Analytical and Instrumental Techniques
This domain tests how spectroscopic and separation methods identify and quantify substances. STPM expects interpretation of spectra and chromatograms, not just recall of instrument names.
Spectroscopic methods
Mass spectrometry gives the molecular ion and fragmentation pattern; the molecular ion peak gives Mr. IR spectroscopy identifies functional groups: C=O around 1700 cm−1, O–H broad around 2500–3300 cm−1, C–H just below 3000 cm−1. NMR gives chemical shift, integration and splitting, indicating different hydrogen environments. UV–Vis absorption arises from electronic transitions, especially in conjugated systems.
Separation and chromatography
Chromatography separates components between a stationary phase and a mobile phase. TLC uses Rf values; GC and HPLC use retention times. Column chromatography and distillation separate by different physical principles.
Worked example
A compound has a molecular ion at m/z 60, a broad IR absorption at 2500–3300 cm−1 and a strong absorption at 1710 cm−1. The broad absorption indicates O–H; the strong peak indicates C=O. With Mr = 60, the compound is consistent with ethanoic acid, CH3COOH.
Traps that cost marks
IR identifies functional groups, not the whole structure. The base peak in mass spectrometry is not necessarily the molecular ion. NMR integration gives the ratio of hydrogens, not their absolute number. Rf depends on solvent and stationary phase. UV–Vis absorption is not specific enough to identify a functional group alone.