Letter pubs.acs.org/ac
Distinguishing the C3 vs SH4 Mass Split by Comprehensive TwoDimensional Gas Chromatography−High Resolution Time-of-Flight Mass Spectrometry Jonathan D. Byer,*,† Kevin Siek,† and Karl Jobst‡ †
Life Science and Chemical Analysis, LECO Corporation, 3000 Lakeview Ave., Saint Joseph, Michigan 49085, United States Laboratory Services Branch, Ontario Ministry of the Environment and Climate Change, 125 Resources Rd., Etobicoke, Toronto, Ontario M9P 3V6, Canada
‡
ABSTRACT: The C3 vs SH4 (0.0034 Da) mass split is considered to be one of the most critical mass splits in petroleomics and is relevant because of the regulatory requirements for sulfur in petroleum fractions. To date, there are two ways to resolve mass splits such as C3 vs SH4: (a) ultrahigh resolution Fourier transform mass spectrometry (FTMS); (b) high-resolution chromatography such as comprehensive two-dimensional gas chromatography (GC×GC). High-resolution chromatography minimizes the mass spectral resolution required to distinguish these key chemical constituents and provides additional sample characterization via isomer separation. High resolution mass spectrometry enables unambiguous chemical formulas determination and structural elucidation. In this paper, we demonstrate the combination of high resolution GC×GC with high resolution time-of-flight mass spectrometry to distinguish the C3 vs SH4 mass split and other common mass splits in a crude oil sample.
T
(GC×GC), a second time-based separation technique that has been used extensively in petroleum analysis for superior class separation.13−19 One can therefore logically ask: Can extremely efficient chromatography be used to minimize the mass spectral resolution requirement? Hughey et al. suggested that if one were to consider FTICRMS to be a one-dimensional isocratic separation then there would be upward of 25 billion theoretical plates.20 A narrow bore GC column of 0.25 mm ID has approximately 4000 theoretical plates per meter of column; therefore, a 30 m × 0.25 mm primary column coupled to a 1 m × 0.25 m secondary column would result in 480 million theoretical plates. Similar to the calculation by Hughey, if one were to consider HRToFMS at 25 000 resolving power to be a one-dimensional isocratic separation, there are more than 100 000 theoretical plates. Thus, the combination of GC×GC-HRToFMS provides a comparable separation in terms of the number of theoretical plates. In this study, we show the ability to distinguish constituents with elemental compositions differing by C3 vs SH4 with isomer separation using GC×GC-HRToFMS.
he aim of petroleomics is the chemical characterization of all of the constituents in a petroleum or crude oil sample.1 The introduction of ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS) represented an important step toward this goal because it provides near unambiguous mass determination, heteroatom class, degree of unsaturation, and carbon number infomation.1−5 One of the key features of high resolution mass spectrometry in petroleomics is the ability to resolve very narrow mass differences (Δm) between isobars;6 e.g., CH4 vs O (0.0364 Da), CH2 vs N (0.0126 Da), and C3 vs SH4 (0.0034 Da) at m/z 300 have full width half maximum (FWHM) resolving power requirements of 8400, 24 000, and 89 000, respectively, for their separation with equal peak intensity. Implicit in the argument for isobars and the need for >50 000 resolving power for the C3 vs SH4 mass split is that these mass differences occupy the same time domain, which is always the case with direct infusion (typically performed with FT-ICRMS). However, if they occupied a different time domain, then the resolving power requirement would be changed. Chromatography can add a time dimension to the paradigm,7,8 but FT-ICRMS data acquisition can take a long time (up to minutes),9 which is incompatible with most chromatographic applications. Time-offlight (ToF) is an inherently fast mass analyzer, and recent high resolution time-of-flight mass spectrometers (HRToFMS) can collect reproducible high resolution mass spectra every 5 ms.10−12 Unlike FT-MS instruments (FT-ICRMS and Orbitrap), resolution and mass accuracy are not strongly linked to the acquisition frequency for ToFMS. A fast acquisition rate opens the realm of two-dimensional gas chromatography © 2016 American Chemical Society
■
EXPERIMENTAL SECTION A Venezuelan crude oil sample was sonicated at room temperature for 20 min and then diluted 1:1 (v/v) in dichloromethane. The resulting mixture was then analyzed by Received: March 22, 2016 Accepted: June 6, 2016 Published: June 6, 2016 6101
DOI: 10.1021/acs.analchem.6b01137 Anal. Chem. 2016, 88, 6101−6104
Letter
Analytical Chemistry comprehensive two-dimensional gas chromatography-high resolution time-of-flight mass spectrometry (GC×GCHRToFMS) using LECO’s Pegasus GC-HRT 4D. Injections were made with a Gerstel MPS2 autosampler coupled to an Agilent 7890B GC, in split mode (50:1) with a constant flow of He at 1.2 mL/min. The primary column was a Rxi-1HT 30 m × 0.25 mm ID × 0.1 μm film (Restek), attached to a secondary column Rtx-35SilMS 0.6 m × 0.25 mm ID × 0.25 μm film (Restek) via a Siltite μ-union (SGE; Trajan). The GC injector was held at 350 °C, and the oven program was as follows: 50 °C (2 min) − (10 °C/min) − 380 °C (5 min), Total 40 min. A secondary oven offset of +15 °C was applied, as well as a modulator offset of +30 °C of the secondary oven. The modulation period was 3 s with a 1 s hot pulse time and 0.5 s cold pulse. The MS transfer line was held at 330 °C. A mass range of m/z 15−535 was used for electron ionization (EI) and m/z 50−535 for chemical ionization (CI) with methane as the reagent gas (0.5 mL/min). The acquisition rate was set to 100 spectra/s, collected in high resolution mode with a resolving power >25 000 FWHM for m/z 218.9851. Data was processed using LECO’s ChromaTOF-HRT software version 1.91. Following deconvolution and peak finding, the software merged the deconvolved spectral signals into a composite mass defect plot. For data that have a chromatographic dimension, the tolerance for merging spectral signals depends on the standard error of the mean m/z for each signal, rather than the mass spectral resolving power. Signals with significantly different measured m/z are validly plotted as independent points in the mass defect plane, even if they would be spectrally unresolvable within a dead coelution. Since the GC×GC-HRToFMS achieved RMS mass accuracy of less than 1 ppm for external mass calibration, a ±0.001 Da or ±1 ppm tolerance, whichever was greater, was set for generating the composite mass defect plot. For each point in this plot, the reported m/z is the intensity-weighted mean m/z across all chromatographic peaks included in the point. Selected rows in the mass defect plot are assigned to formula classes typical of petroleum constituents, subject to upper elemental composition limits of C40H82N3O10S3 and the mass tolerance. Where a row could be assigned to multiple possible classes, the more common class was typically selected, for example, preferring O1 class to N3 class.
Figure 1. (A) Total ion chromatogram of a crude oil sample acquired using comprehensive two-dimensional gas chromatography-high resolution time-of-flight mass spectrometry (GC×GC-HRToFMS), operated in chemical ionization mode with methane, and (B) ensuing Kendrick Mass Defect plot for all identified species shown; the size of the dots indicate intensity. The combined plots illustrate a structured nature and the complementary information contained therein.
■
mass analyzer, and isobars theoretically beyond the mass analyzer’s capabilities can be distinguished. The figure also shows the structured nature of the GC×GC chromatogram and how it complements the Kendrick Mass Defect plot. Figure 2 demonstrates the benefit of this approach as it applies to several common mass splits observed in a crude oil sample including the C3 vs SH4 isobars (Δm = 0.0034 Da). The minimum resolving power at FWHM required to resolve these two masses (exact mass = 223.1481 and 223.1515) is theoretically 66 000; however, Figure 2 clearly shows both isobars recorded accurately and independently, even though the data were collected at 25 000 resolving power at FWHM. Plotting these two masses chromatographically shows their separation in two time dimensions (Figure 3a), and because of the time domain, the masses are independently distinguishable. The distinction of these two masses would not be possible without the use of HRMS because both compound classes share the same mass of 223.15. Additionally, accurate mass data enable unambiguous chemical formula determination and assignment. The chromatogram also highlights the importance of chromatography for the separation of isomers, which enables enhanced speciation not achievable by direct infusion. Additionally, each of these
RESULTS AND DISCUSSION High resolution mass spectrometry permitted unambiguous elemental composition assignments from accurately determined masses. In addition to mass determination, molecules of the same heteroatom class, carbon number (degree of alkylation), and degree of unsaturation (type) are readily differentiated.6 A visualization tool for accurate mass data that has gained popularity in recent years is the Kendrick Mass Defect plot, which defines masses differing by CH2 to the same mass defect.5,21 Figure 1 shows a GC×GC−CI-HRToFMS chromatogram of a crude oil sample and the corresponding Kendrick Mass Defect plot generated by integrating all of the ions in the deconvoluted mass spectra of each of the individual chromatographic peaks determined by the ChromaTOF-HRT software. The advantage of this approach as opposed to integrating the raw data occurs because the spectra are summed based on the intensity-weighted average accurate mass measurement over a chromatographic peak, maintaining the mass spectral integrity in the time domain. Consequently, the mass tolerance of a specific mass can be narrower than the theoretical limit of the 6102
DOI: 10.1021/acs.analchem.6b01137 Anal. Chem. 2016, 88, 6101−6104
Letter
Analytical Chemistry
Figure 2. Integrated mass spectrum, based on spectrally deconvoluted data, of a crude oil sample for all of the identified species. The top mass spectral segment demonstrates one example of the C3 versus SH4 split (0.0034 Da), an important feature in petroleomics that until now has only been resolved by ultrahigh resolution FT-ICRMS.
■
chromatographically resolved constituents has an associated deconvoluted mass spectrum. Figure 3c,d shows the EI and CI mass spectra of the base chromatographic peak for the encircled regions in Figure 3a. The fragmentation in EI and CI information are consistent with the hydrocarbon and sulfur compound class assignments. For rare cases of isobars unresolvable at 25 000 resolving power coeluting in both chromatographic dimensions, achieving chromatographic separation by modifying the column set may be easier and considerably more cost-effective than appropriating additional mass spectral horsepower.
CONCLUSIONS
Comprehensive two-dimensional GC coupled to HRToFMS is a powerful tool for chemical characterization of the GC amenable fraction of crude oil. This technique befits petroleomics because it provides mass determination, heteroatom class, type, and carbon number information similar to direct high-resolution mass spectrometry and, additionally, gives well-defined chromatographic regions that facilitate structural elucidation and speciation. To our knowledge, this is the first report of isomer separation and classification of compounds differing in elemental composition by C3 vs SH4 in crude oil. Future work will compare the chemical information 6103
DOI: 10.1021/acs.analchem.6b01137 Anal. Chem. 2016, 88, 6101−6104
Letter
Analytical Chemistry
Figure 3. (A) Region of the chromatogram shown in Figure 1, but of the extracted ion chromatogram for m/z 223.1481 ± 5 ppm and m/z 223.1515 ± 5 ppm, illustrating the chromatographic separation between these hydrocarbon and sulfur compound classes, respectively. (B) Segment of the Kendrick Mass Defect plot shown in Figure 1, demonstrating the fine structure achievable using GC×GC-HRToFMS. The isobaric masses noted in (A) are within the blue dotted circle. The EI and CI mass spectrum of the most intense chromatographic peaks for the (C) S-class and (D) HC-class compounds identified in (A) and (B). (7) Ortiz, X.; Jobst, K. J.; Reiner, E. J.; Backus, S. M.; Peru, K. M.; McMartin, D. W.; O’Sullivan, G.; Taguchi, V. Y.; Headley, J. V. Anal. Chem. 2014, 86 (15), 7666−73. (8) McKenna, A. M.; Nelson, R. K.; Reddy, C. M.; Savory, J. J.; Kaiser, N. K.; Fitzsimmons, J. E.; Marshall, A. G.; Rodgers, R. P. Environ. Sci. Technol. 2013, 47 (13), 7530−7539. (9) Nikolaev, E. N.; Kostyukevich, Y. I.; Vladimirov, G. N. Mass Spectrom. Rev. 2016, 35 (2), 219−258. (10) Verentchikov, A. N.; Yavor, M. I.; Hasin, Y.; Gavrik, M. A. Tech. Phys. 2005, 50 (1), 73−81. (11) Verentchikov, A. N.; Yavor, M. I.; Hasin, Y.; Gavrik, M. A. Tech. Phys. 2005, 50 (1), 82−86. (12) Klitzke, C. F.; Corilo, Y. E.; Siek, K.; Binkley, J.; Patrick, J.; Eberlin, M. N. Energy Fuels 2012, 26 (9), 5787−94. (13) Phillips, J. B.; Beens, J. J. Chromatogr. A 1999, 856 (1−2), 331− 47. (14) Seeley, J. V.; Seeley, S. K.; Libby, E. K.; McCurry, J. D. J. Chromatogr. Sci. 2007, 45 (10), 650−6. (15) West, C. E.; Pureveen, J.; Scarlett, A. G.; Lengger, S. K.; Wilde, M. J.; Korndorffer, F.; Tegelaar, E. W.; Rowland, S. J. Rapid Commun. Mass Spectrom. 2014, 28 (9), 1023−32. (16) Zhang, W.; Zhu, S.; He, S.; Wang, Y. J. Chromatogr. A 2015, 1380, 162−70. (17) Adam, F.; Bertoncini, F.; Coupard, V.; Charon, N.; Thiébaut, D.; Espinat, D.; Hennion, M. C. J. Chromatogr. A 2008, 1186 (1−2), 236−44. (18) van der Westhuizen, R.; Ajam, M.; De Coning, P.; Beens, J.; de Villiers, A.; Sandra, P. J. Chromatogr. A 2011, 1218 (28), 4478−86. (19) Blomberg, J.; Schoenmakers, P. J.; Brinkman, U. A. J. Chromatogr. A 2002, 972 (2), 137−73. (20) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G.; Qian, K. Anal. Chem. 2001, 73 (19), 4676−81. (21) Kendrick, E. Anal. Chem. 1963, 35 (13), 2146−2154.
derived using GC×GC-HRToFMS and FT-ICRMS for a standard crude oil sample.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone: 269-985-5904. Fax: 269-983-7150. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We thank our colleagues Wei Chen, Kevin McNitt, Terry Goodman, and Steve Robles for software development, as well as Lorne Fell for his comments that greatly improved the manuscript.
■
REFERENCES
(1) Marshall, A. G.; Rodgers, R. P. Acc. Chem. Res. 2004, 37 (1), 53− 9. (2) Marshall, A. G.; Rodgers, R. P. Proc. Natl. Acad. Sci. U. S. A. 2008, 105 (47), 18090−5. (3) Marshall, A. G.; Blakney, G. T.; Beu, S. C.; Hendrickson, C. L.; McKenna, A. M.; Purcell, J. M.; Rodgers, R. P.; Xian, F. Eur. Mass Spectrom. 2010, 16 (3), 367−71. (4) Song, C.; Liu, Y.; Liu, Z.; Wang, W.; Tian, S. Sepu 2015, 33 (5), 488−93. (5) Rodgers, R. P.; Schaub, T. M.; Marshall, A. G. Anal. Chem. 2005, 77 (1), 20A−27A. (6) Lobodin, V. V.; Rodgers, R. P.; Marshall, A. G. In Comprehensive Environmental Mass Spectrometry; Lebedev, A. T., Ed.; ILM Publications: Glendale, AZ, 2012; pp 415−442. 6104
DOI: 10.1021/acs.analchem.6b01137 Anal. Chem. 2016, 88, 6101−6104
