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Negative-ion mode electrospray ionization, ESI(-), with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was coupled to a Partial Least Squares (PLS) regression and variable selection methods to estimate the total acid number (TAN) of Brazilian crude oil samples. Generally, ESI(-)-FT-ICR mass spectra present a power of resolution of ca. 500,000 and a mass accuracy less than 1 ppm, producing a data matrix containing over 5700 variables per sample. These variables correspond to heteroatom-containing species detected as deprotonated molecules, [M - H](-) ions, which are identified primarily as naphthenic acids, phenols and carbazole analog species. The TAN values for all samples ranged from 0.06 to 3.61 mg of KOH g(-1). To facilitate the spectral interpretation, three methods of variable selection were studied: variable importance in the projection (VIP), interval partial least squares (iPLS) and elimination of uninformative variables (UVE). The UVE method seems to be more appropriate for selecting important variables, reducing the dimension of the variables to 183 and producing a root mean square error of prediction of 0.32 mg of KOH g(-1). By reducing the size of the data, it was possible to relate the selected variables with their corresponding molecular formulas, thus identifying the main chemical species responsible for the TAN values.

Asphaltenes are defined as the petroleum fraction insoluble in n-alkanes and soluble in aromatic solvents, such as toluene. Such definition implies that asphaltenes are not a homogeneous material but a mixture of fractions. Asphaltenes represent one of the major contributors to several problematic issues for the petroleum industry. Destabilized asphaltenes can cause arterial clogging within pipelines and wellbores, corrosion and fouling of production equipment, reduction of catalyst activity in refining processes, and other problems. This work describes an investigation of the separation of asphaltenes into three different fractions by adsorption onto silica particles. These fractions (two adsorbed and one non-adsorbed onto silica) were characterized by elemental analysis (C, H, and N), Fourier transform infrared spectroscopy coupled to attenuated total reflectance (ATR−FTIR), proton nuclear magnetic resonance ( 1 H NMR) spectroscopy, and atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry (APPI−FT-ICR MS). APPI−FT-ICR MS and ATR−FTIR accessed chemical information on a molecular level [molecular formula, carbon number (CN) and double bond equivalent (DBE) distributions, and organic groups], whereas 1 H NMR and elemental analysis provided the aromaticity degree and C/H atomic ratio of the samples, respectively. The C/H atomic ratio decreases in the following the order: non-adsorbed > whole asphaltene > adsorbed > irreversibly adsorbed. The irreversible fraction adsorbed had the lowest percentage of aromatic hydrogen compared to other fractions by 1 H NMR. There was a good correlation between the results of NMR and elemental analysis. The efficiency of fractionation on silica particles was proven to be successful by the low concentration of polyaromatic hydrocabons observed for two samples adsorbed onto silica and the increasing of the aromaticity degree and C/H ratio for the non-adsorbed fraction. N 2 , N 2 O, and NO compound classes were selectively separated from whole asphaltene and concentrated in polar fractions (adsorbed fractions onto silica), with their CN and DBE distributions reported. Therefore, this work demonstrated the selectivity of the fractionation method onto silica to retain highly polar compounds and, moreover, extends to the study of the adsorbent surface and how the molecules of the asphaltenes will behave against this change.