Introduction

Crude oil is a very complex mixture of organic compounds consisting of various elemental compositions and chemical structures. The composition of many compounds is not exactly known. Mixtures vary widely depending on the origin of the oil. Crude oil consists mostly (>90%) of hydrocarbons. Remaining compounds in oil mainly contain hetero atom classes with oxygen, sulfur and nitrogen. Commercial oils vary in the composition of the type and amount of compounds as well as compound classes. Crude oil and bitumen can be analyzed by analytical methods like nuclear magnetic resonance (NMR) [1], infrared (IR) [2] spectroscopy or X-ray fluorescence spectroscopy (XRF) [3]. Nevertheless, these methods are limited concerning the information of the elemental composition of compounds in oil. Also GC/LC separations of complex mixtures are difficult and at least time consuming. Polar compounds in crude oil can be detected easily by atmospheric pressure ionization (API) mass spectrometry [4]. Direct infusion experiments of crude oil samples can be carried out by ultra-high resolving power mass spectrometry to achieve mass peak separation and correct annotation of the molecular formula of all peaks in the mass spectrum. Extremely high resolving power is needed to separate isobaric mass peaks which differ only by a few mDa. An important mass difference which has

Application Note # FTMS-44Analysis of sulfur-rich crude oil and bitumen by FTMS

to be resolved in a mass spectrum is 3.4 mDa (difference between C3 and SH4). If radical cations of hydrocarbons are present also the mass differences of 4.4 mDa (difference beween CH and 13C) is observed. The composition of the hetero atomic compounds is a fingerprint for each crude oil. However, compounds containing only sulfur and no other hetero atoms are difficult to detect by electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) due to the fact that these compounds are difficult to be protonated or deprotonated. Therefore, atmospheric pressure photoionization (APPI) [5], which is able to generate radical cations using multi-photon ionization, is used to detect these compounds. Another method for the detection of compound classes CxHyS and CxHyS2 is the chemical modification using methylation reagents.Speciation of heteroatomic compounds like compound class S1 or S2 with specific double bond equivalents (DBE) is necessary for crude oil and bitumen classification on the molecular level. Sulfur containing compound classes like benzothiophenes (DBE 6) or dibenzothiophene (DBE 9) can be identified based on the number of double bond equivalents. Several studies of crude oil and bitumen have been made in positive ion mode by Fourier transform mass spectrometry (FTMS) using APPI. More than 95% of all mass peaks can be assigned with exactly one molecular

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formula with a resolving power of 600,000 at m/z 400 and a mass accuracy better than 0.5 ppm using internal calibration calculated by Composer software.

Experimentals

Sample preparation

Four crude oils and two bitumen samples from the company SINOPEC, China, were analyzed. The spray solutions of the crude oils were prepared without any further purification. 10 mg of the crude oil were dissolved in 200 µL dichloro-methane. These sample stock solutions were diluted 1:300

with 50% methanol, 50% toluene for atmospheric pressure photoionization (APPI) measurements.

Mass Analysis

Mass spectra were acquired with a Bruker solariX Fourier transform mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany) equipped with a 12 T refrigerated actively shielded superconducting magnet (Bruker Biospin, Wissembourg, France). The samples were ionized in posi-tive ion mode using the APPI ion source (Bruker Daltonik GmbH, Bremen, Germany). Sample solutions were continu-ously infused using a syringe at a flow rate of 600 µL h-1. The size of the acquired data sets was 4 MW resulting in a resolving power of 600 000 at m/z 400. The detection mass range was set to m/z 250 – 3000. 300 scans were acquired for each mass spectrum. Spectra were zero-filled to pro-cessing size of 8M data points before sine apodization.

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 clusters. The crude oil spectra were recali-brated internally with the homologous series CnH2n-16 and the bitumen samples with the homologous series CnH2n-16S to improve the mass accuracy.

Molecular formula calculation

The mass formula calculation was done in Composer 1.0.2 (Sierra Analytics, Modesto, CA, USA) using a maximum

Mass peaks are in the range m/z 300-1200

Figure 1: Mass spectra of crude oil and bitumen samples measured in APPI positive ion mode.

400 600 800 1000 1200 1400 m/z

wx-1

wx-2

wx-3

wx-4

wx-5

wx-6

Figure 2: Zoom of all mass spectra at nominal mass m/z 514 to see fine structure of mass spectra.

Significant differences of mass spectra

wx-1

wx-2

wx-3

wx-4

wx-5

wx-6

514.22906 514.26873 514.32294514.35931

514.41694

514.45329

514.49817514.54264

514.09109 514.14190514.18168 514.23582

514.27216

514.32631

514.36272

514.41577514.45671

514.54719

514.26885 514.32294514.35940

514.41697

514.45335

514.49820514.54716

514.09102 514.14195514.18170 514.23580

514.27215

514.32626

514.36270

514.41707514.45343

514.53814

514.17842 514.23257514.26879 514.32287

514.36273

514.41703

514.45339

514.49823 514.54253

514.14205514.17844 514.23248514.27216

514.32632

514.36272

514.41705

514.45336

514.49837 514.54274

514.1 514.2 514.3 514.4 514.5 m/z

formula of CnHhN3O3S3, electron configurations odd and even due to the formation and detection of radical cations and protonated species and a mass tolerance of 0.5 ppm. The double bond equivatents (DBE) vs. carbon number plots as well as the compound class and Van Krevelen plots were also generated with the Composer software.

Results and Discussion

The mass spectra of the crude oil (wx-1, wx-3, wx-5 and wx-6) and bitumen (wx-2 and wx-4) samples measured in

Compound class plot

0

10

20

30

40

50

60

70

HC S S2 S3 N NO O OS OS2

Compound class

rel.

abun

danc

e

wx-1wx-2wx-3wx-4wx-5wx-6

APPI positive ion mode are shown in Figure 1. The mass distribution of all six samples look similar except sample wx-5 which is shifted to higher mass. The mass peaks are in the mass range m/z 300-1200. However, the full mass spectrum gives you only an estimation of the average number of carbons of the detected compounds. A detailed analysis of the single mass peaks is needed to analyze the sample on the molecular level (see Figure 2). The mass spectra have significant differences. However, several peaks are identical but with different peak ratios and intensities.

More than 95% of the mass peaks were annotated with an elemental formula

Figure 3: Zoom of mass spectrum of sample wx-1 at nominal mass m/z 514 with elemental formulae annotation of the most abundant peaks (30 peaks could be annotated in total for nominal mass m/z 514).

C37H70+.

0.25 ppm

C36H6913C+

0.17 ppm

C38H58+.

0.02 ppm

C35H64NO+

0.15 ppm

C37H56N+

0.13 ppm

C36H55N13C+.

0.05 ppm

C37H54O+.

0.05 ppm

C36H52NO+.

0.14 ppm

C36H50S+.

0.09 ppm

C36H46+.

0.19 ppm

C38H42O+.

0.16 ppm

C37H38O2+.

0.04 ppm

C37H38S+.

0.27 ppm

C40H34+.

0.19 ppm

C36H34OS+.

0.18 ppm

C39H30O+.

0.11 ppmC35H30S2+.

0.10 ppm

514.22906

514.26873

514.28661

514.32294

514.35931

514.41694

514.45329

514.49817

514.54264

514.15 514.20 514.25 514.30 514.35 514.40 514.45 514.50 514.55 m/z

Figure 4: Relative abundances of compound classes of all samples (wx-1, wx-3, wx-5 and wx-6 are crude oil samples, wx-2 and wx-4 are bitumen samples).

Relative abundances of the compound classes

Figure 5: Plots of double bond equivalent (DBE) vs. carbon atom number of compound classes S1 and S2 of a) and b) sample wx-3 (crude oil, group 1), c) and d) of sample wx-5 (crude oil, group 2) and e) and f) of sample wx-4 (bitumen, group 3).

More than 95% of the mass peaks could be annotated with an elemental formula. As an example the annotation of peaks with molecular formulae are shown in figure 3 for sample wx-1 for the nominal mass m/z 514.The relative intensities of compound classes in all samples are shown in Figure 4. The six samples can be separated in three groups: group 1 with sample wx-1 and wx-3 (crude oil) containing low amount of sulfur, group 2 with sample wx-5 and wx-6 (crude oil) containing medium amount of sulfur and group 3 with sample wx-2 and wx-4 (bitumen) containing high amount of sulfur. Even compound class S3 with a relative abundance of about 8% is present in samples wx-2 and wx-4.Detailed analyses can be done on the molecular level by plotting the double bond equivalents (DBE) vs. the carbon number (Figure 5). The plots can be done for each compound class. Here, these plots have been generated for sample wx-3, wx-4 and wx-5 for the compound classes S1 and S2 which represent data of each group (group 1: low sulfur, group 2: medium sulfur and group 3: high sulfur). The DBE value is an indication for the aromaticity of the compound and the core structure. For instance dibenzothiophene can be identified in these plots with DBE=9. In group 1 and 2 the average DBE of compound class S1 is mainly between 9 and 15. However, DBE is mainly DBE 6 and 9 in group 3 which indicates high amounts of benzothiophenes (DBE 6) and dibenzothiophenes (DBE 9) in samples of group 3. For all three groups DBE is shifted up to higher DBE values for compound class S2.

Another way beside the compound class plot (Fig. 4) to show the aromaticity and the relative amount of sulfur containing compound classes is the Van Krevelen plot (Figure 6). In addition the relative ratios of compounds of each class displayed with the size and color of the dots, the H/C ratio indicates the degree of unsaturation and indirectly the DBE of the core structure. For instance, the average H/C ratio of class S1 of sample wx-3 (Fig. 5a) is about 1.4 in contrast to sample wx-4 with an average H/C of nearly 1.6 indicating the higher average DBE of sample wx-3.

Conclusions

Ultra-high resolution mass spectrometry can be used to characterize high mass sulfur containing compounds in crude oil and bitumen. These compounds are not accessible by GC due to their low volatility. The relative abundances of the compound classes S1, S2 and S3 in crude oil and bitumen can be measured. DBE vs. carbon number plots can be used to analyze the relative ratios of specific core structures like benzo- or dibenzo thiophenes of these compound classes.

Acknowledgement

The author would like to thank Dr. Maowen Li from the company SINOPEC, China, for providing the crude oil and bitumen samples.

Sulfur containing compound classes

Figure 6: Van Krevelen plot S/C vs. H/C of a) sample wx-3 (group 1) and b) sample wx-4 (group 3) for visualization of aromaticity and relative ratios of sulfur containing compound simultaneously.

Sample wx-3

H/C

0.5

1.0

1.5

2.0

S1

0 0.060.02 0.10 0.12 0.14

S/C ratio

0.04 0.08

S2

a)

Sample wx-4

H/C

0.5

1.0

1.5

2.0

0 0.060.02 0.1 0.12 0.14S/C ratio

0.04 0.08

S1 S2

S3

b)

Sample wx-3

H/C

0.5

1.0

1.5

2.0

S1

0 0.060.02 0.10 0.12 0.14

S/C ratio

0.04 0.08

S2

a)

Sample wx-4

H/C

0.5

1.0

1.5

2.0

0 0.060.02 0.1 0.12 0.14S/C ratio

0.04 0.08

S1 S2

S3

b)

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Keywords

Petroleomics

FTMS

APPI

Instrumentation & Software

solarix 12T

DataAnalysis 4.0

APPI II source

ComposerFor research use only. Not for use in diagnostic procedures.

References

[1] Tomczyk, N. A.; Winans, R. E.; Shinn, J. H.; Robinson, R. C. Energy Fuels 2001, 15, 1498-1504.[2] Saab, J.; Mokbel, I.; Razouk, A. C.; Ainous, N.; Zydowicz, N.; Jose, J. Energy Fuels 2005, 19, 525-531.[3] Barker, L. R.; Kelly, W. R.; Gurthrie, W. F. Energy Fuels 2008, 22, 2488-2490.[4] Marshall, A. G.; Rodgers, R. P. Acc. Chem. Res. 2004, 37, 59-59.[5] Purcell, J. M.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2006, 78, 5906-5912.

Authors

Dr. Matthias Witt, Bruker Daltonik GmbH, Fahrenheitstr. 4, D-28359 Bremen, Germany

Bruker Daltonik GmbH

Bremen · GermanyPhone +49 (0)421-2205-0 Fax +49 (0)421-2205-103 sales@bdal.de

Bruker Daltonics Inc.

Billerica, MA · USAPhone +1 (978) 663-3660 Fax +1 (978) 667-5993 ms-sales@bdal.com

Bruker Daltonics Inc.

Fremont, CA · USAPhone +1 (510) 683-4300 Fax +1 (510) 490-6586ms-sales@bdal.com

www.bruker.com/chemicalanalysis