Evaluating pyrolysis-GC/MS and ¹³C CPMAS NMR in conjunction with a molecular mixing model of the Penido Vello peat deposit, NW Spain

We performed solid state ¹³C cross-polarization magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectroscopy and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) on the Penido Vello peat deposit located in Galicia, NW Spain. Often regarded as complementary techniques, solid state ¹³C NMR and Py-GC/MS are widely used for the characterisation of organic matter. Recently, a molecular mixing model (MMM) was proposed to predict the distribution of C in biochemical components (carbohydrate, protein, lignin, lipid, char) from the ¹³C NMR spectral distribution, thereby allowing a quantitative comparison with Py-GC/MS product abundances. We discuss the application of this model to a peat core, by comparing NMR-MMM results with Py-GC/MS data. The core represents ∼5000 yr accumulation and ranges from fibric (at the surface) to hemic (bottom). The amounts of carbohydrates and lipids predicted by the MMM and calculated from the quantified Py-GC/MS chromatograms are in close agreement. However, the well known low capability of the conventional Py-GC/MS method to provide structural information on proteins from bulk soils, the poor GC amenability of polar compounds and many other possible sources of inaccuracy in Py-GC/MS and NMR-MMM caused discrepancies with predictions made with the MMM. Also, the MMM failed to give good prediction of the C/N ratio of the peat material. Although the NMR-MMM approach does not account for molecular transitions during decomposition, this oversimplification proved to be acceptable. The MMM seems to be a useful tool for the interpretation of NMR spectral distributions in peat material.