number ratio
• Analysis of an unknown compound

  STEP 2: Determine the molecular formula using the mass spec and the empirical formula

  • Use the m/z value for the molecular ion peak to determine the molecular formula
  • e.g. molecular ion peak is at m/z = 164 so molecular mass = 164
  • Find the molecular mass by comparing it with empirical formula mass
• Analysis of unknown compound

  STEP 3: Identify the functional groups using the infrared spectrum and the data sheet
• Analysis of unknown compound

  STEP 4: Analyse the C13 NMR spectrum

  • number of peaks = number of different carbon environments
• Analysis of unknown compound

  STEP 5: Analyse the proton NMR spectrum

  • Analyse the types of proton present and how many of each type
  • Analyse the splitting patterns to find information about adjacent protons
  • Use the data sheet to analyse the chemical shifts for the types of proton
• Analysis of unknown compound

  STEP 6: Combine the information to suggest a structure