Selective ionization by electron-transfer MALDI-MS of vanadyl porphyrins from crude oils

Abstract

Petroporphyrins are biomarkers derived from chlorophyll, bacteriochlorophyll and the heme group, commonly present in crude oils as vanadyl or nickel complexes. These compounds, which despite of the organic matter maturation process preserve the tetrapyrrolic core of their precursors, show low polydispersity, concentration and are commonly used in geochemistry to assess oil lithology, oxidation-reduction conditions, geothermal maturation, conversion of organic matter, and burial depth. The physicochemical characteristics of petroporphyrins make them excellent analytes for MALDI-MS. Electron transfer (ET) ionization in MALDI, using novel electron transfer matrices based on cyano-phenylenevinylenes (e.g. CNPV-CH₃), allows direct observation of labile compounds such as polyaromatics and porphyrin-like compounds, chlorophylls and pigments, as has been previously demonstrated by our group. In this work, we analyze petroporphyrins from two South American heavy oils using CNPV-CH₃ as ET MALDI matrix. Petroporphyrins were isolated by Soxhlet extraction, with acetonitrile, followed by fractionation using normal-phase column chromatography. UV–Vis measurements showed a strong absorption band at 408 nm assigned to porphyrin Soret band for some chromatographic fractions. The fractions enriched with petroporphyrins were subjected to ET MALDI analysis. We demonstrate the suitability of the CNPV-CH₃ MALDI matrix for the efficient and selective desorption and ionization of vanadyl porphyrins, when compared with a commercial MALDI matrix (DCTB) and LDI conditions.

Introduction

Petroporphyrins in crude-oils are widely distributed in homologous series, with ethioporphyrins (Ethio) and deoxophylloerythro ethioporphyrins (DPEP) as the most commonly reported [1], [2], [3]. These compounds consist of a tetrapyrrole core coordinated with vanadium or nickel that may also exhibit side groups such as alkyl chains and aromatic rings with heteroatoms like oxygen, sulfur or nitrogen [4]. These metalloporphyrins derive from biologically functional metal complexes found in plants, bacteria, and algae such as chlorophyll, bacteriochlorophyll, heme groups and related pigments originally present in the sedimentary organic matter from which the fossil fuels were generated [5], [6], [7], [8], [9]. From a geochemical perspective, petroporphyrins are important biomarkers since the thermal maturity of heavy crude oils can be calculated using parameters such as the ratios ΣDPEP/ΣETIO and DPEP/(ETIO + DPEP) [10], [11], [12], [13]. Also, in downstream processes, nickel and vanadium petrophorphyrins are well known as catalysts deactivators, particularly in upgrading processes of heavy and non-conventional oils [9], [14], [15].

Extrography coupled to sequential Soxhlet extraction, is typically used to isolate petroporphyns from crude oils [2], [16], [17]. The petroleum samples, initially adsorbed on a stationary phase such as neutral alumina, clay, or silica gel, are subjected to Soxhlet extraction using a solvent such as acetonitrile to disrupt intermolecular interactions between the crude oil, the stationary phase and the porphyrins. After an exhaustive extraction process, it is possible to release fractions enriched with metal complexes [2]. Recently, Combariza et al. [3] reported the selective extraction (using extrography and Soxhlet isolation with acetonitrile) and analysis (using ultra-high resolution mass spectrometry) of vanadyl petrophorphyrins. Employing this approach, more than 130 molecular formulas corresponding to vanadyl petroporphyrins, with a mass distribution between m/z 402 and 750, were identified.

Several techniques can be used for petroporphyrin characterization after isolation from crude oils. The typical workout starts with detection of a strong UV absorption Soret band, commonly observed at 400–420 nm, which unequivocally indicates the presence of porphyrinic compounds [3], [18]. However, besides the qualitative characterization of porphyrins in heavy crude oils by spectroscopic techniques, nowadays there is significant interest in understanding the structure and function of these species. Mass spectrometry is suitable for the comprehensive characterization of porphyrinic compounds; however due to the high complexity of these fractions, a high mass resolution approach is typically required for analysis [19], [20], [21]. For instance, reports on positive atmospheric pressure photoionization (APPI) coupled to ultra-high resolution mass spectrometry (FT-ICR MS) by Rodgers and co-workers illustrate the possibility of the direct identification of vanadyl porphyrins in raw asphaltenes and South American crude oils [4]. A resolving power higher than 6 × 10⁵, at 500 m/z, and a mass accuracy of <300 ppb, allowed unequivocal molecular assignments for ∼33 porphyrin peaks in each sample. In another report, Zhao and coworkers performed analysis of petroporphyrins in a Venezuelan heavy crude oil, using positive electrospray ionization (ESI) coupled to FT-ICR MS. In this work, three novel families of oxygen-containing vanadyl porphyrins were identified: C_cH_hN₄VO₂, C_cH_hN₄VO₃, and C_cH_hN₄VO₄, which suggest the existence of porphyrinic species with oxidized side groups [13], [22]. Recently, McKenna and coworkers reported the identification of nickel and vanadium porphyrins in samples from volcano-derived asphalt; the authors used solid phase extraction to obtain porphyrin-enriched fractions, which were analyzed by positive ESI FT-ICR MS. Using this approach, it was possible to identify several porphyrin families containing oxygen and sulfur, such as N₄⁵⁸Ni, N₄O₁⁵¹V, N₄O₁S₁⁵¹V, N₄O₂⁵¹V, and N₄O₃⁵¹V [4].

Still, with high resolution MS instruments not widely available, alternative methods for porphyrin molecular analysis become attractive. For example, UV-MALDI is well known as a suitable, low-cost, robust and rapid technique for the analysis of biomolecules, macromolecules, polymers, pigments and organo-metallic compounds [23]. In MALDI gas-phase protonation, deprotonation and adduct formation reactions are common pathways to yield charged biomolecules (e.g. peptides, proteins, carbohydrates). However, not all compounds are suitable to acquire charge through these processes. An alternative ionization byway, the electron transfer (ET) process, allows ionization of organometallic and coordination complexes as well [24], [25], [26]. Vanadium and nickel porphyrins exhibit low ionization potentials (6.5–7.0 eV) [27], [28], [29], [30] resulting in high ionization efficiencies when subjected, for instance, to laser desorption ionization (LDI) [18], [31]. As an example, Xu and co-workers used UV–Vis and LDI TOF MS for the characterization of vanadyl porphyrins in two Chinese crude oils [18]. The authors detected the Soret band around 406 nm in the UV–Vis spectra of the porphyrin extracts and reported a MW distribution centered around m/z 430 and 580 corresponding to petroporphyrins homologous series. The authors suggested that the Tahe crude oil sample exhibits a higher thermal maturity than the Du84 crude oil, due to ∑DPEP/∑ETIO ratios of 0.18 and ∼1.1.

However, the major drawback in LDI MS analysis of porphyrins is the lack of selective ionization [3], [9], [18]; in some cases, LDI may cause fragmentation of labile analytes such as organometallic complexes [19], [32], [33], [34]. Examples of these drawbacks were reported by Wyatt et al., who used LDI-TOF MS for the characterization of Zn and Fe porphyrins and Pd organo-complexes [35]. Matrix-assisted ET processes can circumvent these disadvantages, increasing the survival yields for the molecular ion and avoiding undesirable gas-phase reactions [19], [32], [33], [34]. Commercially available ET matrices, 9-nitroanthracene (9-NA), trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene] malononitrile (DCTB), 1,9-diphenylanthracene (1,9-DPA), terthiophene (TER), and 1,5-diaminonaphtalene (1,5-DAN), can be used for the ionization of fullerenes derivatives, polymers, and porphyrins, among others [36], [37], [38], [39], [40]. These ET matrices have advantages such as good solubility, high UV molar absorptivity and low vapor pressure; however, their application can be restricted due to abundant cluster formation in the low-mass region, specifically between m/z 100–1000, which interferes with the accurate detection of the porphyrinic compounds found in crude oils. Additionally, commercial matrices also exhibit reactivity with analytes, low ionization potentials (for some of them) and vacuum instability, among others.

We have recently reported the use of cyano-phenylenevinylene (CN-PV) derivatives as high-performance ET MALDI matrices for the analysis of porphyrins and phthalocyanines [41], among other analytes. When compared to a reference ET matrix (DCTB), CN-PV efficiently produce abundant molecular ions of the sample at lower laser outputs, and does not react with the analytes or form clusters in the low mass region. In addition, analytical figures of merit such as mass resolution and detection limits are improved as well as molecular ion survival yields. In this context, we report the selective ionization of vanadyl porphyrins, extracted from two heavy South American crude oils, using CNPV-CH₃ ((2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(p-tolyl)acrylonitrile)) as ET MALDI matrix. MALDI has advantages such as high-throughput, exceptional tolerance to impurities, and small amounts of sample, when compared with ionization strategies such as APPI and ESI. Also, operational costs of MALDI-TOF instruments are considerably lower than high-resolution systems such as FT-ICR MS. Our results demonstrate that the novel CNPV-CH₃ matrix enables the selective ionization of vanadyl porphyrins, when compared with DCTB matrix and LDI analysis. At low laser energies, CNPV-CH₃ produce background-clean mass spectra, free of matrix clusters or fragments. We were able to identify ∼100 spectral peaks that were in turn assigned as vanadyl porphyrins or class N₄O₁V₁; these assignments are comparable with results from high-resolution mass spectrometry of similar samples.