Time of flight-secondary ion mass spectrometry (TOF-SIMS) study of diverse asphaltenes

Mass spectrometry has been used to elucidate a large variety of properties of asphaltenes. Here, time-of-flight secondary ion mass spectrometry (TOF-SIMS) is used to probe three diverse asphaltene types with wide ranging fractions of alkyl carbon to the sum of alkyl plus aromatic carbon (Rc), immature source rock asphaltenes (ISA with Rc ∼ 0.75); petroleum asphaltenes (PA with Rc ∼ 0.5); and coal-derived asphaltenes (CDA with Rc ∼ 0.25). In addition, the asphaltenes from a commercial bitumen are examined. Primary ion surface bombardment using the Bi3+ ion yields high energy density of deposition and significant molecular fragmentation with secondary ion formation. Formation of free radical cation fragments is generally suppressed especially for smaller fragments except for specific cases. Possible structures, especially cations of common aromatic compounds, are suggested for fragments with relatively large cross section of formation. Principal component analysis of the fragmentograms allows identification of key properties of the complex fragmentation patterns for the different samples. Comparisons of TOF-SIMS fragmentograms show a fundamental difference for small fragments between CDAs and all petroleum derived asphaltenes with CDAs being dominated by aromatic carbon fragments whereas all petroleum derived asphaltenes show a large fraction of fragments from alkyl carbon. However, the type of alkyl carbon fragments did not exhibit systematic trends with Rc, nor with the extent of chemical reaction or chemical processing of the materials. Consequently, it appears that having an appreciable alkane fraction is a basic property of petroleum derived asphaltenes, but the exact type of alkane carbon for differing samples of these asphaltenes is more idiosyncratic than systematic.