Evaluation of Laser Desorption Ionization Coupled to Fourier Transform Ion Cyclotron Resonance Mass Spectrometry To Study Metalloporphyrin Complexes

In this study, the applicability of positive-ion (+) laser desorption ionization (LDI) as an ionization method for metalloporphyrin complexes was evaluated. The evaluation was performed by analyzing standard compounds and a series of crude oils with various V⁴⁺ and Ni²⁺ contents by (+) LDI and (+) atmospheric pressure photoionization (APPI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The analysis of standard compounds showed that molecular ions were main ionic species in (+) LDI. Fragmented ions because of the loss of CH₃ were also observed. The analysis of crude oils showed that the sensitivity of (+) LDI toward metalloporphyrin complexes is greater than that of (+) APPI. Furthermore, five types of ion VO²⁺ and Ni²⁺ porphyrin complexes (etio, DPEP, rhodo-etio, rhodo-DPEP, and di-DPEP) were observed with (+) LDI, but only three types were observed with (+) APPI. Nickel porphyrins were observed in unfractionated oils by (+) LDI but not by (+) APPI. The summed relative abundance of peaks corresponding to VO²⁺ and Ni²⁺ porphyrins observed by (+) LDI was shown to be correlated with the V⁴⁺ and Ni²⁺ metal contents of the oils in general. However, the abundance of DPEP porphyrins did not correlate well with the metal content because it depends upon the maturity or state of biodegradation of oils. Also, 1 μL of oil sample was sufficient to perform (+) LDI FT-ICR MS analysis. Therefore, (+) LDI FT-ICR MS is a sensitive method to detect metalloporphyrins in petroleum using caution with respect to fragmented ions because of the loss of CH<sub>3