Investigation of Deepwater Horizon Heavy End Environmental Transformation by High Resolution Detection and Isolation Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Because of petroleum's expansive boiling point and carbon number range, two analytical methods are primarily used to unlock composition and structure: 2-dimensional gas chromatography mass spectrometry (GCxGC/MS) and liquid infused Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS). The strength of one method is literally the weakness of the other: GCxGC/MS, despite being able to resolve isomers, cannot analyze polars and molecules larger than ~C35. Alternatively FT-ICR MS can analyze polars and molecules up to C100 but cannot resolve isomers. Fortunately the continuous boiling point and carbon number nature of petroleum allows the techniques to compliment one other and resolve potential ambiguities. Environmental oxidative transformation after an oil spill tends to increase polarity, decreases separation efficiency of GCxGC and increases instrumental resolving power demands. This shifts the onus onto FT-ICR MS for analysis and also demands advances in high resolution isolation (excitation) for structural determination and dynamic range enhancement. Here, we present a detailed compositional analysis of the Deepwater Horizon oil spill contamination transformation products by FT-ICR including the main mechanism of heavy end transport through permeable beach sand. We also show advances in stored waveform inverse Fourier transform (SWIFT) isolation FT-ICR dynamic range enhancement, IRMPD dissociation and the isolation resolving power fundamental limit (coulombic shielding). Finally we show an enhancement in positive ion electrospray sensitivity of weathered crude oil by optimization of the spray, a development that allowed us to observe one of the more atypical transformative signatures of the Deepwater Horizon spill. High resolution isolation (excitation) by stored waveform inverse Fourier transform (SWIFT) is fundamentally limited by long period of excitation time at low voltage (similar to detection being long data acquisition period at low ion number). Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry in general relies upon linearity between the ion cyclotron excitation and the observed response. However, nonlinearities result from non-ideal applied electric and magnetic fields and Coulombic interactions. Here, we report nonlinear response at low excitation electric field magnitude due to Coulombic shielding. The measured ICR signal magnitude exhibits an excitation voltage threshhold that increases monotonically with the number of shielding ions (i.e., nonresonant ions). If shielding ions are not present, ICR signal magnitude versus excitation voltage is linear (e.g., for quadrupole-isolated ions of nearly a single m/z). Finally, we show that shielding results in a reduced cyclotron radius at low excitation voltage, resulting in an increased rate of transient decay; thereby exacerbating response nonlinearity and excitation threshold for long data acquisition period. Of the estimated 4.4 million barrels of crude oil released into the Gulf of Mexico from the Deepwater Horizon (DWH) oil spill, much washed ashore onto sandy beaches from Louisiana to the Florida panhandle. Here comprehensive two dimensional gas chromatography (GCxGC) confirms the source (Deepwater Horizon (DWH), Macondo, Mississippi Canyon Block 252 crude oil), and determines the extent of weathering and detailed molecular level composition of oil spill contamination that reached Pensacola Beach, Florida. Samples collected from an intertidal zone of Pensacola Beach at various sediment depths were analyzed by positive and negative ion electrospray (ESI) ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results display a greater than two-fold increase in number of elemental compositions, particularly oxygen compounds (CcHhN1Ox and CcHhOx classes that serve as molecular tracers of oxidation) relative to the original Macondo wellhead oil. The increase in number of oxygen-containing species is consistent with the five-fold increase in O:C ratio by bulk elemental analysis, and reveals extensive oxidative weathering from biotic and abiotic modification in the marine environment. Time-of-flight (TOF) mass analysis of model compounds strongly suggests that oxidative incorporation of ketones in the parent crude oil is responsible for the increased diversity of oxygen-containing species detected by (+) ESI (from C20 to C80). Carboxylic acids are the dominant oxygen functionality detected by (-) ESI FT-ICR MS analysis (C10 - C50). Tandem mass spectrometry by infrared multiphoton dissociation (IRMPD) pinpoints the location of oxidative modifications to alkyl side chains, confirmed by their labile nature (oxygen loss) in tandem MS fragmentation to generate purely hydrocarbon fragments. Ion exchange separates ketones from carboxylic acids. Subsequent GCxGC mass spectrometry analysis unequivocally verifies the presence of ketones (specifically 2-ones, C6 - C26) in the sediment extracts. Finally, a decrease in carbon number and double bond equivalent (DBE) of petrogenic species with increased depth in the sediment extracts is consistent with the preferential downward migration of compact, aromatic species of higher water solubility. More than 1000 km of Gulf of Mexico (GOM) beaches were oiled after the Deepwater Horizon (DWH) explosion of April 2010. Little is known about transport of the petroleum in the permeable beach sand, particularly the high boiling point, petroleum "heavy ends" (polar compounds and/or those above C35) inaccessible by gas chromatography. Here, we present electrospray (ESI) and atmospheric pressure photoionization (APPI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) of oil extracted from binned sand samples. The sand samples were obtained from vertical sections of sand in the intertidal zone of Pensacola Beach (PB), Florida in the summer and fall of 2010. The native petroleum constituents (petrogenic) show a shift to lower carbon number and double bond equivalent (DBE) at depths above and below the oil "stripe" (the most heavily oiled binned sand depth). The shift to smaller, less aromatic compounds is explained by both the behavior of increasingly weathered crude oils and also the known water solubility of smaller, less aromatic compounds that would accompany vertical migration from both tidal burial and washing. The oxygenated environmental transformation products (CxHyOz) have relative abundance distributions consistent with water solubility and ionization of the proposed functional groups and and highlight chemical separation of the crude oil within the sand. Elemental composition assignment confidence in mass spectrometry is typically assessed by monoisotopic mass accuracy. For a given mass accuracy, resolution and detection of other isotopologues can further narrow the number of possible elemental compositions. However, such measurements require ultrahigh resolving power and high dynamic range, particularly for compounds containing low numbers of nitrogen and oxygen (both 15N and 18O occur at less than 0.4% natural abundance). Here, we demonstrate validation of molecular formula assignment from isotopic fine structure, based on ultrahigh resolution broadband Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Dynamic range is enhanced by external quadrupole and internal stored waveform inverse Fourier transform (SWIFT) isolation to facilitate detection of low abundance heavy atom isotopologues. Broad ionization of the largest possible number of compounds is critical to most Fourier transform ion cyclotron resonance (FT-ICR) crude oil or complex mixture analyses. Here, we show an up-to-three-fold improvement in sensitivity by optimization of acid/analyte concentration in the positive ion electrospray mixture. The results show an increased detection of both oxygen and hydrocarbon classes relative to nitrogen simply by bulk analyte dilution at sufficiently high acid concentration. A similar relative increase in oxygen detection is observed by positive ion electrospray time-of-flight (TOF) analysis of model compounds, particularly with compounds containing ketone functional groups. The model compound work demonstrates that at sufficiently low concentration of analyte and sufficiently high acid concentration, a steady-state is observed in the ratio of detection of nitrogen and oxygen. Here, we report isolation of a largely single stored waveform inverse Fourier transform (SWIFT) isolated crude oil Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) m/z and subsequent fragmentation by infrared multiphoton dissociation (IRMPD) of largely a single crude oil m/z. A sample preparation method that employs higher than typical analyte to acid ratio (2 mg/ml analyte, 0.05% formic acid in spray mixture) was performed to preferentially ionize nitrogen (N1 heteroclass) and simplify m/z species in the FT-ICR cell, ergo an artificially simplified spectra. External quadrupole and internal SWIFT isolation were used to isolate the peak before fragmentation (isolation resolving power of ~11,000). IRMPD was used to fragment the peak at a series of irradiation times between 0.5 seconds and 2.0 seconds with the number of N1 fragment ions between 114 and 200. 13C fragments at irradiation times greater than 0.75 seconds show the presence of a smaller impurity in the FT-ICR cell post-isolation. Recent studies have shown extensive oxidative modification to the Macondo wellhead oil upon release into the Northern Gulf of Mexico as a result of biotic and abiotic environmental processes. The transformations include a ketone signature that is masked in positive ion electrospray by the native petroleum N1 class at high analyte concentration relative to acid modifier in the spray solution. Here, we compare Deepwater Horizon oil spill ketone oxidative transformations to those from the Cosco Bosan oil spill in San Fransisco Bay. In contrast to the Gulf of Mexico samples, the San Francisco Bay oil spill do not contain the magnitude or diversity of positive ion electrospray ketone oxidative products found in the Deepwater Horizon contamination. The data presented lead the authors of this paper to posit that the ketone oxidative transformations are a Gulf of Mexico specific event.