Arif Ahmed

Application of the Mason−Schamp Equation and Ion Mobility Mass Spectrometry To Identify Structurally Related Compounds in Crude Oil

The various components of crude oil were structurally resolved using an atmospheric-pressure solids analysis probe (ASAP) coupled with ion mobility mass spectrometry (IM-MS). An ASAP source was used to broadly fractionate compounds according to their boiling points, thereby simplifying the resulting mass spectra for easier data interpretation. The m/z-mobility plots obtained by IM-MS analysis of crude oil could be used to find the structural relationship between crude oil molecules. That was demonstrated using ion mobility mass spectra from a homologous series of compounds, differing only by the number of alkyl units, found in crude oil. The peaks from this series were linearly aligned in the plot, suggesting a continuous increase of the collisional cross section with an increase of mass values and hence the absence of significant structural differences within the series. In contrast, peaks in a homologous series differing only in the number of pendant hydrogen atoms were not linearly aligned, suggesting a discontinuous increase of the collisional cross section with an increase of mass values and hence significant structural differences due to the addition or removal of hydrogen. Cases in which a slope change was observed at three- or four-peak intervals may be related to the addition of an aromatic ring to existing structures. Overall, ion mobility mass spectrometry demonstrates a useful tool that can be used to elucidate structural relationships between molecules comprising crude oil.

Developments in FT-ICR MS instrumentation, ionization techniques, and data interpretation methods for petroleomics

Because of the increasing importance of heavy and unconventional crude oil as an energy source, there is a growing need for petroleomics: the pursuit of more complete and detailed knowledge of the chemical compositions of crude oil. Crude oil has an extremely complex nature; hence, techniques with ultra-high resolving capabilities, such as Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), are necessary. FT-ICR MS has been successfully applied to the study of heavy and unconventional crude oils such as bitumen and shale oil. However, the analysis of crude oil with FT-ICR MS is not trivial, and it has pushed analysis to the limits of instrumental and methodological capabilities. For example, high-resolution mass spectra of crude oils may contain over 100,000 peaks that require interpretation. To visualize large data sets more effectively, data processing methods such as Kendrick mass defect analysis and statistical analyses have been developed. The successful application of FT-ICR MS to the study of crude oil has been critically dependent on key developments in FT-ICR MS instrumentation and data processing methods. This review offers an introduction to the basic principles, FT-ICR MS instrumentation development, ionization techniques, and data interpretation methods for petroleomics and is intended for readers having no prior experience in this field of study.

Application of Atmospheric Pressure Photo Ionization Hydrogen/Deuterium Exchange High-Resolution Mass Spectrometry for the Molecular Level Speciation of Nitrogen Compounds in Heavy Crude Oils

We report here for the first time the application of atmospheric pressure photo ionization hydrogen/deuterium exchange (APPI HDX) coupled to high-resolution mass spectrometry for molecular level speciation of nitrogen containing compounds in crude oils. The speciation was done based on different combinations of ions produced from nitrogen containing compounds with various functional groups. To prove the concept, 20 nitrogen containing standard compounds were analyzed. As a result, it was shown that the nitrogen containing compound (M) with a primary amine functional group mainly produced a combination of [M - 2H + 2D](•+) and ([M - 2H + 2D] + D)(+) ions, one with a secondary amine including alkylated or phenylated pyrrole a combination of [M - H + D](•+) and ([M - H + D] + D)(+), one with a tertiary amine including N-alkylated or phenylated pyrrole a combination of [M](•+) and [M + D](+), and one with a pyridine functional group mostly [M + D](+) ions. The concept was successfully applied to do nitrogen speciation of resins fractions of two oil samples. Combined with the subsequent investigation of double bond equivalence distribution, it was shown that resins of Qinhuangdao crude oil sample contained mostly alkylated pyrrole and N-alkylated pyrrole type compounds but resins of shale oil extract contained mostly pyridine type nitrogen compounds. It was also shown that the speciation of individual elemental composition was also possible by use of this method. Overall, this study clearly shows that atmospheric pressure photo ionization hydrogen/deuterium exchange (APPI HDX) coupled to high-resolution mass spectrometry is a powerful analytical method to do nitrogen speciation of crude oil compounds at the molecular level.

Elucidating Molecular Structures of Nonalkylated and Short-Chain Alkyl (n < 5, (CH2)n) Aromatic Compounds in Crude Oils by a Combination of Ion Mobility and Ultrahigh-Resolution Mass Spectrometries and Theoretical Collisional Cross-Section Calculations

Ultrahigh-resolution mass spectrometry has allowed the determination of elemental formulas of the compounds comprising crude oils. However, elucidating molecular structures remains an analytical challenge. Herein, we propose and demonstrate an approach combining ion mobility mass spectrometry (IM-MS), ultrahigh-resolution mass spectrometry, and theoretical collisional cross-section (CCS) calculations to determine the molecular structures of aromatic compounds found in crude oils. The approach is composed of three steps. First, chemical structures are suggested based on the elemental formulas determined from ultrahigh-resolution mass spectra. Second, theoretical CCS values are calculated based on these proposed structures. Third, the calculated CCS values of the proposed structures are compared with experimentally determined CCS values from IM-MS data to provide proposed structures. For proof of concept, 31 nonalkylated and short-chain alkyl (n < 5, (CH2)n) aromatic compounds commonly observed in crude oils were analyzed. Theoretical and experimental CCS values matched within a 5% RMS error. This approach was then used to propose structures of compounds in selected m/z regions of crude oil samples. Overall, the combination of ion mobility mass spectrometry, ultrahigh-resolution mass spectrometry, and theoretical calculations was shown to be a useful tool for elucidating chemical structures of compounds in complex mixtures.

Optimization and Application of APCI Hydrogen–Deuterium Exchange Mass Spectrometry (HDX MS) for the Speciation of Nitrogen Compounds

AbstractA systematic study was performed to investigate the utility of atmospheric pressure chemical ionization hydrogen–deuterium exchange mass spectrometry (APCI HDX MS) to identify the structures of nitrogen-containing aromatic compounds. First, experiments were performed to determine the optimized experimental conditions, with dichloromethane and CH3OD found to be good cosolvents for APCI HDX. In addition, a positive correlation between the heated capillary temperature and the observed HDX signal was observed, and it was suggested that the HDX reaction occurred when molecules were contained in the solvent cluster. Second, 20 standard nitrogen-containing compounds were analyzed to investigate whether speciation could be determined based on the different types of ions produced from nitrogen-containing compounds with various functional groups. The number of exchanges occurring within the compounds correlated well with the number of active hydrogen atoms attached to nitrogen, and it was confirmed that APCI HDX MS could be used to determine speciation. The results obtained by APCI HDX MS were combined with the subsequent investigation of the double bond equivalence distribution and indicated that resins of shale oil extract contained mostly pyridine type nitrogen compounds. This study confirmed that APCI HDX MS can be added to previously reported chemical ionization, electrospray ionization, and atmospheric pressure photo ionization-based HDX methods, which can be used for structural elucidation by mass spectrometry. Graphical Abstractᅟ

Optimization and application of atmospheric pressure chemical and photoionization hydrogen–deuterium exchange mass spectrometry for speciation of oxygen-containing compounds

AbstractThis paper presents a detailed investigation of the feasibility of optimized positive and negative atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) and atmospheric pressure photoionization (APPI) MS coupled to hydrogen–deuterium exchange (HDX) for structural assignment of diverse oxygen-containing compounds. The important parameters for optimization of HDX MS were characterized. The optimized techniques employed in the positive and negative modes showed satisfactory HDX product ions for the model compounds when dichloromethane and toluene were employed as a co-solvent in APCI- and APPI-HDX, respectively. The evaluation of the mass spectra obtained from 38 oxygen-containing compounds demonstrated that the extent of the HDX of the ions was structure-dependent. The combination of information provided by different ionization techniques could be used for better speciation of oxygen-containing compounds. For example, (+) APPI-HDX is sensitive to compounds with alcohol, ketone, or aldehyde substituents, while (−) APPI-HDX is sensitive to compounds with carboxylic functional groups. In addition, the compounds with alcohol can be distinguished from other compounds by the presence of exchanged peaks. The combined information was applied to study chemical compositions of degraded oils. The HDX pattern, double bond equivalent (DBE) distribution, and previously reported oxidation products were combined to predict structures of the compounds produced from oxidation of oil. Overall, this study shows that APCI- and APPI-HDX MS are useful experimental techniques that can be applied for the structural analysis of oxygen-containing compounds. Graphical AbstractStructural assignment of oxygen-containing compounds by (+/-) APCI/APPI HDX-MS and their speciation in degraded oil

Data Interpretation Methods for Petroleomics

The need of heavy and unconventional crude oil as an energy source is increasing day by day, so does the importance of petroleomics: the pursuit of detailed knowledge of heavy crude oil. Crude oil needs techniques with ultra-high resolving capa- bilities to resolve its complex characteristics. Therefore, ultra-high resolution mass spectrometry represented by Fourier trans- form ion cyclotron resonance mass spectrometry (FT-ICR MS) has been successfully applied to the study of heavy and unconventional crude oils. The analysis of crude oil with high resolution mass spectrometry (FT-ICR MS) has pushed analysis to the limits of instrumental and methodological capabilities. Each high-resolution mass spectrum of crude oil may routinely con- tain over 50,000 peaks. To visualize and effectively study the large amount of data sets is not trivial. Therefore, data processing and visualization methods such as Kendrick mass defect and van Krevelen analyses and statistical analyses have played an important role. In this regard, it will not be an overstatement to say that the success of FT-ICR MS to the study of crude oil has been critically dependent on data processing methods. Therefore, this review offers introduction to peotroleomic data interpreta- tion methods.

Mechanistic study on lowering the sensitivity of positive atmospheric pressure photoionization mass spectrometric analyses: size-dependent reactivity of solvent clusters.

RATIONALE Understanding the mechanism of atmospheric pressure photoionization (APPI) is important for studies employing APPI liquid chromatography/mass spectrometry (LC/MS). In this study, the APPI mechanism for polyaromatic hydrocarbon (PAH) compounds dissolved in toluene and methanol or water mixture was investigated by use of MS analysis and quantum mechanical simulation. In particular, four different mechanisms that could contribute to the signal reduction were considered based on a combination of MS data and quantum mechanical calculations. METHODS The APPI mechanism is clarified by combining MS data and density functional theory (DFT) calculations. To obtain MS data, a positive-mode (+) APPI Q Exactive Orbitrap mass spectrometer was used to analyze each solution. DFT calculations were performed using the general atomic and molecular electronic structure system (GAMESS). RESULTS The experimental results indicated that methanol significantly reduced the signal in (+) APPI, but no significative signal reduction was observed when water was used as a co-solvent with toluene. The signal reduction is more significant especially for molecular ions than for protonated ions. Therefore, important information about the mechanism of methanol-induced signal reduction in (+) APPI-MS can be gained due its negative impact on APPI efficiency. CONCLUSIONS The size-dependent reactivity of methanol clusters ((CH3 OH)n , n = 1-8) is an important factor in determining the sensitivity of (+) APPI-MS analyses. Clusters can compete with toluene radical ions for electrons. The reactivity increases as the sizes of the methanol clusters increase and this effect can be caused by the size-dependent ionization energy of the solvent clusters. The resulting increase in cluster reactivity explains the flow rate and temperature-dependent signal reduction observed in the analytes. Based on the results presented here, minimizing the sizes of methanol clusters can improve the sensitivity of LC/(+)-APPI-MS.

Which Hydrogen Atom of Toluene Protonates PAH molecules in (+)-Mode APPI MS Analysis?

AbstractA previous study (Ahmed, A. et al., Anal. Chem. 84, 1146–1151( 2012) reported that toluene used as a solvent was the proton source for polyaromatic hydrocarbon compounds (PAHs) that were subjected to (+)-mode atmospheric-pressure photoionization. In the current study, the exact position of the hydrogen atom in the toluene molecule (either a methyl hydrogen or an aromatic ring hydrogen) involved in the formation of protonated PAH ions was investigated. Experimental analyses of benzene and anisole demonstrated that although the aromatic hydrogen atom of toluene did not contribute to the formation of protonated anthracene, it did contribute to the formation of protonated acridine. Thermochemical data and quantum mechanical calculations showed that the protonation of anthracene by an aromatic ring hydrogen atom of toluene is endothermic, while protonation by a methyl hydrogen atom is exothermic. However, protonation of acridine by either an aromatic ring hydrogen or a methyl hydrogen atom of toluene is exothermic. The different behavior of acridine and anthracene was attributed to differences in gas-phase basicity. It was concluded that both types of hydrogen in toluene can be used for protonation of PAH compounds, but a methyl hydrogen atom is preferred, especially for non-basic compounds.

Molecular-level evidence provided by ultrahigh resolution mass spectrometry for oil-derived doc in groundwater at Bemidji, Minnesota

Dissolved organic matter samples extracted from ground water at the USGS Bemidji oil spill site in Minnesota were investigated by ultrahigh resolution mass spectrometry. Principle component analysis (PCA) of the elemental composition assignments of the samples showed that the score plots for the contaminated sites were well separated from those for the uncontaminated sites. Additionally, spectra obtained from the same sampling site 7 and 19 years after the spill were grouped together in the score plot, strongly suggesting a steady state of contamination within the 12year interval. The double bond equivalence (DBE) of Ox class compounds was broader for the samples from the contaminated sites, because of the complex nature of oil and the consequent formation of compounds with saturated and/or aromatic structures from the oxygenated products of oil. In addition, Ox class compounds with a relatively smaller number of x (x<8; x=number of oxygen) and OxS1 class compounds were more abundant in the samples from the contaminated sites, because of the lower oxygen and higher sulfur contents of the oil compared to humic substances. The molecular-level signatures presented here can be a fundamental basis for in-depth analysis of oil contamination.