29: Analytical Chemistry

CHAPTER SUMMARY

CHROMATOGRAPHY

• Solvent (MP), filter paper (water trapped between cellulose fibres is SP)

• Sprayed with chemical that forms coloured compounds (ninhydrin)

• AA are colourless, need to be sprayed

• More soluble in SP: will not move up much

TWO-WAY CHROMATOGRAPHY

• Rotate filter paper and use another solvent

PARTITION COEFFICIENTS

• More soluble in MP: move faster

TYPES OF CHROMATOGRAPHY

• Thin layer

• Gas-liquid

THIN LAYER / ADSORPTION CHROMATOGRAPHY

• SP is a solid that absorbs solute molecules onto its surface

• SP: alumna (Al2O3) or silica on solid support (microscope slide)

• MP: polar or nonpolar solvent

• Al2O3 is polar, solute molecules adsorb

• ADVANTAGES:

  • Quicker than PC

  • Used for smaller samples

  • Cannabis: SP (silica sprayed with dried AgNO3), MP (methylbenzene)

• Depends on solubility on MP and interaction on SP

GAS-LIQUID CHROMATOGRAPHY

• Gas enters column

• SP (high bp nonpolar liquid), MP (inert gas so no chemical can react)

• Same carrier gas, flow rate, SP, temperature

• Detector record retention times (how long to pass through the column) based on interaction with SP only (since MP is inert)

• MP is inert, pushes gas to pass through column, provides pressure

• More time if nonpolar bc SP is nonpolar as they interact more

• LIMITATIONS:

  • Similar retention time

  • Newly discovered compounds may not have a match in the database

• % composition calculated by triangle area, if base too small, measure height only

• Calculations done by GLC, only comparison needed

• USES OF GLC:

  • Testing for steroids

  • Testing for fuels for F1 motor racing

  • Measuring percentage of gasses dissolved in blood sample

  • Combined with mass spectrometry

• Retention time: time it takes for a substance to travel through SP and be detected

• Only depends on interaction with SP

NUCLEAR MAGNETIC RESONANCE (NMR)

• Nucleus of H+ behaves like a tiny magnet

• The proton can spin and create a magnetic field

• Sample put in magnetic field

• Protons either line with the field or spin in the opposite direction

• Difference in E by opposite spinning proton, corresponding to the different E carried by radio waves of EMR

• Nuclei flip between 2 energy levels

• Only atoms with odd mass numbers absorb energy in the range of f that are analysed (1H, 13C, etc.)

• Identify protons in different parts of a molecule

• Size of E gaps depends on molecular environment

• Reference compound: tetramethylsilane Si(CH3)4 because only 1 (very sharp peak) proton environment, inert and volatile liquid

• Against field: higher energy

LOW RESOLUTION NMR

• Single peak

HIGH RESOLUTION NMR

• Chemical shifts are given in ranges

• Ranges overlap

• Heavy shielding of H nuclei by lots of e- in surrounding atoms, peaks shifted beyond their usual range

• Produces grouped peaks

• Magnetic fields generated by spinning nuclei interfere slightly with neighbouring nuclei (spin-spin coupling)

• The exact splitting depends on the number of H atoms on adjacent carbon atoms

• The number of signals a peak splits into equals n + 1, where n is the number of H atoms on the adjacent C atom.

• OH and NH appear as a single peak because it exchanges very rapidly with protons in any traces of water/acids

• If heavy water is used, OH and NH disappear since D has an even mass number

CARBON-13 NMR

• C-13 used instead of C-12 because only odd mass numbers are used

• If even number is used:

  • No signal in NMR

• In proton NMR:

  • Discrete vertical lines without splitting

• Carbon-13 NMR: Heights are not proportional to the number of C atoms present

• Solvent: CDCl3 (small signal near 80 ppm)

  • If CHCl3 used, there will be a disturbance due to the H atom

  • D has an even mass number

• No splitting

• Height is unrelated, refer to chemical shift only.

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