Fundamentals and Applications of Electrospray Ionization Mass Spectrometry for Petroleum Characterization Kuangnan Qian,* Kathleen E. Edwards, John H. Diehl, and Larry A. Green ExxonMobil Research and Engineering Company, 1545 Route 22 East, Annandale, New Jersey 08801 Received May 9, 2004. Revised Manuscript Received July 28, 2004 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). Introduction Since its reintroduction in the mid 1980s by Fenn et al., 1 electrospray ionization mass spectrometry (ESI- MS) has rapidly found a wide range of applications in the analysis of both large- and small-molecule systems. 2-6 Currently, ESI-MS has become an indispensable ana- lytical tool in proteomic and pharmaceutical research. Its applications for petroleum characterization came more than a decade late, partially because ESI is traditionally considered a technique that is most suit- able for molecules that have polar functionalities and petroleum is known to be largely composed of hydro- carbon molecules. Zhan and Fenn first attempted to apply ESI to a range of petroleum products, including gasolines, jet fuels, diesels, and crude oils. 7 Although their quadrupole mass spectrometer applied in the study did not have sufficient mass resolution to separate isobaric molecules and determine their composition, it clearly demonstrated that petroleum products contain many polar molecules that can be ionized by ESI. More in-depth studies came from collaborative research be- tween the National High Magnetic Field Lab at Florida State University and ExxonMobil Research and Engi- neering Company (Annandale, NJ) on high-field Fourier transform ion cyclotron resonance (FTICR) applications to petroleum characterization. A large number of polar compounds in a crude oil were detected, yielding the most-complex mass spectrum ever recorded at the time (∼7 components per nominal mass). 8 High-field FTICR showed its unmatched power in determining elemental compositions of >3000 mass peaks in a single analysis and was applied to characterize metalloporphyrins and basic and acidic species in heavy crude oils under positive- and negative-ion conditions. 9-12 On-line liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS) was recently used to de- termine polycyclic aromatic hydrocarbons (PAHs), mostly 3- to 5-ring condensed aromatics in water. 13,14 Nonpolar petroleum molecules were shown to be ionizable when co-sprayed with Pd and Ag salts. 13-16 The high selectivity of ESI added a significant ad- vantage to the analysis of trace polar compounds in * Author to whom correspondence should be addressed. Telephone: 908-730-2013. Fax: 908-431-1422. 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