Of the estimated 5 million barrels of crude oil released into the Gulf of Mexico from the Deepwater Horizon oil spill, a fraction washed ashore onto sandy beaches from Louisiana to the Florida panhandle. Here, we compare the detailed molecular analysis of hydrocarbons in oiled sands from Pensacola Beach to the Macondo wellhead oil (MWO) by electrospray (ESI) and atmospheric pressure photoionization (APPI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to identify major environmental transformation products of polar, high molecular weight (C >25 ) "heavy ends" (high-boiling species) inaccessible by gas chromatography. The petrogenic material isolated from the Pensacola Beach sand displays greater than 2-fold higher molecular complexity than the MWO constituents, most notably in oxygenated species absent in the parent MWO. Surprisingly, the diverse oxygenated hydrocarbons in the Pensacola Beach sediment extracts were dominant in all ionization modes investigated, (±) ESI and (±) APPI. Thus, the molecular-level information highlighted oxygenated species for subsequent "targeted" analyses. First, time-of-flight mass spectrometry analysis of model compounds attributes the unusually large oxygen signal magnitude from positive electrospray to ketone transformation products (O 1 −O 8 classes). Next, negative electrospray mass spectrometry reveals carboxylic acid transformation products. Two-dimensional gas chromatography with mass spectrometry analysis of anion-exchange chromatographic fractions unequivocally verifies the presence of abundant alkyl ketone fragments in sand extracts, and FT-ICR MS analysis reveals the distribution of high-boiling ketone, carboxylic, and higher numbered (3+) oxygen-containing transformation products too polar to be analyzed by gas chromatography. The results expand compositional coverage of oxygen-containing functionalities beyond the classic naphthenic acid type species to complex/mixed ketone, hydroxyl, and carboxylic acid classes of molecules that have been recently identified in produced water, emulsions, and petroleum production deposits.

Accurate mass measurement requires the highest possible mass resolution, to ensure that only a single elemental composition contributes to the mass spectral peak in question. Although mass resolution is conventionally defined as the closest distinguishable separation between two peaks of equal height and width, the required mass resolving power can be ∼10× higher for equal width peaks whose peak height ratio is 100 : 1. Ergo, minimum resolving power requires specification of maximum dynamic range, and is thus 10-100× higher than the conventional definition. Mass resolving power also depends on massto-charge ratio. Mass accuracy depends on mass spectral signal-to-noise ratio and digital resolution. Finally, the reliability of elemental composition assignment can be improved by resolution of isotopic fine structure. Thus, the answer to the question of "how much is enough mass resolving power" requires that one first specify S/N ratio, dynamic range, digital resolution, mass-to-charge ratio, and (if available) isotopic fine structure. The highest available broadband mass resolving power and mass accuracy is from Fourier transform ion cyclotron resonance mass spectrometry. Over the past five years, FT-ICR MS mass accuracy has improved by about an order of magnitude, based on higher magnetic field strength, conditional averaging of time-domain transients, better mass calibration (spectral segmentation; inclusion of a space charge term); radially dispersed excitation; phase correction to yield absorption-mode display; and new ICR cell segmentation designs.

Since the emergence of high resolving power crude oil mass spectrometry two decades ago, hundreds of publications and presentations have detailed petroleum complex mixtures by electrospray ionization (ESI) mass spectrometry (MS). None of these works have reported or detailed ion suppression (also referred to as ionization biasing or matrix effects) which is a well-known feature of ESI. Here, we show the extreme consequences of ionization biasing within a narrow, 1 order of magnitude concentration range for crude oil mixture direct infusion experiments in positive ion (+) ESI. An oil spill contaminant, a crude oil, and an equimolar model compound mixture were electrosprayed at various analyte and acid modifier concentrations for Fourier transform ion cyclotron resonance (FT-ICR) and time-of-flight (TOF) MS analysis. A 3-fold increase in the number of elemental compositions is achieved by optimization of analyte and acid concentration. At high analyte concentration, oxygen heteroatom class (i.e., CcHhO x species, denoted henceforth simply as O x ) abundance is attenuated and practically nullified. The suppression can be understood from (+) ESI TOF mass analysis of a prepared equimolar model compound mixture, particularly those with ketone functional groups. At sufficiently low concentration of analyte, the relative abundances of nitrogen- and oxygen-containing model compounds no longer vary. For (+) ESI at the flow rates and voltages described in this study, we recommend operating at mass/volume petroleum residue concentration below 0.1 mg/mL in 1:1 (v:v) toluene/methanol with formic acid at 2.5% (v:v).

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