Alkyl carbon number limits the ring-number separation of polycyclic aromatic compounds in petroleum analysis

Applications of High Performance Liquid Chromatography in the Petroleomic Analysis of Crude Oil: A Mini-Review

“…Despite the different stationary phases used in these two works, both found that N1 compounds are found in all the fractions, with the double bond equivalent (DBE) increasing with retention time. One possible explanation for these observations is the amount of alkylation on the N1 species in these gas oil samples. ,, HPLC was also used to separate heavy crude oil by ring type, and fractions were analyzed by both FT-ICR MS and NMR spectroscopy . Data were used to confirm the expected ring-type separation.…”

“…A summary of stationary phases applied to different sample types for group-type separations is provided in Table . Using normal phase HPLC, PAHs are retained by the stationary phase often via a surface adsorption mechanism, which can be influenced not only by a molecule’s ring size, but also by its shape, planarity, and alkyl substituents. , Fundamentally, the molecular characteristics of PAHs control retention, and the electron affinity, ionization potential, and electrostatic interactions of molecules with the stationary phase dominate retention. − Larger ring structures increase retention, but as alkyl chains are added to PAH structures, retention will decrease after a certain number are added, and longer chains decrease retention, as their electron-donating ability decreases with length . In this way, the presence of multiple alkyl chains can complicate elution order, and push molecules into different “ring type” regions.…”

“…In this way, the presence of multiple alkyl chains can complicate elution order, and push molecules into different “ring type” regions. This effect has been observed on both charge-transfer phases, as well as on amino-silica . Studies performed with model compounds lend information about retention mechanisms, and they allow for a better understanding of the separation of oil samples.…”

“…A silver-ion phase was also used as an intermediate separation step between solid phase extraction separation of samples, and GC analysis . Narrow distillation cuts of crude oils were used in an excellent fundamental study of retention mechanisms on amino and cyano stationary phases (using both UV and RI detection), with a focus on the alkyl carbon substituents on the PAHs, and their impact on separation . FT-ICR MS was used for additional characterization of collected fractions, demonstrating that alkyl carbons begin to introduce size exclusion effects into the normal phase retention on these phases.…”

“…Offline fraction collection was used to assess separations on amino and cyano stationary phases, with atmospheric pressure photoionization (APPI) . The use of APPI as an ionization source would enable this separation to be performed online with FT-ICR if so desired.…”