A comprehensive characterization of asphaltenes by FT-ICR MS

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A comprehensive characterization of asphaltenes by FTICR MS precipitated under different n-alkanes solvents Marcos Albieri Pudenzi, Jandyson Machado Santos, Alberto Wisniewski Jr, and Marcos Nogueira Eberlin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02262 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Energy & Fuels

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A comprehensive characterization of asphaltenes

2

by FT-ICR MS precipitated under different n-

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alkanes solvents

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Marcos Albieri Pudenzi*‡, Jandyson Machado Santos‡, Alberto Wisniewski Jr.†, Marcos

5

Nogueira Eberlin‡

6



7

UNICAMP, Campinas, São Paulo, 13083-970, Brazil

8



9

000, Brazil

ThoMSon Mass Spectrometry Laboratory, Institute of Chemistry, University of Campinas,

Federal University of Sergipe, Department of Chemistry, São Cristóvão, Sergipe, 49100-

10 11

ABSTRACT

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Asphaltenes are still problematic fractions and their composition is not fully unveiled as it

13

relates to the original crude oil and the precipitation method. In this work, the composition of

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asphaltenes precipitated with n-heptane and n-pentane from ten crude oils from the Sergipe-

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Alagoas Basin were assessed by Ultra-High Resolution Fourier Transform Ion Cyclotron

16

Resonance Mass Spectrometry (FT-ICR MS) using five ionization procedures (ESI(+) with

17

formic acid, ESI(-) with ammonium hydroxide, ESI(-) with tetramethyl ammonium

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hydroxide, and APPI(+) and APPI(-)) in combination with a 7.2T LTQ FT Ultra Thermo

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Fisher spectrometer. The purpose was to evaluate compositional differences between C5- and

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C7-asphaltenes according to the adopted ionization methodologies. Asphaltenes were

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compared as a function of heteroatomic composition and aromaticity. Chemometrics

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strategies were employed to evaluate the heteroatom classes of higher variation among the

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groups. Ions that were shared by the asphaltenes and the crude oils, and between the

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ionization methods were also evaluated for similarity. Results showed that n-heptane

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asphaltenes are more aromatics than n-pentane asphaltenes; which in turn were found to be

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more acidic and to display a higher heteroatomic complexity. Principal component analysis

27

(PCA) and the evaluation of shared ions showed that APPI(±) is able to detected asphaltene

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1

ions directly from the whole crude oil. It was also found that the use of different ionization

2

methods with and without additives is fundamental to obtain more comprehensive chemical

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profiles in terms of classes and their constituents for crude oils and their asphaltenes, and that

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the specific n-alkane used to precipitate asphaltenes leads to different chemical profiles.

5 6

INTRODUCTION

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The high dependency of fossil fuels has increased the use of heavy crude oils in the

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production of fuels and petrochemical derivatives, but the problems caused by the heavier

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fractions of petroleum, which these oils contain on greater amounts than light oils, are

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becoming critical.[1] Those heavy fractions can cause the formation of emulsions, clogging

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and deposition, which affects directly the oil production chain. Analyzing these heavy

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fractions at the molecular level is not a trivial task. As they show low volatility, high

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tendency of aggregation and high molecular mass (>450 Da), are not amenable to detection

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by standard gas chromatography methodologies.[2,3] Nevertheless, ultra-high resolution mass

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spectrometry has been successfully used for the analysis of either whole heavy crude oils[4-6]

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or their fractions[7-8] through a petroleomics approach.

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Petroleomics can be defined as the comprehensive analysis at the molecular level of crude

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oils, fractions and derivatives to relate chemical composition to physical and chemical

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properties.[1,9] This field of research has mainly relied on ultra-high resolution mass

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spectrometry (UHRMS), especially on Fourier transform ion cyclotron resonance mass

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spectrometry (FT-ICR MS), with eventually other analytical techniques being used to add

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more comprehensiveness to the field, such as GCxGC-MS[2,3], Orbitrap-MS [12,13]

[10,11]

and ion

23

mobility MS.

Whilst having some disadvantages such as the insensitivity to low mass

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compounds and traditional biomarkers and strong difficulties to perform quantitative

25

determinations of individual components, the highest resolution and mass accuracy provided

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by FT-ICR MS make it the most commonly employed mass analyzer for petroleomics

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assessments. The ionization sources, however, have varied considerably and different types

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of oil constituents from a wide range of polarities can be focused depending of the chosen

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technique. Electrospray ionization (ESI)[14] and atmospheric pressure photoionization

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(APPI)[15] have been the most commonly employed, since they preferentially ionize polar and

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mid polar compounds, respectively. Analyses that employ ESI and APPI FT-ICR MS result,

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therefore, on comprehensive and detailed data on the mid to high polar heteroatom

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composition of oils that have been shown to enable deeply understanding of crude oil

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properties.[16,17]

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The ultra-high resolution and high mass accuracy m/z values obtained by FT-ICR MS enable

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attribution of molecular formulae to ions with high confidence levels, and from these

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molecular formulae it is possible to visualize class distribution via selective plots. One of the

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most important of such plots is the class chart, which sums the relative abundances of

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compounds having the same heteroatom numbers in order to facilitate comparisons between

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the heteroatomic content of different crude oils. Another very important plot is the one that

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relates carbon number (Cn) versus double bond equivalents (DBE) as:

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DBE= C - (H/2) + (N/2) + 1

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where C=number of carbons, H=number of hydrogens and N=number of nitrogen atoms in

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the molecular formula

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DBE vs Cn plots allow organizing constituents of the same heteroatomic class as a function

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of carbon number and unsaturation/cyclization levels. Thus, it is possible to track how the

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aromaticity and cyclization level of the oil vary in terms of molecular weight as measured by

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Cn. Such trends have been shown to be directly related and to provide reliable information on

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a diverse number of geochemical properties.[6,18]

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In the petroleomics field, many different applications have been explored, but the analysis of

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asphaltenes is still a great challenge, both on the analytical and on data interpretation

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fronts.[19-21] Asphaltenes are defined as the n-alkane insoluble and toluene soluble fraction of

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crude oils. Their definition is, therefore, not directly related to their chemical composition,

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but simply to their solubility. Due to a complex chemical composition, aggregation tendency,

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low solubility and high boiling points, the challenges to characterize asphaltenes at the

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molecular level are great and their actual composition is still a matter of debate.[19-21] It is

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generally accepted, however, that asphaltenes are highly aromatic compounds, with several

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heteroatoms in their composition and few and small alkyl chains.[19-21] [19, 21, 22]

Asphaltenes from

, and eventually other alkanes

[23-25]

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crude oils are often precipitated using n-heptane

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but the chemical composition of these precipitates have never been compared via FT-ICR MS

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petroleomic approaches. Gu and collaborators[23], using mostly physicochemical assays and

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bulk spectroscopy analysis, have compared asphaltenes precipitated by using three different

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n-alkanes, concluding on highest aromaticity and MW for n-heptane precipitation when

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compared to propane and n-pentane. For that work, however, the authors did not characterize

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the asphaltenes at the molecular level of high-resolution mass spectrometry techniques.

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Crude oils from the Sergipe-Alagoas Basin (SEAL) were selected due to their unique

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intrinsic characteristics[4], such as high amounts of C7-asphaltenes (10%), aromatics (20%),

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high acidity (overall TAN>5 mg KOH g-1) and variable API gravity (16-30 °API (Table

7

1)).[4] These unique set of characteristics make these oils interesting samples to FT-ICR MS

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petroleomic investigation since: a) the range of API gravity suggests crude oils with high

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content of polar, heavier compounds that are more easily detected with atmospheric pressure

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ionization techniques, especially ESI; b) the high level of acidity suggests high concentration

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of oxygenated compounds such as phenols and naphthenic acids, which are easily detected by

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ESI(-) and APPI(+) FT-ICR MS and have been shown to function as geochemistry markers

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for oil migration and maturation,[25,26] c) the high aromaticity makes the samples prompt to

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APPI(±) ionization; d) the large amount of asphaltenes aids on getting precipitates with a

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more comprehensive content of chemicals. Here, the composition of asphaltenes obtained by

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precipitation with n-heptane and n-pentane are assessed by FT-ICR MS using ESI(+) with

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formic acid, ESI(-) with ammonium hydroxide, ESI(-) with tetramethylammonium

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hydroxide, APPI(+), and APPI(-) as ionization methods for a more comprehensive analysis of

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the effects produced by n-alkane precipitants on asphaltene composition.

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EXPERIMENTAL SECTION

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Samples. Ten crude oil samples from different wells, which represent the total area of

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the Sergipe Block at the Sergipe-Alagoas Basin were kindly provided by Petroleo Brasileiro

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S.A. (Petrobras). They were received in glass vials from the Sergipe-Alagoas Exploration and

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Production Operations Unit (UO-SEAL) and were labeled as UO-SEAL01-10, which will be

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shortened to UO01-10 in this work (Table 1).

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Asphaltene precipitation. Asphaltene precipitation was performed through a

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modification of ASTM D3279-12 methodology.[27] Briefly, 40 mL of the n-alkane of choice

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(namely n-pentane or n-heptane) was added stepwise (4×10 mL) to 250 mg of the crude oil

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sample to obtain the corresponding C5-asphaltenes (Asp_Pent) or C7-asphaltenes (Asp_Hept).

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The oil-solvent mixtures were shortly stirred, and centrifugated during 5 min at 2500 rpm

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rotation frequency in an FANEM Excelsa® (II) 206BL centrifuge, obtaining the asphaltenes

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by precipitation. The precipitated asphaltenes was washed three more times in the same

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condition described above to eliminate possible contaminants and maltenes residues. The

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asphaltenes were dried for 6 hours on stove and had their masses determined gravimetrically.

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All solvents were used as received. The mass of the asphaltenes and API gravity of each oil

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are shown on Table 1. Sample UO01 presented a higher Asp_Hept content when compared to

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Asp_Pent, opposite the trend of the other samples. The °API seems to be not related with

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asphaltene content in this case[4], probably due to the oil source material and/or following

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alteration, as reported by Evdokimov.[28,29]

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Table 1. Amount of precipitated asphaltenes and API gravity for each crude oil. The content

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of asphaltenes are shown in m/m percentage relative to the whole crude oil mass SAMPLE NAME

C5-Asphaltenes,

C7-Asphaltenes,

(% m/m)

(% m/m)

°API

UO01

27.0

12.85

18.64

UO02

30.0

6.00

2.47

UO03

24.0

12.87

10.22

UO04

16.4

13.14

7.11

UO05

27.0

8.53

3.93

UO06

23.5

11.80

5.07

UO07

22.4

11.92

7.58

UO08

23.4

12.78

5.54

UO09

22.7

12.23

4.52

UO10

23.3

15.16

5.83

13 14

Petroleomics analysis by FT-ICR MS. MS analysis was performed using a 7.2T

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LTQ FT Ultra mass spectrometer (Thermo Scientific, Bremen, Germany) equipped with

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IonMax ESI and APPI ionization sources operated on both positive and negative ion modes.

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For ESI analysis 1 mg of the crude oil or asphaltenes was diluted on 1 mL of a 1:1(v/v).

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methanol:toluene mixture. To improve the ionization efficiency, some additives were added:

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for positive ion mode, formic acid (HForm) and for negative ion mode ammonium hydroxide

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Another

set

of

samples

for

ESI(-)

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1

(NH4OH).

analysis

was

prepared

using

2

tetramethylammonium hydroxide (TMAH). All additives were added in methanol at 0.1% v/v

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concentration prior to mixing with samples diluted in toluene. For APPI analysis, samples

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were diluted only in toluene to reach final concentrations of 1 mg mL-1 for crude oils and of 2

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mg mL-1 for asphaltenes.

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Ionization conditions were set as follows: source voltage 3.6 kV for positive ion mode

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and -3.1 kV for negative ion mode, tube lens voltage at ±160 V, capillary temperature at 280

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°C and syringe flow of 5 µL min-1 for ESI and 20 µL min-1 for APPI. Spectra were acquired

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on 200-1000 Da mass range with 400,000@400 resolving power for ESI and 750,000@400

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for APPI. A hundred scans were averaged for each analysis. Data treatment was carried out

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through homemade software PetroMS[30] and the principal component analyses (PCA) were

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conducted using Pirouette 3.11 (Infometrix, Inc., Woodinville, WA) with mean-centered data

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of relative abundance of heteroatom classes. Several PCAs were performed to investigate the

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ionization behavior of the crude oils and their asphaltene fractions: one PCA was performed

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for each of the five ionization strategies and another PCA was performed using the combined

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data for all ionization strategies. Classes that were not present in some samples were

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considered to have zero of intensity in those samples for the PCA analysis.

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The results were first compared inside each ionization strategy, to understand the

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ionization behavior of the crude oil and their types of asphaltenes, and then overall compared.

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RESULTS AND DISCUSSION

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ESI(+) FT-ICR MS

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Figure 1 shows the ESI(+) FT-ICR mass spectra for the UO04 crude oil (a) and its

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asphaltenes precipitated in n-pentane (c) or n-heptane (e). Attributed ions are marked with

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color dots, whereas the DBE x Cn plots are shown in (b) and in (d), respectively. It is

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observed that the MS profile of asphaltenes is drastically different from that of crude oil, with

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a significant decrease of absolute intensity. as additive in ESI(+) of Asp_Pent provided lower

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relative intensity for the N1 class and higher relative intensity of oxygen and sulfur

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containing classes (NxOx, Ox and OxSx) in comparison to the crude oil. The detection of

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more diverse classes in Asp_Pent can be explained by a signal-enhancement effect produced

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by the extraction of major N1 class compounds during the asphaltene washing. In crude oils,

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the presence of N1 compounds usually suppress the ESI(+) ionization of NxOx, Ox, and

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OxSx compounds, and so reducing their absolute intensity. This finding suggests that the N1

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class compounds found in crude oil are less aromatic when compared to these oxygen and

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sulfur compounds appearing in Asp_Pent, since N1 compounds are soluble in the n-alkane

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solvent whereas NxOx, Ox and OxSx compounds are not. This characteristic was confirmed

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by the DBE vs Cn plots (b) and (d), which show a higher dispersion on Cn for crude oil, in

8

contrast to Asp_Pent plots that show compounds mainly in low Cn regions for each DBE.

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The remaining N1 compounds that appears in Asp_Pent are also highly aromatic and in less

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number than those in crude oil. Remarkably, no attributed ions resulted from the Asp_Hept

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analysis using formic acid in ESI(+) (Figure 1 (e)), indicating a much lower basicity of the

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compounds that precipitate with n-heptane as compared to n-pentane. Similar trends as those

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seen in Figure 1 were also observed for all other samples. ESI(+) FT-ICR MS analysis

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showed, that constituents from both asphaltene display quite low basicity, which is more

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pronounced when n-heptane is used (Asp_Hept, Figure 1(e)), mainly due to their high

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aromatic stability as shown by the DBE trends which makes protonation unfavorable.

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1 2 3

Figure 1. (a) ESI(+) FT-ICR mass spectra for the UO04 crude oil, and their asphaltenes precipitated in (c) n-pentane and (e) n-heptane using formic acid (ESI_HForm) as additive. DBE versus carbon number plots for the N1 class of (b) crude oil and (d) Asp_Pent.

4 5

ESI(-) FT-ICR MS.

6

For ESI(-), two additives were evaluated separately: NH4OH that favors naphthenic

7

acids (O2 class) ionization or TMAH that favors carbazols (N1 class). Figure 2 shows

8

representative ESI(-) FT ICR mass spectra and plots for the crude oil UO05 sample , and its

9

Asp_Pent and Asp_Hept using NH4OH as additive. The decrease of ion absolute intensities

10

from crude oil to asphaltenes was also observed, but for ESI(-), the Asp_Hept analysis

11

resulted in attributed ions, when no ions could be attributed using ESI(+), (Figure 1(e)).

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Asp_Pent spectrum displayed overall higher intensity and greater variety of the oxygen and

13

sulfur containing classes compared to crude oil with a subtle decrease of the O2 class. For

14

most samples, the number of O2 class ions increases from crude oil to Asp_Pent and, even

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so, the relative intensity of this class diminishes. This trend is the result of the lower ESI(-)

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ionization efficiency of the O2 compounds present in asphaltenes when compared to those in

17

the crude oil.

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Asp_Hept displays lower variety of classes and lower intensity of the O2 class as compared

19

to Asp_Pent. This trend shows that in the crude oil, most O2 class compounds have low

20

aromaticity, being more soluble in n-heptane as compared to n-pentane.

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The increase in oxygen and sulfur containing classes (NxOx, Ox and OxSx) from crude oil to

22

asphaltenes was also noted in ESI(-) using TMAH, but in this case, Asp_Hept showed lower

23

complexity of heteroatomic classes when compared to Asp_Pent (Figure S2 in Supporting

24

Information for the UO06 sample). Lower absolute intensities were also found in the

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asphaltenes when compared to the crude oil. On the other hand, the CH class ionized by ESI(-

26

) in crude oil is absent in both asphaltenes, revealing the low aromaticity and polarity of this

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CH class, being soluble in both n-alkanes. Concerning the N1 class, the compounds with

28

highest intensity were concentrated on higher DBE for each carbon number when moving

29

from crude oil to Asp_Pent and finally to Asp_Hept. This trend shows the higher aromaticity

30

of Asp_Hept (Figure S3). The O2 class also displayed lower intensity, but did not concentrate

31

on the high-DBE-low-carbon-number regions as the N1 class. This result suggests that highly

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aromatic carboxylic acids are either absent or have low ionization efficiencies in ESI(-) using

2

TMAH as additive (Figure S3).

3

Another important fact is that the trends of the O2 and N1 classes observed in sample UO06

4

were also noticed in the rest of the analyzed samples. This result strongly suggests that

5

asphaltene acidity is mainly due to pyrrolic derivatives rather than carboxylic acids and that

6

the best ESI (-) method for asphaltene analysis is, therefore, to use TMAH as the additive

7

since it favors the ionization of the N1 class. These findings also show that, if present, the

8

carboxylic group can be found on asphaltenes in multifunctional molecules as indicated by

9

the higher abundances of the OxSx and NxOx classes.

10 11 12 13

Figure 2. (a-c) Mass spectra and (d-f) class charts of UO05 for ESI(-) using NH4OH as additive. Crude oil plots are shown on (a) and (d), Asp_Pent on (b) and (e) and Asp_Hept on (c) and (f).

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APPI(+) FT-ICR MS

1 2

APPI(+) FT-ICR MS provided overall low intensity spectra for asphaltenes (Figure S4), with

3

heteroatomic class composition varying little between crude oil and their asphaltenes. As

4

Figures 3(a-c) show, the major differences were noted for the N1O1, N1O2 and O1 classes,

5

which

6

oil