TG–FTIR, LC/MS, XANES and Py-FIMS to disclose the thermal decomposition pathways and aromatic N formation during dipeptide pyrolysis in a soil matrix

Understanding the thermal decomposition pathways of amino acids and peptides is crucial for, e.g. fuel and tobacco industry, and environmental science, especially if analytical pyrolysis is applied to study the composition of organic N compounds. Results of pyrolysis-field ionization mass spectrometry (Py-FIMS) applied to soil samples were suspected to be falsified by heteroaromatic N-compounds absent in the samples but formed during analyses. To disclose the decomposition pathways and derive a correction factor for the formation of heteroaromatic-N from amide-N during temperature-resolved Py-FIMS measurement, we spiked a soil sample with a model dipeptide (Gly–Asp), heated this sample stepwise to 700 °C, and investigated volatile and solid pyrolysis products by TG–FTIR, LC/MS, synchrotron-based C- and N-XANES and Py-FIMS. The Gly–Asp decomposition was dominated by water cleavage reactions, forming piperazines and pyrazines, and by intermediary formation and later degradation of polymers having 2,5-furandione, succinimide and maleimide as its subunits. The XANES spectra revealed that the pyrolysis of Gly–Asp led to the formation of imide-N, nitrile-N, imine-N and other aromatic structures in the solid phase. The volatiles were composed of 45% amide-N, 18% aromatic-N, 3% imidic-N, 2% nitrile-N, and 32% unassigned N. Thus, a correction factor 0.4 (18:45) was derived to calculate how much aromatic-N was formed during the thermal degradation of the dipeptide. In conclusion, all six complementary analytical methods, which were applied in conjunction for the first time, were crucial to characterize the pyrolytic decomposition pathways of the Gly–Asp, and this assembly of methods can be recommended for various research areas to disclose thermal decomposition pathways of amides and amino acids.