From mass to structure: modifiedvan Krevelen diagram and adjusted indexes for high-resolution mass data of organic matter
Dear Editor:
The development of high-resolution mass spectrometry (HRMS) has led to deeper insight into the molecular characterization of organic matter. However, a large amount of data obtained by HRMS cannot be immediately converted into specific substances, especially for Fourier transform ion cyclotron resonance mass spectrometry. Some methods have been developed to further explore the information obtained by HRMS by interpreting the data and predicting structures based on exact molecular weight and element constituent patterns.1 Of these methods, the van Krevelen diagram is widely used to classify of organic matter by plotting atomic ratios of H/C and O/C in two-dimensional ordinations.2 In addition, some index are employed to evaluate molecular characterizations, such as the nominal oxidation state of carbon (NOSC)3 and the aromaticity index (AI, AImod and Xc),4,5. While the traditional van Krevelen diagram and the indexes were mainly targeted at natural organic matter (NOM), the calculation equations normally considered C, H, O, N, P, and S factors, or just C, H, and O. However, for organic matter, particularly environmental contaminants and artificial chemicals, halogen elements are important elements and their chemical and toxicological properties cannot be overlooked. 6-8 Moreover, the application of HRMS in environmental pollutants and anthropogenic organics is increasing, 8,9 and the ability to more accurately interpret large amounts of data is essential. Therefore, we adapted the van Krevelen diagram and the other indexes with halogen elements for the analysis of organic matter with complex constituents.
To verify the modified van Krevelen diagram and the adjusted indexes, chemical formulas of 2235 organics were evaluated, which originated from a web database of disinfection byproducts (http://dbps.com.cn/#/main) . The results were compared and depicted as follows:
The van Krevelen diagram
The van Krevelen diagram is normally depicted as a two-dimensional plot using the molar ratio of hydrogen to carbon (H/C) as the ordinate and the molar oxygen-to-carbon ratio (O/C) as the abscissa.2 The H/C ratio separates compounds according to the degree of saturation, whereas O/C ratios separate compounds according to O classes.10 Halogens are generally bonded to organic matter by replacing hydrogen, so their contribution to valence bond saturation should be equivalent to that of hydrogen. For halogen-containing organics, if only the H/C ratio is considered, the material structure will be misjudged. For example, the H/C ratio of trichloromethane is 1, and it is classified as an unsaturated compound by the traditional van Krevelen diagram; however, trichloromethane is a typical saturated organic matter. When halogen is introduced to the calculation of the H/C ratio, H(X)/C (H(X) is the sum of H and halogen elements) of trichloromethane is 4, which is an apparent characteristic of saturated compounds. Fig. 1(a) and (b) show the traditional van Krevelen diagram with H/C versus O/C and the modified van Krevelen diagram with H(X)/C versus O/C, respectively. The distribution of organics clearly shifts upward in the diagram. In particular, compounds with H/C ratios lower than 0.5 are classified as condensed aromatic hydrocarbons,11,12while almost no condensed aromatic compounds are found in the 141 compounds with H/C ratios lower than 0.5 in the database. These compounds are mostly multihalogen substituted organics with H(X)/C ranging from 0.67 to 4. The above results indicate that using the modified van Krevelen diagram plotted with H(X)/C versus O/C could provide a more accurate classification for the organic matter detected by HRMS.
The oxidation of organic carbon
Three indexes are used to evaluate the oxidation state of organic carbon, including Cox,3 NOSC,13 and CHO.11 The equations of these three indexes are calculated as follows:
Cox = (2O+2S-H+3N-5P)/C (1) NOSC = -((-Z+4C+H-3N-2O+5P-2S)/C)+4 (2) CHO = (2O-H)/C (3)
In these three equations, C, O, H, N, P, and S represent the stoichiometric numbers of the elements C, O, H, N, P and S, respectively. Z in Eqn. (2) is the net charge of organic matter. Cox and CHO are special cases of NOSC. When Z=0, Cox = NOSC, and when the stoichiometric numbers of the heteroatoms are zero, CHO = NOSC. However, even NOSC ignores the contribution of halogen to the oxidation state of organic carbon. According to the definition of NOSC and Cox,3,13 we recommend the equation used to adjust NOSC, which is depicted as NOSC’: NOSC’ = -((-Z+4C+H-3N-2O+5P-2S-X)/C)+4. In the equation of NOSC’, X is the sum of stoichiometric numbers of halogen elements (F, Cl, Br, I).