Anal. Chem. 2004, 76, 2511-2516
Two- and Three-Dimensional van Krevelen Diagrams: A Graphical Analysis Complementary to the Kendrick Mass Plot for Sorting Elemental Compositions of Complex Organic Mixtures Based on Ultrahigh-Resolution Broadband Fourier Transform Ion Cyclotron Resonance Mass Measurements Zhigang Wu, Ryan P. Rodgers, and Alan G. Marshall*
Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310-4005, and Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306
Ultrahigh-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry has resolved and identified the elemental compositions of over 10 000 organic constituents of coal and petroleum crude oil. A plot of Kendrick mass defect versus Kendrick nominal mass sorts compounds into homologous series according to compound class (i.e., numbers of N, O, and S heteroatoms), type (number of rings plus double bonds), and degree of alkylation (number of CH2 groups), to yield unique elemental assignments from ultrahigh-resolution mass measurements in the 200-900 Da range. Interpretation of such a vast compilation requires a simple (preferably graphical) means to differentiate between complex organic mixtures of different origin or processing. In an extension of the recently revived van Krevelen plot, each elemental composition is projected onto two or three axes according to its H/C, O/C, and/or N/C atomic ratios. The H/C ratio separates compounds according to degree of saturation, whereas O/C or N/C ratios separate according to O and N classes. We show that the threedimensional van Krevelen diagram can completely separate different classes in pyridine-extracted coal or petroleum samples and can also graphically distinguish fossil fuels according to their nature (coal vs petroleum), maturation (coals of different rank), and processing (the same coal at two stages of liquefaction). The van Krevelen diagram thus appears well suited to amplifying and exposing compositional differences within and between complex organic mixtures. Ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) makes it possible to resolve up to 10 000 different elemental compositions in a single complex * To whom correspondence should be addressed. E-mail: marshall@ magnet.fsu.edu.. 10.1021/ac0355449 CCC: $27.50 Published on Web 03/26/2004
© 2004 American Chemical Society
organic mixture, such as petroleum crude oil1 or coal.2 For each spectral peak, ICR frequency is converted to mass-to-charge ratio, m/z, by a simple two-parameter equation requiring at least two known m/z values in the spectrum,3,4 preferably as internal standards (see below). The internal standards may be known compounds added to the sample or may consist of an alkylation series of high-mass defect common to samples of a given origin. The charge state may be determined from the m/z spacing between ions differing in composition by 12Cn versus 12Cn-113C.5 At the present mass resolving power, m/∆m50% ≈ 300 000, in which ∆m50% is mass spectral peak full width at half-maximum peak height, it is possible to resolve close mass doublets (notably, elemental compositions differing by C3 vs SH4, 0.0034 Da) up to ∼900 Da. The corresponding mass accuracy (
