B American Society for Mass Spectrometry, 2017 J. Am. Soc. Mass Spectrom. (2017) 28:2401Y2407DOI: 10.1007/s13361-017-1764-2

RESEARCH ARTICLE

Application of Atmospheric Solids Analysis Probe MassSpectrometry (ASAP-MS) in Petroleomics: Analysisof Condensed Aromatics Standards, Crude Oil,and Paraffinic Fraction

Lilian V. Tose,1 Michael Murgu,2 Boniek G. Vaz,3 Wanderson Romão1,4

1Petroleomic and Forensic Chemistry Laboratory, Department of Chemistry, Federal University of Espírito Santo, 29075-910,Vitória, ES, Brazil2Waters Technologies of Brazil, Alameda Tocantins 125, 27° Andar, Barueri, SP CEP: 06455-020, Brazil3Chemistry Institute, Federal University of Goiás, 74001-970, Goiânia, GO, Brazil4Federal Institute of Education, Science, and Technology of Espírito Santo, 29106-010, Vila Velha, ES, Brazil

Abstract. Atmospheric solids analysis probe mass spectrometry (ASAP-MS) is apowerful tool for analysis of solid and liquid samples. It is an excellent alternative forcrude oil analysis without any sample preparation step. Here, ASAP-MS in positiveion mode, ASAP(+)-MS, has been optimized for analysis of condensed aromatics(CA) standards, crude oil, and paraffinic fraction samples using a Synapt G2-SHDMS. Initially, two methodologies were used to access the chemical compositionof samples: (1) using a temperature gradient varying from 150 to 600 °C at a heatingrate of 150 °Cmin–1, and (2) with constant temperature of 300 and 400 °C. ASAP(+)-MS ionized many compounds with a typical petroleum profile, showing a greatersignals range of m/z 250–1300 and 200–1400 for crude oil and paraffin samples,

respectively. Such performance,mainly related to the detection of highmolecular weight compounds (>1000Da),is superior to that of other traditional ionization sources, such as ESI, APCI, DART, and DESI. Additionally, theCA standards were identified in both forms: radicals, [M]+•, and protonated cations, [M + H]+, with minimumfragmentation. Therefore, ASAP was more efficient in accessing the chemical composition of nonpolar and polarcompounds. It is promising in its application with ultrahigh resolution MS instruments, such as FT-ICR MS andOrbitrap, since molecular formulas with greater resolution and mass accuracy (<1 ppm) would be assigned.Keywords: ASAP(+)-MS, Hydrocarbons, Crude oil, Condensed aromatics

Received: 16 March 2017/Revised: 17 July 2017/Accepted: 17 July 2017/Published Online: 7 August 2017

Introduction

A mbient ionization mass spectrometry (AMS) has beendeveloped as a rapid, easy, inexpensive, simple, and

green analysis of materials at ambient pressure. It is a gentleway to generate ions. AMS is the simplest and most efficientalternative to transfer analytes directly from their natural environ-ments to high vacuum system of mass spectrometers [1]. In thelast decade, AMS has grown rapidly, and as a consequence therehas been an explosive increase in the number of applications that

include Bomics^ sciences, such as proteomics and metabolomics,as well as petroleomics [1]. By petroleomics, it is possible tocharacterize organic compounds in crude oil and their fractions atthe molecular level, and to correlate the chemical compositionwith their behavior in the production, extraction, and refiningprocesses of crude oil [2, 3].

Some AMS methods have already been applied inpetroleomics, such as easy ambient sonic-spray ionization(EASI) [4], Venturi easy ambient sonic-spray ionization(VEASI) [5], low-temperature plasma (LTP) [6], laser-inducedacoustic desorption chemical ionization (AP/LIAD-CI) [7, 8],laser-induced acoustic desorption combined with chemical ioni-zation by the cyclopentadienyl cobalt radical cation(LIAD/CpCo) [9], direct analysis in real time (DART) [10–12],atmospheric solids analysis probe (ASAP) [13–15], Figure 1a, b,

Electronic supplementary material The online version of this article (doi:10.1007/s13361-017-1764-2) contains supplementary material, which is availableto authorized users.

Correspondence to:Wanderson Romão; e-mail:wandersonromao@gmail.com

desorption atmospheric pressure chemical ionization (DAPCI)[16], and desorption electrospray ionization (DESI) [17].

The main applications in petroleomics using AMS areanalysis of saturated hydrocarbon mixtures [9, 10, 17], petro-leum model-molecule compounds (ellipticine, perylene,diphenylbenzoquinoline, benzo(ghi)perylene, coronene,rubrene, 9-phenylanthracene, benzo[c]benzofuran, benzo-[c]benzothiophene, and the hormone estradiol [7]; 5,10,15,20-tetraphenyl-21H,23H-porphine [10], 5,10,15,20-tetraphenyl-21H,23H-porphine vanadium (IV) oxide [10], and fullerene[12]), naphthenate deposits [12], some petroleum constituents(hydronaphthalenes, thiophenes, alkyl-substituted benzenes,pyridines, fluorenes, and polycyclic aromatic hydrocarbons[13]) [16], and classification [6], and characterization of crudeoil samples [4, 7, 8], and their respective distillation cuts [5].

Different ionization mechanisms may be present dependingon the type of ambient ionization source employed. Basically,for EASI [4], VEASI [5], and DESI [17] sources, an ionizationmechanism via proton transfer ([M + H]+ and [M – H]–) isusually present. On the other hand, in procedures using LTP[6], DAPCI [16], and AP/LIAD-CI [7], ionization of the analytecan occur simultaneously, by proton or electron (M•+) transfer aswell as by hydride abstraction ([M – H]+). The two first ioniza-tion mechanisms [([M + H]+/[M –H]– and M•+] are also presentin DART [10] and ASAP [15] sources, Figure 1c; however,depending on the polarity of the analyte, a specific ionizationmechanism can be triggered.

Among the AMS sources mentioned above, ASAP [13–15,18–24] is an ambient ionization source that provides simple andfast analysis without any sample preparation, Figure 1a, b. Thesample is simply applied to the glass capillary probe of theASAP under controlled heating without any sample preparation,Figure 1b. In addition, one of main advantages of ASAP forpetroleomics is its ability to use a temperature gradient duringthe procedure, which can simulate the fractionation or distillationof a typical crude oil sample. Thus, mass spectra are generatedwith greater amplitude of the dynamic range of m/z [15].

In 2014, Lv et al. [14] employed GC-MS and ASAP-TOF-MS to analyze mineral coal, in which they noted the presenceof alkanes, arenes, and organic compounds containing oxygen-ated species with mass values higher than m/z 500. Whu et al.[15] conducted experiments with paraffin, isoparaffins,naphthenates, and condensed aromatic (CA) standards byASAP-MS. Cyclic paraffins and CA compounds were mainlyionized by a charge-transfer mechanism; on the other hand,saturated fractions showed a greater range of compounds withm/z 400–1600. Wang et al. [13] also analyzed model com-pounds of pyrene, anthracene, and phenanthrene, in whichASAP generated radical cations, [M+•], with the formation offragments by cleavage or rearrangement. Such studies haveallowed the application of this methodology to the identifica-tion of mineral coal.

Herein, ASAP-MS in positive-ion mode, ASAP(+)-MS,has been optimized for analysis of five CA standards:(benz[a]anthracene, coronene, polyaromatic hydrocarbon(PAH) additional component mix, 2,9-dipropylanthra[2,1,9def:6,5,10d’e’f’] diisoquinoline 1,3,8,10(2H,9H) tetrone;and N,N’-bis(3-pentyl)perylene-3.4.9.10-bis(dicarboximide)), aswell as analysis of a crude oil and a paraffinic fraction using aSynapt G2-S HDMS. Two forms of ion acquisition were evalu-ated: the first (1) using a temperature gradient in the glasscapillary probe of ASAP (from 150 to 650 °C at a heating rateof 100 °Cmin-1); and the second (2 from an isothermal procedureat T = 350 and 400 °C.

ExperimentalReagents and Samples

Methanol, sodium trifluoroacetate (NaTFA), L-arginine, andmethanoic acid (HCOOH) analytical grade purity >99.99%,were purchased from Sigma-Aldrich Brasil Ltda., São Paulo,Brazil, and used for the FT-ICR MS calibration. Sodium for-mate and Leu-enk, also analytical grade purity >99.99%

Figure 1. Photographs of (a) the ASAP-MS system employed, and (b) the ASAP probe. (c) The ionizationmechanism via proton ([M+ H]+) and electron (M•+) transfers

2402 L. V. Tose et al.: Application of ASAP-MS in Petroleomics

(Sigma-Aldrich Brasil Ltda., São Paulo, Brazil), were used tocalibrate Synapt G2-S HDMS.

One crude oil and one saturated hydrocarbon fraction(CENPES/Petrobras, Rio de Janeiro, Brazil) were used in thiswork. Five CA standards were supplied by Sigma-AldrichChemicals. They were: (1) benz[a]anthracene; (2) coronene;(3) PAH additional components mix; (4) 2,9-dipropylanthra[2,1,9def:6,5,10d’e’f’] diisoquinoline 1,3,8,10(2H,9H) tetrone;and (5)N,N’-bis(3-pentyl)perylene-3,4,9,10-bis(dicarboximide).

The crude oil was characterized according to the stan-dards of the American Society for Testing and Materials(ASTM) to determine the API degree (ASTM D1298-99)and the kinematic viscosity (ASTM D7042-04). The char-acterization data obtained for the crude oil are as follows:API degree = 17.9, and viscosity = 1600 mm2 s-1 at 30 °C[25, 26]. The paraffin (food grade) was obtained directlyfrom a commercial process.

ASAP(+)-MS

ASAP-MS analysis was performed using a mass spectrometerby Q-TOF MS Synapt G2-S HDMS (high-definition massspectrometer; Waters, Manchester, UK, Figure 1a). This equip-ment had hybrid geometry, comprised of quadrupole and time-of-flight (Q-TOF) analyzers. The CA standards (1–5), crudeoils, and paraffin samples were analyzed using positive ioniza-tion mode, ASAP(+)-MS. Approximately 1 mg of sample wasdeposited directly onto a glass capillary and inserted into theASAP source, Figure 1b.

The conditions of operation of the ASAP probe were: tem-perature gradient from 150 to 650 °C at 100 °C min–1 and atconstant temperature of 350 °C (for crude oil and paraffinsamples); and from 100 to 350 °C or at constant temperatureof 400 °C for CA standards. Other parameters were: capillaryvoltage 3000 V; corona discharge 9000 nA; temperature of drygas 150 °C; and m/z range 200−1500. The resolving power

Table 1. CAs Standard Molecules (named 1–5) Analyzed by ASAP(+)MS

Standard measured

m/z

Theoretical

m/z

Error

(ppm)

[M+H]+

M+•

DBE

Standard 01 Benz[a]anthracene 228.101

229.105

228.093

229.101

35.07

17.46

[C18H12] • +

[C18H12 + H] +

13

Standard 02 Coronene 300.100 300.093 23.33 [C24H12] • + 19

301.108 301.102 19.93 [C24H12 + H] + 19

Standard 03 PAH Additional Components

Mix

253.102 253.101 3.95 [C20H13] • + 15

Standard 04 2,9 Dipropylanthra[2,1,9-

def:6,5,10d’e’f’]diisoquinoline

474.173 474.157 33.74 [C30H22N2O4 ]•+ 21

1,3,8,10(2H,9H)tetrone 475.177 475.165 25.25 [C30H22N2O4 + H ]+

Standard 05 N,N’-Bis(3-pentyl)perylene-

3,4,9,10-bis(dicarboximide)

530.231

531.238

530.220

531.228

20.75

18.82

[C34H30N2O4] •+

[C34H30N2O4 + H] +

21

L. V. Tose et al.: Application of ASAP-MS in Petroleomics 2403

(m/Δm50%) ranged from 38,000 to 47,000 at m/z 400, whereΔm50% is the full peak width at half-maximum peak height. Allmass spectra were acquired and processed using MassLynx 4.1software (Waters Corporation).

Results and DiscussionCAs Standards Analysis

ASAP(+) easily ionizes CA standards. Supplementary Figure 1S(Supporting Information) shows the ASAP(+) mass spectra forfive commercial CA standards identified on a temperature gra-dient of 100–350 °C at 50 °C min–1. In general, the CA stan-dards were mostly ionized by electron transfer (M+•) followedby proton transfer ([M + H]+). The values of experimental mass,molecular formula, double bond equivalent (DBE), and masserror of all molecules identified are shown in Table 1. ASAP(+)formed two ion types (M+• and [M + H]+), where nine peakswere detected instead the five expected. In all cases, DBE rangedfrom 13 to 21, with an average mass error of 30 ppm.

The ionization mechanism in an ASAP source occurs anal-ogously to APCI, in which the compounds are volatilized by aheated stream of nitrogen, and submitted to a proton transferreaction near to corona discharge, where protons, electrons, andradicals are formed, to generate [M + H]+ and [M]+• ions,Figure 1c. The peculiarity of this source is its ability to ionizepolar and nonpolar compounds simultaneously, reachinghigher amplitude of molecular weight distribution [27]. More-over, the ASAP(+) source provided the ionization of CA stan-dards with minimal fragmentation and sample preparation.

It is important to highlight that there was a subtle fragmen-tation for standard 3, producing the daughter ions of m/z 226and 202, Supplementary Figure 1S. However, such a phenom-enon was more pronounced for standards 4 and 5, whereseveral signals are observed at the m/z 110–390 region, whichis the ion ofm/z 390, the high-abundance fragment. The higher

lability of these two molecules is related to their naphthenicfraction (dicarboxyamide group). The fragment of m/z 390 isobserved for standards 4 and 5 (from 531 → 390 Da and 474→ 390 Da transitions), which are produced by cleavage of thelateral chain of the dicarboxyamide group with neutral losses ofC7H11NO2 and C4H6NO (141 and 84 Da, respectively).

With the aim of evaluating the influence of the temperatureof the ASAP probe on the thermal stability of CA standards,standards 4 and 5 were submitted to an isothermal heating at400 °C, Supplementary Figure 2S (see in SupportingMaterial),where molecules of m/z 474 and 531, [C30H22N2O4]

+• and[C34H30N2O4+H]

+ ions, respectively, are clearly degraded, asevidenced by the extensive fragmentation observed in theregion of m/z 100–250.

Crude Oil Analysis

A typical crude oil was also analyzed by ASAP(+) MS,Figure 2a–l, as a function of heating temperature of the capillaryprobe (150 to 650 °C). A more comprehensive molecular com-position of the sample was obtained, where the ASAP(+) sourceenabled the separation of molecules according to their boilingpoint, volatilizing and ionizing each set of compounds at aspecific temperature provided by a heating rate of 100 °C min–1. In all cases, a typical crude oil distribution is clearly observed,where the m/z range is enlarged with the increase in heatingtemperature of probe, ranging fromm/z 250–400 at 150–200 °Cto m/z 1000–1300 at 600–650 °C. Generally, when the massspectra are integrated from two sets of temperature intervals(150–350 °C and 400–650 °C), the resulting ASAP(+) massspectra display a distribution of m/z 250–780 (Figure 2l), andm/z 650–1350 (Figure 2k), respectively. In general, the resolvingpower of the mass spectra was around m/Δm50% ≈ 39,000 form/z 894 and 47,000 for m/z 478.

In 2014, Dalmaschio et al. [28] investigated the chemicalcomposition of 12 cuts and distillation residues produced from

Figure 2. ASAP(+) mass spectra for a crude oil as a function of heating temperature [from 150 to 600 °C (a–j)] and from theintegration of two set of temperature ranges: (k) 400 to 600 °C; and (l) 150 to 300 °C

2404 L. V. Tose et al.: Application of ASAP-MS in Petroleomics

one Brazilian offshore acidic crude oil sample (TAN = 3.19 mgKOH g–1) submitted to primary characterization using an in-house designed distillation process. The boiling points changedfrom 100.4 to 372.9 °C, where sulphur-, nitrogen-, and oxygen-containing compound classes (naphthenic acids, O2 class, andphenols, O1 class, sulfides, S1 class, and pyridines, N class)were identified with m/z ranges of 150–450 and carbon num-bers of C8 to C25. The average molecular weight distribution,Mw, shifted to higher m/z values (Mw = 169 to 321 Da) as afunction of distillation cut temperature. Correlating the dataobtained byDalmaschio et al. [28] to those reported in Figure 2,we observed that only the first m/z range of ASAP(+)-MS, T =150–200 °C, Figure 2i, corresponds to a typical petroleum cutprofile, such as that obtained at 305.9–372.9 °C. Species withcarbon number lower than C20 (m/z < 250) were not commonlyidentified by ASAP-MS via temperature gradient experiments;these compounds are typical of light petroleum cuts, such asgasoline, kerosene, and diesel. On the other hand, this problemcan be bypassed by using the isothermal heating mode on theASAP probe for ion acquisition.

The ASAP(+) mass spectrum was, therefore, obtained forthe crude oil at a constant temperature of 400 °C, Supplemen-tary Figure 3S (Supporting Information). The molecular com-pounds detected in crude oil ranged from m/z 200 to 900,

indicating that the lower molecular weight compounds (C20–C60) are easily detected using an isothermal procedure. On theother hand, a procedure using temperature gradient, Figure 2,favors the access, simultaneously, to the chemical compositionof low and high molecular-weight compounds (C20–C100). Thehigh molecular weight compounds are exclusively detected inFigure 2a–e, and correspond to the heavy fractions of crude oilsuch as resins and asphaltenes, as well as heavy basic nitrogencompounds that are typically present in vacuum residue [29].

Paraffin Analysis

Figure 3a–d show the ASAP(+) mass spectra for paraffinsample as a function of heating temperature of ASAP probe(150–600 °C). The paraffin has a distribution of m/z rangingfrom 200 to 1400. In general, the resolving power of the spectrawas around m/Δm50% ≈ 37,000 at m/z 1028. The ASAP(+)mass spectrum of paraffin in temperature interval of 600 to 400°C, Figure 3a, b, allows the selective identification of highmolecular weight HCs compounds containing m/z 800 to1300, whereas the Figure 2c, d show the distribution of lowmolecular mass HCs compounds withm/z 200–700 at 150–300°C. Integrating now mostly all the ASAP(+) spectra (i.e., thoseobtained from 200 to 500 °C, Figure 3e) note that a bimodalchemical profile is observed in the m/z range of 200 to 1300.

Figure 3. ASAP(+) of the paraffin A sample at heating temperature of 150 to 600 °C (a–d) and 200 to 500 °C (e)

L. V. Tose et al.: Application of ASAP-MS in Petroleomics 2405

The ASAP(+) mass spectra was also obtained for the paraf-fin at constant temperature around 400 °C, SupplementaryFigure 4S (Supporting Information). In this case, the HCs areonly accessed from m/z 200 to 1000.

ConclusionCA standards, crude oils, and saturated fractions were easilyionized via atmospheric solids analysis probe mass spectrometry(ASAP-MS), which accessed a more complete chemical com-position compared with other typical ionization sources (APCI,ESI, DART, DESI, etc.). From a temperature gradient (150 to650 °C) applied to the ASAP probe, a chemical profile as afunction of the boiling point of the sample can be obtained, thusconferring information about the refining process as well as theproduction or quality of the crude oil. ASAP favored the detec-tion of the highest amplitude of compound distribution, rangingfromm/z 200 to 1400 for saturated fractions andm/z 200 to 1300for crude oil. ASAP also accessed the chemical composition ofCA standards with minimum fragmentation and sample prepa-ration. In general, the coupling of the ASAP technique withhigh-resolution mass analyzers, such as FT-ICR MS andOrbitrap, is a promising analytical tool in petroleomics, allowingdefinition of the elemental composition (CcHhNnOoSs), and thedegree of unsaturation, i.e., double bond equivalent (DBE), ofmolecules, based on their the mass-to-change ratio (m/z).

AcknowledgmentsThe authors are grateful to CAPES (23038.007083/2014-40)and FAPES (73309516/16) for financing the research.

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