Contrasting Perspective on Asphaltene Molecular ... - ACS Publications

Energy & Fuels 2008, 22, 1765–1773

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Contrasting Perspective on Asphaltene Molecular Weight. This Comment vs the Overview of A. A. Herod, K. D. Bartle, and R. Kandiyoti Oliver C. Mullins,*,† Bruno Martínez-Haya,‡ and Alan G. Marshall§ Schlumberger-Doll Research, Cambridge, Massachusetts 02139; Departamento de Sistemas Fisicos, Quimicos y Naturales, UniVersidad Pablo de OlaVide, 41013 SeVille, Spain; and National High Magnetic Field Laboratory, Florida State UniVersity, Tallahassee, Florida 32310-4005 ReceiVed NoVember 28, 2007. ReVised Manuscript ReceiVed February 11, 2008

Asphaltene molecular weight (Asphaltenes, HeaVy Oils and Petroleomics; Springer: New York, 2007) continues to be the subject of a longstanding debate in the literature. A paper (Energy Fuels 2007, 21, 2176-2203) recently published (referred to as HBK) claims that asphaltene molecular weights are bimodal with one component in the roughly megadalton range and a second component in the roughly 5 kDa range. These claims are in sharp contrast to results published from a variety of measurements with the overall conclusions that asphaltene molecular weights are monomodal with a most probable 750 Da ((200) with a fwhm 500–1000 Da. In this report, we provide a summary of the four molecular diffusion techniques and seven mass spectral techniques from many groups around the world that are all in accord with the 750 Da most probable mass. Moreover, here we discuss why HBK reported anomalously large asphaltene molecular weights along with the unique claim of a bimodal distribution. In particular, the size exclusion chromatography (SEC) results that yield megadalton masses were performed with the solvent N-methylpyrrolidinone which is known to flocculate up to 50% of the asphaltenes. The megadalton mass is likely large asphaltene aggregates or flocs. In a previous referenced paper from the HBK labs, the better solvent for asphaltenes, tetrahydrofuran, did not give the megadalton peak in their SEC experiments as they stated; we suspect because the asphaltenes were suitably dissolved (although still with some aggregation). The corresponding discussion treats the known hierarchy of asphaltene aggregation at very low concentration in a good solvent, toluene. In addition, the mass spectral method used in HBK, laser desorption ionization, is shown herein and in the literature to yield anomalously large molecular weights for asphaltenes and polycyclic aromatic hydrocarbons due to gas phase aggregation if (1) the laser power is too high, (2) the surface concentration of asphaltenes is too high, or (3) if the ions are collected too quickly (i.e., from a dense plasma). Properly accounting for these potential pitfalls, one obtains the same most probable 750 Da molecular weight as from all of the other techniques. Finally, ESI MS is shown herein and in ample literature to be readily able to detect large masses (the primary reason ESI led to a Nobel Prize); the absence of large mass species in ESI MS of asphaltenes is because they are not present. The congruence of so many molecular diffusion techniques and mass spectral techniques is a powerful advance for asphaltene science.

Introduction The most important attribute of any chemical is its constituent elements. Fortunately, for asphaltenes,1 there is no dispute about elemental composition. The second most important attribute of a chemical species is its molecular weight. Size counts in chemistry. For a chemical mixture, the moments of the molecular weight distribution are of critical interest. Unfortunately, in asphaltene science, there is a persistent debate about molecular weight. In particular, there was recently a publication purporting to be a characterization of heavy hydrocarbons.2 (We will refer to this paper as HBK.) However, the findings regarding asphaltene molecular weight in HBK are in gross disagreement * To whom correspondence should be addressed. † Schlumberger-Doll Research. ‡ Universidad Pablo de Olavide. § Florida State University. (1) Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Asphaltenes, HeaVy Oils and Petroleomics; Springer: New York, 2007. (2) Herod, A. A.; Bartle, K. D.; Kandiyoti, R. Characterization of Heavy Hydrocarbons by Chromatographic and Mass Spectrometric Methods: An Overview. Energy Fuels 2007, 21, 2176–2203.

with almost all new measurements and many not so new measurements of asphaltene molecular weight. In the HBK overview, the authors primarily reviewed their own work. For example, the first 40 references in HBK are to their coauthored work. Consequently, in our view HBK mandates this Article to include a broader context. This paper is structured as follows: first, we frame the debate on asphaltene molecular weight. In doing so, we provide a short survey of analytical methods which have been employed to address asphaltene molecular weight; this large body of work is shown to be consistent and at significant variance with important conclusions in HBK. We then address specific claims in HBK regarding asphaltene molecular weight; the anomalous claims are readily explained in terms of well-known and demonstrated tendencies of asphaltenes to aggregate. We finally review in greater detail some of the large body of consistent literature showing that the number average molecular weights of crude oil asphaltenes are ∼750 u ((200 u) with a fwhm (full width half maximum) of 500 u to 1000 u (u is Da or amu). This debate has had an extensive history.2–5 Consequently, we make an effort here to be as clear

10.1021/ef700714z CCC: $40.75  2008 American Chemical Society Published on Web 04/05/2008

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as possible. We thank Professors Herod, Bartle, and Kandiyoti for engaging in this debate; indeed it is not easy to do so, yet we believe the readership will benefit from the clarity. The Debate In HBK, primarily two methods of measurement are used for determination of asphaltene molecular weight: laser desorption ionization mass spectrometry (LDI) and size exclusion chromatograpy (SEC), also known as gel-permeation chromatography (GPC). From both their LDI and SEC data they claim there is a “low molecular weight” component of asphaltenes in the tens of kilodaltons range. From the SEC data alone they claim there is a second, high molecular weight component of asphaltenes in the megadalton range. They are alone in this claim. We believe their measurements on asphaltenes are dominated by artifacts of asphaltene aggregation so that their interpretation of their data needs to be critically reexamined. We will treat the problems with LDI and SEC for asphaltenes below in some detail. In the large body of asphaltene literature, there have been two primary methods used to obtain asphaltene molecular weight; mass spectrometry (MS) and molecular diffusion. Figure 1 provides a pictorial overview of these different techniques and summarizes the key results.6 This instructive figure shows general agreement on asphaltene molecular weight among a wide variety of methods. In mass spectrometry, the ionization or the volatilization/ionization step is of central concern especially for heavy and/or sticky materials such as asphaltene. Consequently, it is important to compare results from different ionization methods. Five different ionization methods are shown in Figure 1 to yield comparable data. John Fenn won a Nobel prize for invention of one of these methods, electrospray ionization (ESI).7 In a comprehensive review, ESI ionization methods used on asphaltenes found most of the asphaltenes between 400 and 800 Da with a range of 300–1400 Da.8 In addition, recently published work on LDI of asphaltenes shows that (1) if LDI is performed improperly, artificially large apparent molecular weights are obtained and (2) if LDI is performed properly, then LDI results on asphaltenes are in line with all other MS techniques employed for asphaltenes.9–12 The (3) Badre, S.; Goncalves, C. C.; Norinaga, K.; Gustavson, G.; Mullins, O. C. Molecular size and weight of asphaltene and asphaltene solubility fractions from coals, crude oils and bitumen. Fuel 2006, 85, 1. (4) Morgan, T. J.; Millan, M.; Behrouzi, M.; Herod, A. A.; Kandiyoti, R. On the limitations of UV-fluorescence spectroscopy in the detection of high-mass hydrocarbon molecules. Energy Fuels 2005, 19, 164. (5) Mullins, O. C. Rebuttal to comment by professors Herod, Kandiyoti, and Bartle on “Molecular size and weight of asphaltene and asphaltene solubility fractions from coals, crude oils and bitumen”. Fuel 2006, 86, 309–312. (6) Akbarzadeh, K.; Hammami, A.; Kharrat, A.; Zhang, D.; Allenson, S.; Creek, J.; Kabir, S.; Jamaluddin, A.; Marshall, A. G.; Rodgers, R. P.; Mullins, O. C.; Solbakken, T. Asphaltenes-Problematic But Rich in Potential. Oilfield ReV., Summer 2007, 22–43. (7) Cho, A.; Normile, D. Nobel Prize in Chemistry: Mastering Macromolecules. Science 2002, 298, 527–528. (8) Rodgers, R. P. Marshall, A. G. Petroleomics: Advanced Characterization of Petroleum Derived Materials by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). Chapter 3 in ref 1. (9) Hortal, A. R.; Martínez-Haya, B.; Lobato, M. D.; Pedrosa, J. M.; Lago, S. On the determination of molecular weight distributions of asphaltenes and their aggregates in laser desorption ionization experiments. J. Mass Spectrom. 2006, 41, 960. (10) Martínez-Haya, B.; Hortal, A. R.; Hurtado, P. M.; Lobato, M. D.; Pedrosa, J. M. Laser desorption/ionization determination of molecular weight distributions of polyaromatic carbonaceous compounds and their aggregates. J. Mass Spectrom. 2007, 42, 701–713. (11) Hortal, A. R.; Hurtado, P. M.; Martínez-Haya, B.; Mullins, O. C. Molecular weight distributions of coal and petroleum asphaltenes from laser desorption ionization experiments. Energy Fuels 2007, 21, 2863–2868.

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results of HBK which also employs LDI are in gross disagreement with all of these MS results including the LDI work in refs 9–12. Figure 1 also shows four different molecular diffusion methods that have been performed on asphaltenes, all with consistent results and in good agreement with all MS results, excepting those of HBK. For example, the first molecular diffusion measurements of asphaltenes reported asphaltene molecular weights with an average of 750 g/mol with a width of 500–1000 g/mol, fwhm.13,14 The debate is framed. Size Exclusion Chromatography We now consider the SEC results of HBK on asphaltenes. Asphaltenes are defined as a solubility class; typically n-heptane insoluble, toluene soluble. In HBK it is claimed based on SEC that asphaltene molecular weight is bimodal. (They have similar claims about related carbonaceous materials that are plausibly spurious.) They indicate that asphaltenes have a roughly 10 kDa peak or component and a megadalton peak with very little in between! That is, they claim the molecular weight of asphaltenes is bimodal and basically discontinuous; their conclusion was obtained from SEC data only. Their LDI data does not show the megadalton peak. The claims of HBK strain credulity on two counts: First, it seems very unlikely that both the megadalton and kilodalton materials would have the same solubility. [N.B. Asphaltene is defined as a solubility class.] Moreover, HBK had inconsistent SEC data with different solvents and chose a solvent known to flocculate asphaltenes, thus strongly indicating the impact of asphaltene aggregates. Second, nature does not generate discontinuous, bimodal distributions of molecular weight in thermal degradation processes especially within a single solubility class. Asphaltenes are produced in a thermal catagenesis processsheat over geologic time degrades kerogen, producing crude oil. The distribution of alkanes produced in this process is broad and continuous. This is known from every gas chromatogram ever taken on crude oils. (Of course, biodegradation can alter this distribution.) Two-dimensional gas chromatography (GCxGC) elucidates the continuous alkane distribution ever more clearly.15 In addition, GCxGC shows that branched and normal alkanes, alkylcyclopentanes, alkylcyclohexanes, and alkyl aromatics all show continuous molecular weight distributions. High-temperature GC shows that even the heaviest alkanes are present in a continuous largely monomodal distribution.16 The distribution of aromatic compounds in crude oils17 and in asphaltenes18–20 is broad and continuous. It is extremely unlikely that such a natural thermal degradation (12) Hurtado, P.; Hortal, A. R.; Martínez-Haya, B. MALDI detection of carbonaceous compounds in ionic liquid matrices. Rapid Commun.Mass Spectrom. 2007, 21, 3161–3164. (13) Groenzin, H.; Mullins, O. C. Asphaltene Molecular Size and Structure. J. Phys. Chem. A 1999, 103, 11237–11245. (14) Groenzin, H.; Mullins, O. C. Molecular sizes of asphaltenes from different origin. Energy Fuels 2000, 14, 677. (15) Reddy, C. M.; Nelson, R. K.; Sylva, S. P.; Xu, L.; Peacock, E. A.; Raghuraman, B.; Mullins, O. C. “Identification and quantification of alkenebased drilling fluids in crude oils by comprehensive two-dimensional gas chromatography with flame ionization detection. J. Chromatogr. A 2007, 1148, 100–107. (16) Roehner, R. M.; Fletcher, J. V.; Hanson, F. V.; Dahdah, N. F. Comparative Compositional Study of Crude Oil Solids from the Trans Alaska Pipeline System Using High-Temperature Gas Chromatography. Energy Fuels 2002, 16 (1), 211–217. (17) Mullins, O. C.; Mitra-Kirtley, S.; Zhu, Y. Electronic absorption edge of petroleum. Appl. Spectrosc. 1992, 46, 1405. (18) Mullins, O. C.; Zhu, Y. First observation of the Urbach tail in a multicomponent organic system. Appl. Spectrosc. 1992, 46, 354. (19) Ruiz-Morales, Y.; Mullins, O. C. Polycyclic Aromatic Hyodrocarbons of Asphaltenes Analyzed by Molecular orbital Calculations with Optical Spectroscopy. Energy Fuels 2007, 21, 256.

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Figure 1. Asphaltene molecular weight through mass spectrometry and molecular diffusion. There is convergence in results from all techniques from many groups around the world; asphaltenes are small molecules with an average molecular weight of ∼750 Da. LDI measurements which had not been in accord with all other MS measurements nor with any asphaltene diffusion techniques have been shown to suffer from gas phase aggregation. Recent LDI measurements performed properly are now in accord with all other measurements.

process would produce such a peculiar discontinuous, bimodal distribution of asphaltenes and in sharp contrast to known continuous distributions of various alkanes and aromatics. How do you get from the megadalton peak to the kilodalton peak (or vice versa) without having intervening masses in a thermal degradation process? HBK arrives at the peculiar bimodal molecular weight distribution exclusively from SEC data. SEC is a chromatographic method that separates primarily by size. Large species (20) Ruiz-Morales, Y.; Wu, X.; Mullins, O. C. Electronic Absorption Edge of Crude Oils and Asphaltenes Analyzed by Molecular Orbital Calculations with Optical Spectroscopy. Energy Fuels 2007, 21, 944.

are too big to fit into the pores of the column packing material so they go through the column rapidly. Small species can enter the pores of the stationary phase, thus getting trapped in the stationary phase for a while. The range of pore sizes in SEC columns results in a continuous trend; smaller species elute later. SEC does not discriminate on what the species is, whether molecule, dimer, nanoaggregate, cluster, or floc. Bigger species elute faster (although adhesion interferes with quantitative analysis). HBK shows SEC elutions of asphaltenes with a very fast eluting peak. HBK claims without any supporting evidence that this peak is due to asphaltene molecules as opposed to some asphaltene aggregate structure. HBK then asserts that the elution


time corresponds to molecular megadaltons. Might this peak be due to some sort of aggregation (thus explaining the unlikely bimodal distribution)? HBK employed NMP (N-methylpyrrolidinone, also called N-methylpyrrolidone) as an eluting solvent.2,4 In a related paper by these authors,4 it is stated that, “the aggregation of small molecules in an NMP solvent has been discounted”. However, NMP not only aggregates asphaltenes, NMP also flocculates a significant fraction of asphaltene.3,21 For example, S. I. Andersen and co-workers report in their abstract that 9–53% of [petroleum] asphaltenes are insoluble in NMP.21 We think that the “giant” megadalton asphaltene “molecules” reported in HBK2,4 are the expected asphaltene flocs that one collects on filter paper.3,21 Moreover, in an earlier paper HBK indicated that use of tetrahydrofuran (THF) as an eluting solvent caused the giant molecule peak to vanish.4 It is likely that THF actually dissolves the megadalton aggregates. In any event, HBK had SEC results that were contradictory. They chose to rely on the demonstrably poor solvent for asphaltenes, NMP, and then they interpreted all data as if this poor solvent actually created a true molecular solution of asphaltene, the presence of asphaltene flocs notwithstanding. As a final note, SEC relates elution time to molecular weight provided that there is a valid standard for this time-to-molecular weight comparison. There is no such standard for asphaltenes in SEC due to differing adherence characteristics. In addition to considering insoluble asphaltene fractions, it is important to consider the nature of the “dissolved” or really colloidally suspended asphaltene fraction. Toluene is a good solvent for asphaltenes and is actually used as the solubility standard for the definition of asphaltenes. However, asphaltenes in toluene have been shown to form a variety of aggregation structures. At ultralow concentrations of asphaltene in toluene (30 µg/L), one has a true molecular solution as shown by Andrews and co-workers.22 At 50 mg/L asphaltene in toluene, asphaltenes have been shown by fluorescence intensity methods to form dimers.23 At ∼150 mg/L, asphaltenes in toluene have been shown to form aggregates using high-Q ultrasonics by G. Andreatta and co-workers.24–26 In addition, the high-Q ultrasonics work implies that these asphaltene structures are nanoaggregates.24–26 Both the concentrations of formation of the nanoaggregates and their size have been confirmed in several studies. NMR diffusion measurements performed by Freed et al. at Schlumberger-Doll Research clearly show the formation of the nanoaggregates at very low concentrations and show the small, sharp reduction in diffusion constant upon nanoaggregate formation.27 AC conductivity performed by Eric Sheu (now at Lawrence Livermore Laboratories) and co-workers confirm these (21) Ascanius, B. E.; Garcia, D. M.; Andersen, S. I. Analysis of asphaltenes subfractionated by N-methyl-2-pyrrolidone. Energy Fuels 2004, 18, 1827. (22) Andrews, A. B.; Guerra, R.; Sen, P. N.; Mullins, O. C. Diffusivity of Asphaltene Molecules by Fluorescence Correlation Spectroscopy. J. Phys. Chem. A 2006, 110, 8095. (23) Goncalves, S.; Castillo, J.; Fernandez, A.; Hung, J. Absorbance and fluorescence spectroscopy on the aggregation behavior of asphaltenetoluene solutions. Fuel 2004, 83, 1823. (24) Andreatta, G.; Bostrom, N.; Mullins, O. C. High-Q ultrasonic determination of the critical nanoaggregate concentration of asphaltenes and the critical micelle concentration of standard surfactants. Langmuir 2005, 21, 2728. (25) Andreatta, G. Bostrom, N. Mullins, O. C. Ultrasonic spectroscopy on asphaltene aggregation. Chapter 9 in ref 1. (26) Andreatta, G.; Goncalves, C. C.; Buffin, G.; Bostrom, N.; Quintella, C. M.; Arteaga-Larios, F.; Perez, E.; Mullins, O. C. Nanoaggregates and structure-function relations in asphaltenes. Energy Fuels 2005, 19, 1282. (27) Freed, D. E. Lisitza, N. V. Sen, P. N. Song, Y.-Q. Asphaltene molecular composition and dynamics from NMR diffusion measurements. Chapter 11 in ref 1.

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results.28 These asphaltene nanoaggregates have now been seen directly in (live) crude oil in oil reservoirs using in situ fluid analysis by Mullins and co-workers.29 This analysis obtained the asphaltene aggregate size in a 3000 ft column of crude oil by measuring asphaltene gravitation segregation described by Archimedes (negative) buoyancy of asphaltene nanoaggregates in the Boltzmann distribution.29 At several grams per liter, asphaltenes are thought to form clusters of nanoaggregates as shown by studying flocculation kinetics upon addition of n-heptane as shown independently by Yudin and Anisimov30 and by Oh and Deo.31 HBK states in the abstract: “There is no confirmable experimental evidence, either from our work or in the literature, showing that aggregation occurs under the dilute conditions prevailing during SEC, using NMP as an eluent.” Note that there is no measure of the asphaltene concentration that is operative in the SEC studies. The reason that there are few studies of asphaltene in NMP is because NMP is a poor solvent for asphaltenes and NMP flocculates a significant fraction of asphaltenes.3,21 There are virtually no studies of asphaltene aggregation in methyl alcohol because it is a very poor solvent for asphaltenes. However, one cannot conclude that there are no asphaltene aggregates upon addition of asphaltene to methyl alcohol. There are many studies showing asphaltene aggregation in toluene, a good solvent for asphaltenes.23–31 HBK presumes that asphaltenes do not aggregate in NMP; they do not claim to show this. It is important to realize asphaltenes are a polydisperse mixture of compounds. Potentially, in a poor solvent, some fraction can dissolve as aggregates; perhaps another fraction dissolves as a true molecular solution (possibly the very polar components) and another fraction flocculates. In light of the complex aggregation hierarchy that occurs for asphaltenes in toluene, there is simply no credibility to the assumption that asphaltenes do not form aggregates in NMP. The strange, bimodal distribution of asphaltenes that only shows up in the SEC interpretation of HBK is likely due to asphaltene aggregation. Vapor Pressure Osmometry HBK did not use this method for molecular weight determination; nevertheless, we address this per referees’ request. VPO has been used as a simple and inexpensive method, thus very popular, for molecular weight determination of various nonvolatile petroleum components. VPO works reasonably well for large alkanes. However, VPO molecular weight results for asphaltenes always exceed those of other methods cited herein by a factor of 5-10. VPO depends on the colligative property which is the reduction of vapor pressure due to the presence in solution of a nonvolatile component. However, VPO does not determine whether the nonvolatile component is a molecule or some type of aggregate. Moreover, VPO signal can become rather small if concentrations much below 1% are used. All VPO studies of asphaltenes acknowledge the aggregation problem and try to get around this by various methods including (28) Sheu, E. Y. Long, Y. Hamza, H. Asphaltene self-association and precipitation in solvents - AC conductivity measurements. Chapter 10 in ref 1. (29) Mullins, O. C.; Betancourt, S. S.; Cribbs, M. E.; Dubost, F. X.; Creek, J. L.; Andrews, A. B.; Venkataramanan, L. The colloidal structure of crude oil and the structure of oil reservoirs. Energy Fuels 2007, 21, 2785– 2794. (30) Yudin, I. K. Anisimov, M. A. Dynamic light scattering monitoring of asphaltene aggregation in crude oils and hydrocarbon solutions. Chapter 18 in ref 1. (31) Oh, K. Deo, M. D. Near infrared spectroscopy to study asphaltene aggregation solvents. Chapter 19 in ref 1.


(1) plotting apparent molecule weight vs concentration and extrapolating, (2) using elevated temperature (presuming that aggregation should be reduced at elevated temperature), and (3) comparing different apparent molecular weights for asphaltenes in different solvents. The primary problem for the application of VPO to asphaltenes is clearly aggregation. The problem is greatly exacerbated by the likely hierarchy of asphaltene aggregation in the concentration range below typical VPO concentrations. Specifically, it appears asphaltenes form dimers at less than 10-4 mass fraction,23 form nanoaggregates at ∼10-4 mass fraction,24–28 and form clusters of nanoaggregates at a few times 10-3 mass fraction.30,31 All of these concentrations are below typical VPO concentrations of 10-2 mass fraction. This hierarchy precludes extrapolation of VPO results to obtain accurate or even approximate molecular weights. Moreover, it is quite plausible that asphaltene nanoaggregate formation is in part entropy driven similar to aqueous micelle formation of many surfactants. Increasing temperature would not reduce aggregation in such circumstances. The fact that VPO often is in error by a factor of 5-10 argues that the extrapolation studies get past clustering but not past nanoaggregates, and that there are roughly 5-10 asphaltene molecules per nanoaggregate. This conclusion is known from NMR studies and corroborated by high-Q ultrasonics 24–26 and asphaltene gravity segregation measurements.29 Laser Desorption Ionization Mass Spectrometry HBK obtains roughly 10 kDa as the “lighter” asphaltene component. This is still much larger than results obtained in nearly all other investigations. The apparent discrepancy between LDI MS and mass spectrometry by all other ionization methods has recently been deciphered by Martínez-Haya and co-workers at the University Pablo de Olavide in Seville, Spain.9–12 LDI starts with a laser pulse irradiation on a surface where asphaltene has been deposited. Time of flight is then used to determine “molecular” mass. The problem is that if a dense plasma plume is formed upon desorption either by using high laser power and/ or by using high asphaltene mass/area densities, then gas phase aggregation takes place quite efficiently.9–11 The resulting cluster ions can become much larger than the actual molecular ions. The gas phase formation of these clusters is supported by the finding that the yield of heavy ions (>1000 g/mol) is sensitive to changes in the plume dynamics induced by altering the operating conditions of the ion source. For instance, a sizable enhancement of the aggregation efficiency is observed if the ion-extracting electric field is left on continuously, in comparison to pulsed delayed extraction. Under continuous extraction, the asphaltene ions are immediately accelerated through the dense plume, thereby picking up and adhering with more asphaltene molecules.9–11 These effects of gas phase aggregation apply to asphaltenes as well as to standard aromatic compounds.11 Figure 2 (top panel) shows a LDI spectrum of coronene exhibiting gas phase aggregation effects, where clusters as large as hexamers are observed. The LDI measurements with model PAHs show that larger pericondensed fused aromatic ring systems are more susceptible to in-plume aggregation effects.10,11 Figure 2 (middle panel) compares LDI spectra of a pure asphaltene sample obtained at low laser power and high laser power. Note the huge difference in apparent molecular weight. Notably, a similar kind of effect was already observed in the systematic LDI investigation of R. Tanaka and co-workers.32 Those authors attributed the enhancement of the yield at the high masses to ionization efficiency effects and, consequently, concluded that the distribution obtained at high laser power ensured a more homogeneous


Figure 2. LDI TOF mass spectrum of asphaltene obtained at low laser power and high laser power and with pulsed ion extraction. Top: LDI of coronene shows clustering, which occurs readily with (1) high laser power, (2) high surface mass density, and/or (3) continuous ion extraction. Middle: LDI mass spectra for pure asphaltenes deposited on a surface show that, at large laser power, asphaltene aggregation takes place. Accurate asphaltene molecular weights are obtained only for low laser power. Bottom: Dilution of asphaltenes in a matrix greatly mitigates asphaltene aggregation. Somewhat elevated laser powers with dilute asphaltenes still give reasonably accurate molecular weight distributions.

detection of the asphaltenes and was more reliable. However, Martínez-Haya and co-workers have shown that the potential differences in ionization efficiencies for the different components of the asphaltene class have a minor effect on the main features of the molecular weight distributions measured with the LDI technique. On one hand, the dependence of the recorded molecular weight distribution on laser power becomes negligible when the asphaltenes are sufficiently diluted in the sample.9,12 This is illustrated in Figure 2 (bottom panel), showing that LDI spectra of asphaltenes diluted in a matrix show a drastic suppression of the signal from high mass peaks at high laser power. The main reason for this observation is that aggregation is suppressed as a consequence of the reduction of the frequency of many-body collisions in the plume. Furthermore, similar mass distributions were observed by the group of Martínez-Haya in LDI measurements on pure asphaltenes, in MALDI measurements with the asphaltene diluted in a matrix,9,10 or diluted in an inorganic Cu salt,10 or even in MALDI measurements performed on asphaltenes solved in a liquid matrix.12 In each of these cases, the ionization mechanism is expected to differ, at least partially. Other literature reports using LDI on asphaltenes also led to average molecular weights below 1000 g/mol, in agreement with Martínez-Haya and co-workers.33,34 Hence, it is very likely that HBK suffers from the aggregation effects shown in Figure 2, due to a lack of control of laser power and/ or of asphaltene surface mass density, leading to a misinter(32) Tanaka, R.; Sato, S.; Takanohashi, T.; Hunt, J. E.; Winans, R. E. Analysis of the molecular weight distribution of petroleum asphaltenes using laser desorption-mass spectrometry. Energy Fuels 2004, 18, 1405.


pretation of the collected spectra. It should be noted that in LDI/ MALDI experiments it is very common to look for the “sweet” spots of the sample, where the ion signal becomes optimum. Asphaltene LDI samples are typically very inhomogeneous, and it is likely that the highest intensity is recorded on the densest regions. In those regions, aggregation effects are also expected to be most effective, since the density of the desorption plume is also maximized thereby leading to large aggregation artifacts. Very recently, a variation of the standard LDI measurement was performed by Drew Pomerantz (Schlumberger), Professor Dick Zare (Stanford University), et al. using two-step laserdesorption/laser-ionization mass spectrometry (L2MS).35 In this experiment, the desorption and ionization steps are decoupled by using an IR laser for desorption followed by a UV laser for ionization. Because the IR laser photon energy is well below the ionization potential for any component of asphaltenes, L2MS produces a desorbed plume of neutral molecules. In this measurement, aggregation is greatly reduced. This technique is shown to detect porphyrin model compounds with minimal fragmentation and no detectable aggregation. Additionally, this separation of the desorption and ionization steps results in a robust experiment: no changes in the measured mass spectrum of asphaltenes or model compounds are detected as the power of the desorption laser, the power of the ionization laser, the time delay between laser pulses, and the sample concentration are varied over reasonable ranges. Especially important is the invariance of the measured mass spectrum with desorption power: high desorption power can be used, suggesting efficient desorption of all components of asphaltenes, without aggregation. Independent of measurement parameters, this experiment consistently returns an asphaltene MW distribution peaking in the range 500–600 amu.35 General Mass Spectrometry Results Here, we briefly consider results from various laboratories regarding asphaltene molecular weight. First, Boduszynski at Chevron originally employed field ionization mass spectrometry (FIMS) on crude oil asphaltenes and obtained average molecular weights of roughly 800 g/mol.36 Rodgers, Marshall, and coworkers at the National High Magnetic Field Laboratory (NHMFL) at Florida State University have created a unique facility to investigate petroleum.37 Primarily, they employ electrospray ionization (ESI) as the method of choice. ESI merits special consideration. This soft ionization technique was invented by John Fenn for which he was awarded the Nobel Prize in 2002.7 The huge impact of ESI is due to (1) the ionization technique is very soft so that fragmentation does not occur and (2) very large species (see below) can be lofted into the vapor phase.7 The basis of ESI is solvent evaporation, not heavy compound vaporization. The title of John Fenn’s Nobel Prize lecture is “Electrospray Wings for Molecular Elephants”.38 (33) Miller, J. T.; Fisher, R. B.; Thiyagarajan, P.; Winans, R. E.; Hunt, J. E. Subfractionation and characterization of Mayan asphaltene. Energy Fuels 1998, 12, 1290. (34) Yang, M.-G. Eser, S. ACS reprints. ACS New Orleans Meeting. 1999; No. 768. (35) Pomeranz, A. E. Hammond, M. R. Morrow, A. L. Mullins, O. C. Zare, R. N. Manuscript in preparation. (36) Boduszynski, M. M. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; American Chemical Society: Washington, DC, 1981; Chapter 7. (37) Marshall, A. G.; Rodgers, R. P. Petroleomics: The Next Grand Challenge for Chemical Analysis. Acc. Chem. Res. 2004, 37, 53–59. (38) John B. Fenn 2002 Nobel Lecture: “Electrospray Wings for Molecular Elephants,” http://nobelprize.org/nobel_prizes/chemistry/laureates/ 2002/fenn-ecture.html.

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In fact, Fenn performed the first analysis of petroleum by ESI with low-resolution mass spectrometry.39 At the NHMFL, ESI is routinely employed for ionization. There, the mass to charge ratio is determined by Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS).40 The NHMFL mass spectroscopy group employs up to a 14.5 T magnet at present and has the highest mass accuracy and highest mass resolution mass spectrometer in the world.41 This formidable combination of ESI with FT-ICR MS provides an extremely powerful tool to probe crude oil and its components.8,37,42 Asphaltenes are not close to pushing the limits of this methodology. Rodgers, Marshall, and co-workers have measured the molecular weight of various carbonaceous species such as asphaltenes,8,43 heavy oils,44–46 and interfacially active components of crude oil.47 The corresponding extensive results on the molecular weight of crude oil asphaltenes are summarized in their book chapter8 (and very briefly in Figure 1). They find the bulk of asphaltenes are between 400 and 800 Da with a range of 300–1400 Da.8 Other groups have used ESI FT-ICR MS on heavy petroleum with similar results.48 In addition, several groups have used other ionization methods and have obtained comparable results. Atmospheric pressure photoionization (APPI) and atmospheric pressure chemical ionization (APCI) measurements have been performed at the Institut Francais du Petrole by Merdrignac and co-workers.49 Their average asphaltene molecular weight varied between 500 and 800 Da. APPI ionizes both polar and nonpolar aromatics, whereas ESI is limited to polars. These results are in close agreement with other APCI measurements.50 Qian at ExxonMobil and co-workers also used field desorption (FD) (39) Zhan, D. L.; Fenn, J. B. Electrospray Mass Spectrometry of Fossil Fuels. Int. J. Mass Spectrom. 2000, 194, 197–208. (40) Marshall, A. G.; Hendrickson, C. L.; Jackson, G. S. Fourier Transform Ion Cyclotron Resonance Mass Spectrometry: A Primer. Mass Spectrom. ReV. 1998, 17, 1–35. (41) Schaub, T. M.; Hendrickson, C. L.; Blakney, G. T.; Quinn, J. P.; Senko, M. W.; Marshall, A. G. Performance Characteristics of a 14.5 Tesla LTQ FT-ICR Mass Spectrometer. Proceedings of the 55th American Society for Mass Spectrometry Annual Conference on Mass Spectrometry, Indianapolis, IN; American Society for Mass Spectrometry, Poster MPD063, 3– 7 June, 2007. (42) Rodgers, R. P.; Schaub, T. M.; Marshall, A. G. Petroleomics: Mass Spectrometry Returns To Its Roots. Anal. Chem. 2005, 77, 20A–27A (43) Klein, G. C.; Kim, S.; Rodgers, R. P.; Marshall, A. G.; Yen, A.; Asomaning, S. Mass Spectral Analysis of Asphaltenes. I. Compositional Differences between Pressure-Drop and Solvent-Drop Asphaltenes Determined by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Energy Fuels 2006, 20, 1965–1972. (44) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Resolution of 11,000 Compositionally Distinct Components in a Single Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrum of Crude Oil. Anal. Chem. 2002, 74 (16), 4145–4149. (45) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Qian, K.; Robbins, W. R. Identification of Acidic NSO Compounds in Crude Oils of Different Geochemical Origins by Negative Ion Electrospray Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Org. Geochem. 2002, 33, 743– 759. (46) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Reading Chemical Fine Print: Resolution and Identification of 3000 Nitrogen-Containing Aromatic Compounds from a Single Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrum of Heavy Petroleum Crude Oil. Energy Fuels 2001, 2, 492–498. (47) Stanford, L. A.; Rodgers, R. P.; Marshall, A. G.; Czarnecki, J.; Wu, X. A. Compositional Characterization of Bitumen/Water Emulsion Films by Negative- and Positive-Ion Electrospray Ionization and Field Desorption/Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Energy Fuels 2007, 21, 963–972. (48) Muller, H.; Andersson, J. T.; Schrader, W. Characterization of HighMolecular Weight Sulfur-Containing Aromatics in Vacuum Residues Using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Anal. Chem. 2005, 77, 2536–2543.


ionization.51 While these results are slightly higher than typical ESI results there is only a small discrepancy (see Figure 1).51 At this point, it is appropriate to correct two HBK misstatements about FT-ICR MS. First, HBK states: “However, collection and transmission into the ion trap of high-mass components from polydisperse mixtures (coal liquids and petroleum asphaltenes) seems to be incomplete.” First, whenever singly charged petroleum-derived ions of >1200 Da are observed in the linear ion trap, those ions are noncovalent multimers, because they can readily be dissociated into small oligomers or monomers at collision energies insufficient to break covalent bonds.52 Thus, high-mass ions are in fact readily collected in the external linear ion trap. Second, very similar molecular weight distributions are typically observed for FT-ICR and linear quadrupole ion trap mass spectra. Thus, the absence of highMW components in FT-ICR mass spectra is not due to lack of transmission of ions from the external ion trap to the FT-ICR cell. Second, HBK asserts: “... the practical upper limit of FTMS instruments in ESI mode appears to be m/z ∼1200 for singly-charged ions.” First, the upper mass limit for FT-ICR MS is determined by m/z, not m. Thus, the behavior of doubly charged ions of mass 2m is essentially the same as for singly charged ions of mass m. Second, we have in fact observed ions of up to m/z ∼ 12 000 at 9.4 T by FT-ICR MS. A particularly direct example is shown in Figure 3 (bottom), demonstrating broadband detection (2500 < m/z < 5000) of a mixture of dozens of singly charged carbon cluster ions.53 Thus, the FTICR MS upper m/z limit is clearly well above the observed upper mass range for singly charged ions seen in FT-ICR mass spectra of asphaltenes (e.g., the APPI spectrum in Figure 3, top). Third, given the high heteroatom content of species observed by electrospray ionization, one would expect to see multiply charged high-mass ions if they were present in petroleum (e.g., multiply charged DNA of up to 100 000 000 Da has been seen by FT-ICR MS54), whereas in fact virtually all electrosprayed ions from petroleum samples are singly charged, implying that very large masses are not present. It is important to recognize that mass spectrometry yields the molecular weight distribution of ions, not the precursor neutrals in the original sample. Variation in ionization efficiency (49) Merdrignac, I. Desmazieres, B. Terrier, P. Delobel, A. Laprevote, O. Proceedings, Heavy Organic Deposition, “Analysis of raw and hydrotreated asphaltenes using off-line and on-line SEC/MScoupling,” Los Cabos, Baja California, Mexico, 2004. (50) Cunico, R. I.; Sheu, E. Y.; Mullins, O. C. Molecular weight measurement of UG8 asphaltene by APCI mass spectroscopy. Pet. Sci. Technol. 2004, 22, 787. (51) Qian, K.; Edwards, K. E.; Siskin, M.; Olmstead, W. N.; Mennito, A. S.; Dechert, G. J.; Hoosain, N. E. Desorption and Ionization of Heavy Petroleum Molecules and Measurement of Molecular Weight Distributions. Energy Fuels 2007, 21, 1042–1047. (52) Smith, D. F.; Schaub, T. M.; Rahimi, P.; Teclemariam, A.; Rodgers, R. P.; Marshall, A. G. Self-Association of Organic Acids in Petroleum and Canadian Bitumen Characterized by Low- and High-Resolution Mass Spectrometry. Energy Fuels 2007, 21, 1309–1316. (53) Purcell, J. M. Hendrickson, C. L. Dunk, P. Kroto, H. W. Marshall, A. G. Carbon Cluster Structural Characterization by Gas-Phase Ion-Molecule Reactions in an FT-ICR Mass Spectrometer. Proceedings of the 55th American Society for Mass Spectrometry Annual Conference on Mass Spectrometry, Indianapolis, IN; American Society for Mass Spectrometry, Poster MPD068, 3–7 June, 2007. (54) Chen, R.; Cheng, X.; Mitchell, D. W.; Hofstadler, S. A.; Wu, Q.; Rockwood, A. L.; Sherman, M. G.; Smith, R. D. Trapping, Detection, and Mass Determination of Coliphage T4 DNA Ions of 108 Da by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Anal. Chem. 1995, 67, 1159–1163. (55) Groenzin, H. Mullins, O. C. Asphaltene molecular size and weight by time-resolved fluorescence depolarization. Chapter 2 in ref 1.


Figure 3. Top: Positive-ion atmospheric pressure photoionization 14.5 T FT-ICR broadband mass spectrum of Canadian bitumen asphaltenes. All ions are singly charged, as evidenced by the unit m/z spacing between 12Cn and 13C12Cn-1 isotopic variants with the same elemental composition. Only peaks whose magnitude exceeds 6σ of baseline noise are reported. Note the monomodal mass distribution, with average molecular weight of ∼800 Da. (Data provided by Brandie M. Ehrman, Tanner M. Schaub, and Ryan P. Rodgers from the NHMFL ICR Program.) Bottom: Broadband FT-ICR mass spectrum of singly charged carbon cluster positive ions formed by laser ablation of carbon in an external ion source. Ion accumulation and transfer are the same as for electrosprayed ions (e.g., petroleum and coal-derived samples). Note the large masses readily obtained for the carbon clusters. The zoom inset shows the C278 monoisotopic peak and its isotopic variants. Reprinted with permission from ref.53 Copyright 2007 American Society for Mass Spectrometry.

for different chemical classes can be profound. For example, positive (or negative) ion electrospray ionizes primarily organic bases (or acids) and is thus “blind” to the other ∼90% of the mixture (hydrocarbons, thiophenes, etc.). Atmospheric pressure photoionization increases the coverage to >50% for asphaltenes, primarily by ionizing aromatic species. Both methods are relatively free of fragmentation and aggregation. As noted above, the measure of MW for petroleum-based materials with laser desorption ionization can suffer from aggregation and fragmentation; these problems can be greatly mitigated with low sample density and low laser power density conditions. Low-resolution (non-ICR) mass spectrometry is also valuable, because it offers higher sensitivity and less mass discrimination. Therefore, studies involving FT-ICR-MS frequently employ (56) Ralston, C. Y.; Wu, X.; Mullins, O. C. Quantum yields of crude oils. Appl. Spectrosc. 1996, 50, 1563. (57) Wargadalam, V. J.; Norinaga, K.; Iino, M. Size and shape of a coal asphaltene studied by viscosity and diffusion coefficient measurements. Fuel 2002, 81, 1403.


low-resolution mass analysis first, to check that the mass distribution obtained by high-resolution FT-ICR MS is valid.8] Molecular Diffusion Measurement of asphaltene molecular diffusion has been very instructive regarding many asphaltene properties, but none so important as the asphaltene molecular weight (cf. Figure 1). The first asphaltene molecular diffusion measurements were performed by Groenzin and Mullins at Schlumberger-Doll Research13,14 and, in later work, with many co-workers.3,55 These early diffusion measurements indicate that petroleum asphaltene average molecular weights are 750 Da with a fwhm of 500-1000 Da. These diffusion measurements were performed using time-resolved fluorescence depolarization (TRFD), in which the excitation laser pulse polarizes the excited-state molecular ensemble. The rate of rotational molecular diffusion is determined as the measured rate of ensemble fluorescence depolarization with time. As correctly pointed out by HBK, the molecules must fluoresce for TRFD to work. The point missed in HBK is that very small quantum yields are compatible with making TRFD measurements. Fluorescence quantum yields of crude oils and asphaltenes obey the energy gap law.56 Thus, bigger chromophores have red-shifted fluorescence19,20 and are thus characterized by smaller quantum yields.56 Consequently, by tuning the excitation laser to long wavelength, one does not excite large quantum yield components, thereby enabling one to detect small quantum yield components.56 Thus, TRFD studies have been performed on low quantum yield components routinely.3,13,14,55 HBK claims that the “giant” molecules do not fluoresce. We note that solids suffer huge quenching effects even exhibited with many laser dyes. The putative lack of fluorescence observed in HBK is likely to be due to quenching in solid flocs. Subsequent to the TRFD studies on asphaltene molecular diffusion, there have been three other diffusion methods applied to asphaltenes; Taylor dispersion (TD), NMR translational diffusion, and fluorescence correlation spectroscopy (FCS). TD was used by Iino and co-workers at Tohoku University.57 In this technique, an initial perpendicular plane of asphaltene solution is prepared in a capillary. Laminar flow is initiated. The width of the measured parabolic asphaltene solution along the capillary is reduced by increased translational diffusion. The spatial location of the asphaltene solution is determined by an optical absorption measurement, that is, by asphaltene color. The strong coloration of asphaltenes is one of their canonical properties. As made clear by those authors in their paper, TD results agree exactly with those from TRFD for the exact same asphaltene samples.57 Translation diffusion based on NMR pulsed field gradient techniques has been employed by Freed and co-workers.27 Translational diffusion of molecules in a field gradient gives rise to local out-of-phase spins thereby reducing echo amplitude. NMR measurements of asphaltene diffusion constants are in reasonable agreement with the TRFD studies but have a somewhat larger width to the distribution.27 The NMR measurements were made in ∼50 mg/L asphaltene in toluene so there may have been some dimer formation.23 Of (58) Guerra, R.; Ladavac, K.; Andrews, A. B.; Sen, P. N.; Mullins, O. C. Diffusivity of Coal and Petroleum Asphaltenes Monomers by Fluorescence Correlation Spectroscopy. Fuel 2007, 86, 2016–2020. (59) Schneider, M. H.; Andrews, A. B.; Mitra-Kirtley, S.; Mullins, O. C. Asphaltene Molecular Size by Fluorescence Correlation Spectroscopy. Energy Fuels 2007, 21, 2875–2882. (60) Buenrostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. The overriding chemical principles that define asphaltenes. Energy Fuels 2001, 15, 972.

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course, the NMR measurements rely on the molecules having hydrogen. In addition, FCS measurements were made by Andrews at Schlumberger-Doll Research and Guerra at Harvard University and co-workers.22,58,59 FCS employs confocal imaging to measure the fluorescence intensity autocorrelation function vs time for roughly a cubic micron of an ultradilute asphaltene solution. The autocorrelation function depends on the rate of translational diffusion of molecules into and out of the imaged cubic micron of solution. The FCS results match closely the TRFD studies as made clear by the authors.22,58,59 In total, these diffusion measurements correspond to molecules with one of the following properties: fluorescence, color, and/or hydrogen. The only excluded “molecules” are nonfluorescent, type 1A natural diamonds! Conclusion The debate over asphaltene molecular weight has been going on too long; recent literature (HBK2) differs by orders of magnitude from other recent literature. The SEC work of HBK (ref 2) that purports to show the existence of “giant” megadalton asphaltene molecules likely suffers from substantial molecular aggregation because (1) the solvent they used is known to flocculate up to half of an asphaltene samples and is thus a very poor solvent for asphaltenes, (2) they do nothing to rule out any of the hierarchical asphaltene aggregate structures that are known to exist in a good solvent, toluene, let alone a poor solvent, and (3) they had fundamentally contradictory SEC data resulting from the solvent THF that lacked the “giant” molecule peak.4 There is no basis for preferring the molecular interpretation of the NMP data over the THF data. The unnatural discontinuous bimodal molecular weight distribution they infer from their NMP data is likely due to small aggregates and possible molecules in the lower mass peak and something approaching flocs in the large mass peak. The LDI work of HBK also likely suffers from asphaltene aggregation. The LDI work of Martínez-Haya and co-workers shows that laser surface power density, asphaltene surface mass density, and ion source operation all must be carefully controlled in order to avoid extensive gas phase aggregation for asphaltenes as well as for known polycyclic aromatic hydrocarbons. The LDI work in HBK lists almost no control over these parameters explaining the much larger masses they report for asphaltenes vs other references herein. We conclude that HBK suffers from substantial gas phase aggregation effects and that their molecular weight interpretation is in error. Fortunately, the current state of affairs for asphaltene molecular weight is that all ionization techniques for mass spectrometry yield concordant data including FIMS, FDMS, ESI FT-ICR MS, APPI MS, APCI MS, FD MS, and now LDI MS (when performed so as to avoid gas phase aggregation). In addition, all four reported methods for determination of asphaltene molecular diffusion constants, both translation and rotation, are in agreement, including TRFD, TD, NMR, and FCS. Moreover, all mass spectrometric methods are in agreement with all molecular diffusion methods. Petroleum asphaltenes have a number average molecular weight of ∼750 u ((200 u) with a range (fwhm) of 500-1000 u. There is now effort to clarify the small remaining differences among (61) Mullins, O. C.; Rodgers, R. P.; Weinheber, P.; Klein, G. C.; Venkatramanan, L.; Andrews, A. B.; Marshall, A. G. Oil Reservoir Characterization via Crude Oil Analysis by Downhole Fluid Analysis in Oil Wells with Visible-Near-Infrared Spectroscopy and by Laboratory Analysis with Electrospray Ionization-Fourier Transform Ion Cyclotron Resonance Mass Spectroscopy. Energy Fuels, 2007, 21, 256.


the different techniques and to clarify the variations in molecular weight of asphaltenes from different sources such as petroleum, bitumen, coal, and resid. Differences in molecular architecture play a large role in explaining these differences.60 There is a substantial effort to understand asphaltene molecular architecture in view of the molecular weight results. In fact, aggregation effects in LDI and other experimental approaches, far from representing a drawback,


provide a unique means to probe the supramolecular properties of asphaltenes and to study the hierarchy of cluster structures associated with them. In addition to impacting flow assurance,6 this new understanding of asphaltenes is likely to impact understanding of oil reservoir architecture, the largest unresolved issue today in deepwater development of oil.29,61 EF700714Z

Contrasting Perspective on Asphaltene Molecular ... - ACS Publications

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