Ionizing Aromatic Compounds in Petroleum by Electrospray with

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Ionizing Aromatic Compounds in Petroleum by Electrospray with HCOONH4 as Ionization Promoter Jincheng Lu, Yahe Zhang, and Quan Shi Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b00022 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on February 28, 2016

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Analytical Chemistry

Ionizing Aromatic Compounds in Petroleum by Electrospray with HCOONH4 as Ionization Promoter Jincheng Lu, Yahe Zhang, Quan Shi* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249 China

Abstract Electrospray ionization (ESI) coupled with Fourier ion cyclotron resonance mass spectrometry (FT-ICR MS) has been successfully used for molecular characterization of petroleum. However ESI cannot ionize non-polar components which generally are dominant in petroleum fraction. Here we introduce a novel approach for aromatic compounds molecular characterization. Aromatics in petroleum fractions were ionized to [M+H]+ by positive-ion ESI

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with HCOONH4 as an ionization promoter; combining with high resolution FT-ICR MS, aromatic hydrocarbons and heteroatoms in petroleum fractions can be simultaneously analyzed. The method is easily available and has potential for the characterization of aromatic compounds in any other matrix.


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Introduction Electrospray ionization (ESI) coupled to Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has become a powerful technique for detailed molecular characterization of polar species in petroleum. However, non-polar species cannot be ionized directly in common ESI. Some chemical derivatization methods have been developed to transform non-polar species to polar species which can be ionized by ESI. Fox example, saturates were oxidized to alcohols by ruthenium ion catalyzed oxidation;[1, 2] sulfur compounds were converted to methylsulfonium salts;[3] sulfides were selectively oxidized into sulfoxides.[4] Although these derivatization methods were effective for some non-polar species characterization, the operation is time consuming and laborious. To ionize poly aromatic hydrocarbons (PAHs), the complexation of Ag(I) cation to non-polar molecules was used to achieve the direct analysis of PAHs by ESI(+).[5, 6] Silver[7] and lithium[8] cationization were found to be sample approaches for the rapid speciation of sulfur-containing species in crude oils by positive-ion ESI FT-ICR MS. Atmospheric pressure photoionization (APPI) so far is considered as the most feasible technique for the characterization of aromatic compounds in petroleum.[9-11] However, the simultaneous generation of radical cations and protonated molecules complicated the APPI mass spectra, leading to challenges with regard to the mass resolving power. In this work, we introduce a more easy way than recent available approaches for the aromatic compounds characterization by positive-ion ESI FT-ICR MS with HCOONH4 as the ionization promoter.



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Experimental section Sample Description. A fluid catalytic cracking (FCC) diesel was obtained from PetroChina Liaohe refinery. The aromatic fraction was obtained using a solid phase extraction separation method developed by the Sinopec Research Institute of Petroleum Processing.[12] A FCC slurry oil was obtained from PetroChina Fushun refinery. Sample Preparation. The analytical-reagent-grade toluene and methanol were distilled twice and kept in glass bottles with ground glass stoppers. Formic acid (~98%) and ammonium hydroxide (28vol% NH3 in water) were purchased from Sigma Aldrich. Each of the samples was dissolved in a 1:3 (v/v) solution of toluene/methanol at a concentration of 0.2 mg/mL. Eight microliter of formic acid were added to every 1 mL of sample solution while 20 µL of ammonium hydroxide were added to every 1 mL of sample solution. The reagents and sample solutions were sonicated for a few seconds before ESI analysis. Each of the samples was dissolved in toluene at a concentration of 0.5 mg/mL before APPI analysis. The sample solution was infused by a syringe pump via an Apollo II electrospray source at a flow rate of 180 µL/h. Instrumentation, Data Acquisition and Processing. ESI. The conditions for positive-ion formation were -4.0 kV spray shield voltage, -4.5 kV capillary column introduction voltage, and 320V capillary column end voltage. Dry gas temperature was 200 ˚C. APPI. The conditions for positive-ion formation were -1.5 kV spray shield voltage, -2.1 kV capillary column introduction voltage, and 320 V capillary column end voltage. Dry gas


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temperature was 200 ˚C. The nebulizing gas temperature was 350 ˚C. FT-ICR MS. The MS analysis was performed using a Bruker Apex-ultra FT-ICR mass spectrometer equipped with a 9.4 T superconducting magnet. Ions accumulated for 0.1 s in a hexapole with 2.4 V direct-current (DC) voltage and 200 Vp-p radiofrequency (RF) amplitude. The optimized mass for Q1 was 100 Da. Hexapoles of the Qh interface were operated at 5 MHz and 200 Vp-p RF amplitude, in which ions accumulated for 0.2 s. The delay was set to 0.7 ms for fluid catalytic diesel aromatics (1.0 ms for slurry oil) to transfer the ions to the ICR cell by electrostatic focusing of transfer optics. The ICR was operated at 18 dB attenuation, 100-700 Da mass range, and 4M data size. The time domain data sets of 32 acquisitions were co-added. Mass Calibration and Data Analysis. Mass spectra were internally calibrated using extended homologous alkylation series (protonated molecules) of high relative abundance in a heavy oil mixture within the mass range of 100−500 Da and recalibrated with protonated molecules in the mass spectra, which exhibited relative high abundance in (+) ESI and (+) APPI mass spectra. Mass spectrum peaks with relative abundance greater than 6 times the standard deviation of the baseline noise were exported to a spreadsheet. Data analysis was performed using a custom software. The detail of data processing was described elsewhere.[13, 14]



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Results and Discussion Aromatic of Fluid Catalytic Cracking Diesel. Figure 1 shows the ESI(+) FT-ICR MS results of the aromatic fraction of the diesel. Mass peaks with high relative abundance are [M+H]+ of HC and N1 class species (Figure 1A). Four class species including HC, N1, O1, and O2 were detected. The relative abundance of these class species are shown in Figure 1B. The distribution of carbon number versus double bond equivalent (DBE) of HC and N1 class species were shown in Figure 1C and Figure 1D, respectively. It is well known that HC almost cannot be ionized by ESI both in positive and negative modes. However, with the participating of HCOONH4, HC class species are generated in high abundance in the mass spectrum. The most abundant HC class species has a carbon number of 17 with DBE value of 10, which corresponding to trimethyl phenanthrenes (molecular weight: 220). These compounds were confirmed by gas chromatography-mass spectrometry (GC-MS) analysis (See Figure S1 in Supporting Information). However the relative abundance of alkyl benzenes (DBE=4) and alkyl naphthalenes (DBE=7) from ESI MS are much lower than that detected by GC-MS. This implies the ESI ionization has different efficiency and/or mass discrimination of the FT-ICR MS on low mass end leads to the low response of small molecules. For N1 class species, the most abundant compounds have a DBE value of 9 with carbon number of 14~16. The compounds should be C2-C4 alkyl carbazoles, which were mostly present in the resin fraction and abundant in FCC diesels.[15] Since ESI(+) can ionize HC class species with HCOONH4, it is no surprise that the non-basic nitrogen compounds were detected in positive ESI. In other words, basic and non-basic nitrogen compounds could be analyzed simultaneously. The ionization of oxygen containing compounds could be also attribute to the existing of aromatic structures instead of oxygen function groups.


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Figure 1. Analysis results of the aromatic fraction from diesel by positive-ion ESI FT-ICR MS with HCOONH4: the mass spectrum(A), relative abundance of assigned class species(B), iso-abundance plots of DBE as a function of the carbon number for HC (C) and N1 (D).

Comparative Analysis of FCC Slurry Oil by ESI and APPI. FCC slurry oil is known as having high concentration of polycyclic aromatic compounds (PAHs). To validate that HCOONH4 promoted ESI is an effective approach for aromatic compounds analysis, a comparative analysis of the FCC slurry oil was performed between ESI and APPI ionization. The results were shown in Figure 2. Although there is a low mass as well as a severe high mass cut-off in the ESI mass spectrum, the mass spectrum of ESI(+) is generally similar to that obtained by APPI(+) (Figure 2A). The relative abundance of assigned class species is shown in Figure 2B. The HC, N1, N1O1, N1O2, O1, O2, and S1 are assigned in ESI(+) with HCOONH4. More class species are detected in ESI than that in APPI, such as N1O1, N1O2, and O2 class species.



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B +APPI APPI (+)

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Figure 2. Mass spectra of the FCC slurry oil obtained by positive-ion APPI and positive-ion ESI FT-ICR MS with HCOONH4 (A), and relative abundance of assigned class species (B). Figure 3 shows the plots of DBE as a function of the carbon number for HC, N1, O1, S1 classes derived from APPI and ESI FT-ICR MS of the slurry oil. The results are consistent in the distribution of HC, N1, O1, and S1 class species. The little difference is the distribution range of DBE and carbon number. The range of DBE is 3-32 and carbon number is 10-45 for HC in APPI while the range of DBE is 4-30 and carbon number is 15-40 in ESI. The dominant HC compounds are polycyclic aromatic hydrocarbons. For example, DBE value of 12, 13, 15 are corresponding to pyrenes, chrysenes, and benzopyrenes, respectively. The most abundant N1 class species are compounds with DBE value of 12, corresponding to benzocarbazoles. Since the results on molecular composition of ESI are comparable with that of APPI detectable compounds and more class species were detected in ESI ionization, we can conclude that the HCOONH4 promoted ESI is a more proper approach for the molecular characterization of aromatic compounds in petroleum, as well as in other complex mixtures. The additional


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benefits of the HCOONH4 promoted ESI include but are not limited to the following: it generates just protonated ions, lowering mass resolving power demand for the mass spectrometers than that using APPI; the ESI is operating at low temperature avoiding the possible dehydrogenation of aromatic compounds, which could be happened in the APPI analysis, especially for heavy petroleum fractions. 36

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Figure 3. Plots of DBE as a function of the carbon number for HC, N1, O1, and S1 class species assigned from APPI and ESI FT-ICR MS of the slurry oil.

Ionization Mechanism of Aromatics in ESI with HCOONH4. Aromatic hydrocarbons are non-polar species which cannot be ionized in the normal ESI analysis even with the electrolyte such as either ammonium or formic acid. HCOONH4 has



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flavonoids/terpenoids[16] and lipids.[17] Here, HCOONH4 should be acting as a charge carrier to facilitate the charge-separation process of positive-ion ESI, facilitating the formation of [M+NH4]+ adducts. With the evaporating of solvent, NH3 was released from the [M+NH4]+ adducts and leads to the formation of protonated ion of [M+H]+. The proposed pathway of the [M+H]+ ion formation in positive-ion ESI for aromatic compounds is shown in Figure 4. This speculation is supported by the fact that [M+NH4]+ could be found in the mass spectrum when using a low temperature drying gas for the ESI operation (not shown). However, the mechanism of the ionization should be investigated in further studies.

Figure 4. Proposed ionization pathway of aromatics in (+) ESI with HCOONH4.


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Conclusions We describe a simple and feasible method for the molecular characterization of aromatic compounds in petroleum by positive-ion ESI with HCOONH4 as ionization promoter. Both non-polar and low-polar compounds can be investigated simultaneously. The method was found to be an alternative of APPI ionization for non-polar compounds. When coupled with high resolution mass spectrometry, this is a promising approach for the molecular analysis of complex mixtures such as petroleum, coal liquids, PAHs in aerosols, soils, and waters, as well as other environmental samples.

Associated Content Supporting Information Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author *E-mail: [email protected].

Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC 21236009, and 21376262)

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Ionizing Aromatic Compounds in Petroleum by Electrospray with

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