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).