Energy Fuels 2010, 24, 6067–6073 Published on Web 10/14/2010
: DOI:10.1021/ef101139f
One- and Two-Step Ultraviolet and Infrared Laser Desorption Ionization Mass Spectrometry of Asphaltenes P. Hurtado, F. Gamez, and B. Martı´ nez-Haya* Departamento de Sistemas Fsicos, Quı´micos y Naturales, Universidad Pablo de Olavide, 41013 Seville, Spain Received August 23, 2010. Revised Manuscript Received September 20, 2010
The determination of reliable molecular-weight distributions of complex asphaltene fractions has challenged modern mass spectrometry. This investigation extends previous studies on the application in this field of one-step laser desorption/ionization (LDI) and two-step laser desorption/post-ionization (L2MS) techniques employing ultraviolet (UV) and mid-infrared (IR) wavelengths. In the LDI experiments, single-resonant UV (280 nm) or IR (3.45 μm) laser pulses are applied for desorption-ionization of a model asphaltene sample. In the L2MS approach, asphaltene neutrals are postionized in the gas phase by a high power UV (266 nm) laser pulse. The study confirms that one-step UV-LDI easily induces aggregation of the asphaltenes. In contrast, in one-step IR-LDI and two-step UV/UV-L2MS and IR/ UV-L2MS, aggregation is largely suppressed and the measurements reproduce correctly the known molecular-weight distribution of the model asphaltenes, with a bulk of constituents within m/z 200-1000. It is shown that the two-step methods also provide valuable information about the dynamics of the laser desorption process.
pressure ionization,11-13 field desorption,11,14,15 and laser desorption/ionization (LDI).11,13,16-23 Laser-based MS techniques have important advantages for asphaltene studies. They provide a simplified sample treatment, not requiring the addition of matrix or cationizing agents, high throughput and sensitivity, a large dynamic range capable of detecting families of compounds of different abundance, and a high mass resolution [M/ΔM ≈ 40 000 in current commercial time-of-flight (TOF) MS equipments]. In typical LDI measurements of asphaltenes with ultraviolet (UV) laser pulses, aggregation of neutral and ionized species occurs readily in the gas phase, leading to an apparent molecular-weight distribution (MWD) shifted to heavier masses.11,17-20 To reduce aggregation effects, the density of the desorbing plume must be kept sufficiently low by means of
1. Introduction Asphaltenes are commonly defined as the n-heptaneinsoluble, toluene-soluble fraction of crude oil or coal. Because of their technological relevance, much effort has been devoted to characterize the molecular structure of the complex and broad range of polar and nonpolar compounds encompassed by such a phenomenological definition.1,2 Modern mass spectrometry (MS) techniques have contributed crucially to the understanding of the structural and physicochemical properties of asphaltenes. Ultra-high-resolution Fourier transform ion cyclotron resonance (FT-ICR) measurements have identified the stoichiometry of thousands of polar compounds produced by different vaporization and ionization techniques.3-11 The FT-ICR experiments are in concordance with systematic MS studies based on atmospheric
(12) Cunico, R. I.; Sheu, E. Y.; Mullins, O. C. Pet. Sci. Technol. 2004, 22, 787. (13) Apicella, B.; Alfe, M.; Ciajolo, A. Combust. Sci. Technol. 2010, 182, 640. (14) Boduszynski, M. M. In Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; American Chemical Society: Washington, D.C., 1982; Vol. 195, Chapter 7, pp 119-135. (15) Qian, K.; Edwards, K. E.; Siskin, M.; Olmstead, W. N.; Mennito, A. S.; Dechert, G. J.; Hoosain, N. E. Energy Fuels 2007, 21, 1042. (16) Tanaka, R.; Sato, S.; Takanohashi, T.; Hunt, J. E.; Winans, R. E. Energy Fuels 2004, 18, 1405. (17) Hortal, A. R.; Martı´ nez-Haya, B.; Lobato, M. D.; Pedrosa, J. M.; Lago, S. J. Mass Spectrom. 2006, 41, 906. (18) Martı´ nez-Haya, B.; Hortal, A. R.; Hurtado, P. M.; Lobato, M. D.; Pedrosa, J. M. J. Mass Spectrom. 2007, 42, 701. (19) Hortal, A. R.; Hurtado, P. M.; Martı´ nez-Haya, B.; Mullins, O. C. Energy Fuels 2007, 21, 2863. (20) Hurtado, P. M.; Hortal, A. R.; Martı´ nez-Haya, B. Rapid Commun. Mass Spectrom. 2007, 21, 3161. (21) Pomerantz, A. E.; Hammond, M. R.; Morrow, A. L.; Mullins, O. C.; Zare, R. N. J. Am. Chem. Soc. 2008, 130, 7216. (22) Pomerantz, A. E.; Hammond, M. R.; Morrow, A. L.; Mullins, O. C.; Zare, R. N. Energy Fuels 2009, 23, 1162. (23) Sabbah, H.; Morrow, A. L.; Pomerantz, A. E.; Mullins, O. C.; Tan, X.; Gray, M. R.; Azyat, K.; Tykwinski, R. R.; Zare, R. N. Energy Fuels 2010, 24, 3589.
*To whom correspondence should be addressed. E-mail: bmarhay@ upo.es. (1) Asphaltenes, Heavy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007. (2) Mullins, O. C. Energy Fuels 2010, 24, 2179. (3) Marshall, A. G.; Rodgers, R. P. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 18090. (4) McKenna, A. M.; Purcell, J. M.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2010, 24, 2929. (5) Corilo, Y. E.; Vaz, B. G.; Simas, R. C.; Lopes Nascimento, H. D.; Klitzke, C. F.; Pereira, R. C. L.; Bastos, W. L.; Santos Neto, E. V.; Rodgers, R. P.; Eberlin, M. N. Anal. Chem. 2010, 82, 3990. (6) McKenna, A. M.; Purcell, J. M.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2009, 23, 2122. (7) Fernandez-Lima, F. A.; Becker, C.; McKenna, A. M.; Rodgers, R. P.; Marshall, A. G.; Russell, D. H. Anal. Chem. 2009, 24, 9941. (8) Shi, Q.; Hou, D.; Chung, K. H.; Xu, Ch.; Zhao, S.; Zhang, Y. Energy Fuels 2010, 24, 2545. (9) Hur, M.; Yeo, I.; Park, E.; Kim, Y. H.; Yoo, J.; Kim, E.; No, M. H.; Koh, J.; Kim, S. Anal. Chem. 2010, 82, 211. (10) Pinkston, D. S.; Duan, P.; Gallardo, V. A.; Habicht, S. C.; Tan, X. L.; Qian, K. N.; Gray, M.; Mullen, K.; Kentt€amaa, H. I. Energy Fuels 2009, 23, 5564. (11) Mullins, O. C.; Martı´ nez-Haya, B.; Marshall, A. G. Energy Fuels 2008, 22, 1765. r 2010 American Chemical Society
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sample dilution and attenuation of the laser pulse energy. In fact, the low critical nano-aggregation and flocculation concentrations of asphaltenes constitute major practical properties, affecting their viscosity, heat transfer, lubrication, and efficiency in the combustion of the fuel.1,24,25 The marked aggregation tendency of asphaltenes has also been the primary cause for a long misunderstanding concerning their MWD and molecular structure, as determined with different experimental techniques.11 MS experiments from different groups seem to have shed light into this controversy, converging to coal and petroleum asphaltenes having the bulk of constituents within the m/z 200-1000 range.3-10,12-15,17-23 It seems timely to stress that the present paper does not intend to reopen the debate around the actual molecular weight of asphaltenes, which is to a large extent settled (e.g., see refs 1, 2, 11, and 17-19). Our experiments have been conducted on an asphaltene sample (UG8) that has been extensively characterized in numerous previous investigations, leading to a MWD peaking at m/z 500-600 and having the bulk of constituents at masses smaller than 1000. Previous studies have demonstrated the feasibility of twostep laser mass spectrometry (L2MS) for the characterization of polyaromatic hydrocarbons26-31 and, more recently, also asphaltenes.21-23 In the latter experiments, far-infrared (farIR, 10.6 μm) CO2 laser pulses were applied for the desorption of neutral asphaltene compounds, followed by ionization in the gas phase with a delayed UV pulse. The IR laser desorption step does not produce an appreciable yield of asphaltene aggregates. The L2MS MWD of UG8 asphaltenes peaked at ca. m/z 60021,22 and showed a remarkable overall similarity to those typically obtained in the FT-ICR measurements on electrosprayed ions.3-5 In this study, we extend the applicability of the LDI and L2MS techniques in this field, employing UV (280 nm) and mid-IR (3.45 μm) laser pulses for the desorption step. These two wavelengths are resonant with electronic and vibrational (C-H stretch) excitations of the asphaltenes, respectively. It is shown that this methodology yields reliable MWDs of asphaltenes, similar to those reported previously with a far-IR CO2 laser for desorption. Hence, it is concluded that L2MS may be applied for asphaltene analysis with a broad range of UV and IR lasers. From a more fundamental point of view, it will be discussed that the evolution of the L2MS signal as a function of the time delay between the desorption and ionization pulses provides information about the dynamics of the UV and IR laser desorption processes.
refs 11, 12, and 19, where the extraction procedure is described; see also references therein). In particular, the same UG8 asphaltene fraction was analyzed in the previous far-IR L2MS investigations.21,22 For the present study, UG8 pellets were prepared by pressing a fine powder mixture of the asphaltene and KBr salt with a 1:5 weight ratio. 2.2. LDI and L2MS Methods. LDI and L2MS measurements were performed in a customized TOF mass spectrometer employed in a number of previous experiments on asphaltenes.17-20 Only a brief description of the equipment is provided here. The laser is focused on the sample pellet, which is inserted in the repeller plate of the ion source. Lenses with focal lengths of 25 cm for the UV and 20 cm for the IR were employed. The sample was rotated during the course of the measurement, so that a fresh area of the sample is exposed continuously. The positive adduct ions, either desorbed from the sample or produced by laser post-ionization, are accelerated with a net voltage of 4.2 kV into the mass spectrometer operated in linear TOF mode. In the LDI measurements, the extracting voltage is applied with a 1 μs delay with respect to the laser pulse. In the L2MS measurements, the extracting voltage is applied continuously to separate any potential LDI background ions from the post-ionized neutrals. UV laser radiation comes from a Nd:YAG laser (Minilite II, Continuum) and from a tunable dye-laser system (ScanMate, Lambda Physik). Tunable IR laser radiation is obtained from a Nd:YAG-pumped optical parametric oscillator/amplifier (OPO/OPA, LaserVision). Laser desorption is induced with UV (280 nm, 1-10 μJ/pulse, and 7 ns pulse width) or IR (3.45 μm, 0.5-3 mJ/pulse, and 7 ns pulse width) laser pulses. Post-ionization of the desorbed neutrals is achieved by means of two-photon absorption induced by 266 nm laser pulses (3 mJ/pulse and 7 ns pulse width), focused in the plume at 3.0 ( 0.5 mm downstream from the sample surface. The delay between the desorption and the ionization lasers was varied within 1-100 μs. All mass spectra were averaged over 128 laser shots to produce a sufficient signal-to-noise ratio. 2.3. Spectrophotometry. The UV-vis absorption spectrum of a thin film of the UG8 asphaltenes deposited on quartz from a toluene solution was measured (after complete solvent evaporation) in an optical fiber CCD spectrometer (DH2000, Ocean Optics). The IR spectrum was measured on a thin UG8/KBr pellet in a FTIR spectrometer (Jasco 480).
3. Results and Discussion Asphaltenes are visually black or dark-brown and are known to have a broad optical absorbance in the visible and near-UV ranges because of their polyaromatic cores.32 This is well-illustrated by the optical absorption spectrum of the UG8 asphaltenes employed in this study, shown in Figure 1. This property represents a substantial advantage for laser MS analysis, because it allows for resonant electronic excitation of the molecules, leading to desorption/ionization, with a variety of wavelengths and without the addition of a chromophore matrix. The present experiments employ 280 nm laser pulses for desorption (or desorption/ionization), and 266 nm pulses for post-ionization. Both wavelengths are associated with a strong absorbance by the asphaltenes, as can be appreciated in Figure 1. The polyaromatic and related functional groups of asphaltenes also display a remarkable absorbance in the mid- and far-IR. This property has been employed in the past to characterize their chemical composition and aggregation propensity, as well as for analytical purposes.33-35 Pomerantz and
2. Experimental Section 2.1. Sample Asphaltene. The UG8 asphaltenes from Kuwaiti crude oil have constituted in a model system for numerous physicochemical and MS investigations in the last few years (e.g., see (24) Andreatta, G.; Bostrom, N.; Mullins, O. C. Langmuir 2005, 21, 2728. (25) Indo, K.; Ratulowski, J.; Dindoruk, B.; Gao, J. L.; Zuo, J. L.; Mullins, O. C. Energy Fuels 2009, 23, 4460. (26) Kovalenko, L. J.; Maechling, C. R.; Clemett, S. J.; Philipoz, J. M.; Zare, R. N.; Alexander, C. M. O. Anal. Chem. 1992, 64, 682. (27) Focsa, C.; Mihesan, C.; Ziskind, M.; Chazallon, B.; Therssen, E.; Desgroux, P.; Destombes, J. L. J. Phys: Condens. Matter 2006, 18, S1357. (28) Mihesan, C.; Ziskind, M.; Therssen, E.; Desgroux, P.; Focsa, C. Chem. Phys. Lett. 2006, 423, 407. (29) Mihesan, C.; Ziskind, M.; Chazallon, B.; Therssen, E.; Desgroux, P.; Gurlui, S.; Focsa, C. App. Surf. Sci. 2006, 253, 1090. (30) Mihesan, C.; Ziskind, M.; Therssen, E.; Desgroux, P.; Focsa, C. J. Phys.: Condens. Matter 2008, 20, 025221. (31) Faccinetto, A.; Thomson, K.; Ziskind, M.; Focsa, C. Appl. Phys. A: Mater. Sci. Process. 2008, 92, 969.
(32) Ruiz-Morales, Y.; Mullins, O. C. Energy Fuels 2009, 23, 1169. (33) Wilt, B. K.; Welch, W. T. Energy Fuels 1998, 12, 1008. (34) Rodrigues Coelho, R.; Hovell, I.; Lopez Moreno, E.; Lopes de Souza, A.; Rajagopal, K. Pet. Sci. Technol. 2007, 25, 41. (35) Tay, F. H.; Kazarian, S. G. Energy Fuels 2009, 23, 4059.
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Figure 2. One-step UV-LDI (top) and IR-LDI (bottom) mass spectra of the UG8 asphaltenes. The UV (280 nm) and IR (3.45 μm) energies per laser pulse applied to obtain each spectrum are indicated. Note that the values given in the bottom axis are mass units divided by 1000.
Figure 1. UV and IR absorption spectra of the UG8 asphaltenes. The UV spectrum was measured on a thin asphaltene film deposited on quartz. The IR spectrum was measured on a thin asphaltene/KBr pellet. The arrows indicate the laser wavelengths employed in the present LDI and L2MS experiments for desorption, 280 nm and 3.45 μm (2900 cm-1), and post-ionization, 266 nm.
of ionic and neutral molecules.37 Operating with diluted asphaltene samples was shown to be crucial in UV-LDI. Only at sufficient dilution, the MWD obtained in the mass spectrum corresponded to the monomeric compounds and became roughly independent of the laser power deposited on the sample. Because the present experiments are performed on a pellet of concentrated asphaltene powder, pronounced aggregation effects can be expected in the LDI process. The UV-LDI spectra shown in Figure 2 demonstrate that this is indeed the case. The UV-LDI measurements invariably lead to a broad adduct distribution, with a maximum around m/z 1000 and a long tail extending to a few thousand mass units. Such distribution deviates from the known MWD of the UG8 asphaltenes. Given the relatively high concentration of UG8 in the sample pellet, aggregates are formed during the LDI process at all laser pulse energies above the ionization threshold. 3.2. IR-LDI (Bottom of Figure 2). IR laser desorption MS constitutes a consolidated tool in chemical and biomolecular analyses.27,38-43 The IR-LDI technique has been extensively applied for the resonant desorption of polyaromatic compounds in solid samples.27-29 However, we are not aware of any previous application to the characterization of complex
co-workers21,22 demonstrated the feasibility of laser desorption of asphaltenes with far-IR wavelengths (the 10.6 μm output of a CO2 laser). The present study extends those previous studies by employing a mid-IR laser wavelength of 3.45 μm (2900 cm-1), in this case resonant with the C-H stretching modes of the asphaltenes, as shown in Figure 1. Our experiments encompass four types of laser mass spectrometric measurements, namely, UV-LDI at 280 nm, IRLDI at 3.45 μm, and UV/UV-L2MS and IR/UV-L2MS employing those same two wavelengths for desorption and 266 nm laser pulses for post-ionization. In the following section, the main results obtained with each technique are discussed. As already anticipated, the measurements are performed on the UG8 asphaltene fraction, which has constituted a model system for numerous investigations in the last few years,11,12,19 including L2MS studies,21,22 leading to a MWD peaking around m/z ∼ 500-600. 3.1. UV-LDI (Top of Figure 2). Most of the previous studies of our group on UV-LDI of asphaltenes have been devoted to outline strategies aiming to avoid aggregation effects in the dense plume formed in the early stages of laser desorption.17-20 Aggregation may arise from the covalent association of asphaltene photofragments36 and the non-covalent clustering
(38) Dreisewerd, K.; Berkenkamp, S.; Leisner, A.; Rohlfing, A.; Menzel, C. Int. J. Mass Spectrom. 2003, 226, 189. (39) Menzel, Ch.; Dreisewerd, K.; Berkenkamp, S.; Hillenkamp, F. Int. J. Mass Spectrom. 2001, 207, 73. (40) Rousell, D. J.; Dutta, S. M.; Little, M. W.; Murray, K. K. J. Mass Spectrom. 2004, 39, 1182. (41) Zhigilei, L. V.; Garrison, B. J. J. Appl. Phys. 2000, 88, 1281. (42) Leisner, A.; Rohlfing, A.; Berkenkamp, S.; Hillenkamp, F.; Dreisewerd, K. J. Am. Soc. Mass Spectrom. 2004, 15, 934. (43) Little, M. W.; Laboy, J.; Murray, K. K. J. Phys. Chem. C 2007, 111, 1412.
(36) Cristadoro, A.; R€ader, H. J.; M€ ullen, K. Rapid Commun. Mass Spectrom. 2007, 21, 2621. (37) Rodgers, R. P.; Marshall, A. G. In Asphaltenes, Heavy Oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; Chapter 3, pp 63-93, DOI: 10.1007/0-38768903-6.
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spectrum. The desorbed asphaltene neutrals were postionized by 266 nm pulses of 3 mJ energy. At this latter wavelength, resonance-enhanced 1 þ 1 two-photon ionization of the polyaromatic asphaltene components takes place.21-23,44 Figure 3 shows a representative series of UV/UV-L2MS spectra of the UG8 asphaltene. It can be noted that the resolution of the mass spectra is sufficient as to display a progression of well-resolved peaks within the m/z 300-800 range, with a regular spacing of ca. 24 mass units. Similar peak progressions have been observed in previous LDI studies, where it was assigned to asphaltene molecules incorporating additional rings in their polyaromatic cores.45 A significant signal of the asphaltenes is only detected at time delays between the desorbing and ionizing laser pulses within 5-35 μs. Remarkably, two groups of desorbed asphaltenes are detected, a first group of faster adducts within the time delay window of 5-22 μs and a second group of slower adducts within 28-40 μs. A much weaker adduct signal is observed at intermediate time delays (23-27 μs). This feature can be appreciated in Figure 5, which shows the integrated ionic yield of asphaltene detected at the different time delays. The presence of the fast and slow desorption components was systematically observed in independent experimental runs, and its origin is discussed below. It is gratifying to observe that the MWD of the faster asphaltene component resembles the known MWD of the UG8 asphaltene, peaking at m/z 500-600. The second group of desorbed products, detected at the longer delay times, displays a somewhat broader MWD with a more significant contribution from aggregates at m/z 1000. As a consequence, the average molecular weight of the detected ionic adducts increases from 650-750 in the LDI spectra taken within the time delay window of 5-22 μs to 900-1000 for those taken within 30-40 μs. In any case, the low fluence applied for desorption UV/UV-L2MS techniques aids at maintaining a much lower degree of aggregation than in one-step UV-LDI (Figure 2). In UV-LDI, higher laser fluences are mandatory to produce ions, with the collateral effect of also increasing plume density and enhancing clustering collisions. 3.4. IR/UV-L2MS (Figure 4). The two-step IR/UV-L2MS experiments are illustrated in Figure 4. In this case, the 3.45 μm laser pulses applied for desorption were attenuated to 0.5 mJ energy (5 J cm-2 fluence). In this way, the LDI ion yield from the IR laser alone was negligible in comparison to the ion yield of the two-step process. An advantage of this approach is the significantly smaller sample consumption in comparison to one-step IR-LDI. The desorbed asphaltene neutrals were again post-ionized by delayed 266 nm and 3 mJ pulses. Figure 4 shows that IR/UV-L2MS yields reliable mass spectra for the asphaltenes, without a significant signal from aggregates at larger masses. Such mass spectra are in fact qualitatively similar to those from the present IR-LDI and UV/UV-L2MS measurements and those obtained in the earlier far-IR/UV-L2MS measurements.21,22 Two main differences are observed between the L2MS measurements performed with the IR and UV desorbing lasers. First, the use of IR laser pulses leads to a reduced mass resolution as a consequence of the more bulky volume of material removed from the sample, leading to a broader spatial and kinematic spread of the ions and, possibly also, to space charge effects.38,41 Second, IR laser desorption
Figure 3. Two-step UV/UV-L2MS spectra of the UG8 asphaltenes at different time delays between the desorption (280 nm, 1.5 μJ) and the post-ionization (266 nm, 3 mJ) lasers. The corresponding time delay is indicated in each panel.
carbonaceous mixtures, such as asphaltenes. Figure 2 shows IR-LDI mass spectra of the UG8 asphaltenes recorded at two different laser energies. The wavelength is tuned at 3.45 μm, leading to resonant excitation within the C-H stretching band of the molecules (see Figure 1). Pulse energies of 1-3 mJ, corresponding to fluences (laser energy deposited per unit area) of ca. 10-30 J cm-2, were required to obtain a well-defined MWD of the asphaltenes. Off-resonance irradiation demanded higher laser energies for ion detection and was not explored here in a systematic way. Interestingly, the IR-LDI mass spectrum peaks at m/z 500-600, as expected for the UG8 sample, and displays only a weak signal from asphaltene aggregates at m/z above 1000 at all of the laser energies applied. Suppression of asphaltene aggregation in IR-LDI, in contrast to UV-LDI, is likely to be related to two main effects: (i) the less efficient formation of reactive photofragments in comparison to UV-LDI and (ii) the overheating and subsequent explosive vaporization of the asphaltene sample by the IR laser pulse,38-41 which would reduce the formation of non-covalent clusters. These aspects are discussed below in some detail. 3.3. UV/UV-L2MS (Figure 3). Two-step UV/UV-L2MS experiments were carried out with pairs of UV pulses time delayed from each other. Pulses of 280 nm were applied for desorption, at an energy of 1.5 μJ (a fluence of ∼0.015 J cm-2), lying just below the threshold for ion appearance in the mass
(44) Haefliger, O. P.; Zenobi, R. Anal. Chem. 1998, 70, 2660. (45) Rizzi, A.; Cosmina, P.; Flego, C.; Montanari, L.; Seraglia, R.; Traldi, P. J. Mass Spectrom. 2006, 41, 1232.
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of a weak second component of slow products, eventually spread over a broad range of ejection times, would be difficult to detect and cannot be ruled out. Nevertheless, a tentative explanation for the observed single-component desorption mechanism is provided below. 3.5. UV and IR Laser Desorption and Aggregation Mechanisms. The mass spectra of the UG8 asphaltenes obtained within this work display distinct features that suggest different desorption mechanisms induced by the UV and IR lasers. As one main effect of analytical relevance, UV-LDI on concentrated samples produces a large fraction of asphaltene aggregates at masses above m/z 1000. Such a high mass ion yield is largely suppressed in the IR-LDI and UV/UV-L2MS and IR/UVL2MS spectra. Furthermore, the two-step UV/UV-L2MS experiments have revealed the production of at least two groups of desorbed asphaltene molecules that leave the irradiated volume with different axial speeds. In contrast, a singlecomponent desorption process has been observed in the IR/ UV-L2MS measurements. The photochemical and desorption mechanisms responsible for these effects are complex, and their rationalization at a microscopic level lies well beyond the scope of the present work. Nevertheless, some speculative considerations are outlined below, which are based on the current knowledge in the field. As already advanced earlier in this section, aggregation in UV-LDI may arise from the covalent association of asphaltene photofragments and also from the non-covalent clustering of ionic and neutral molecules. Previous UV-LDI measurements on model polyaromatic compounds have shown that dehydrogenated photofragments undergo rapid covalent oligomerization at sufficiently high plume densities.36 A similar route for aggregate formation is also plausible for asphaltenes, although its rate remains difficult to predict because of the higher complexity of asphaltenes (e.g., the presence of side chains) in comparison to the model compounds employed in the previous study. On the other hand, the strong trend for noncovalent aggregation is a recognized property of asphaltenes.1 Clusters of proven non-covalent character have been observed in electrospray measurements,37 demonstrating that asphaltenes have an intrinsic propensity to aggregate in both solution and the gas phase (without the action of UV radiation). We conclude that in the LDI measurements, both covalent and noncovalent aggregates are likely to be formed. Systematic experiments including controlled collisional fragmentation of the aggregates formed in LDI are encouraged to discern between covalent and non-covalent species. Such experiments are, however, outside the scope of the present study. In a scenario in which the covalent oligomerization route dominates, it is straightforward to rationalize the observation that aggregation is dramatically reduced when IR wavelengths are employed for desorption; photofragments would be largely suppressed, and the only reactive species would arise from less efficient thermal mechanisms. In addition, there is also a plausible argumentation for a less efficient stabilization of noncovalent clusters in IR-LDI in comparison to UV-LDI. IR laser desorption typically results in thermal confinement conditions;38-41 i.e., a negligible dissipation of heat away from the irradiated sample volume takes place during the action of the laser. Severe thermal confinement results in the overheating of the absorbing material beyond its boiling temperature, leading to explosive vaporization.38,41 Such a desorption mechanism would account for the suppression of aggregates observed in the present IR-LDI measurements. The time required for energy dissipation scales with the square of the penetration
Figure 4. Two-step IR/UV-L2MS experiments of the UG8 asphaltenes at different time delays between the desorption (3.45 μm and 0.5 mJ) and the post-ionization (266 nm and 3 mJ) lasers. The corresponding time delay is indicated in each panel.
Figure 5. Integrated signal of asphaltene ions detected in the twostep UV/UV-L2MS (0) and IR/UV-L2MS (O) experiments at different delays between the desorption and post-ionization laser pulses. Both curves are normalized to have a maximum yield value of unity. Whereas the UV/UV-L2MS measurement leads to two components of ejected material, in the IR/UV-L2MS measurements only the faster of the two components is observed.
leads to a single component of ejected material, detected with post-ionization delays of ca. 4-22 μs. As can be appreciated in Figure 5, a negligible signal from asphaltene adducts was detected at time delays longer than 25 μs in the IR/UV-L2MS experiments. This result was corroborated by several repetitions of the experiment under similar conditions. The presence 6071
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depth and, for the present case, can be estimated to be on the order of microseconds for the IR laser pulses and to lie in the sub-microsecond range for the UV pulses.46 Hence, whereas thermal confinement conditions hold in the present IR laser desorption measurements, partial heat dissipation may take place in the UV laser desorption process. Partial stress confinement conditions38,41 may also affect the present IR-LDI measurements (but not the UV-LDI measurements).47 Stress confinement occurs when the dissipation time of the pressure waves formed upon the local heating on the material is longer than the laser irradiation time. The result is that violent spallation of the material takes place at the initial stages of desorption, plausibly further contributing to the disaggregation of asphaltene clusters. When two-step UV/UV-L2MS or IR/UV-L2MS is used, aggregation effects are largely suppressed. This can be mainly attributed to the small laser fluences applied for desorption in these measurements, leading to diluted plumes with reduced collisional cooling and stabilization of asphaltene clusters. Furthermore, it has been argued that the neutral adducts must display a smaller propensity to aggregate in comparison to the ionized asphaltenes produced in LDI.21 This would be a consequence of the weaker intermolecular interactions between the neutrals in comparison to the stronger charge and dipolar interactions introduced by the ionic compounds. Such tentative argumentation would also be consistent with previous UV-LDI experiments, showing that an enhancement of the clustering of the asphaltenes takes place when the ionic asphaltenes are accelerated through the dense desorbing plume by means of the application of continuous extraction voltages.17,18 Multi-component velocity distributions, qualitatively similar to the ones observed in the present UV/UV-L2MS experiments, have been observed in previous LDI investigations of low-molecular-weight species.27,42,48,49 A mechanistic interpretation has emerged that assigns the faster component to a rapid hyperthermal vaporization and the slower component to a subsequent thermal ejection of the remaining irradiated material, in the form of slower adducts, after some heat dissipation has occurred.27 This idea would be consistent with the MWD presently observed for the fast and slow components of the desorbed asphaltenes. The monomeric MWD of the fast component agrees with overheating and rapid vaporization, preventing extensive aggregation of the asphaltenes. The enhanced
aggregation observed in the second group of neutral asphaltenes can be traced back to the presumably lower temperatures and higher densities achieved in the desorbing plume. In fact, the integrated ion yield at the maximum of the slow desorption component is roughly 1.5 times greater than that of the fast component (see Figure 5). In the present IR/UV-L2MS measurements, a single component of desorbed asphaltenes is observed within a range of laser delays similar to that of the fast component in the UV/ UV-L2MS measurements. The absence of the slower component of desorbed products under the present working condition results may be related to the large penetration depth of the IR laser in the asphaltene sample. Because thermal confinement conditions apply,46 most of the irradiated sample volume would be uniformly overheated and vaporized, thereby leaving a reduced amount of activated material for a second wave of ejected material. 4. Summary and Conclusions The main aim of this study has been to provide an extended overview on the feasibility and limitations of LDI and L2MS laser MS for the characterization of the MWD of asphaltenes. Whereas recent investigations have consolidated these techniques in the field,17-22 the present experiments demonstrate the wide range of laser wavelengths that may be applied in this type of analysis. In particular, the combination of these results with the previous investigations shows that different types of lasers ranging from the near-UV to the far-IR may be employed. Aggregation of the asphaltene adducts in the plume remains as the main effect interfering with an accurate MWD determination. In this sense, this study has corroborated that aggregation occurs readily in UV-LDI in concentrated asphaltene samples. This is in agreement with previous investigations that established that high dilution of asphaltenes is required for UV-LDI measurements.17-19 According to the present results, this caution is not so relevant in IR-LDI, in which the formation of aggregates is largely reduced. The suppression of cluster formation when IR laser desorption is employed presumably results from two complementary effects: (i) a drastic reduction of reactive fragments, leading to covalent oligomerization, and (ii) non-covalent aggregates that are not stabilized because of the overheating of the material under the thermal and partial stress confinement conditions expected from the IR laser irradiation of the sample. Hence, IR-LDI arises as a relatively simple method for the characterization of the MWD of asphaltenes and related carbonaceous compounds. When two-step UV/UV-L2MS or IR/UV-L2MS is employed, aggregation effects are largely suppressed. Both types of L2MS experiments can be conclusively used to obtain reliable MWD of monomeric asphaltenes, without clustering of adducts even in dense samples. L2MS experiments are also weakly dependent upon the laser energy employed for desorption, as long as it is maintained below the LDI threshold. The lack of aggregation effects in the L2MS measurements is to a large extent a consequence of the small laser fluences applied for desorption. This results in a low-density desorption plume with limited collisions and cooling of the asphaltenes, preventing cluster formation. UV/UV-L2MS presents several advantages with respect to its IR/UV-L2MS counterpart. On one hand, a broader range of UV lasers than IR lasers are commercially available, allowing for a simpler and less expensive implementation of the technique. At least for asphaltenes, the UV wavelength employed is not crucial because of
(46) The time scale for heat dissipation is given by τth ∼ Lp2DT-1, where Lp denotes the laser penetration depth and DT denotes the thermal diffusivity of the irradiated material.38,41 Lp can be expected to be on the order of several micrometers at 3.45 μm and in the sub-micrometer range at 280 nm. On the basis of typical thermodynamical parameters of asphalt, DT can be estimated to lie within the range of 0.5-1 10-6 m2 s-1. It follows that τth is on the order of microseconds for the IR and in the sub-microsecond range for the UV. These values are in both cases larger than the 7 ns duration of the laser pulses, and thermal confinement conditions are fulfilled, although to a lesser extent in the case of the UV pulses. (47) The time scale for the dissipation of stress because of pressure waves formed upon the local heating of the material is given by τs ∼ Lpvs-1, where Lp denotes the laser penetration depth and vs denotes the speed of sound in the material.38,41 Considering Lp on the order of several micrometers in our IR-LDI measurements and vs ∼ 2000 m s-1 for asphaltenes, values for τs within 1-3 ns are obtained. Because the 7 ns duration of the laser pulses applied in the present experiments lies within the same order of magnitude, partial stress confinment conditions might be expected in the IR-LDI process. (48) Natzle, W. C.; Padowitz, D.; Sibenert, S. J. J. Chem. Phys. 1988, 88, 7975. (49) Tsai, S. T.; Chen, C.-H.; Lee, Y. T.; Wang, Y.-S. Mol. Phys. 2008, 106, 239.
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the broadband absorption of the molecules (Figure 1). In addition, a higher mass resolution is obtained than with the IR/ UV-L2MS technique. UV/UV-L2MS measurements should be performed at short time delays between the desorption and post-ionization lasers. In this way, the leading pulse of the desorbed material is probed, and aggregation effects are more easily avoided. We close by remarking that the development of the laser MS techniques explored in the present study would largely benefit from systematic research devoted toward obtaining a deeper insight into the underlying photochemical and desorption
mechanisms. The understanding of such mechanisms should guide the application of laser energy deposition conditions, providing optimum analytical sensitivity while maintaining a negligible interfering signal from asphaltene clusters. Acknowledgment. The authors are thankful to Klaus Dreisewerd (M€ unster, Germany) for discussion during the elaboration of the manuscript. The research leading to these results has received funding from the Regional Government of Andalucı´ a (Projects P07-FQM-02600 and P09-FQM-4938) and from the Ministry of Science and Innovation of Spain (Projects CTQ2009-10477 and CSD2009-00038).
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