Vanadyl (VO) porphyrins and sulfur-containing vanadyl (VOS) porphyrins of a wide carbon number range (C(26) to C(52)) and Z-number range (-28 to -54) were detected and identified in a petroleum asphaltene by atmospheric pressure photonionization (APPI) and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). APPI provides soft ionization of asphaltene molecules (including VO and VOS porphyrins), generating primarily molecular ions (M(+.)). The ultra-high mass resolving power (m/Delta m(FWHM) approximately 500 K) of FTICR-MS enabled resolution and positive identification of elemental formulae for the entire family of VO and VOS porphyrins in a complicated asphaltene matrix. Deocophylerythro-etioporphyrin (DPEP) is found to be the most prevalent structure, followed by etioporphyrins (etio)- and rhodo (benzo)-DPEP. The characteristic Z-distribution of VO porphyrins suggests benzene and naphthene increment in the growth of porphyrin ring structures. Bimodal carbon number distributions of VO porphyrins suggest possible different origins of low and high molecular weight species. To our knowledge, the observation of VOS porphyrins in a petroleum product has not previously been reported. The work is also the first direct identification of the entire vanadyl porphyrin family by ultra-high resolution mass spectrometry without chromatographic separation or demetallation.

Field desorption (FD) ionization is studied for the measurement of the molecular weight distribution of heavy petroleum, vacuum residua, and asphaltene derived from them. Hydrocarbon polymers, heavy petroleum distillation cuts, and chemical and solubility fractions of heavy petroleums were examined to determine the ionization efficiency. FD showed effective ionization of molecules of a wide molecular weight range (300−5000 Da) with different chemical compositions (saturates versus aromatics) and solubility (maltene versus asphaltene) properties. Electrospray ionization (ESI) was explored and compared with FD. Previous studies had shown that ESI is selective toward the ionization of polar molecules (mostly basic and acidic molecules) while FD is a more universal ionization method. The comparison revealed general agreement between the two techniques. Small variation in the molecular weight distribution of polar and nonpolar molecules was observed. ESI yields lower average molecular weights. The difference increases with the boiling point.

We report here the first high resolution mass spectrometric evidence of nickel porphyrins in petroleum. A petroleum asphaltene sample is fractionated by a silica-gel cyclograph. Nickel content is enriched by approximately 3 fold in one of the cyclograph fractions. The fraction is subsequently analyzed by atmospheric pressure photoionization (APPI) Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) with an average mass resolving power of over 500 K (M/DeltaM(fwhm)). Similar to vanadyl porphyrins, monocylcoalkano-type (presumed to be deocophylerythro-etioporphyrin DPEP) Ni porphyrins are found to be the most abundant family followed by etio, bicycloalkano-type, and rhodo-monocylcoalkano-type Ni porphyrins. A Z number ranging from -28 to -44 and a carbon number ranging from 26 to 41 were observed. A significant amount of nickel and vanadyl geoporphyrins are in more condensed tetrapyrrolic cores than just chlorophyll-derived DPEP- and etioporphyrins. Ni has a higher etio/DPEP ratio and rhodo-etio/rhodo-DPEP ratio than does VO.

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We are reporting a study of the fundamental and application aspects of positive-ion electrospray ionization (ESI+) for petroleum characterization. In the positive-ion mode, basic nitrogen molecules (typically pyridinic molecules) form predominantly protonated molecule ions. Most other molecular types, such as paraffins, naphthenes, aromatics, and thiophenoaromatics, are largely not ionized by ESI. Metalloporphyrins and neutral nitrogens show little response. The effects of acid and analyte concentrations, and instrumental conditions, were evaluated. Increasing the acid concentration promotes the ionization of the low-molecular-weight bases but has minimal impact on high-molecular-weight species. Analyses of low-and high-molecular-weight petroleum fractions and their mixtures showed minimal mass discrimination. Cone and extraction voltages have a significant impact on ESI responses, particularly on aliphatic amines. Linear dynamic range exists at lower analyte concentrations (<10 -5 M).