Fractionation of polar compounds from crude oils by hetero-medium pressure liquid chromatography (H-MPLC) and molecular characterization by ultrahigh-resolution mass spectrometry

Polar compounds from three Brazilian crude oils were isolated and separated into seven fractions according to the polarity (low polarity-LP, low medium polarity-LMP, high medium polarity-HMP, high polarity-HP, basic-BAS, acidic-AC) by using hetero-medium pressure liquid chromatography (H-MPLC). Chemical characterization of all fractions was performed by mass spectrometry using an electrospray ionization source. In positive ion mode, the N-1 compound class was detected with high relative abundance in all samples. Interestingly, the unsaturation degree of this heteroatom class decreased from fractions LP, LMP, HMP to the HP, BAS and AC fractions. On the other hand, the type of compound classes found in negative ion mode was more diverse, revealing that nitrogen species (N-1) corresponding to carbazoles, were eluted in the less polar fraction (LP). Conversely, NxOy (x = 1, y = 1-4) heteroatom classes were found with a higher relative abundance in LMP, HMP, HP and BAS fractions. Overall, the H-MPLC reduced the complexity of the polar fraction contained in whole crude oils and allowed to extend the chemical characterization since the number of molecular formulas assigned in all fractions exceeds significantly to those detected in the whole crude oils. This separation methodology is an interesting alternative to other techniques such as gas chromatography (GC, GCxGC), liquid/liquid extraction, extrography, solid phase extraction and SARA (saturates, aromatics and resins) separation. MPLC not only isolate polar compounds but also fractionate them by the degree of polarity without previous de-asphalting steps. This separation approach, along with the molecular characterization of polar compounds by ESI-Orbitrap MS is also a tempting option to perform geochemical assessment of crude oils and source rocks.