Spray Injection Direct Analysis in Real Time (DART) Ionization for

May 10, 2016 - Negative- and positive-ion direct analysis in real time (DART) ionization coupled to Fourier transform ion cyclotron resonance mass spe...
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Spray Injection Direct Analysis in Real Time (DART) Ionization for Petroleum Analysis Limin Ren,† Yehua Han,*,† Yahe Zhang, Yanfen Zhang, Xianghai Meng, and Quan Shi* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China S Supporting Information *

ABSTRACT: Negative- and positive-ion direct analysis in real time (DART) ionization coupled to Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was applied to characterize crude oil and its fractions. Crude oil samples dissolved in toluene were directly infused into a spray needle, which produced a continuous and long-time stable ion current for FT-ICR MS analysis to obtain mass spectra with a broad dynamic range and high signal-to-noise ratio. A comparison between negative-ion electrospray ionization [ESI(−)] and negative-ion DART for crude oil analysis was presented. The DART(−) ionized almost all of the compound classes found in ESI(−), while it exhibited high selectivity on naphthenic acids, which enabled the characterization of naphthenic acids in petroleum with a low total acid number (TAN). The method is suitable for the analysis of naphthenic acids in petroleum distillation cuts, even with a very high boiling point. Sulfides in petroleum were likely oxidized to sulfoxides and exhibited high selectivity in positive-ion DART, indicating that it can potentially be used for the molecular characterization of sulfides in petroleum.

1. INTRODUCTION Crude oil is a complex mixture containing potentially billions of elemental compounds. Many analytical techniques have been developed and used to characterize the chemical composition and mass distribution of crude oil and its fractions. Among them, high-resolution mass spectrometry is a powerful tool to achieve molecular composition. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) possesses ultrahigh mass resolving power and mass accuracy, enabling routine identification of very small mass differences.1 The ultrahighresolution power of FT-ICR MS provides new opportunities for in-depth analysis of the composition of crude oil at the molecular level. FT-ICR MS has been successfully used for petroleum analysis, leading to a science of “petroleomics”.2−4 Soft ionization techniques play an important role in the FTICR MS application on petroleum analysis, which include electrospray ionization (ESI),5−11 atmospheric pressure photoionization (APPI),6,12−14 low-voltage electron ionization (EI),15,16 nanospray desorption electrospray ionization (nanoDESI),17 field desorption ionization (FD),16,18,19 atmospheric pressure chemical ionization (APCI),20,21 atmospheric pressure laser ionization (APLI),22,23 matrix-assisted laser desorption ionization (MALDI),6,24 laser desorption atmospheric pressure photochemical ionization (LD/APPCI),25 and direct analysis in real time (DART) ionization.26−28 However, even using FTICR MS coupled with various soft ionization techniques, the complex chemical composition of crude oil and its fractions has not yet been completely understood. One problem is that none of the available ionization techniques can ionize all of the compounds in crude oil simultaneously.6,14 ESI is the most commonly used ionization source and especially efficient at ionizing polar compounds with nitrogen or oxygen atoms; however, it has serious tendency toward ion suppression caused by contaminants in solvents and/or the ionization source. For example, in the negative-ion ESI mass spectra, naphthenic acids © XXXX American Chemical Society

are seriously suppressed by C16 and C18 fatty acids, which are generally considered as contaminants from solvents.8,29,30 In comparison to ESI, an outstanding advantage of DART is its relatively lower tendency of ion suppression and matrix effect, resulting in wider applicability for screening of targeted compounds in complex matrices,31,32 such as forensics,33 metabolomics,34 pharmacokinetics,35,36 and food and beverage safety analysis.37,38 Since it was introduced in early 2005, DART has been a successful ambient ionization method in various applications.39 Using helium or nitrogen as the working gas, DART could produce heated metastable plasma that impinge on samples, causing desorption and ionization of analytes in the ambient atmosphere. The application of DART coupled to FT-ICR MS for petroleum analysis has been reported by Rummel et al.28 and Lobodin et al.26 However, the ionization feature of DART for the whole petroleum component has not be well investigated. In this paper, a spray injection was used for DART analysis (spray-DART). Spray-DART was coupled with FT-ICR MS to characterize petroleum fractions. The selectivity and capabilities of the technique for petroleum analysis were investigated by comparing to positive- and negative-ion ESI results.

2. MATERIALS AND METHODS 2.1. Samples and Reagents. Four crude oils (crude oils 1, 2, 3, and 4) were obtained from Sinopec Qingdao refinery. Detailed information on the crude oils used in this study is listed in Table 1. Three straight run vacuum distillation gas oil (VGO) fractions with different boiling ranges (400−450, 450−500, and 500−540 °C, respectively) were distillated from crude oil 1, obtained from the side draws of a commercial vacuum distillation unit. Another set of VGO fractions and a bottom vacuum residue were also applied to evaluate Received: January 4, 2016 Revised: April 24, 2016

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DOI: 10.1021/acs.energyfuels.6b00018 Energy Fuels XXXX, XXX, XXX−XXX

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with a transient time of about 2.3 s, resulting in a resolving power of roughly 420 000 resolving power at m/z 400. The signal-to-noise ratio and dynamic range were improved by summing 128 time domain transients for each spectrum. The changes from ESI to DART analysis were as follows: (a) switching off the voltages related to the generation of the charged solvent droplets and (b) switching off the heater of the desolvation gas to bring down the glass transfer capillary to 350 °C). Therefore, the abundance of Ox classes can also be explained by partial oxidation of hydrocarbons during the DART process. However, the carbon number distributions of oxygencontaining compounds from distillate fractions (as shown in Figure 7) indicate that the oxidation of hydrocarbons can be ignored in this study, because alkanes have higher carbon numbers than naphthenic acids in a narrow distillate fraction. Additionally, the carbon number and DBE distributions of N1 species were also compared (as shown in Figure S3 of the Supporting Information). The carbon number and DBE distributions of N1 classes assigned from the two ionization methods agreed well with each other, although the summed relative abundance of N1 classes were significantly different, as shown in Figure 3. 3.3. Potential Application of DART(+) for the Analysis of Sulfides in Petroleum as a Selective Approach. A Kuwait crude oil with a high sulfur content was analyzed by DART in positive-ion mode. The FT-ICR mass spectrum and relative abundance of assigned class species are shown in Figure 5. According to the results from Roman et al.,27 nitrogencontaining compounds presented high relative abundance and paraffins could be ionized in oxidized form with positive-ion DART. Figure 5b showed that the hydrocarbon class species were ionized in this study but the oxidized hydrocarbons were not observed. The presence of hydrocarbon, S1, and S2 class species indicates that the selectivity of DART(+) is more like APPI(+) and could ionize more class species than ESI(+).51

Surprisingly, the most abundant class species found in the spectrum was O1S1, instead of N1. Initially, we considered that the O1S1 class species correspond to sulfoxides, which exist in crude oil with a large dispersity in concentration and could be detected by ESI in positive-ion mode. However, the model compound experiments (see Figure S4 of the Supporting Information) indicated that sulfides were detected in the DART mass spectra as O1S1 class species, while thiophenic compounds generate weak or no O1S1 class species. Collision-induced dissociation (CID) results (shown in Figure S5 of the Support Information) indicate that the O1S1 class species are not adduct ions; the product O1S1 ion of butyl sulfide has identical CID mass spectra with that of butyl sulfoxide. This indicates that sulfide could be oxidized to sulfoxide in the DART ionization process. Figure 5c shows the plot of DBE versus carbon number for O1S1 class species. Most abundant compounds have DBE values of 1−6, which agree well with the DBE value distribution of sulfides in our previous studies.52,53 Some O1S1 class species could be derived from the oxidation of sulfides; others should originally exist in the crude oil as sulfoxides [see the ESI(+) result of the crude oil in Figure S4 of the Supporting Information, the DART(+) results of the crude oil and saturate fraction in Figure S6 of the Supporting Information, and the DART(+) results of the distillate fraction and its sulfidic fraction in Figure S7 of the Supporting Information]. To characterize sulfides in petroleum, we have developed two analysis approaches to distinguish sulfides from thiophenic compounds in petroleum in the past few years. One of them is using selective oxidation to convert sulfides to sulfoxides, which enables the characterization by ESI(+),52 and the other approach is separating sulfides from petroleum by a methylation/demethylation process.53 Both of these methods are time-consuming and need high experimental operating skills. The new finding in this study implies that DART(+) can be potentially used for the molecular characterization of sulfides in petroleum. 3.4. Applicability of DART for Petroleum Distillate Fractions. Lobodin et al.26 found that the desorption/ E

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Figure 7. Plots of DBE versus carbon number for O2 class species from DART (top) and ESI (bottom) in negative (−) mode coupled with FT-ICR MS of VGOs.

sulfides from the thiophenic compound matrix. The finding is significant for further application of molecular characterization for sulfur compounds in petroleum. Spray-DART is verified to be capable of ionizing petroleum fractions with boiling points significantly higher than the DART source temperature.

ionization compounds in DART could have boiling points significantly higher than the DART source temperature. We also investigated the capability of spray-DART for petroleum distillate cuts with various boiling ranges. Three VGO fractions with different boiling ranges (400−450, 450−500, and 500− 540 °C, respectively) were examined by both DART and ESI in negative-ion mode. The temperature of the DART source was set at 400 °C, which was lower than the boiling ranges of the three VGO fractions. ESI(−) and DART(−) FT-ICR mass spectra of the three VGO fractions are shown in Figure 6, and the plots of DBE versus carbon number for O2 class species are shown in Figure 7. As shown in Figure 6, mass distribution ranges shifted to the high end with the increasing boiling ranges of the fractions. Except for the difference of nitrogen compounds mass peaks between DART(−) and ESI(−), the abundant mass peaks that correspond to naphthenic acids are presenting similar mass distribution ranges and profiles between the two ionization techniques (can also be seen in Figure 7). This is agree with the results by Lobodin et al.,26 indicating that spray-DART is capable of ionizing petroleum fractions with boiling points significantly higher than the DART source temperature. A vacuum distillate petroleum residua (>560 °C) was also introduced into spray-DART. Using the same operation parameters as used for the distillated fraction analysis, no stable signal was detected in the mass spectrum (as shown in Figure S8 of the Supporting Information).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00018. Additional information as noted in the text (Figures S1− S8) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions †

Limin Ren and Yehua Han contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21205137, 21236009, and 21376262), the Beijing Natural Science Foundation (2132041), and the Science Foundation of China University of Petroleum, Beijing (YJRC-2013-08).

4. CONCLUSION Spray injection with DART ionization was coupled to FT-ICR MS to characterize the molecular composition of crude oils and petroleum distillate fractions. In comparison to ESI, DART has a low requirement for solvents and more class species capability. DART(−) has high ionization selectivity to acidic compounds, which enables the direct analysis of naphthenic acids in petroleum fractions with a very low TAN value. Sulfides in petroleum can be oxidized to O1S1 class species in DART(+) ionization, leading to a direct characterization of

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DOI: 10.1021/acs.energyfuels.6b00018 Energy Fuels XXXX, XXX, XXX−XXX