Applications of quantitative 1H- and 13C-NMR spectroscopy in drug analysis

The usefulness of 1H and 13C Fourier transform (FT) nuclear magnetic resonance spectroscopy (1H- and 13C-NMR) as quantitative methods stems from the potential direct relationship between the area under an NMR peak and the number of the particular type of nuclei that give rise to the signal, though it is necessary, especially for quantitative 13C-NMR, to take some precautions. The experimental limitations that have to be overcome in order to obtain quantitative 13C-NMR spectra are associated with the relaxation time, the nuclear Overhauser effect (NOE), and the NMR instrument itself (filter characteristics, power level of the exciting pulse, dynamic range, digital resolution). Practical problems aside, 13C-NMR has a greater potential than 1H-NMR for the study of organic systems. The sensitivity of 13C chemical shifts to small differences in molecular environment, coupled with a large chemical shift range, gives a "chromatographic" separation of resonances of interest, and has made 13C-NMR an attractive method for analysing complex mixtures. Some applications of quantitative 1H- and 13C-NMR spectroscopy in drug analysis are discussed.