Application Note FTMS-56 Reproducibility of Crude Oil Characterization by Flow Injection APPI-FT-ICR Mass Spectrometry Introduction The oil industry requires detailed information on the composition of crude oil for refinery and catalytic processes. The term used to describe the analysis of this extremely complex mixture at the molecular level is petroleomics. Due to the high chemical complexity of crude oil, the mass spectra of petroleum samples are also very complex, and ultrahigh resolution of > 500,000 is essential to resolve all peaks in the mass spectrum. Fourier Transform Ion Cyclotron Resonance (FT-ICR MS) mass spectrometry is a well-established method in petroleomics. FT-ICR MS is capable of achieving ultrahigh resolution as well as extremely high mass accuracy, enabling assignment of all peaks in the mass spectrum with their correct molecular formula, even at masses up to m/z 800 [1-3]. Authors Matthias Witt Bruker Daltonik GmbH, Bremen, Germany Keywords Petroleomics solarix XR FTMS APPI II source APPI Composer 1.0.6 Instrumentation and Software Ionization methods such as electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photo ionization (APPI) or laser desorption ionization (LDI) are typically combined with FT-ICR MS to determine the molecular composition of crude oils using positive- and negative-ion mode. Depending on the ionization method, compound classes such as hydrocarbons, O x, S x, SO x, N x, and NO x are ionized with differing efficiencies.
Therefore, the mass spectrum of a crude oil using a specific ionization method can be used as a fingerprint for crude oil characterization [4]. High reproducibility of mass spectrometric results is essential to enable use of these data for reliable characterization of crude oil at the molecular level or identification using statistical methods such as principle component analysis or unsupervised clustering. In addition, highly stable ionization conditions are crucial for reproducibility in the detection of relative abundances of compound classes and the ratio of radical cations to protonated species [5]. This ratio is mainly influenced by the dopant used for the photoionization process [6]. Flow injection analysis is a proven method for increasing the reproducibility of chemical analyses. In this study, flow injection analysis was combined with APPI-FT-ICR mass spectrometry for characterization of crude oil samples. Forty-five replicate measurements were performed on the same crude oil sample over 8 hours. Each measurement required around 9 minutes and samples were introduced by flow injection using an autosampler and a 100 µl sample loop. To enable detection of polar compounds and nonpolar compounds (such as hydrocarbons compounds with a low proton affinity), APPI was used as the ionization method in positive-ion mode. Using this ionization method, radical cations and protonated species were principally detected. However, detection of both protonated species and radical cations resulted in very complex spectra. Experimental Sample preparation: A North Sea crude oil sample (light crude oil with a API gravity of 32.9 API, asphaltene content 1.9%) was measured without any purification by flow injection using APPI-FT-ICR MS. The sample was kindly provided by the Norwegian Petroleum Directorate, Stavanger, Norway. Crude oil (10 mg) was dissolved in 990 µl dichloromethane. This stock solution was diluted 1:100 in 50% toluene / 50% methanol + 0.1% formic acid to give a final concentration of 100 ppm. Five vials were each filled with 1.5 ml of the sample solution. The five vials were used in rotation for sample injection (1st injection from vial 1, 2 nd injection from vial 2, 3 rd injection from vial 3, 4 th injection from vial 4, 5 th injection from vial 5, 6 th injection from vial 1, 7 th injection from vial 2 and so on) to minimize the aging effect of the sample during the measurements. In total, 900 µl sample solution was injected from each vial. Mass analysis: Mass spectra were acquired with a Bruker solarix XR Fourier transform ion cyclotron resonance mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany) equipped with a 12 T refrigerated actively shielded superconducting magnet (Bruker Biospin, Wissembourg, France) and the new dynamically harmonized analyzer cell (ParaCell ). The samples were ionized in positive-ion mode using the APPI ion source (Bruker Daltonik GmbH, Bremen, Germany) equipped with a krypton lamp at 10.6 ev. The sample was introduced to the mass spectrometer by flow injection using a G1367A well plate autosampler (Agilent, Santa Clara, CA, USA) with a 100 µl sample loop and a G1311A pump (Agilent, Santa Clara, CA, USA). The flow was set to 100 µl/min during the injection and 0.2 min after injection the flow was reduced to 10 µl/min over 0.6 min. The flow of 10 µl/min was maintained for 7.2 min and then increased over 1 min to 100 µl/min. The final mass spectrum was acquired in 7.7 min by adding 128 single scans. The mass range was set to m/z 150 2000 using 8M data points with a transient length of 3.3 s resulting in a resolving power of 900,000 at m/z 400 in magnitude mode. The ion accumulation time was set to 30 ms. Ramped excitation (20% at m/z 147 to 35% at m/z 3000) was used for ion excitation before detection. Mass calibration: The mass spectra were calibrated externally with arginine clusters in positive-ion mode using a linear calibration. A 10 µg ml -1 solution of arginine in 50% methanol was used to generate the arginine clusters. Single scans were aligned during the measurement with a single mass online calibration using mass m/z 500.437653 (C 37 H 56 ) to align single scans to this mass. Spectra were recalibrated internally with the N 1 series in DataAnalysis 4.2 (Bruker Daltonik GmbH, Bremen, Germany). The RMS mass error of the internal calibration was better than 150 ppb for all measurements. Molecular formula calculation: The mass formula calculation was performed using Composer 1.0.6 (Sierra Analytics, Modesto, CA, USA) with a maximum formula of C n H h N 3 O 3 S 3, electron configurations odd and even (due to the formation and detection of radical cations and protonated species), and a mass tolerance of 500 ppb. The relative abundances of all compound classes were calculated using the Composer software. Results and Discussion The mass spectra obtained in positive-ion mode of the 5 th, 15 th, and 25 th injections are shown in Figure 1. Identical mass distributions were observed for the different measurements with a maximum abundance of the mass distribution at m/z 390. No additional peaks (siloxanes or silicones) from the septum of the vial were observed in any of the injections. Masses up to m/z 1100 were detected. After internal calibration of the mass spectra, molecular formulas of all peaks in the spectra were calculated using the Composer software. Based on this data, relative abundances of compound classes were calculated for all 45 replicate measurements (see Table 1).
Mass spectra of different crude oil injections 300 400 500 600 700 800 900 1000 m/z Figure 1: Mass spectra of the 5 th, 15 th and 25 th injection of the North Sea crude oil using APPI in positive-ion mode. Table 1: Average values and absolute and relative standard deviations of detected compound classes relative abundances. Relative abundance of compound classes and standard deviations
The relative standard deviations of abundant (> 5%) compound classes such as HC, N 1, O 1 were very low (< 2%). This demonstrates not only the reproducibility of flow injection analysis but also the reproducibility of the APPI FT-ICR mass spectrometric results. Slightly higher standard deviations were observed for low-abundance compound classes such as N 1 O 1 and O 1 S 1. The presence of these oxygen-containing compound classes could partly be due to chemical reactions of highly reactive species in the APPI ion source. The higher standard deviation of such species could therefore be attributed to ionization effects of APPI as well as their low abundance. The relative abundances of compound classes N 1, O 1 and S 1 as radical cations for all injections are plotted in Figure 2. A small increase (< 3%) in the relative abundance of compound class N 1 was observed after 25 injections. This could be due to aging effects on the sample. The reproducibility of the APPI method was also studied on the basis of the ratio of radical cations to protonated species. This ratio is very sensitive to ionization conditions in the APPI source, which must be kept constant for reproducible results from one injection to the next [6]. The results for the compound classes HC, N 1 are shown in Table 2 and plotted in Figure 3. The higher fluctuation of the class N and S relative to the HC class indicates that the protonation is a first order reaction. Relative abundance of compound classes of all injections Figure 2: Relative abundance of compound classes N 1, O 1 as radical cations. Table 2: Average values and absolute and relative standard deviations of the ratio of radical cations to protonated species for compound classes HC, N 1. Ratio of radical cations to protonated species and standard deviations Ratio Injection 1 Injection 2 Injection 3 Injection 43 Injection 44 Injection 45 Average Standard deviation Standard deviation [%] HC/HC[H] 3.51 3.49 3.48 3.60 3.66 3.63 3.53 0.05 1.5 N/N[H] 3.64 3.43 3.49 3.78 3.82 3.82 3.62 0.11 3.0 S/S[H] 3.54 3.43 3.40 3.59 3.56 3.65 3.50 0.07 1.9
Ratio of radical cations to protonated species Figure 3: Ratio of radical cations to protonated species of compound classes HC, N 1. Conclusion Mass spectra of crude oil can be measured with high reproducibility using flow injection APPI-FT-ICR mass spectrometry. Abundant (> 5% relative abundance) compound classes are detected with relative standard deviations of less than 2%. Low-abundance (< 2% relative abundance) compound classes have relative standard deviations below 10%. Therefore, these results can be used to get a semi-quantitative overview of the chemical composition of crude oils. The low relative standard deviation (< 3%) in the ratio of radical cations to protonated species for the compound classes HC, N 1 indicates that the ionization conditions in the APPI source are very reproducible. This high-level of reproducibility could be used for quality control processes at refineries or to establish the origin of unknown samples.
Bruker Daltonics is continually improving its products and reserves the right to change specifications without notice. Bruker Daltonics 11-2014, FTMS-56, 1833835 References [1] Rodgers, R. P.; Schaub, T. M.; Marshall, A. G. Anal. Chem. 2005, 21A. [2] Marshall, A. G.; Rodgers, R. P. Acc. Chem. Res. 2004, 37, 53-59. [3] Cho, Y.; Ahmed, A.; Islam, A., Kim, S. Mass Spectrom. Rev. 2014, doi: 10.1002/mas.21438 [4] Lobodin, V.V.; Nyadong, L.; Ruddy, B.M.; Quinn, J.P.; Hendrickson, C.L.; Rodgers, R.P. and Marshall, A.G., Ambient Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry for Comprehensive Chemical Fingerprinting of Petroleum and Deposits, Gulf of Mexico Oil Spill and Ecosystem Science Conf., New Orleans, LA, January 19-23 (2013). [5] Cai, S. S.; Hanold, K. A.; Syage, J. A. Anal. Chem. 2007, 79, 2491-2498. [6] Robb, D. B.; Covey, T. R.; Bruins, A. P. Anal. Chem. 2000, 72, 3653-3659. For research use only. Not for use in diagnostic procedures. Bruker Daltonik GmbH Bremen Germany Phone +49 (0)421-2205-0 Fax +49 (0)421-2205-103 ms.sales.bdal@bruker.com Bruker Daltonics Inc. Billerica, MA USA Phone +1 (978) 663-3660 Fax +1 (978) 667-5993 ms.sales.bdal@bruker.com www.bruker.com