Gasoline, Kerosene, and Diesel Fingerprinting via Polar Markers

Article pubs.acs.org/EF

Gasoline, Kerosene, and Diesel Fingerprinting via Polar Markers Renato Haddad,†,§ Thaís Regiani,† Clécio F. Klitzke,† Gustavo B. Sanvido,† Yuri E. Corilo,† Daniella V. Augusti,‡ Vânya M. D. Pasa,‡ Rita C. C. Pereira,‡ Wanderson Romaõ ,† Boniek G. Vaz,† Rodinei Augusti,‡ and Marcos N. Eberlin*,† †

ThoMSon Mass Spectrometry Laboratory, Institute of Chemistry, University of Campinas (UNICAMP), 13083-970 Campinas, São Paulo (SP), Brazil ‡ Department of Chemistry, Federal University of Minas Gerais (UFMG), 31270-901 Belo Horizonte, Minas Gerais (MG), Brazil ABSTRACT: Venturi easy ambient sonic-spray ionization mass spectrometry working in the liquid mode (VL-EASI-MS) is shown to provide rapid and reliable characterization of crude oil distillates. With no extraction or pretreatments, samples of gasoline, kerosene, diesel, and admixtures of gasoline/diesel and gasoline/kerosene were directly analyzed by VL-EASI(+)-MS. Homologous series of natural N-heteroaromatics were detected in their protonated forms providing typical profiles of polar markers. For gasoline, VL-EASI(+)-MS detects a homologous series of mainly C1−C5 alkyl pyridines. For kerosene, a typical series of alkylated tetrahydroquinolines is detected. For diesel, the V-EASI(+)-MS profile is much richer because of the detection of several classes of many N-heteroaromatics. VL-EASI(+)-MS is also shown to provide typical spectra for petrochemical gasolines detecting specific antioxidants. Admixtures of gasoline/kerosene and gasoline/diesel are also clearly characterized with limits of detection of 10 and 1% (v/v), respectively. Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) has been used to confirm assignments of the polar markers detected by VL-EASI(+)-MS.

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INTRODUCTION Gasoline, kerosene, and diesel are mainly composed of complex mixtures of homologous series of hydrocarbons (HCs) obtained from petroleum refining.1 Gasoline is a light crude oil product containing a HC distribution from C4 and C12 carbon atoms. Kerosene contains HCs from C8 to C18 carbon atoms, whereas diesel is much heavier and is comprised of HCs with C8−C40 carbon atoms. The characterization and quality control of such complex HC mixtures are therefore normally performed via gas chromatography separation,2 which provides typical profiles of the increasingly heavier and diverse HC homologous series of gasoline, kerosene, and diesel. Crude oils are also characterized by gas chromatography via HC profiles. However, alternatively, this highly complex mixture can also be comprehensively characterized by direct mass spectrometry (MS) analysis without pre-separation steps via profiles of their polar components. These natural components have been shown to function as natural markers for major crude oil properties, such as type, origin, and biodegradation, in a field that has became known as petroleomic MS.3 Recently, ambient mass spectrometric techniques requiring no pre-separation or sample preparation protocols have been introduced,4 and the application of this technique to fuel analysis has been demonstrated.5 We have shown that comprehensive analysis of crude oils6 can be performed via direct desorption and ionization performed at ambient conditions using one of such techniques: easy ambient sonic-spray ionization (EASI).7 EASI-MS has also been widely and successfully used to the direct analysis of biodiesel.8 More recently, we have introduced a simpler variant of EASI, named Venturi EASI (V-EASI)9 able to operate in dual mode for liquid (VL-EASI) or solid (VS-EASI) samples. The technique simplifies analysis by eliminating electrical pumping and © 2012 American Chemical Society

incorporating solution self-pumping because of the Venturi effect. In the VL-EASI mode, it uses a high stream of nitrogen or gas to produce both self-pumping of the analyte solution and proper ionization via sonic spray10 (Figure 1). Additionally, adulteration of fuel is a common illegal practice worldwide, and gasoline has been adulterated by the addition of kerosene or diesel.11 Herein, we test the use of VL-EASI as a direct method of characterization and quality control of three major crude oil distillates: gasoline, kerosene, and diesel.

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EXPERIMENTAL SECTION

Reagents and Samples. Samples of gasoline (C type) and diesel (one of each raw material) were supplied by the Analytical Center at the University of Campinas. A commercial sample of kerosene was purchased at King, Ltd. (Rio de Janeiro, Brazil). All of the samples were within the specifications of the Brazilian Agency for petroleum and fuels (ANP regulation 309/2001). Gasoline was adulterated by the addition of diesel at the following proportions (%, v/v): 99:1, 90:10, 75:25, and 50:50. Blends of gasoline and kerosene were prepared at the 90:10, 75:25, and 50:50 (%, v/v) proportions. The samples of gasoline, diesel, kerosene, and admixtures (500 μL each) were dissolved in 1 mL of methanol [with 0.1% (m/v) of formic acid], and the resulting solutions were transferred to Eppendorf tubes to be analyzed by the V-EASI-MS device (Figure 1). VL-EASI-MS. The V-EASI source used was constructed as described in details elsewhere.9 Briefly, in one of the holes of the T-shaped pipe (stainless steel), a silica capillary with a diameter of 100−125 μm was connected. By the Venturi effect, the solutions in the Eppendorf tubes were self-pumped at a flow rate of roughly 20 μL min−1 and sprayed directly into the entrance of the mass spectrometer (a single quadrupole instrument, model 2010EV, Shimadzu Corp., Japan) Received: February 17, 2012 Revised: April 23, 2012 Published: April 30, 2012 3542

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Figure 1. V-EASI-MS setup used for the analysis of diluted methanolic solutions of crude oil distillates. analyzer. The mass spectra were recorded in the positive-ion mode and using a mass range of m/z 100−800. Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). The samples (50 μL) were dissolved in 1 mL of methanol [with 0.1% (m/v) of formic acid], and the resulting solutions were directly infused, by means of a 500 μL microsyringe (Hamilton, Reno, NV) at a flow rate of 5 μL min−1, into the electrospray ionization (ESI) source. The Fourier transfer ion cyclotron ressonance mass spectrometer (model 7 T LTQ FT Ultra, ThermoScientific, Bremen, Germany) was set to operate in the positive-ion mode and in the m/z range of 100−800. The ESI source conditions were as following: gas pressure of 0.3 psi, capillary voltage of 3.1 kV, and ion-transfer capillary of 270 °C. Mass spectra were acquired by summing up 100 microscans and processed using the Xcalibur 2.0 software (ThermoScientific, Bremen, Germany). Using a custom algorithm developed specifically for petroleum data processing (the PetroMS software),6 the MS data were handled and the elemental composition of the compounds were attributed by the measurement of m/z values. To help summarize, visualize, and interpret these remarkably complex MS data, classical plots of the carbon number versus the double-bond equivalent (DBE) values were built for the components detected under these conditions (polar constituents, mainly mono-nitrogenated N compounds).

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RESULTS AND DISCUSSION Natural Markers. Diluted methanolic solutions (ca. 1%, v/ v) of gasoline, kerosene, and diesel were directly analyzed with no sample pretreatment or extraction procedures, and VL-EASI mass spectra in the positive-ion mode were acquired (panels a− c of Figure 2). For gasoline (Figure 2a), a characteristic series of marker ions of m/z 94, 108, 122, 136, 150, and 164 was detected. As known from previous petroleomics studies12 and confirmed via the molecular formula provided by highresolution and high-accuracy FT-ICR MS measurements (see below), these ions correspond to the protonated molecules of a homologous series of C1−C5 alkyl pyridines with the general formula Py−(CH2)nCH3, with n = 0−4 and a DBE of 4. Shi at al.13 also verified that mono-nitrogenated heteroaromatics are usually the dominant species in petroleum products, with an abundance of up to 93%. Marshall et al.14 showed that such N compounds are among the most stable polar species present in crude oils, being very difficult to remove, even after catalytic treatments. The VL-EASI(+)-MS data of kerosene (Figure 2b) was also characteristic. Contrary to gasoline that displays basically a single homologous series of C1−C5 alkyl pyridine polar markers, kerosene displays two overlapping homologous series of polar markers. The series of alkyl pyridines detected in gasoline is also detected in kerosene, but the maximum of the Gaussian distribution is shifted accordingly to higher masses

Figure 2. VL-EASI(+)-MS of fresh methanolic solutions of (a) gasoline, (b) kerosene, and (c) diesel.

Figure 3. V-EASI(+)-MS of methanolic solutions of a gasoline sample with the detection of an “artificial marker”, the antioxidant additive N,N′-di-sec-butyl-p-phenylenediamine.

(most abundant ions of m/z 122, 136, and 150). However, the most typical series of polar markers for kerosene is formed by 3543

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Figure 4. V-EASI(+)-MS of methanolic solutions of gasoline/kerosene admixtures: (a) 50:50, (b) 75:25, and (c) 90:10 (%, v/v).

Figure 5. V-EASI(+)-MS of methanolic solutions of gasoline/diesel admixtures: (a) 50:50, (b) 75:25, and (c) 99:1 (%, v/v).

cyclic tetrahydroquinolines (mostly the ions of m/z 176, 190, 204, 218, 232, and 246) from C2 to C7 alkyl groups and DBE of 5 (because of an aromatic ring plus a saturated ring). Diesel also displayed a very characteristic, unique, and rich VL-EASI(+)-MS profile with a much greater diversity of polar markers (Figure 2c). As shown by FT-ICR MS analysis (see below), this high complex profile with a Gaussian distribution centered at m/z ≈ 400 is mainly comprised of a homologous series of N-polycyclic heteroaromatic compounds. Artificial Markers. Figure 3 shows an interesting VLEASI(+)-MS profile for a gasoline sample, with the characteristic series of marker ions of m/z 94, 108, 122, 136, 150, and 164 for the C1−C5 alkyl pyridines. However, an additional and quite abundant ion of m/z 221 is also detected. FT-ICR MS data show a molecular formula corresponding to a known gasoline antioxidant: N,N′-di-sec-butyl-p-phenylenediamine. This compound is commercialized as “Santoflex” (Figure 3) and is a common antioxidant added to the commercial Brazilian gasoline.15 This result indicates, therefore, that direct VLEASI(+)-MS is also able to identify additives in petrofuels and that these additives may also function as artificial markers. The VL-EASI(+)-MS/MS of the Santoflex ion of m/z 221 (not shown) was also acquired and compared to that of an standard, and great similarity was observed with major fragment ions of m/z 164 and 135. Adulteration. To verify whether these profiles of polar markers would be useful in recognizing fuel adulterations, particularly gasoline adulterated with kerosene or diesel, admixtures of gasoline/diesel and gasoline/kerosene were prepared and analyzed. Figures 4 and 5 show the V-EASI(+)

Figure 6. Plot of I190/I122 (see the text) versus the kerosene concentration (%, v/v) in kerosene/gasoline admixtures.

mass spectra of these “blends”. Note that, when compared to the pure samples, these admixtures are readily recognized by VL-EASI(+)-MS. A limit of detection of ca. 5% (v/v) was determined for kerosene addition to gasoline. For diesel addition to gasoline, VL-EASI(+)-MS was much more sensitive, easily detecting adulteration at levels as low as 1% (v/v) (Figure 5c). 3544

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Figure 7. (a) FT-ICR MS of a methanolic solution of diesel obtained and (b) plot of the relative intensities (%) versus DBE built from the FT-ICR MS data. Putative structures of the N class components are also indicated.

roMS),6 molecular formula attribution was possible for the marker ions. Similar to that verified for gasoline and kerosene, the most abundant ions observed in VL-EASI(+)-MS of diesel (Figure 7a) were found to correspond to mono-N-polycyclic heteroaromatics. Figure 7b shows a plot of the relative intensity (%) versus DBE for these molecules and their putative structures. The most abundant species, which present DBE values of ca. 7−10, were previously reported.13 In addition, the natural markers (N compounds; Figure 2b) for kerosene (DBE of 5) are also detected in diesel but at lower concentrations. Note that the natural markers (class N compounds; Figure 2a) for gasoline (DBE of 4) are remarkably absent in diesel. In petroleomic MS, Kendrick17 and van Krevelen18 diagrams are classical plots used to visualize trends in crude oil composition. Another useful plot is that in which DBE is ploted versus carbon number, and such plots were therefore constructed for the VL-EASI(+)-MS data of gasoline, kerosene, and diesel (panels a−c of Figure 8). Note that each sample displays characteristic distributions in such plots. The HC

Additionally, using the absolute intensities for peaks corresponding to the ions of m/z 190 and 122 (I190/I122) as a measure for kerosene concentration and plotting I190/I122 versus the actual kerosene concentration (%, v/v) in the admixture, a quite linear relationship (R2 of 0.97) was obtained (Figure 6). Therefore, the following equation can be used to estimate the concentration of kerosene present in a given sample of commercial gasoline y = 0.061x + 0.011

where y = I190/I122 and x = kerosene concentration (%, v/v). Skrobot and co-workers,16 for instance, applied chemometry on chromatographic data to determine the content of assorted oil products (naphtha, thinner, or kerosene) added to commercial gasoline with a limit of detection of ca. 20%. To confirm the identity of the polar markers, VL-EASI(+)MS was also performed using high-resolution and high-accuracy FT-ICR MS (Figure 7). After proper data processing using homemade software for petroleomic data treatment (Pet3545

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Figure 8. Plots of DBE versus C (carbon number) for (a) gasoline, (b) kerosene, (c) diesel, (d) admixture of 80:20 gasoline/kerosene (%, v/v), and (e) admixture of 80:20 gasoline/diesel (%, v/v).

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CONCLUSION Direct analysis of crude oil distillates via MS using VL-EASI, an ambient ionization technique, detects characteristic set of polar markers, providing therefore an efficient methodology for rapid characterization and quality control of major crude oil distillates: gasoline, diesel, and kerosene. The methodology uses no demanding and time-consuming sample pretreatment or pre-separation steps. It also provides proper monitoring and detection of gasoline adulteration by the addition of diesel and kerosene. Polar additives or impurities can also be readily detected, as shown by the detection of a common gasoline antioxidant. Such additives could be alternatively used as artificial markers in a type of “refinery labels” for the certification of origin. Abnormal ions would also indicate adulteration via, for instance, the addition of low-quality solvents. Petrochemical gasoline samples could also be readily recognized via the lack of such natural markers. On-site

homologous series of polar markers for gasoline ranges from C5 to C15 (for total carbon numbers), with a maximum around C8 (Figure 8a) and maximum DBE of 4. For kerosene (Figure 8b), polar markers range from C8 and C16 centered around C12, with a maximum DBE of 5. Diesel displays a much broader (C15−C50) polar marker distribution centered at C27, with a maximum DBE from 6 to 9. Panels d and e of Figure 8 show plots of DBE versus C (carbon mumber) for gasoline/ kerosene and gasoline/diesel admixtures, respectively, at a proportion of 80:20 (%, v/v). Both profiles are dramatically distinct in comparison to that of gasoline (Figure 8a). Finally, note the much greater dispersion of points in the plot of Figure 8e (gasoline/diesel) in comparison to that of Figure 8d (gasoline/kerosene). 3546

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gasoline analysis, such as in gas stations, could also be performed using portable mass spectrometers equipped with portable, even disposable VL-EASI sources.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: 55-19-35213049. E-mail: eberlin@iqm. unicamp.br. Notes

The authors declare no competing financial interest. § In memoriam.

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ACKNOWLEDGMENTS The authors thank the Brazilian research foundations FAPESP, FAPEMIG, CNPq, and FINEP and the Brazilian petroleoum agencies ANP and Petrobras for assistance and financial support.

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DEDICATION This paper is especially dedicated (in memoriam) to Dr. Renato Hadaad for his seminal work on the early developments and applications of the EASI technique7.

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