And Matrix-Assisted Laser Desorption/Ionization

Egyptian Journal of Petroleum (2016) xxx, xxx–xxx

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FULL LENGTH ARTICLE Laser Desorption/Ionization-Time of Flight (LDI-TOF) and Matrix-Assisted Laser Desorption/ Ionization - Time of Flight (MALDI – TOF) mass spectrometry of an Algerian asphaltene

T. Fergoug a, C. Boukratem a,c, B. Bounaceur b, Y. Bouhadda a,* a Laboratoire de Chimie Physique des Macromole´cules et interfaces biologiques Faculte´ des Sciences, De´partement de Biologie, Universite´ de Mascara, Mascara 29000, Algeria b Laboratoire de Chimie Physique Macromole´culaires, Faculte´ des Sciences, De´partement de Chimie, Universite´ d’Oran, Oran 31000, Algeria c Faculte´ des Sciences, De´partement de Chimie, Universite´ d’Oran, Oran 31000, Algeria

Received 21 May 2016; revised 15 September 2016; accepted 19 October 2016

KEYWORDS Abstract Both LDI-TOF and MALDI-TOF Mass spectroscopy experiments of an Algerian Asphaltene; asphaltene derived from a deposit were performed. LDI mass experiments were conducted for both LDI-TOF mass linear and reﬂectron modes under laser wavelength/attenuation variation. The different LDI-Mass spectroscopy; spectra show that mass distribution depends on experimental condition for masses below 1000 amu MALDI-TOF mass and that the average molecular weight is around 650 for the polar fraction and beyond 1000 amu spectroscopy; for non-polar ones. The use of different matrices as CHCA, HABA and Dithranol changes slightly Laser attenuation; the aspect of the spectra. Mass distribution Ó 2016 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction the flocculation and deposition phenomena observed during recuperation and transportation processes [3]. They represent Asphaltenes are petroleum organic compounds that are insol- a class of material with varieties of molecular structures, rather uble in light paraffinic solvents and soluble in aromatic ones than a substance with a well-defined chemical formula [4–7]. such as toluene [1,2]. They are famous for their negative During these two past decades, various scientific works were impact on the oil industry because they are responsible of focused around the asphaltene chemical structure characterization and the latest conclusions confirm the continental-like * Corresponding author. structure claimed by Mullins and coworkers through their E-mail addresses: [email protected], bouhaddayoucef@ new modified-Yen model [8–10]. The new model is called univ-mascara.dz (Y. Bouhadda). Yen-Mullins model which suggests that an average molecular Peer review under responsibility of Egyptian Petroleum Research weight of the order of 700–800 amu for an asphaltene molecule Institute. is considered as the most viable compared to the Archipelago http://dx.doi.org/10.1016/j.ejpe.2016.10.017 1110-0621 Ó 2016 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: T. Fergoug et al., Laser Desorption/Ionization-Time of Flight (LDI-TOF) and Matrix-Assisted Laser Desorption/ Ionization - Time of Flight (MALDI – TOF) mass spectrometry of an Algerian asphaltene, Egypt. J. Petrol. (2016), http://dx.doi.org/10.1016/j.ejpe.2016.10.017 2 T. Fergoug et al. model [11] and permits to predict satisfactory asphaltene gra- of the aromatic structure. From these chemical structures and dients in oil reservoirs with the use of Flory-Huggins-Zuo using the aromaticity, average molecular weight ranging from equation of state. The main reason for the discrepancy 550 to 900 amu is deduced. So, in this work the LDI/MALDI between the two models is surely related to the sensitivity of TOF mass spectroscopy is used to confirm the molecular the characterization techniques used, the asphaltenes origin, weight deduced from the combination of the three methods their chemical structure polydispersity and their high tendency cited above and to search for the best experimental conditions to self-associate and flocculate even in dilute solutions [9–11]. for the determination of representative molecular weight for Anyway their real molecular weight (MW) value and their these asphaltenes. chemical structure still being the subject of various investiga- tion [12–14]. 2. Materials and methods To date, different techniques as tensiometry [6], mass spectrometry [15] and fluorescence spectroscopy [16] are considered 2.1. Sample extraction suitable and credible for MW determination however amongst them mass spectroscopy seems to be the most advanced tech- The asphaltene was extracted from the deposit sample by nique to overcome the difficulties related to the asphaltene n-heptane precipitation in a ratio of 40 ml n-heptane to 1 g peculiarities because of its ionization processes diversity and of the deposit. The mixture was stirred at 60 °C for 2 h and the high sensitivity of its detectors. In fact, from the basic elec- let to stand overnight at ambient temperature under stirring. tron ionization (EI) to the most sophisticated ones as Electro- The insoluble part was obtained by filtration through a spray ionization (ESI), laser and matrix laser desorption/ 0.45 lm membrane pore diameter and subsequently dissolved ionization (LDI/MALDI), atmospheric pressure photo/chemi- in enough amount of hot toluene (50 °C). The toluene cal ionization (APP/CI) and Fourier Transform Ion Cyclotron mixture was then re-filtered with the same kind of membrane Resonance Mass Spectrometry (FT-ICR/MS) this technique and the obtained solution was then dried in a Rotavapor. hasn’t cease progressing and many complex systems as humic The retrieved asphaltene powder was further air-dried in an substances, asphaltene and bitumen were revealed and their autoclave at 140 °C for one week to evaporate the trace compositional complexity outlined [17–22]. Recently, many amount of toluene. interesting LDI/MALDI time of flight mass spectrometry works were performed and different systematic procedures 2.2. Apparatus were proposed to set up the best experimental conditions to determine the asphaltene average molecular weight distribution [17–24]. The authors clearly highlighted the met difficul- The mass Spectrometer used is a Reflex VI Bruker Daltonics ties in obtaining reproducible resolved spectra because the using laser desorption ionization time of flight technique. It’s molecular weight distribution depends on many experimental equipped with a reflectron mode to obtain a better resolution conditions as Laser wavelength/power, method of preparation, for masses >10,000 and can also be used with a linear mode etc. [17–24]. It was shown that the matrices are not for ionic species with high molecular mass 8000. The SCOUT necessary for obtaining representative MS spectra of MTP system enables via computer an automated introduction asphaltene, they are their own matrices. Acevedo et al. [24] of the sample on the sample holder with an XY accuracy of l concluded from LDI-TOF MS experiments that asphaltenes 4 m. The desorption/ionization unit contains a pulsed Nitro- k are a mixture of A1-insoluble and A2-soluble type molecules gen Laser ( = 337 nm) with an attenuation system. It consists which could be identified as continental and archipelago type to extract and accelerate the derived laser-impact ions which molecules. They proposed that these types of asphaltene will be then separated on the bases of the time of flight needed associated with some trapped compounds (metalloporphyrins, to fly across the tube. Also another laser system (Nd-YAG) k free radicals, resins, and other crude oil components) represent with = 266 nm was used. With this latter system, post ion- the entire asphaltene sample. With these 3 types molecules ization sequence analysis can be done by using the laser to l they interpreted the bimodal form observed on the LDI-TOF ablate the sample at ablation onset and after a few s the sec- spectra. ond laser ionize the neutral molecules. In this work analysis The aim of this paper is to study LDI and MALDI mass under laser ablation condition were performed on asphaltene spectra of an Algerian asphaltene derived from a deposit. This sample without matrices at first. Subsequent, MALDI-MS asphaltene sample was already characterized by Raman spec- experiments with different matrices were conducted. The inves- a trometry and X-ray diffraction measurements where it was tigated matrices were -cyano-4-hydroxycinnamic acid possible to estimate an average estimate of the diameter of aro- (CHCA), 2-(4-hydroxy-phenylazo) benzoic acid (HABA) and matic sheet forming the asphaltene (La = 11.85 A˚) [25]. Also 1,8,9-anthracenetriol (Dithranol). from solid-state NMR, an average value of aromaticity was extracted from the analysis of the NMR spectra 2.3. Preparation (Ar = 57.5%) [26]. With this information in hand and from the theoretical calculation done by Ruiz-Morales [27] on some For LDI experiments the asphaltene is dissolved in toluene (at À representative polyaromatic hydrocarbon PAH using quantum a concentration of 0.001 g L 1) and directly deposited on the calculation, a chemical formula for the aromatic portion of sample holder for laser treatment. In the case of MALDI asphaltene was proposed and hence an average molecular experiments equal volumes of asphaltene and matrix were pre- weight for this sample can be deduced. In fact, according to pared for experiments. An external calibration of the mass Ruiz-Morales [28], aromatic sheet of a diameter equal to Spectrometer were performed with PEG 600 and PEG 1500, 11.54 A˚to 11.97 A˚corresponds to aromatic chemical formulae each spectra correspond to a cumulative of a minimum of ranging from C40H16 to C24H14 depending on the compactness 300 laser shoot (LS).

3. Results The occurrence of such fragmentation is questionable since when the laser energy decreases less desorption must be As already defined in introduction section, Asphaltene repre- achieved, and also less fragmentation. The peaks at (300– sents a complex mixture of compounds fulfilling the same sol- 400u) appear to be inevitable even at soft experimental condi- ubility criterion. Knowing that solubility of asphaltene is tions and may correspond to the side hydrocarbon chains governed by the balance between polarity and molecular brought by the aromatic portion of an asphaltene molecule weight [29], it is interesting to start experiments with the reflec- or to the trapped compounds (metalloporphyrins, free radi- tron mode because it is thought that it permit to scan the max- cals, resins, and paraffins) mentioned earlier by Acevedo imum domain of molecular weights. et al. [24]. Anyway, the molecular mass values of the large In this first experiment (Nd-YAG) laser with k = 266 nm band are of the same order as ones published by Pomerantz was used with no matrices employed (LDI) and a laser atten- et al. [20] and lighter than ones earlier reported in the literature uation level of 68% was chosen. The LDI Mass spectrum for by Acevedo et al. [17] and Tanaka et al. [23] with the same an attenuation level of 68% is visible in Fig. 1, intense peaks technique for other asphaltenes. appear at low molecular weight values (200–300u) suggesting The large band seen in Fig. 4 is interesting because its shape the occurrence of fragmentation but at higher molecular seems to result from the superposition of several bands and the weight domain no clear peaks are resolved except a relatively range (500–3500u) merits more attention. Matrices effect was weak large band. This behavior reveals that the starting exper- investigated with three different matrices for the mass domain imental conditions may be severe for this asphaltene sample raging from 500 to 4500u. The presence of a matrix is intended and that soft ones must be employed. primarily to vanish any fragmentation effects and hence bring To achieve this, one can either increase the laser attenua- out to us more details about what really exist in the sample. tion level or change the laser wavelength. For the second Indeed matrices will absorb the excess of energy and pre- experiment we begin by increasing the laser attenuation from serve the asphaltene molecules from laser heat. Figs. 5–7 con- 68% to 100% and maintaining the same laser wavelength. In tain the spectra for the sample with the corresponding matrices k Fig. 2, a different spectrum is obtained under these relative soft under the following experimental conditions ( = 337 nm, conditions and weaker probable fragmentation peaks are reflectron mode, 50% attenuation). The results indicate that observed at the 200–400 m/z range. matrices influence slightly the general aspect of the spectra To explore the laser wavelength effect the pulsed Nitrogen which agree with the conclusions of Tanaka et al. [23] and Ace- Laser (k = 337 nm) is used. Lowering the laser energy is sup- vedo et al. [17] about the use of matrices for asphaltene mole- posed to reduce the formation of fragments, however it can cules because we obtain almost the same feature with regard to lead to a different desorption/ionization mechanism and a pos- ones obtained without matrices. sible lack of some signals with regard to ones observed with However, it is important to underline that with matrices (Nd-YAG) laser. For the pulsed Nitrogen Laser best resolved bimodal spectra are obtained and the peaking (k = 337 nm), we performed experiments with two different is improved for the 700–900u range. From the MALDI spec- attenuation levels (40% and 80%). The 40% attenuation level tra, the molecular weight of asphaltene are generally situated LDI/TOF spectrum is visible in Fig. 3 and one can note that around 850 and 1300 but the complexity of the bands supports the fragmentation peaks are still present with almost the same the probable existence of other hidden average molecular band at higher mass values observed in Figs. 1 and 2 however weight distributions. The 1300 signal is interesting because it for the case of 80% attenuation level, the spectrum (see Fig. 4) may represent a dimer signal formed by two asphaltene mono- shows a significant effect on the shape and position of the band mers of 650 uma. This idea is supported by LDI mass works around 500–2000 (bimodal form) and surprisingly several conducted by Daaou et al. [30] on other Algerian asphaltene intense peaks at 200–400u. samples where they pointed out that the spectra corresponding

Figure 1 LDI Mass spectrum of DP45 asphaltene with (Nd-YAG) laser at k = 266 nm, 68% attenuation level and reﬂectron mode.

Figure 2 LDI Mass spectra of DP45 asphaltene with (Nd-YAG) laser at k = 266 nm, 100% attenuation level and reﬂectron mode.

Figure 3 LDI Mass spectra of DP45 asphaltene with Nitrogen laser at k = 337 nm, 40% attenuation level and reflectron mode. to the asphaltenes extracted from the crude oil with different Figs. 3 and 4 (Reflectron mode) disappeared attesting that polar solvents are quite different from their parent asphaltene the fragments may have an apolar nature and a new asymmet- (Toluene extracted). rical molecular weight distribution appears from 250 to 900u The parent asphaltenes spectrum contains two maxima at with a band maximum centered around 600. This reveals the 1000 and 2000u whereas for all sub-fractions an average mass contribution of a significant amount of polar asphaltenes in of 500u was observed. the sample and confirms the hypothesis about the occurrence To expand the exploration of experimental conditions we of several hidden molecular mass distributions. performed LDI experiments with the linear mode instead of reflectron one. This change can just alter the resolution but still a good alternative to investigate molecular masses especially 4. Discussion the polar ones. Figs. 8 and 9 contain the spectra for an attenuation of 100% for laser wavelength value of 266 nm and 80% To interpret the obtained results, one has to keep in mind that attenuation for less energetic laser with 337 nm wavelength several influencing factors related to the used laser such as value in linear mode. wavelength, attenuation, irradiated area and time pulses, affect The LDI spectrum at 266 nm in Fig. 8 (linear mode) seems LDI experiments. Furthermore, other parameters as matrix to be unaffected with mode change because almost the same nature and residual gas pressure, mass analyzer type and detec- aspect is observed compared to Fig. 2 (reflectron mode) with tor type can also change the results. These parameters will a little increase in intensities. However, for 337 nm laser under influence the mechanism of ablation/desorption process, ion- linear mode a completely different spectrum is observed in ization of molecules, their time of flight and hence the aspect Fig. 9. In fact the intense fragmentation signals observed in of mass spectra. This statement is true for any studied well

Figure 4 LDI Mass spectra of DP45 asphaltene with Nitrogen laser at k = 337 nm, 80% attenuation level and reﬂectron mode.

Figure 5 MALDI Mass spectra of DP45 asphaltene with Nitrogen laser at k = 337 nm, 50% attenuation level, HABA matrix and reflectron mode. defined system, so with a natural complex system such as desorption efficiency is observed when the laser intensity asphaltene it is not surprising to obtain intricate results. Beside reaches a critical value. Because an energy transfer from the this, for MALDI experiments, according to Guenther et al. matrix to the internal vibrational modes of the analyte molecule [31], because of the complexity and incomplete inconsistency occurs ejection of intact molecules takes place. The second of the experimental results, no generally accepted comprehen- model known as the pressure pulse theory [33] proposes that sive and universal model of MALDI mechanisms has been a pressure gradient is created normal to the surface and by proposed yet. In the case of MALDI experiments, two theories momentum transfer from collisions with fast-moving matrix have been developed to explain desorption of large molecules. molecules desorption of large molecules might be enhanced. The first one named thermal-spike model [32] proposes that as From these results, it is evident that experimental condi- a consequence of local heating at low laser fluence, the tions influence significantly the aspect of asphaltene LDI- matrix molecules sublime from the surface and a fast rise in TOF mass spectra and one can question if these MW values

Figure 6 MALDI Mass spectra of DP45 asphaltene with Nitrogen laser at k = 337 nm, 50% attenuation level, CHCA matrix and reﬂectron mode.

Figure 7 MALDI Mass spectra of DP45 asphaltene with Nitrogen laser at k = 337 nm, 50% attenuation level, Ditranol matrix and reflectron mode. correspond really to ones attributed to asphaltene. In fact, According to several asphaltene researchers [20–24], Acevedo et al. [24] have mentioned in their LDI experiments asphaltene molecules are their proper matrices since no that the bimodal form observed is due to the existence in the decisive modification were brought by the addition of asphaltene sample of trapped compounds and that it disap- the used matrices. For the first LDI experiment with pears when the sample is fractionated into pure A1-insoluble (Nd-YAG) laser at 68% of attenuation the whole distribution and A2-soluble fractions. However, Pomerantz, et al. have of the asphaltene matter should be revealed for the corre- assured in their LDI MS experiments that the molecular sponding experimental condition. However, the presence of distributions observed for asphaltene are not the results fragmentation peaks indicate that more than a sufficient of parasitic reactions occurring in the desorption/ionization ejection energy transfer occurs between asphaltene molecules chamber. playing in the same time the function of matrix and analyte.

Figure 8 LDI Mass spectra of DP45 asphaltene with (Nd-YAG) laser at k = 266 nm, 100% attenuation level and linear mode.

Figure 9 LDI Mass spectra of DP45 asphaltene with Nitrogen laser at k = 337 nm, 80% attenuation level and linear mode.

Even when the operating conditions changed via attenuation matrices is that they permit us to better resolve the distribu- of the laser energy with the pulsed Nitrogen Laser tion of the less polar fraction and to center the distribution of fragmentation peaks subsist. These observations highlight masses around 850u. the necessity to explore the process of ejection of asphaltene Furthermore, It is useful to underline the effect of the detec- molecules from the erupted aggregates generated by the laser tors in Figs. 8 and 9. From the corresponding spectra, it is shot, at least theoretically. From the MALDI results, it is obvious to state that the polar asphaltene fraction is centered obvious to note that CHCA, Dithranol and HABA matrices around 600u and contains the lighter asphaltene molecules have a signiﬁcant effect different from one brought by and that the more heavier ones situated beyond 1000u are asphaltene themselves. So from our point of view, the claim probably less polar. Finally, these molecular weight values that for asphaltene molecules the matrices are useless deduced from this technique are good estimates since they [23,24] is not completely true and one must always perform match satisfactorily with the one obtained from the combina- experiments with matrices because their contribution can be tion of three different methods (Raman, NMR and Quantum helpful as we report on our work. The contribution of the calculation) [25,26].

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