analytical techniques

• Chromatography is a process used to separate a mixture of substances
• In thin-layer chromatography, the mobile phase is a solvent and the stationary phase is typically a layer of silica gel or alumina on a piece of glass
• In thin-layer chromatography, a spot of the substance being analyzed is put on the pencil baseline before the stationary phase is placed into the solvent
• The Rf value (or retention factor) can be calculated after chromatography is completed because different compounds have different solubilities in the solvent and different attraction to the stationary phase
• In gas-liquid chromatography, the mobile phase is a carrier gas (such as helium) and the stationary phase is a liquid with a high boiling point which is adsorbed onto a solid
• Retention time is the time taken for a sample to travel from the injector to the detector in gas-liquid chromatography
• Factors affecting the retention time in gas-liquid chromatography:

  • Boiling point: compounds with higher boiling points will condense sooner in the column so the retention time will be longer
  • Solubility in the liquid stationary phase: more soluble compounds will have a longer retention time
  • Temperature: the higher the temperature of the column, the shorter the retention times of all the compounds will be
• A gas-liquid chromatogram can be used to work out the percentage composition of a mixture by calculating the area under a peak
• Carbon-13 NMR Spectroscopy involves the alignment of C-13 nuclei with or against a magnetic field, with the number of peaks showing the number of carbon environments
• When using a C-13 NMR spectrum to predict structures of compounds, consider the number of carbon environments and the functional groups present
• Proton (1H) NMR Spectroscopy involves hydrogen nuclei aligning with or against a magnetic field, with the number of peaks in the splitting pattern indicating the number of adjacent protons
• The n+1 rule in proton NMR spectroscopy states that the number of peaks in the splitting pattern is equal to the number of adjacent protons + 1
• TMS (Tetramethylsilane) is used as a standard for chemical shift measurements during NMR spectroscopy because it has 12 hydrogen atoms in the same environment, contains both carbon and hydrogen atoms, is non-toxic, inert, and produces a single peak in carbon-13 NMR
• Deuterated solvents, like CDCl3, are used in NMR to avoid hydrogen atoms producing peaks on the spectrum
• To identify O-H and N-H protons in NMR, shake the sample with D2O and run a second proton NMR to compare the spectra