Although crude acids are minor constituents in petroleum, they have significant implications for crude oil geochemistry, corrosion, and commerce. We have previously demonstrated that a single positive-ion electrospray ionization (ESI) high-field Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) experiment can resolve and identify 3000 chemically different elemental compositions of bases (basic nitrogen compounds) in a crude oil. Here, we show that negative-ion ESI high-field FT-ICR MS can selectively ionize and identify naphthenic acids without interference from the hydrocarbon background. When combined with prechromatographic separation, ESI FT-ICR MS reveals an even more detailed acid composition. An average mass resolving power, m/∆m 50% g 80 000 (∆m 50% is mass spectral peak full width at half-maximum peak height) across a wide mass range (200 < m/z < 1000), distinguishes as many as 15 distinct chemical formulas within a 0.26 Da mass window. Collectively, more than 3000 chemically different elemental compositions containing O 2 , O 3 , O 4 , and O 2 S, O 3 S, and O 4 S were determined in a South American heavy crude. Our data indicates that the crude acids consist of a mixture of structures ranging from C 15 -C 55 with cyclic (1-6 rings) and aromatic (1-3 ring) structures. The acid composition appears to be simpler than that of the corresponding hydrocarbon analogues.

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The presence of naphthenic acids in crude oils is of concern in the petroleum industry due to their corrosivity to refinery units. It is desirable to determine the ring type and carbon number distributions because the corrosivity of naphthenic acids is dependent on the sizes and structures. The characterization of naphthenic acids is also of interest to geochemical studies, particularly migration and biodegradation, and to refinery wastewater treatment for environmental compliance. We have evaluated chemical ionization, liquid secondary ion mass spectrometry (fast ion bombardment), atmospheric pressure chemical ionization (APCI), and electrospray ionization in both positive and negative ion modes for the determination of molecular distribution of acids without derivatization. Negative-ion APCI using acetonitrile as a mobile phase yields the cleanest spectra with good sensitivity among the ionization techniques evaluated. The selectivity of negative-ion APCI for naphthenic acids has also been demonstrated by comparing results for a whole crude oil with those for the isolated acid fraction. APCI also holds a great potential for on-line liquid chromatography-mass spectrometric (LC/MS) to separate acids by high-performance liquid chromatography (HPLC) followed by mass spectrometric characterization of acids.

Twenty-five years ago, Boduszynski et al. conducted a comprehensive study of heavy oil composition and concluded that crude oil composition increases gradually and continuously with regard to aromaticity, molecular weight, and heteroatom content from the light distillates to non-distillables (the Boduszynski continuum model). Previous exhaustive characterization of heavy vacuum gas oil by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) provided compositional data that strongly supports the continuum model. However, when the molecular formulas obtained by FT-ICR MS for the distillates and asphaltenes from the same parent crude oil are plotted as double bond equivalents (DBE) versus carbon number, a gap appears between the compositional space of "asphaltenes" and "maltenes", in contradiction to the Boduszynski−Altgelt model. Here, a heavy distillate cut (atmospheric equivalent boiling point of 523−593 °C) is fractionated according to the number of aromatic rings by HPLC-2. The C7-deasphalted whole oil (C7-DAO), its pentane soluble/insoluble fractions, and each of their ring number fractions are comprehensively characterized by atmospheric pressure photoionization (APPI) FT-ICR MS and tandem mass spectrometry (MS/MS). The HPLC-2 fractions from both the C5-soluble and C5insoluble C7-DAO represent a gradual and continuous progression that fills the compositional "gap" in carbon number and aromaticity between asphaltenes and maltenes as a function of the increasing aromatic ring number, as predicted by Boduszynski. MS/MS results indicate that each ring number fraction comprises both island and archipelago structural motifs. FT-ICR MS reveals a continuum in carbon number and aromaticity. The C5-insoluble C7-DAO components have a similar structure but with higher-order fused ring core structures and are composed of a higher proportion of archipelago structures than the C5-soluble C7-DAO components. Thus, fractionation by the aromatic ring number of "maltenic" and "asphaltenic" species from the C7solubles from a high boiling distillate validates the compositional continuum of petroleum components, and MS/MS exposes the aromatic building blocks of "maltenic" and "asphaltenic" species (structural continuum) that comprise island and archipelago structural motifs.