Select and interpret spectroscopic and chromatographic techniques to determine structure and concentration

What this dot point is asking

Analytical chemistry answers two questions: what is in a sample (qualitative analysis) and how much is present (quantitative analysis). Different instruments probe different aspects of a molecule, so chemists often combine several techniques to build a full picture. You should be able to match a technique to the information it provides and interpret the data it generates.

Mass spectrometry measures the mass-to-charge ratio of ions. A sample is vaporised and ionised, often breaking into fragments, and the ions are separated by mass. The peak at the highest mass, the molecular ion peak, gives the molar mass of the original molecule. The pattern of smaller fragment peaks gives clues about the structure, since particular fragments correspond to particular groups. Mass spectrometry is also used to determine relative isotopic abundances and hence relative atomic mass.

Infrared (IR) spectroscopy identifies functional groups. Bonds absorb infrared radiation at characteristic frequencies that make them vibrate, so the wavenumbers at which absorptions occur reveal which bonds are present. A broad absorption around 3200 to 3550 wavenumbers suggests an $O-H$ group, while a strong absorption near 1700 wavenumbers indicates a carbonyl ($C=O$) group. The complex region of the spectrum, called the fingerprint region, is unique to each compound and can confirm identity by comparison with reference spectra.

Ultraviolet-visible (UV-visible) spectroscopy is mainly used for quantitative analysis of coloured solutions. The amount of light absorbed at a particular wavelength is proportional to concentration, as described by the Beer-Lambert relationship. By measuring the absorbance of standards of known concentration, you build a calibration curve and read off the concentration of an unknown.

Nuclear magnetic resonance (NMR) spectroscopy gives detailed structural information about the carbon-hydrogen framework. Proton ($^1H$) NMR reveals the different chemical environments of hydrogen atoms. The number of distinct peaks shows how many environments there are, the area under each peak (integration) shows the relative number of hydrogen atoms in each environment, and the chemical shift indicates the type of environment. Carbon-13 NMR similarly shows the number of distinct carbon environments.

Chromatography separates the components of a mixture based on their different affinities for a stationary phase and a mobile phase. Components that interact more strongly with the stationary phase move more slowly. In thin-layer and paper chromatography, the retention factor ($R_f$) is the distance moved by the spot divided by the distance moved by the solvent front, and it helps identify components by comparison with standards. High-performance liquid chromatography and gas chromatography give precise separations and can be coupled to a mass spectrometer for both separation and identification.

Volumetric analysis (titration) is a classic quantitative method. A solution of known concentration (the standard) is reacted with a measured volume of the unknown until an indicator or instrument shows the equivalence point. From the balanced equation and the volumes and concentrations involved, you calculate the concentration of the unknown using mole ratios.

In exams, justify your choice of technique by stating exactly what information it provides, and when interpreting spectra, link each piece of data to a specific structural feature before drawing a conclusion.