FT-ICR MS analysis of asphaltenes: Asphaltenes go in, fullerenes come out

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Fuel xxx (2014) xxx–xxx

JFUE 8043 No. of Pages 10, Model 5G

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Contents lists available at ScienceDirect

Fuel

journal homepage: www.elsevier .com/locate / fuel

FT-ICR MS analysis of asphaltenes: Asphaltenes go in, fullerenes comeout

http://dx.doi.org/10.1016/j.fuel.2014.04.0490016-2361/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors at: Federal Institute of Education, Science and Technol-ogy of Espírito Santo, 29106-010 Vila Velha, ES, Brazil. Tel.: +55 27 3149 0833(W. Romão). Tel.: +55 62 3521 1016x261 (B.G. Vaz).

E-mail addresses: [email protected] (B.G. Vaz), [email protected](W. Romão).

Please cite this article in press as: Pereira TMC et al. FT-ICR MS analysis of asphaltenes: Asphaltenes go in, fullerenes come out. Fuel (2014),dx.doi.org/10.1016/j.fuel.2014.04.049

Thieres M.C. Pereira a, Gabriela Vanini a, Lílian V. Tose a, Felipe M.R. Cardoso b, Felipe P. Fleming b,Paulo T.V. Rosa c, Christopher J. Thompson d, Eustáquio V.R. Castro a, Boniek G. Vaz e,⇑,Wanderson Romão a,f,⇑a Petroleomic and Forensic Chemistry Laboratory, Department of Chemistry, Federal University of Espírito Santo, 29075-910 Vitória, ES, Brazilb Petróleo Brasileiro S/A – PETROBRAS, CENPES, Rio de Janeiro, RJ, Brazilc Chemistry Institute, University of Campinas, Campinas, Brazild Bruker Daltonics, 40 Manning Road, Billerica, MA 01821, United Statese Chemistry Institute, Federal University of Goiás, 74001-970 Goiânia, GO, Brazilf Federal Institute of Education, Science and Technology of Espírito Santo, 29106-010 Vila Velha, ES, Brazil

h i g h l i g h t s

�We describe FT-ICR MS analysis of asphaltene using different ionization sources.� LDI/MALDI facilitated clustering reactions to form characteristic fullerene ions.� The clusterization of asphaltenes showed a dependency on the laser power density.� APCI/APPI should be more suitable to access the chemical composition of asphaltene.

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a r t i c l e i n f o

Article history:Received 10 January 2014Received in revised form 13 April 2014Accepted 15 April 2014Available online xxxx

Keywords:PetroleomicsAsphaltenesIonization sourceFT-ICR MSFullerene clusters

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a b s t r a c t

Asphaltenes analysis still remains a challenge due to their unknown molecular structure and self-associa-tive behavior. FT-ICR MS analyses using five different ionization methods, ESI, APCI, APPI, LDI and MALDI,in both positive and negative mode were performed. A characteristic and typical behavior was observedfor the experiments using LDI(±) and MALDI(±)FT-ICR analyses: a broadband mass spectrum ranging fromm/z 500 to 3000 was observed with a 24 Da interval. This phenomenon was attributed to fullerene clusterformation. The most abundant allotropic form of fullerene, C60, was observed at m/z 719.99914. Thesespecies were not observed in APCI(+)/APPI(+) analyses. This last analysis of asphaltene samples dopedwith the C60 solution promptly reveals the typical fullerene ions. LDI(+) analyses of pure C60 showed con-densation products. These results indicate a clusterization of asphaltenes in the LDI/MALDI plume, whichshowed a highly dependency on the laser power density. The dispersion of LDI results on carbonaceussamples in the literature is associated with the lack of control of these parameters. However, to accessthe sample composition, APPI and APCI should be more suitable.

� 2014 Elsevier Ltd. All rights reserved.

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1. Introduction

Asphaltenes are defined by their solubility in toluene and theirinsolubility in normal paraffinic solvents (e.g., n-heptane). Compo-sitionally, asphaltenes characteristically have a high heteroatomiccontent (N, O, S) and polar nature. Despite its importance, asphalt-

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enes analysis is challenge majorly due to the difficulties imposed tothe analytical methods by its aggregation properties [1].

Asphaltenes are intimately related to several operational issuesin both upstream and downstream oil production, putting itsbehavior in the center of attention and attracting research invest-ments [1–5].

Spectrometric analysis of asphaltene fractions remains chal-lenging because their molecular properties cause limited solubilityand subsequent self-aggregation [6,7]. Fluorescence quenchingmeasurements indicated that the critical nanoaggregate concen-tration (CNAC) is around 60 mg/L in toluene [1].

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Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR MS) provides unsurpassed mass resolution and accuracy,enabling the complex petroleum mixtures and asphaltenes to beanalyzed on a molecular level for comparison with the bulk mea-surements [8,9]. Accurate mass measurements [10,11] define theunique elemental composition (CcHhNnOoSs) and DBEs (doublebond equivalent), facilitating material classification by heteroatomcontent and the degree of aromaticity [12–14].

Out of the several atmospheric pressure ionization (API)sources, electrospray ionization (ESI) is particularly efficient ingenerating acceptable data for polar constituents with high hetero-atom content. The combination of positive and negative ESI withFT-ICR MS analyses of asphaltenes was reported by Klein et al[9]. The authors identified the differences between asphaltenescollected from different geographic locations using C7 precipitationand live oil depressurization experiments [9]. Using 1 mg mL�1

samples, they detected constituents with DBE values up to 37.Although ESI generates abundant ions from several polar species,substituted hydrocarbon cycloalkanes and aromatic species remainundetectable. Another problem for asphaltene analysis by ESI is thecompatibility of the solvents required by solution preparation(methanol, acetonitrile, or water) with the solvents that solubilizethe asphaltenes (benzene and toluene). Modifying the solvent sys-tems to enable the ESI destabilizes the asphaltene solutions result-ing in precipitation.

Atmospheric pressure chemical ionization (APCI) and atmo-spheric pressure photoionization (APPI) are suitable to asphalt-enes and model-asphaltene compound analyzes, especiallyunder selected measurement conditions [15–17]. These methodsefficiently ionize polycyclic aromatic hydrocarbons (PAHs) withor without heteroatoms, producing radical cations, [M]+�, andprotonated molecules, [M + H]+ [18]. APCI has been used in com-bination with collision-activated dissociation (CAD) to describethe preferred fragmentation pathways for the island and archi-pelago models [19]. Furthermore, when analyzing asphaltenesamples, the loss of varying alkyl chain lengths (between oneand twelve carbon, mainly methyl groups followed by ethyl,propyl, butyl, etc.) was observed as the dominant pathway, sug-gesting the existence of ‘‘island’’-type structures [20,5]. Changesto hydrocarbon and sulfur compounds during deep hydrotreat-ment processes were observed using APPI and were successfullyapplied to identify and catalog several vanadyl porphyrin struc-tures [21].

Laser desorption/ionization (LDI) techniques were developed inthe early 1970s and were further developed throughout the 1980s,with the introduction of matrix-assisted laser desorption/ioniza-tion (MALDI) technique. MALDI became an established ionizationmethod for mass spectrometry analysis of macromolecular com-pounds. These techniques were applied to analyze asphaltenesand heavy crude oil fractions [22–25]. It is well known that param-eters such as the laser wavelength, pulse width, laser fluency,potential reactions between ions and molecules, and the chemicalcharacteristics of the analyte, influence desorption/ionizationmechanism [25].

Although ESI, APPI, APCI, MALDI and LDI ionization methodshave been applied to asphaltene characterization [26,27], each ion-ization method access a subset of the global asphaltene composi-tional and structural diversity and do not correspond to the bulkproperties of asphaltenes. Therefore, understanding the effectscaused by the ionization parameters during the asphaltenes analy-sis by different ionization methods is crucial. The analysis of oneBrazilian asphaltene sample using different ionization sources,such as ESI, APCI, APPI, LDI and MALDI was performed. The resultssuggest that asphaltene analysis through LDI or MALDI ionizationmethods can lead to molecular reactions and subsequent cluster-ization and formation of fullerenic species.

Please cite this article in press as: Pereira TMC et al. FT-ICR MS analysis ofdx.doi.org/10.1016/j.fuel.2014.04.049

2. Experimental

2.1. Reagents

Toluene, pentane, tetrahydrofuran (THF) and carbon disulfide(CS2) (analytical grades with purity higher than 99.5%) were sup-plied by Vetec Química Fina Ltda, Brazil. Methanol, ammoniumhydroxide (NH4OH), sodium trifluoroacetate (NaTFA), L-arginine,formic acid (HCOOH), 2,3-dihydroxybenzoic acid (DHB), sinapicacid, a-cyano-4-hydroxycinnamic acid (CHCA) and fullerene werepurchased from Sigma–Aldrich Chemicals, USA. All reagents wereused as received.

2.2. Sample characterization

One sample of crude oil was obtained and characterized accord-ing to the standards of the American Society for Testing and Mate-rials (ASTM) to determine the API degree (ASTM D1298-99) [28],the total acid number (ASTM D664-09) [29], the kinematic viscos-ity (ASTM D7042-04) [30], and the total sulfur (ASTM D4294) [31].The characterization data obtained for the crude oil are as follows:API degree = 27.6, TAN = 0.17 mg KOH g�1, viscosity = 24.96 mm2/sat 30 �C, and total sulfur = 0.51 wt%. The emulsified water and sed-iment fraction was ca. 1.21% v/v. Subsequently, the oil sample wasfractionated using the saturates, aromatics, resins, and asphaltenes(SARA) method (ASTM 6560-00) [32]. The weight percent recoveryfor each fraction was as follows: saturates, 48.9 wt%; aromatics,30.9 wt%; resins, 18.0 wt%; and asphaltenes, 2.2 wt%. The asphal-tene sample was characterized using ESI, APCI, APPI, LDI and MAL-DI(±)FT-ICR MS.

2.3. FT-ICR MS analysis

FT-ICR MS analysis was performed using a 9.4 T Q-FT-ICR MShybrid (Solarix, Bruker Daltonics Bremen, Germany) equipped withcommercially available ESI/APCI/APPI [33–35] and MALDI sourcesset to operate over m/z 200–1000 (for ESI, APCI and APPI sources;Bruker Daltonics) or m/z 200–3000 (for LDI and MALDI sources).The asphaltene sample was analyzed using positive and negativeionization modes.

The asphaltene sample was diluted to ca. 0.25 mg mL�1 forESI(+) and 1 mg mL�1 for ESI(�) in 50:50 (v/v) toluene/methanolcontaining 10% w/v of HCOOH for ESI(+) and NH4OH for ESI(�).The same concentrations were utilized in APCI(±) measurements,but the solvent was changed to pentane/carbon disulfide [36,37].For the APPI(±) measurements, the sample was dissolved in ca.0.10 mg mL�1 toluene for APPI(+) and 1 mg mL�1 toluene forAPPI(�). To dissolve the entire sample, the solution was sonicatedduring 5 min. The resulting solution was directly infused into theionization source at 4–10 ll min�1 for ESI(±), 10 ll min�1 forAPPI(±) and 15–20 ll min�1 for APCI(±) (see Table 1 for furtherdetails). A Krypton photoionization lamp (k = 124 nm and 10 eV)and a corona discharge (5000 nA) were used in the APPI(±) andAPCI(±) sources, respectively. The API sources conditions, such asthe capillary voltage, end plate offset, transfer capillary tempera-ture, vaporization temperature, nebulizer gas pressure and dryingflow, are described in Table 1.

For the LDI(±) analyses, the asphaltene sample was diluted toca. 1 mg mL�1 (for both ionization modes) in three different sol-vents: toluene, CS2, and THF. The asphaltene solution was spotted(1 lL) directly onto a LDI plate and dried. For the MALDI(±) analy-ses, three freshly prepared saturated DHB, CHCA and sinapinic acidmatrix solutions in methanol were subsequently added to thespots. The LDI/MALDI source conditions were as follows: the plateoffset was �100 V, the deflector plate was �220 V, the Nd:YAG

asphaltenes: Asphaltenes go in, fullerenes come out. Fuel (2014), http://



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Table 1Ionization source parameters optimized for FT-ICR mass spectral (9.4 T) analysis of asphaltene samples.

ESI APPI APCI MALDI/LDI

+ � + � + � + �

Concentration (mg mL�1) 0.25 1.0 0.1 1.0 0.25 1.0 1.0 1.0Flow (lL min1) 4 10 10 10 20 15 – –Mass range (m/z) 200–1000 200–1000 200–1000 200–1000 200–1000 200–1000 200–3000 200–3000Capillary voltage (V) �3500 3500 �3000 2500 �2000 3000 – –End plate offset (V) �500 �500 �500 �500 �500 �500 �500 �500Transfer capillary Temperature (�C) 250 250 250 250 250 250 – –Vaporization Temperature (�C) – – 320 320 320 320 – –Nebulizer gas pressure (bar) 1 1 1.5 1.5 2 2 – –Drying Flow (L min�1) 4 4 4 4 2 2 – –Skimmer (V) 30 �50 60 �75 50 �75 15 15Collision voltage (V) �60 30 �50 50 �50 35 �0.5 �0.5Ion accumulation time (s) 0.8 0.5 0.3 0.3 0.3 0.8 0.5 0.5Time-of-flight (ms) 0.9 0.8 0.9 0.8 0.8 0.8 0.8 0.8

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laser had a 100 Hz frequency, wavelength of 355 nm (third har-monic that corresponds to 3.5 eV), the foci ranged from mediumto small (100–250 lm in a spot diameter of 800 lm), and the max-imum energy was 40% (full power corresponds to 250 lJ)respectively.

Generally, a 0.3–0.8 s ion accumulation time in the hexapolewas followed by ion transfer to the analyzer cell (ICR) throughthe multipole ion guide system (another hexapole). Each spectrumwas acquired using 200 scans of time-domain transient signals in 4mega-point time-domain data sets (Table 1). The front and backtrapping voltages in the ICR cell were �0.60 V and �0.65 V for neg-ative ionization mode and +0.80 V and +0.85 V for positive ioniza-tion mode. All mass spectra were externally calibrated using L-arginine and NaTFA solutions (m/z 200–1200) after internal recal-ibration using the most abundant homologous alkylated com-pounds in each sample.

The resolving power (m/Dm50% from 450,000 to 550,000 at m/z400, where Dm50% is the full peak width at half-maximum peakheight) and <1 ppm mass accuracy provided the unambiguousmolecular formula assignments for all singly charged molecularions. The mass spectra were acquired and processed with a customalgorithm specifically developed for petroleum data using Com-poser software (Sierra Analytics, Pasadena, CA, USA). For each spec-trum, an automated analysis was used to assign the formulas to thepeaks with a signal-to-noise ratio (S/N) greater than 3. The ele-ments allowed were 12C, 1H, 16O, 14N, 32S, and 13C. The maximumallowed formula error was 1 ppm, and the mass limit for empiri-cally assigning the elemental formulas was 500 Da. The formulaswith atomic masses greater than 500 Da were assigned throughthe detection of a homologous series. If no chemical formulamatched a m/z value within the allowed error, the peak was notincluded in the list of elemental formulas. For each elemental com-position, CcHhNnOoSs, the heteroatom class, the type [double bondequivalents (DBE) = the number of rings plus the double bondsinvolving carbon], and the carbon number, c, were tabulated togenerate the relative abundance distributions of the heteroatomclass, van Krevelen diagram and the graphical DBE versus the car-bon number images.

For the NC versus DBE plot, DBE was the number of rings addedto the number of double bonds in each molecular structure. Thearomaticity of a petroleum component can be deduced directlyfrom its DBE value according to Eq. (1):

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DBE ¼ c � h=2þ n=2þ 1 ð1Þ 287
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where c, h, and n are the numbers of carbon, hydrogen, and nitrogenatoms, respectively, in the molecular formula.

3. Results and discussion

3.1. FT-ICR mass spectra for positive ionization mode

Fig. 1a–d display the spectra of a typical Brazilian asphalteneacquired using four different ionization sources: ESI(+), APCI(+),APPI(+), and LDI(+). In general, the ESI(+), APCI(+), and APPI(+)-FT-ICR mass spectra (Fig. 1a–c) show broadband profiles from m/z 200–800 with an average molar mass distribution centered atapproximately m/z 480, 448 and 460, respectively. Generally, FT-ICR mass spectrometers have an average resolution of 430,000 to520,000 at m/z values ranging from 400 to 490. The magnifiedregion near m/z 400 indicates that ESI(+) mostly detects hetero-atom-containing compounds (correspond to ions [C30H13NO + H]+,DBE = 25, and [C31H17N + H]+, DBE = 24, respectively). APCI(+) andAPPI(+) sources produced more ions than ESI(+) (14 for APCI(+)and 16 for APPI(+)) due to their ability to produce radical cations[M]+� and protonated molecules [M + H]+ simultaneously). Theyalso efficiently ionize polycyclic aromatic compounds ([C30H13-

NO + H]+, DBE = 25 or [C32H20]�+, DBE = 23) (Fig. 1b–d). Therefore,FT-ICR MS may distinguish different compounds by selecting dif-ferent ionization methods. Consequently, the different ionizationmethods available to analyze the selected asphaltene sampleshould be compared [39,40].

Gaspar et al. [39] investigated the chemical composition ofasphaltene fraction samples using five distinct atmospheric pres-sure ionization techniques (ESI, H-ESI, APCI, APPI and APLI). Similarto our work, higher number of assigned molecular formulas areobserved for APCI(+) and APPI(+) sources and a lowest number ofassignments for ESI(+).

Some works have also reported the characterization of asphal-tene fractions using only APPI(+) [13,18,40,41]. In 2011, Tachonet al. [40] isolated and characterized organic compounds fromSARA fractions using a hybrid mass spectrometer: APPI(+)QqTOF-MS. The fraction precipitated in heptane (ca. 13 wt%) had a500 Da Mw. In 2012, Cho et al. [41] also observed similar resultswhen analyzing heavy oil products using a 15 T APPI(+)-FT-ICRMS; Mw values >600 Da were observed. Romao et al. analyzedten Brazilian asphaltene fractions from crude oil samples withAPI degrees ranging from 13 to 31. In all cases, a <500 Da Mw

was observed.A characteristic behavior was observed during the LDI(+)-FT-ICR

MS experiments using toluene as a solvent (Figs. 1d and 2c). Sev-eral peaks with a broad Gaussian profile from m/z 500 to 2000and an m/z centered at 900 Da were observed with a 24 Da interval.These peaks can be associated with fullerenes allotropic forms,with the C60 peak appearing at m/z 720.0008. This fullereneappearance may be due to the presence and the preferential




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Fig. 1. FT-ICR mass spectra of a typical asphaltene sample using (a) ESI(+), (b) APCI(+), (c) APPI(+), and LDI(+) at (d) m/z 200–2000, as well as an expanded region at (e) m/z200–550. For the LDI(+) experiments, toluene was used to prepare the asphaltene solution (1 mg mL�1) with a laser pulse energy of 40%.



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ionization of these compounds in the bulk asphaltenic matrix ordue to its cluster formation during the LDI and MALDI processes[42,43].

In an attempt to minimize and suppress fullerene ionizationand therefore increase the abundance of the asphaltene fraction,the type of solvent used to prepare the asphaltene solution(1 mg mL�1) was varied. Fig. 2a and b display the LDI(+)FT-ICRMS mass spectra from samples prepared in CS2 and THF. In bothcases, a distribution of compounds that corresponded to fullereneclusters (Cn molecules where n ranges from C44 (m/z 527.9996) toC234 (m/z 2808.0022) was observed. At this point, the range massprofile was broader, and the m/z shifted toward larger m/z values.Another alternative methodology, MALDI(+)FT-ICR MS, was evalu-ated. Essentially, three saturated matrices solutions (CHCA, DHBand sinapinic acid) were prepared and added to the spots contain-ing asphaltenes dissolved in toluene (1 mg mL�1). The MALDI(+)FT-ICR mass spectra are displayed in Fig. 2d and f. In the experimentswith all three matrices, the fullerene compounds were detected(most abundant ions appeared within the m/z 200–900 region).Both the matrices and the occurrence of fullerene cluster formationsuppressed the ionization of organic compounds present in theasphaltene fractions.

Fig. 2. LDI(+)FT-ICR mass spectra for asphaltene samples prepared in (a) CS2, (b) THF andICR mass spectra for the asphaltene sample using (d) CHCA, (e) DHB and (f) synaptic acidsample was dissolved in toluene. Laser pulse energy of 40% was also used in all the exp


The fullerene cluster may be formed during the ionization pro-cesses performed in the LDI(+) and MALDI(+)FT-ICR analysis. It iswell known that polyaromatic hydrocarbons (PAH) readily aggre-gate after desorption under favorable conditions, includingreduced pressure (10�6 mbar) and high laser power [42]. These fac-tors may synergistically contribute to fullerene cluster formationwith an increasing number of fused rings in the original sample.However, the peaks with spacing of 12, 24 and 26 u were easilydistinguished above 500 Da due to the growth of PAH via the inser-tion of a CH2 bridge, followed by hydrogen elimination (spacing of12 Da), a sequential addition of C2 as an ethylene bridge (spacing of24 Da), or via sequential addition of C2H2 (mass increment of26 Da) [42]. Growth reactions may have occurred in the laserplume due to high concentrations of PAH present in the asphaltenefractions.

To better understand the mechanism behind the fullerene clus-ter formation during the LDI(+)/MALD(+) experiments, a typicalcommercial fullerene and asphaltenes doped with fullerene solu-tions (1 mg mL�1) were directly infused into APCI(+) (Fig. 3a andd), APPI(+) (Fig. 3b and e), and LDI(+) (Fig. 3c) sources. In theAPCI(+) and APPI(+)FT-ICR MS experiments, similar chemical pro-files were observed. For the pure commercial fullerene solution, a

(c) toluene. In all cases, the same concentration was used (1 mg mL�1). MALDI(+)FT-as the organic matrices. For the MALDI(+)FT-ICR mass experiments, the asphaltene

eriments.




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Fig. 3. (a) APCI(+), (b) APPI(+) and (c) LDI(+)FT-ICR MS for the fullerene solution; and (d) APCI(+), (e) APPI(+)FT-ICR MS for the asphaltene sample doped with fullerene C60.



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parent peak at 720 Da (C60�+ at m/z 720), with the typical isotopic

pattern dominates the APCI(+) and APPI(+) spectra. In theLDI(+)FT-ICR MS of standard fullerene solution, in addition to thepeak at 720 Da, fullerene cluster in the range of m/z 720–2000and fragmentation (m/z 400–720) are observed in Fig. 3c. This isin agreement with results reported by Apicella and co-works[42], showing clustering and fragmentation phenomena in theLDI asphaltene analysis.

The laser applied in low vacuum LDI and MALDI conditionscould induce clusterization of large PAH from the samples. Thisphenomenon was reported [42,44], but only in asphaltene’s molec-ular models. In Fig. 4, it is showed that this interesting phenome-non also happens with crude oil sample.

The clusterization phenomenon takes place when the laserpower is set to 40% or higher. Using a laser power of 60%, all signalsof crude oil (m/z 200–1000) are suppressed. The LDI(+)FT-ICR spec-tra of the crude oil acquired with a laser power above 40% providesclear evidence of an extensive fullerene clusters formation (see the

Fig. 4. LDI(+)FT-ICR mass spectra of crude oil acquir


typical peak of m/z 720) in desorption plume from the crude oilsample. These results are in good agreement when a pure commer-cial fullerene solution (1 mg mL�1) is also submitted to differentlaser pulse power (from 20 to 80%, see Fig. 1S, Supplementarymaterial). However, the fullerene clusters formation can be mini-mized when diluted solutions are analyzed, lower than0.5 mg mL�1, Fig. 2S).

In summary, these results indicate that pure and small alkyl-substituted poly-nuclear aromatics are easily ionized under LDIconditions. They start to polymerize at relatively low laser energiesand continue to polymerize at higher laser energies. The fragmen-tation profile observed on the spectra is related to the bridged aro-matics that ionize at low laser energy, but start to fragment atmoderate laser energy. Non-aromatics are difficult to ionize usinglaser desorption, except in the presence of an appropriate matrix,(MALDI conditions).

An alternative laser-based ionization technique is laser desorp-tion laser ionization mass spectrometry (L2MS). In this technique,

ed as function of different laser pulse energies.




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desorption and ionization are separated in time and in space andare performed with separate lasers [45,25]. In the desorption step,a pulsed infrared (IR) laser desorbs analytes from the surface ofsome substrate, such as glass or quartz. In the ionization step,gas-phase analytes are photoionized, typically by either resonantenhanced multiphoton ionization (REMPI) at 266 nm or single-photon ionization (SPI) at 157 nm.

Recently, Zare et al. [25] showed that mass spectra of asphalt-enes and PAHs model compounds acquired with L2MS contain nodetectable aggregate peaks. It is suggested that aggregation is min-imized in L2MS because the IR laser is sufficient only to break non-covalent bonds in the solid sample, resulting in desorption ofisolated neutral molecules. The plume of desorbed neutrals lacksions that cause ion-induced dipole attractive forces leading toaggregation in the gas phase. Fragmentation occurs occasionallyin REMPI-L2MS (two photon energy of 9.3 eV) and rarely in SPI-L2MS (single photon energy of 7.9 eV) due to the photon energyto be only slightly above the ionization potential of these mole-cules. The minimization of fragmentation and aggregation impliesthat L2MS may be an attractive ionization technique for analysis ofcomplex mixtures, where fragmentation and aggregation found intraditional ionization techniques, such as LDI and MALDI, result inmass spectra that evidence even the same phenomenon.

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3.2. FT-ICR mass spectra for negative ionization mode

Fig. 5a–d display the ESI(�), APCI(�), APPI(�) and LDI(�)FT-ICRmass spectra acquired from a typical Brazilian asphaltene sample.For the ESI(�), APCI(�), APPI and LDI(�)FT-ICR MS (Fig. 5a–c), abroad-band mass profile (m/z 280–800) was observed at Mw of553, 546, 458 and 520, respectively. Essentially, the FT-ICR MSexperiments in positive ionization mode indicated that Mw valueswere all high.

In the LDI(�)FT-ICR MS experiments, a more distinct chemicalprofile was obtained, validating the LDI(+)FT-ICR MS experiments.Two broad-band distributions were observed: the first and lessintense distribution included m/z 400–700, and the second

Mw = 553

Mw = 546

Mw = 458

Mw = 520

Fig. 5. FT-ICR mass spectra of a typical asphaltene sample collected using (a) ESI(�), (b) A483 reveals the selective ionization of [C36H10NO – H]� at m/z 482.1558 from the APCI(


distribution exhibited more intensity from m/z 600 to 1400. There-fore, a distribution of fullerene clusters formed during the ioniza-tion process. Notably, during the LDI(�) experiments, the ionsuppression phenomenon was less pronounced for asphaltenecompounds, improving the data processing.

3.3. Class distribution, DBE versus number of carbon and van Krevelendiagrams

The similarities and differences between the five ionizationmethods can be investigated via the compound class distribution.Fig. 6 displays the distribution of polar and nonpolar compoundclasses obtained from the ESI(+), APCI(+), APPI(+) and LDI(+)FT-ICR MS. ESI(+) seems to efficiently promoted the ionization of basicnitrogen compounds, as protonated cations ([M + H]+) and the het-eroatomic compound classes were detected in the following orderof abundance: N[H] > N2[H] > NO[H] > N2O[H]. However, APCI(+),APPI(+) and LDI(+) promoted ionization via two mechanisms, pro-tonation and electron transfer, producing [M]+� and [M + H]+ ions.Consequently, the compound classes generated by these sourceswere closely related. The order of relative abundance for APCI(+)was as follows: N[H] > N > HC[H] > HC > N2[H] > NO > N2 > N2-

O > O[H] > O. A similar distribution profile was observed with theAPPI(+) source. Finally, LDI(+) selectively ionized the HC andHC[H] classes. These classes include organic compounds that arepresent in the asphaltene fraction which did not assign to fullereneclusters (Cn compounds). To perform the visualization of non-ful-lerene compounds, the quadrupole window was partially closed(from m/z 200–3000 to m/z 100–520), and the LDI(+)FT-ICR massspectra were reacquired (see Fig. 3S).

Fig. 7 illustrates the compound classes distribution obtainedfrom the ESI(�), APCI(�), APPI(�) and LDI(�)FT-ICR MS. Similarto the ionization mechanism of ESI(+), ESI(�) promotes the ioniza-tion of acidic nitrogen compounds. The most abundant classesdetected via the proton abstraction mechanism, ion [M–H]�, wereas follows: N2 > N2O > NO > N > NO2 > N2O2 > O3 > O4. Other lessabundance polar classes included N2O4, NO3 and NO4. However,

PCI(�), (c) APPI(�), and (�)LDI at (d) m/z 200–2000. The magnification of m/z 482–�) and APPI(�) sources.




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Fig. 6. Class distribution for a typical asphaltene fraction generated from ESI(+),APCI(+), APPI(+) and LDI(+)FT-ICR MS data.



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for the APCI(�), APPI(�) and LDI(�) sources, two ionization mech-anisms were observed: proton abstraction and electron transfergenerated [M–H]� and M��, respectively. Consequently, theobserved classes were duplicated. The class profiles obtained fromthe APCI(-) and APPI(-) data were very similar, displaying theextensive range of detected classes. The NO, NO[H], N2O[H] andN2O classes were the most abundant. The nonpolar species wereonly detected in the LDI(�) data because the generation of negativearomatic hydrocarbon ions with APCI(�) and APPI(�) was not pre-ferred [42].

Fig. 8a–d present the DBE versus the carbon number plots forthe N[H] class, which was the most abundant class of polar com-pounds detected in the ESI(+), APCI(+), APPI(+) and LDI(+)FT-ICRMS data for the asphaltene sample. ESI(+) (8a) reveals more nitro-gen-containing compounds when compared to the other ionizationsources (the carbon numbers ranged from C20 to C70, and the DBE

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Fig. 7. Class distribution of a typical asphaltene fraction generat


ranged from 10 to 40). The most abundant compounds display acarbon number of C50 and a DBE of 29. Therefore, ESI(+) may beemployed to selectively identify the polar components in theasphaltene sample, providing detailed data concerning nitrogen-containing compounds: pyridines, quinoline, and benzoquinoline,as well as their alkylated and hydrogenated derivatives. APCI(+)and APPI(+) (8b-c) display a similar profile, enabling the ionizationof compounds with narrow carbon numbers but with high DBE val-ues (C20–C45 and DBEs of 18–36). These two sources were the mostsuitable to ionize low polarity compounds. LDI(+) (8d) alsoimproved the compound ionization of molecules with a low carbonnumber and a high aromaticity (DBE = 6–36). For the HC[H] class,the DBE versus the carbon number were plotted for the APCI(+),APPI(+) and LDI(+) data showing similar results (Fig. 4S); the car-bon number ranged from C20 to C45, and the DBEs ranged from10 to 38. Additionally, the slopes of the lines determined by DBE/carbon number ratio were calculated by linear regression for pro-tonated basic nitrogen (N[H]) compounds classes. An increasingof the inclination of the DBE/carbon number line is observed inthe following order: LDI(+) > APPI(+) > APCI(+) > ESI(+), Fig. 8.Therefore, LDI(+) and APPI(+)-FT-ICR MS selectively identify highlyaromatic components in the asphaltene samples.

The van Krevelen diagram is another important graphical toolthat was first employed by Kim et al. [46] to distinguish condensedhydrocarbons from lipids and other components using FT-ICR massspectrometry. van Krevelen diagrams consist of iso-abundance sur-face as a function of (e.g.) the H/C ratio and O/C ratio for each com-pound containing the relevant atoms. In a van Krevelen diagram,the heteroatom classes were plotted separately on the x- (e.g., O/C ratio distinguished classes with different numbers of O atoms)and y-axes (the DBE difference was spread out along the H/C ratio).The homologous alkylation series were composed of compoundsthat contained the same number of rings plus double bonds, aswell as the same number of heteroatoms; but different numbersof CH2 groups. The van Krevelen plot facilitates the visual

ed from ESI(�), APCI(�), APPI(�) and LDI(�)FT-ICR MS data.




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Fig. 8. DBE versus carbon number plots for nitrogen-containing compounds (N[H] class) generated from (a) ESI(+), (b) APCI(+), (c) APPI(+), and (d) LDI(+)FT-ICR MS data. Alaser pulse energy of 40% was used for LDI(+) experiments.



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separation of heteroatom classes, DBE types, and alkylationpatterns.

Fig. 9a–d compare the Nx profile classes detected during ESI(�),APCI(�), APPI(�) and LDI(�)FT-ICR MS analysis. The ESI(�) data(9a) revealed a larger magnitude of nitrogenous compounds incomparison with the other three ionization methods. The com-pounds with the largest relative abundance displayed an H/C of�0.7, indicating they were aromatic compounds. APPI(�) (9c)

Fig. 9. van Krevelen diagrams for Nx-containing species generated from


detected fewer compounds than the APCI(�) (9b) and ESI(�)FT-ICR MS (9a). In contrast, LDI(�)FT-ICR MS (9d) providedcompounds with H/C �0.1, suggesting that only highly aromaticcompounds were ionized due to the ionization process in theLDI(�) source (9d), as previously discussed.

For the NxOy compounds in Fig. 10, the APCI(�) accessed a largerange of species (N2O or NO, NO2, NO3, NO4, 10b) except for theN2O class; which performed better in the ESI(�) source (10a).

(a) ESI(�), (b) APCI(�), (c) APPI(�) and (d) LDI(�)FT-ICR MS data.




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Fig. 10. van Krevelen diagrams for NxOy-containing species observed using (a) ESI(�), (b) APCI(�), (c) APPI(�) and (d) LDI(�)FT-ICR MS.



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Generally, the most abundant compounds were found in the H/Cregion of �0.7 (Fig. 10b). APPI(�) results show a similar profileto those from the APCI(�). No NxOy compounds were detectedusing LDI(�).

For Ox classes compounds (O1, O2, O3 and O4) (Fig. 5S), theAPCI(�) provided a greater abundance of detected species, inwhich the majority compounds showed an H/C of �0.7. However,in APPI(�), the most intense compounds had an H/C of �0.5, andthe LDI(�) only accessed O2 class compounds with an H/C of�0.3. Generally, due to its chemical ionization mechanisms in thegas phase, the APCI source ionizes both highly aromatic and non-aromatic species. However, the ESI source only favors the detectionof compounds with low hydrogen deficiencies, specificallycompounds containing few aromatic rings.

4. Conclusion

Different ionization methods for asphaltene analyses were com-pared and discussed. The molecules detected through the five dif-ferent ionization sources revealed substantial differences in thecontent and in the aromaticity of the detected classes. ESI ionizesnitrogenous and highly aromatic compounds. APCI and APPIdisplayed similar ionization profile, with similar amplitudes forthe carbon number and DBE. These last two techniques ionizelow-polarity compounds, such as hydrocarbons. However, for APPIand APCI, it was observed the formation of [M+H]+ and e [M]+�,which explain the complexity of the spectra obtained. All ionsources described a portion of the real sample, but each individualsource revealed specific singularities of the sample.

LDI displayed a characteristic chemical profile. It was due to thehigh density laser plume that favored clustering reactions to formfullerene ions, especially when condensed aromatic structures arepresent. In the positive mode, ionic suppression phenomenon wasmore pronounced than in the negative one. However, in both cases,a homologous series of compounds corresponding to fullerene


clusters (Cn molecules where n ranges from C44 (m/z 528.0011)to C234 (m/z 2808.0022)) were observed. The ionization usingMALDI source also induced fullerene formation from the asphal-tene sample.

This study demonstrated that APCI and APPI are able to access awide range of compound classes, DBEs and carbon number distri-butions within the evaluated ionization sources. Although the APCIand APPI results were very informative, a multidimensional analyt-ical platform, including selective separation methods, might benecessary to analyze ultra-complex samples such as crude oil orits fractions. It demonstrates that none of the available ionizationtechniques fully assesses the content of complex samples, suchas asphaltenes from crude oil.

5. Uncited reference

[38].

Acknowledgments

We acknowledge FAPEG and PETROBRAS for their financial sup-port. GV acknowledges FAPES and CAPES for fellowships. EVRCacknowledges CNPq for fellowships.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.fuel.2014.04.049.

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FT-ICR MS analysis of asphaltenes: Asphaltenes go in, fullerenes come out

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