Application Note # FTMS-44
Analysis of sulfur-rich crude oil and bitumen by FTMS
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
to be resolved in a mass spectrum is 3.4 mDa (difference
between C 3 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
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.
with 50% methanol, 50% toluene for atmospheric pressure
photoionization (APPI) measurements.
Experimentals
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 positive ion mode using the APPI ion source (Bruker Daltonik
GmbH, Bremen, Germany). Sample solutions were continuously 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 processing size of 8M data points before sine apodization.
Mass Analysis
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 dichloromethane. These sample stock solutions were diluted 1:300
Mass peaks are in the range m/z 300-1200
wx-1
wx-2
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 recalibrated internally with the homologous series CnH2n-16 and
the bitumen samples with the homologous series CnH2n-16S
to improve the mass accuracy.
wx-3
wx-4
wx-5
wx-6
Molecular formula calculation
400
600
800
1000
1200
1400
The mass formula calculation was done in Composer 1.0.2
(Sierra Analytics, Modesto, CA, USA) using a maximum
m/z
Figure 1: Mass spectra of crude oil and bitumen samples measured
in APPI positive ion mode.
Significant differences of mass spectra
wx-1
514.45329
514.35931
514.32294
514.22906 514.26873
wx-2
514.41694
514.49817
514.54264
514.36272
514.27216
514.09109
514.14190
514.18168
514.45671
514.32631
514.23582
514.41577
wx-3
514.54719
514.45335
514.35940
514.26885
514.32294
wx-4
514.41697
514.49820
514.54716
514.36270
514.27215
514.09102
514.14195
514.18170
514.23580
514.45343
514.32626
514.41707
wx-5
514.53814
514.45339
514.36273
514.17842
514.26879
514.23257
wx-6
514.45336
514.36272
514.14205514.17844
514.1
514.2
514.23248
514.27216
514.32632
514.3
514.49823 514.54253
514.41703
514.32287
514.41705
514.4
514.49837 514.54274
514.5
m/z
Figure 2: Zoom of all mass spectra
at nominal mass m/z 514 to see fine
structure of mass spectra.
More than 95% of the mass peaks were annotated with an elemental formula
514.45329
C36H50S+.
0.09 ppm
C36H46+.
0.19 ppm
C37H38S+.
0.27 ppm
C37H56N+
0.13 ppm
C38H42O +.
0.16 ppm
C40H34+.
0.19 ppm
C36H6913C+
0.17 ppm
C37H54
0.05 ppm
C35H64NO +
0.15 ppm
C39H30O +.
0.11 ppm
C36H52NO +.
0.14 ppm
514.22906
514.20
514.54264
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).
514.41694
514.32294
514.49817
514.28661
514.25
C37H70+.
0.25 ppm
514.35931
C37H38O 2+.
0.04 ppm
514.26873
514.15
C36H55N13C+.
0.05 ppm
O +.
C36H34OS+.
0.18 ppm
C35H30S2+.
0.10 ppm
C38H58+.
0.02 ppm
514.30
514.35
514.40
514.45
514.50
514.55
m/z
Compound class plot
70
wx-1
wx-2
60
wx-3
wx-4
50
wx-5
rel. abundance
wx-6
40
30
20
10
0
HC
S
S2
S3
N
NO
Compound class
formula of CnHhN3O3S 3, 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
O
OS
OS2
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).
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.
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).
S1
0.5
0
0.02
0.04
Sulfur containing compound classes
2.0
a)
0.12
Sample wx-4
S1
0.14
b)
S2
1.0
S3
H/C
H/C
0.10
1.5
1.0
0
2.0
S2
S1
0.5
0.08
S/C ratio
Sample wx-3
1.5
0.06
0.5
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0
0.02
0.04
S/C ratio
0.06
0.08 0.1
S/C ratio
0.12
0.14
2.06: 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
Figure
ratios of sulfur containing compound
simultaneously.
Sample
wx-4
b)
1.5
S1
H/C
1.0
S2
S3
More than 95% of the mass peaks could be annotated
with0.5
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
all samples
0
0.02 0.04of compound
0.06
0.08classes
0.1 in0.12
0.14
are shown in Figure 4. The six samples
can
be
separated
S/C ratio
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 S 3
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 S 3 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.
Authors
Dr. Matthias Witt, Bruker Daltonik GmbH, Fahrenheitstr. 4, D-28359
Bremen, Germany
Keywords
Instrumentation & Software
Petroleomics
solarix 12T
FTMS
DataAnalysis 4.0
APPI
APPI II source
Composer
For research use only. Not for use in diagnostic procedures.
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to change specifications without notice. © Bruker Daltonics 06-2011, FTMS-44 #280761
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[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.
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References