Interpreting Chemical Structures of Compounds in Crude Oil Based on the Tandem Mass Spectra of Standard Compounds Obtained at the Same Normalized Collision Energy

In this study, the tandem mass spectra of 25 standard compounds, obtained at a normalized collision energy of 50, were used to investigate the structures of crude oil. The product ion distributions of these standard compounds were dependent on the number and length of their alkyl side chains. Long alkyl side chains (C_nH_2n, n ≥ 5) were easily lost during molecular collision, leaving behind methyl groups on the aromatic cores. In contrast, compounds with short interlinking alkyl chains (C_nH_2n, n ≤ 1) were not significantly fragmented, and a decrease in the double bond equivalent (DBE) value was observed when compounds had saturated rings. Based on the derived observations, the possible structures of S₁-class compounds with DBE values of 6, 7, 8, and 9 were suggested. It was also determined that, in the tandem mass spectra of crude oil fractions, (1) a series of peaks separated by CH₂ groups were generated from isomers with different numbers of alky side chains, (2) the number of methyl groups left after molecular collision was positively correlated to the quantity of branched molecules, and (3) significant fragmentation did not occur for aromatic cores linked by short alkyl chains (C_nH_2n, n ≥ 2), and hence, archipelago structures with short alkyl linkage could not be excluded based solely on tandem mass spectra. This study clearly shows that a systematic analytical approach using a well-defined set of standard compounds combined with tandem mass spectrometry can significantly improve our understanding on the chemical structures of crude oil compounds.