Evaluation of the quantitative reliability of the 13C CP/MAS technique for the analysis of coals and related materials

The percentages of aromatic carbon in three representative solid coal samples and one pitch sample have been determined by three different n.m.r. techniques. The first two involve the use of cross-polarization/magic-angle spinning (CP/MAS) and in one case, the magnetization curves were fitted to a theoretical expression and the fit parameters used to calculate the percentage of aromatic carbon. In the second case, the percentage of aromatic carbons was obtained from a choice of contact time in the cross-polarization pulse sequence which gave maximum signal intensity. The third technique used MAS together with a simple 90° carbon pulse sequence with a 5 × T₁, (for the carbon-13 nuclei) recycle time and was assumed to give the absolute quantitative values. It was found that the results obtained using cross-polarization techniques could be substantially different from those obtained using 90° pulses and that caution must be exercised in the application of these techniques to the quantitative analysis of solid carbonaceous fuels.