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.

No abstract

Coal tar has been considered as a potential energy alternative because of dwindling supplies of petroleum. To determine if the coal tar could be refined and upgraded to produce clean transportation fuels, detailed investigation of its composition is necessary, particularly for identifying the acidic components that account for about one-quarter of the weight of the coal tar. A middle-temperature coal tar (MTCT) and its fractions were characterized by gas chromatography–mass spectrometry (GC–MS) and negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with different ion transmission modes for high- and low-mass ions. Analytical results of narrow distillation fractions from FT-ICR MS agreed reasonably well with those from GC–MS, although each technique has its own advantages and disadvantages. In this work, FT-ICR MS was demonstrated to be capable of characterizing small molecules of <100 Da using appropriate operation conditions, thus yielding mass distributions to compare to GC–MS results. A continuous distribution in double bond equivalent (DBE) and carbon number was observed with the distillates of increasing boiling point, while the composition of the distillation residue was much more complex than that of distillates. Acidic compounds containing 1–7 oxygen atoms were observed in the MTCT by FT-ICR MS, with O1 and O2 classes being dominant. Various phenolic compounds with 1–4 aromatic rings were identified on the basis of literature references, including some molecules having structures resembling known biomarkers in petroleum and coal.