Effect of the Ionization Source on the Targeted Analysis of Nickel and Vanadyl Porphyrins in Crude Oil

We compare the performance of atmospheric pressure photoionization (APPI), laser desorption ionization (LDI), and electron-transfer matrix-assisted laser desorption ionization (ET-MALDI) for the analysis of petroporphyrin (PP)-enriched extracts. APPI, one of the most used ionization sources for crude oil analysis because of its low matrix and ion suppression effects, provides a broad picture of the crude oil extract, including PPs. APPI analysis resulted in a complex spectrum with more than 12000 radical cations where signals from high ionization energy (IE) species with abundant heteroatoms (NxOySz) predominate, masking the PP target group. LDI shows species with aromatic cores or conjugated functionalities particularly susceptible to UV laser excitation and ionization. A reduction in N-containing compounds (NxOy, NxSz) and an increase in PPs signals indicate some selectivity in LDI. ET-MALDI resulted in a less complex spectrum with 3500 radical cations mainly from aromatic species, including NiPP and VOPPs. PPs' selective ionization in ET-MALDI occurs via thermodynamically favored charge exchange reactions between the matrix radical cations and the analytes. ET-MALDI results in fewer ions per nominal mass than APPI and LDI, a situation benefiting signal resolution and mass accuracy in FT-ICR-MS. Identifying more than 350 PPs in crude oils (N4VO, N4VO2, N4VO3, N4VOS, and N4Ni) was possible by combining isotopic structure analysis and data refinement using the Kendrick mass defect (KMD) plots. The PP compositional space in ET-MALDI includes 269 species corresponding to N4VO and N4Ni classes, in contrast with 65 in APPI and 53 LDI. The compound classes N4VOS, N4VO2, and N4VO3 were not observed in APPI or LDI.

Automated summary

In this study, the performance of APPI, LDI, and ET-MALDI for analyzing PP-enriched extracts was compared. ET-MALDI produced a less complex spectrum with fewer ions, leading to improved signal resolution and mass accuracy in FT-ICR-MS.