Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)The extent of unsaturation level in a compound, commonly measured in terms of its number of double bond equivalents (DBEs), reflects the total number of pi bonds plus rings and can be calculated if the molecular formula is known. The extent of unsaturation can also be calculated via DBE/C, where C is the number of carbon atoms. For hydrocarbons, if DBE/C is > 0.67, the structure corresponds unequivocally to a condensed aromatic (CA) compound, but this criterion cannot be applied directly to heteroatom containing compounds. A new parameter, the aromaticity index (AI) was recently proposed to solve this problem, and has been applied to establish a region of CA compounds in high resolution mass spectrometry (MS) van Krevelen diagrams to characterize natural organic matter (NOM), such as humic acids (HAs). In this study, we report a reformulated AI (rAI) that also considers formulae that accommodate non-aromatic (NA) and non-condensed aromatic (NCA) cyclic carbonyl compounds. We also propose a correction for AI with respect to N and P containing formulae and show an application of the rAI index for the analysis and data treatment of a certified reference material (CRM) of NOM. (C) 2016 Elsevier Ltd. All rights reserved.95 2933Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP), Brazil [2013/15644-0]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)