Laser Desorption Single-Photon Ionization of Asphaltenes: Mass Range, Compound Sensitivity, and Matrix Effects

Molecular-level characterization of petroleum asphaltenes is important for addressing reservoir concerns such as connectivity and flow assurance. Laser desorption single-photon ionization mass spectrometry (LDSPI-MS) has emerged as a favored technique for asphaltene analysis, because of its ability to detect these samples with minimal artifacts from fragmentation, aggregation, and multiple charging. However, questions persist regarding the sensitivity of LDSPI-MS to different components of asphaltenes, the molecular weight range detectable with the technique, and the importance of matrix effects. We present LDSPIMS mass spectra of mixtures of model compounds and asphaltenes in various matrices to assess the significance of these effects. We observe that LDSPI-MS has comparable sensitivity for all studied model compounds and for asphaltenes, including compounds with molecular weight exceeding 1500 Da. In addition, only a minimal matrix effect is observed, as expected from considerations of the desorption and ionization mechanisms. The results add confidence to the previous conclusion of an LDSPIMS study, in particular that (i) 1500 Da represents a maximum molecular weight for asphaltenes and (ii) the lack of fragmentation implies the dominance of island geometries in asphaltenes. ■ INTRODUCTION Defined by their solubility in toluene and insolubility in heptane, asphaltenes are the most enigmatic component of crude oil. This fraction of petroleum is relevant to several industrial concerns, such as flow assurance and reservoir characterization. However, a detailed understanding of these processes has historically been impaired because knowledge about the molecular structure and weight distribution of asphaltenes has been lacking and a topic of some controversy. To address this deficiency, several recent experiments have aimed to measure fundamental molecular properties of asphaltenes, such as their molecular mass distribution and dominant molecular architecture. Laser mass spectrometry, including laser desorption/ ionization (LDI) and two-step laser desorption laser ionization (LMS), has emerged as a powerful means of assessing both molecular weight and molecular architecture in nonvolatile molecules and mixtures such as asphaltenes. LDI experiments employ one laser pulse, typically in the UV or IR wavelength region, to desorb and ionize molecules contained in the solid sample. However, obtaining an artifact-free signal from any sample prepared in this manner requires a careful combination of optimized instrumental parameters. Without such considerations, the resulting spectra can be obscured by excessive fragmentation, plasma-phase reactions resulting in detectable ion clusters, or, worse, the absence of any signal at all. Two-step laser desorption laser ionization mass spectrometry (L2MS) addresses these issues by separating both processes temporally and spatially, allowing individual optimization of each. Coupling UV resonance-enhanced multiphoton ionization (REMPI) to laser desorption mass spectrometry has led to successful organic analyses, because it combines selectivity, sensitivity, and rapidity of measurement. More recently, vacuum ultraviolet single-photon ionization (SPI) has been proposed as a universal soft ionization method for organic compounds and has been applied to study asphaltenes as well as surfaces, materials, aerosols, drugs, and peptides. LMS with REMPI and especially SPI ionization has been found to detect asphaltenes with minimal fragmentation and without the plasma-phase aggregation often found in LDI, suggesting that LMS is particularly well-suited for analyzing the molecular composition of asphaltenes. Previously, we measured the laser desorption single-photon ionization mass spectrometry (LDSPI-MS) mass spectra of model compounds and asphaltene samples. The results suggested that asphaltenes have a wide range of molecular masses, peaking at ∼700 Da and extending to a maximum of ∼1500 Da. In addition, model compounds with island geometries displayed fragmentation patterns similar to asphaltenes, whereas model compounds with archipelago geometries displayed fragmentation patterns distinct from asphaltenes, suggesting that asphaltenes are dominated by island geometries. To gain further confidence in these results, it must be demonstrated that LDSPI-MS does not suffer from some potential artifacts common in mass spectrometric Received: February 9, 2012 Revised: April 17, 2012 Published: April 30, 2012 Article