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.