Article
Selective Analysis of Sulfur-Containing Species in a
Heavy Crude Oil by Deuterium Labeling Reactions
and Ultrahigh Resolution Mass Spectrometry
Xuxiao Wang and Wolfgang Schrader *
Received: 3 November 2015; Accepted: 9 December 2015; Published: 17 December 2015
Academic Editor: Laszlo Prokai
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany;
[email protected]
* Correspondence: [email protected]; Fax: +49-208-306-2973
Abstract: A heavy crude oil has been treated with deuterated alkylating reagents (CD3 I and C2 D5 I)
and directly analyzed without any prior fractionation and chromatographic separation by high-field
Orbitrap Fourier Transform Mass Spectrometry (FTMS) and Fourier Transform Ion Cyclotron
Resonance Mass Spectrometry (FT-ICR MS) using electrospray ionization (ESI). The reaction of
a polycyclic aromatic sulfur heterocycles (PASHs) dibenzothiophene (DBT), in the presence of
silver tetrafluoroborate (AgBF4 ) with ethyl iodide (C2 H5 I) in anhydrous dichloroethane (DCE)
was optimized as a sample reaction to study heavy crude oil mixtures, and the reaction yield
was monitored and determined by proton nuclear magnetic resonance spectroscopy (1 H-NMR).
The obtained conditions were then applied to a mixture of standard aromatic CH-, N-, O- and
S-containing compounds and then a heavy crude oil, and only sulfur-containing compounds were
selectively alkylated. The deuterium labeled alkylating reagents, iodomethane-d3 (CD3 I) and
iodoethane-d5 (C2 D5 I), were employed to the alkylation of heavy crude oil to selectively differentiate
the tagged sulfur species from the original crude oil.
Keywords: selective analysis; sulfur; crude oil; deuterium labeling; ultrahigh resolution;
mass spectrometry
1. Introduction
The demand for affordable and reliable energy leads to a continuous focus on fossil-based
materials, and nowadays the trend is shifting to heavier petroleum resources. One of the huge
disadvantage of heavy crude oils as energy supply is that they contain rich heteroatoms, such as
sulfur, nitrogen and oxygen. The sulfur content is detrimental to refining processes and harmful to
the environment after combustion, and thus must be removed. Various stringent legislations and
regulations have been implemented to limit the sulfur content of fuels [1]. A better understanding of
heavy crude oil composition is necessary for that aim.
Heavy petroleum is a supercomplex [1] mixture of hydrocarbons containing various amounts of
heteroatoms (N, O and S), and it challenges and meanwhile promotes the development of analytical
techniques [2]. Mass spectrometry has been established as the most powerful and promising method
to characterize such complex mixtures [3,4]. FT-ICR MS provides sufficient mass resolving power
and mass accuracy to identify each of the thousands of different molecules and their elemental
compositions from the most complex mixtures [5]. Recently, a new commercially available type of
the high-field Orbitrap FTMS, the Orbitrap Elite [6–8], has been evaluated and employed successfully
to analyze the petroleum samples with a resolving power of up to 900,000 at m/z 400 [9,10].
Int. J. Mol. Sci. 2015, 16, 30133–30143; doi:10.3390/ijms161226205
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Int. J. Mol. Sci. 2015, 16, 30133–30143
Electrospray ionization (ESI) [11] coupled with ultrahigh resolution mass spectrometry [9,12,13]
is an excellent method employed to ionize polar species in crude oil. It efficiently ionizes the acidic or
basic molecular species in petroleum by deprotonation or protonation to form [M ´ H]´ or [M + H]+
ions [11,12,14] and even highly condensed polyaromatic compounds can be ionized by ESI as radical
cations [15].
However, PASHs are not basic enough to be efficiently ionized by ESI. Thus, some achievements
have been made to enhance ionizing efficiency of nonpolar sulfur species in ESI process.
Muller et al. [16] developed a derivatization method by forming the polar sulfonium salts in solution
prior to ESI, but the selectivity toward sulfur aromatics is achieved by relying on chromatographic
separations. Purcell et al. [17] derivatized a vacuum bottom bitumen residue by this derivatization
procedure, and observed not only sulfur compounds but also detected nitrogen and other heteroatom
containing classes at high abundance by positive ESI FT-ICR MS.
Metal complexation methods, such as Pd2+ electrospray mass spectrometry (ESI-MS) [18]
and especially Ag+ ESI-MS [19], have also been developed for the detection of sulfur-containing
compounds in petroleum by the formation of positive metal-complexes [20]. However, the relatively
flexible coordination sphere of the metal ions allows them to coordinate with a variety of ligands even
the solvents with the coordination numbers from 2 to 6, which on some level, increases the complexity
of crude oil analysis [19,21,22].
Here, on the basis of previous research, we combine the silver coordination chemistry and
nucleophilic chemistry of heteroatoms compounds with ESI-MS, and introduce the deuterium
labeling technique to develop a method that allows determination of sulfur-containing species in
a whole crude oil with high selectivity, and avoiding the complexity from the transition metal
complexation. When using the derivatization procedure with methyl iodide in a crude oil sample,
where the elements C, H, O, N, S and a few metals are usually present, it is difficult to separate
the derivatized from the non-derivatized compounds. The difference cannot really be established
because the derivatizing group contains the same elements as the compounds present in a crude
oil sample and only the increase in signal intensity of sulfur compounds can be an indication of
the reaction. Here, the usage of a deuterated reactant for the derivatization is introduced which
allows to clearly distinguish the reacted from potentially unreacted compounds and allows an
unambiguous characterization.
2. Results and Discussion
Ultrahigh-resolution MS has an unparalleled advantage for crude oil analysis, but as of yet, not
all compositions present in heavier petroleum can be completely and accurately analyzed by any
single available analytical method. Thus, simplification methods [23,24] and methods for selective
analysis need to be developed to gain a deeper insight into crude oil composition. It has been shown
that Ag+ ions kinetically coordinate preferentially to sulfur atoms instead of oxygen or nitrogen atoms
to form metal-ligand bonds [20,25]. This Ag+ selectivity towards sulfur species can now be used for
selective reactions with derivatization agents to tag sulfur heterocycles in crude oil. To achieve this
goal, the reaction has been investigated in detail in three steps: (1) The reaction was carried out using
a standard sulfur-containing compound. Here, dibenzothiophene (DBT) was chosen as a probe to
optimize the reaction condition and monitored by proton nuclear magnetic resonance (1 H-NMR) and
ultrahigh resolution MS; (2) In the second stage the obtained conditions were applied for a simple
mixture of standard aromatic CH-, N-, O- and S-containing compounds to investigate the selectivity;
(3) Finally, the conditions were applied to a heavy crude oil.
It is worth noting that the feeding sequence plays an important role on the selectivity towards
sulfur. Here, a certain amount of AgBF4 was added into the mixture of standards or crude oil, by
which Ag+ is first allowed to selectively coordinate with sulfur atoms to form complex Ag+ adducts
over a short time [26], then the alkylating reagent was added dropwise to the system and the S-C
covalent bond was formed by the strong driving force of the precipitation of silver iodide (AgI).
30134
Int. J. Mol. Sci. 2015, 16, 30133–30143
Int.derivatization
J. Mol. Sci. 2015, 16,procedure
page–page allows detection of sulfur compounds with very high selectivity, and
This
meanwhile avoids the extra complications arising from silver complexation [22,27] and silver natural
109Ag) [20]. The implementation of multi-chemical methods
isotopes
(51.84%107107
Agand
and48.16%
48.16% 109
isotopes
(51.84%
Ag
Ag) [20]. The implementation of multi-chemical methods
combined with the ultrahigh resolution mass spectrometry (FT-ICR MS and Orbitrap FTMS) with
combined with the ultrahigh resolution mass spectrometry (FT-ICR MS and Orbitrap FTMS) with
electrospray ionization offers an efficient and feasible approach to detect the sulfur-containing
electrospray ionization offers an efficient and feasible approach to detect the sulfur-containing species
species directly from the heavy petroleum without any time-consuming fractionation and
directly from the heavy petroleum without any time-consuming fractionation and separation.
separation.
2.1. Ethylation of Dibenzothiophene (DBT)
2.1. Ethylation of Dibenzothiophene (DBT)
The standard derivatization reaction that was previously introduced [16] was carried out under
The standard derivatization reaction that was previously introduced [16] was carried out under
the following condition: containing 10´−22 and 4 ˆ 10−3´3 mmol sulfur and 1 mmol of CH3 I, were
the following condition: containing 10 and 4 × 10 mmol sulfur and 1 mmol of CH3I, were
dissolved in 3 mL of dry DCE. A solution of 1 mmol AgBF4 in 2 mL of DCE was then added.
dissolved in 3 mL of dry DCE. A solution of 1 mmol AgBF4 in 2 mL of DCE was then added. While
While
the initial
reaction
reported
Muller
et al.
only
implemented
the initial
reaction
that that
was was
reported
by by
Muller
et al.
[16][16]
waswas
only
implemented
forfor
thethe
derivatization
with
CH
I;
here
an
additional
derivatization
agent,
C
H
I,
was
studied
as
ethylating
3
2
5
derivatization with CH3I; here an additional derivatization agent, C2H5I, was studied as ethylating
agent.
InIn
addition,
bothreagents
reagentsininthe
thedeuterated
deuterated
form
directly
agent.
addition,the
thereaction
reactionwas
wascarried
carried out
out using
using both
form
to to
directly
study
thethe
reaction
product
andand
distinguish
derivatized
from
non-derivatized
compounds
in the
study
reaction
product
distinguish
derivatized
from
non-derivatized
compounds
in crude
the
oilcrude
sample
that
was
not
chromatographically
simplified.
oil sample that was not chromatographically simplified.
1 H-NMR
Figure1.1. 1H-NMR
spectra
(top)(top)
and HRMS
(electrospray
ionization
Fourier transform
cyclotronion
Figure
spectra
and HRMS
(electrospray
ionization
Fourier ion
transform
resonance
mass
spectrometry
(ESI
FT-ICR
MS))
spectra
(bottom)
of
5-ethyldibenzo[b,d]thiophenium
cyclotron resonance mass spectrometry (ESI FT-ICR MS)) spectra (bottom) of 5-ethyldibenzo[b,d]
tetrafluoroborate.
thiophenium
tetrafluoroborate.
3
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The molar ratio of DBT, CH3 I (or C2 H5 I) and AgBF4 remains 1:2:2, and the yield of crude
The molar ratio of DBT,
CH3I (or C2H5I) and AgBF4 remains 1:2:2, and the yield of crude product
product determined
by 1 H-NMR was 70% for CH3 I and 87% for C2 H5 I. Similar experimental results
1
determined by H-NMR was 70% for CH3I and 87% for C2H5I. Similar experimental results were
were obtained by Acheson et al. [28], so C H5 I was chosen for the continuous studies. In regard to
obtained by Acheson et al. [28], so C2H52I was
chosen for the continuous studies. In regard to
selectivity, the reaction needed to be altered. Instead of adding equal molar ratio of CH I and AgBF
selectivity, the reaction needed to be altered. Instead of adding equal molar ratio of CH33I and AgBF44
(1 mmol) that are 250 times the concentration of sulfur, as mentioned by Muller et al., [16] the reaction
(1 mmol) that are 250 times the concentration of sulfur, as mentioned by Muller et al., [16] the
condition depicted in Scheme 1 were optimized to produce a high yield of sulfonium salt.
reaction condition depicted in Scheme 1 were optimized to produce a high yield of sulfonium salt.
When the molar ratio of DBT, AgBF and C H5 I remains 1:2:8, i.e., the amount of AgBF4 is only
When the molar ratio of DBT, AgBF44 and C22H
5I remains 1:2:8, i.e., the amount of AgBF4 is only 2
2 times more than DBT, the yield determined by 1 H-NMR increases significantly (99%), as shown in
times more than DBT, the yield determined by 1H-NMR increases significantly (99%), as shown in
Figure 1 (top).
Figure 1 (top).
Scheme 1. Ethylation of dibenzothiphene (DBT), reaction carried out at room temperature (rt).
Scheme 1. Ethylation of dibenzothiphene (DBT), reaction carried out at room temperature (rt).
The 11H-NMR spectrum of the above salt is simple because of the molecular symmetry (see
The
H-NMR
of the above
saltintegral
is simple
molecular
symmetry
Figure
1 top).
The 4 spectrum
groups of protons
with the
ratio because
2:2:2:2 inof
thethe
downfield
belong
to the
(see
Figure
1
top).
The
4
groups
of
protons
with
the
integral
ratio
2:2:2:2
in
the
downfield
aromatic rings; the 2 groups of protons with the integral ratio 2:3 in the upfield are attributedbelong
to the
to
the
aromatic
rings;
the
2
groups
of
protons
with
the
integral
ratio
2:3
in
the
upfield
are
attributed
to
ethyl group. With the appropriate splitting according to the relevant environments of each proton,
the
ethyl
group.
With
the
appropriate
splitting
according
to
the
relevant
environments
of
each
proton,
the structure of 5-ethyldibenzo[b,d]thiophenium tetrafluoroborate was ambiguously determined.
structure
of 5-ethyldibenzo[b,d]thiophenium
1the
H-NMR
(300 MHz,
CDCl3): δ 8.41 (d (doublet), J = tetrafluoroborate
7.6 Hz, 2H, 2 × Ph),was
8.12ambiguously
(d, J = 7.1 Hz,determined.
2H, 2 × Ph),
1 H-NMR (300 MHz, CDCl ): δ 8.41 (d (doublet), J = 7.6 Hz, 2H, 2 ˆ Ph), 8.12 (d, J = 7.1 Hz, 2H,
3 2 × Ph), 7.76 (t, J = 7.9 Hz, 2H, 2 × Ph), 4.21 (q (quartet), J = 7.1 Hz, 2H,
7.87 (t (triplet), J = 7.6 Hz, 2H,
Ph),2CH
7.87
(t (triplet), J = 7.6 Hz, 2H, 2 ˆ Ph), 7.76 (t, J = 7.9 Hz, 2H, 2 ˆ Ph), 4.21 (q (quartet),
22 ׈CH
3), 0.90 (t, J = 7.1 Hz, 3H, 3 × CH2CH3).
J = 7.1
Hz,
2H,
2 ˆ CH
(t, J = 7.1 Hz, 3H, 3 ˆ CH2 CH3tetrafluoroborate
).
2 CH
3 ), 0.90
The ethylated
DBT
salt,
5-ethyldibenzo[b,d]thiophenium
was obtained as
The needles.
ethylated
DBTdetermined
salt, 5-ethyldibenzo[b,d]thiophenium
tetrafluoroborate
was 1obtained
colorless
It was
by the positive HRMS (ESI FT-ICR
MS) in Figure
(bottom) as
at
colorless
needles.
It
was
determined
by
the
positive
HRMS
(ESI
FT-ICR
MS)
in
Figure
1 (bottom)
m/z 213.07322 (calcd 213.07325) for C14H13S+. A tiny peak of [M + H] was also observed
at m/z
at m/z 213.07322
(calcd 213.07325)
C14by
H13
S+loss
. A oftiny
[M +under
H] was
also observed
at
185.04197,
and explanation
might be for
given
the
the peak
ethyl of
group
ESI condition.
It was
m/z
185.04197,
and
explanation
might
be
given
by
the
loss
of
the
ethyl
group
under
ESI
condition.
shown, that when using a heated nebulizer the derivatization group can be removed during the
It was shown,
that[1].
when using a heated nebulizer the derivatization group can be removed during
ionization
process
the ionization process [1].
2.2. Ethylation of a Mixture of Standards (ANTH, DBT, ACR and DBF)
2.2. Ethylation of a Mixture of Standards (ANTH, DBT, ACR and DBF)
Based on the results of the reaction of a pure standard sulfur compound, the conditions were
Based on the results of the reaction of a pure standard sulfur compound, the conditions were
applied to a mixture of standards (antracene (ANTH), dibenzothiphene (DBT), acrinine (ACR) and
applied to a mixture of standards (antracene (ANTH), dibenzothiphene (DBT), acrinine (ACR) and
dibenzofuran (DBF)). The HR ESI-MS spectrum of the ethylated mixture of standards was obtained
dibenzofuran (DBF)). The HR ESI-MS spectrum of the ethylated mixture of standards was obtained
as shown in Figure 2. No corresponding signals of derivatized ANTH, ACR and DBF were observed
as shown in Figure 2. No corresponding signals of derivatized ANTH, ACR and DBF were observed
(calcd. m/z values at 207.11683, 208.11208 and 197.09609), and only DBT was selectively ethylated
(calcd. m/z values at 207.11683, 208.11208 and 197.09609), and only DBT was selectively ethylated
among this standards mixture. Besides the peaks from DBT, a very minor signal [M + H]++ from
among this standards mixture. Besides the peaks from DBT, a very minor signal [M + H] from
underivatized ACR at m/z 180.08084 was detected. Additionally, minor traces of the disubstituted
underivatized ACR at m/z 180.08084 was detected. Additionally, minor traces of the disubstituted
product of DBT was observed as well. From the results obtained above, we can see, even in the
product of DBT was observed as well. From the results obtained above, we can see, even in
presence of other polyromantic heterocyclic compounds, the nucleophilic substitution of sulfur
the presence of other polyromantic heterocyclic compounds, the nucleophilic substitution of sulfur
aromatic compound works with high selectivity towards sulfur species due to activation by the
aromatic compound works with high selectivity towards sulfur species due to activation by the
silver ions.
silver ions.
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Int. J.
J. Mol.
Mol. Sci.
Sci. 2015,
2015, 16,
16, page–page
page–page
Int.
Figure
spectrum
of of
aofmixture
of the
ethylated
standards
(anthracene
(ANTH),
DBT,
Figure 2.
2. ESI
ESIFT-ICR
FT-ICR
MS
spectrum
aa mixture
of
standards
(anthracene
(ANTH),
FT-ICRMS
MS
spectrum
mixture
of the
the ethylated
ethylated
standards
(anthracene
(ANTH),
acridine
(ACR)(ACR)
and
dibenzofuran
(DBF)).
DBT, acridine
and
dibenzofuran
(DBF)).
(ACR) and dibenzofuran (DBF)).
2.3. Methylation and Ethylation
2.3.
Ethylation of
of aa Heavy
Heavy Crude
Crude Oil
Oil
Deuterated alkylating
alkylating
reagents
(CD
II and
utilized
to
the
C2C
D
I)55I)
were
utilized
to alkylate
the whole
heavyheavy
crude
alkylatingreagents
reagents(CD
(CD
and
C252D
D
I) were
were
utilized
to alkylate
alkylate
the whole
whole
heavy
3 I33and
crude
oil instead
of the
reagents
to
the
in
crude
oil
instead
of the standard
reagents
to distinguish
the compounds
presentpresent
in a crude
that oil
contain
the standard
standard
reagents
to distinguish
distinguish
thecompounds
compounds
present
inaaoil
crude
oilthat
that
contain
N-,O-S-elements
and
Oelements
from
the
derivatized
ones.
An
adjustment
for
CD
3
I
had
to
CH-,
N-,CH-,
S- and
from
the
derivatized
ones.
An
adjustment
for
CD
I
had
to
be
made
to
a
and O- elements from the derivatized ones. An adjustment
for CD3I had tobe
be
3
made to
a molar
ratio
of
crude
and
CD
molar
ratio
of crude
oil,
andAgBF
CD3 I44 of
1:2:10.
ratio
ofAgBF
crude4 oil,
oil,
AgBF
and
CD33IIof
of1:2:10.
1:2:10.
Positive
was
performed
for
the untreated
untreated
and
methylated
heavy
Positive ESI
ESI Orbitrap
Orbitrap FTMS
FTMS analysis
analysis was
was performed
performed for
for the
untreatedand
andmethylated
methylatedheavy
heavy
crude
oil.
For
a
better
signal-to-noise
ratio,
up
to
500
scans
in
the
mass
range
of
m/z
150–1200
crude
For
a
better
signal-to-noise
ratio,
up
to
500
scans
in
the
mass
range
of
m/z
150–1200
crude oil. For a better signal-to-noise ratio, up to 500 scans in the mass range of m/z 150–1200were
were
accumulated,
and
difference
of
spectra
before
and
after
methylation
can
be
inin
were
accumulated,
a difference
of mass
the mass
spectra
before
methylation
be
seen
accumulated,
and aaand
difference
of the
the
mass
spectra
before
andand
afterafter
methylation
cancan
beseen
seen
Figure
3.
noting
that
the
change
of
class
distribution
before
and
in
Figure
3.is
is worth
noting
the dramatic
change
of heteroatom
distribution
before
and
Figure
3. It
It
isItworth
worth
noting
thatthat
the dramatic
dramatic
change
ofheteroatom
heteroatom
classclass
distribution
before
andafter
after
deuterium
labeled
methylation
in
44 is
Before
the
reaction,
the
after
deuterium
labeled
methylation
in Figure
4 is immense.
the derivatization
reaction,
the
deuterium
labeled
methylation
in Figure
Figure
is immense.
immense.
BeforeBefore
thederivatization
derivatization
reaction,
themore
more
polar
compounds
are
as
Here,
majority
of
assigned
belong
more
compounds
are detected
as expected.
Here,
the majority
of that
signals
that
were assigned
polar polar
compounds
are detected
detected
as expected.
expected.
Here, the
the
majority
ofsignals
signals
thatwere
were
assigned
belong
to
class,
with
signals
from
11SS11[H]
inand
very
amounts
from
the
belong
to11[H]
the N
class,additional
with additional
signals
from
N1 Sand
in minor
very
minor
amounts
from
to the
the N
N
[H]
class,
additional
signals
from the
the N
Nthe
[H]
and
very
minor
amounts
from
the
1 [H] with
1 [H] in
O
1
S
1
[H]
class.
Note
that
the
protonated
molecules
that
were
detected
are
distinguished
by
[H].
the
class.
Note
protonated
molecules
thatwere
weredetected
detectedare
are distinguished
distinguished by [H].
O1SO
1[H]
class.
Note
thatthat
thethe
protonated
molecules
that
[H].
1 S1 [H]
Radial
ions
would
be
described
without
Radial
[H].
Radial ions
ions would
would be
be described
described without
without [H].
[H].
Figure 3. ESI(+) Orbitrap FTMS mass spectrum of heavy crude oil (left) and deuterium labeling
Figure 3.
3. ESI(+)
(left) and
and deuterium
deuterium labeling
labeling
Figure
ESI(+) Orbitrap
Orbitrap FTMS
FTMS mass
mass spectrum
spectrum of
of heavy
heavy crude
crude oil
oil (left)
methylated heavy crude oil (right).
methylated
heavy
crude
oil
(right).
methylated heavy crude oil (right).
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16, page–page
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Figure
4.
Class distribution
distribution for
for heavy
heavy crude
crude oil
oil from
from direct
direct analysis
analysis (left)
(left) and
and heavy
heavy crude
crude oil
oil
after
Figure
Figure 4.
4. Class
Class
distribution
for
heavy
crude
oil
from
direct
analysis
(left)
and
heavy
crude
oil after
after
deuterium
labelled
methylation
(right).
deuterium
deuterium labelled
labelled methylation
methylation (right).
(right).
After
compound classes
classes that
that were
were assigned
assigned changed
changed drastically.
drastically. Now,
Now, instead
After methylation,
methylation, the
the compound
instead
of
the
nonopolar
SS11 and
S
22 classes
which
are
the polar
polar
compounds
that
are
usually
ionized
bybyESI,
ESI,
the
nonopolar
S
classes
which
are
of the
polarcompounds
compoundsthat
thatare
areusually
usuallyionized
ionizedby
ESI,
the
nonopolar
Sand
and
S
classes
which
1
2
usually
not
ionized
by
ESI
are
detected
at
high
abundance.
The
deuterated
version
of
the
usually
not
ionized
by
ESI
are
detected
at
high
abundance.
The
deuterated
version
of
are usually not ionized by ESI are detected at high abundance. The deuterated version of the
derivatization
readily distinguish
detected
derivatization agent
agent now
now also
also makes
makes it
it possible
possible to
to readily
distinguish that
that the
the sulfur
sulfur species
species detected
are
not
only
from
the
crude
oil.
derivatization reaction
reaction and
and not
not only
only from
from the
the crude
crude oil.
oil. After
After the
the reaction
reaction the
are arising
arising from
from the
the derivatization
derivatization
reaction
and
the
sulfur
as
S-methyl
(d
3
)
sulfonium
salts.
A
small
amount
of
S
1
compounds
(2.6%)
sulfur species
species are
are present
present
as
S-methyl
(d
3
)
sulfonium
salts.
A
small
amount
of
S
1
compounds
(2.6%)
present as S-methyl (d3
salts.
1
can
can also
also be
be detected.
detected.
The
in
Figure
5.
are shown
shown in
in Figure
Figure 5.
5. The
[H] class
class in
in the
the non-derivatized
non-derivatized sample
The details
details of
of the
the results
results are
are
shown
The N
N111[H]
sample
generated
generated from
from protonation
protonation has
has aa DBE
DBE range
range between
between 44 and
and 24
24 with
with aa carbon
carbon number
number distribution
distribution
from
86. The
The N
N111SSS111[H]
[H]
class
meanwhile
has
DBE
range
of
5–25
and carbon
carbon number
number of
of 10–83.
from 11
11 to
to 86.
86.
[H]class
classmeanwhile
meanwhilehas
hasaaaDBE
DBErange
range of
of 5–25
5–25 and
10–83.
The
The deuterated
deuterated SS111[H]
[H] class
class spans
spans aa range
range of
of DBE
DBE (2–25)
(2–25) and
and carbon
carbon number
number (9–84),
(9–84), where
where the
the
compounds
with
DBE
value
of
2
are
assumed
to
be
the
aliphatic
sulfides
containing
a
ring
or
withDBE
DBEvalue
valueofof
2 are
assumed
tothe
be aliphatic
the aliphatic
sulfides
containing
or aa
compounds with
2 are
assumed
to be
sulfides
containing
a ring aorring
a double
double
bond.
The
SS22[H]
class
has
aa DBE
range
3–25
and
doubleThe
bond.
The deuterated
deuterated
[H]
class
hasrange
DBEof
range
of
3–25
and carbon
carbon number
number
(8–73).
bond.
deuterated
S2 [H] class
has
a DBE
3–25of
and
carbon
number
(8–73). (8–73).
Figure 5.
Double bond
bond equivalent
carbon number
[H]
Figure
the
N
N
5. Double
Double
bond
equivalent (DBE)
(DBE) versus
versus carbon
number plots
plots for
for the
the N
N111[H]
[H] and
and N
N111SS111[H]
classes from
the
classes
oil
(left)
and
for
the
D
[H] and
and D
D333SS22[H]
[H] classes
classes from
from the
from the
the protonated
protonated heavy
heavy crude
crude oil
oil (left)
(left)and
andfor
forthe
theD
D333SSS111[H]
deuterated methylated
methylated heavy
Note
deuterated
crude
oil
(right).
Note
that
the
[H]
indicates
a
protonated
molecule.
heavy crude oil (right).
that the [H] indicates a protonated molecule.
Additional
Additional ESI(+)
ESI(+) FT-ICR
FT-ICR MS
MS analyses
analyses were
were performed
performed on
on the
the deuterated
deuterated ethylated
ethylated heavy
heavy
crude
crude oil,
oil, as
as shown
shown in
in Figure
Figure 6.
6. Here,
Here, the
the results
results of
of the
the ethylation
ethylation reaction
reaction are
are consistent
consistent with
with those
those
30138
66
Int. J. Mol. Sci. 2015, 16, 30133–30143
Additional
FT-ICR MS analyses were performed on the deuterated ethylated heavy crude
Int. J. Mol.
Sci. 2015, ESI(+)
16, page–page
Int. J. Mol. Sci. 2015, 16, page–page
oil, as shown in Figure 6. Here, the results of the ethylation reaction are consistent with those from
frommethylation
the methylation
reaction.
The nonpolar
sulfur-containing
compounds
1, S2 S
and
1O1 species)
the
reaction.
The nonpolar
sulfur-containing
compounds
(S1 , S2(S
can
1 O1S
from the methylation
reaction.
The nonpolar
sulfur-containing
compounds
(Sand
1, S2 and
Sspecies)
1O1 species)
canselectively
be selectively
detected
by ESI(+)
and
differentiated
by C
the
C
2
D
5
-group
from
the
original
crude
be
detected
by
ESI(+)
and
differentiated
by
the
D
-group
from
the
original
crude
oil.
2 5C2D5-group from the original crude
can be selectively detected by ESI(+) and differentiated by the
oil.comparison
In comparison
with
the methylation
procedure,
a small difference
in
S
1
O
1
class
(2.5%)
In
with
the
methylation
procedure,
a
small
difference
in
S
O
class
(2.5%)
distribution
1 1
oil. In comparison with the methylation procedure, a small difference
in S1O1 class (2.5%)
distribution
appears
which
coulddifferent
result from
different
reactivities
of the
alkylating
reagents.
Stillthe
in
appears
which
could
result
from
reactivities
of
the
alkylating
reagents.
Still in
both cases
distribution appears which could result from different reactivities of the alkylating
reagents.
Still in
both
cases
the
S
1
class
is
the
dominant
class
in
both
alkylation
reactions,
followed
by
the
S
2
class.
S
class
is
the
dominant
class
in
both
alkylation
reactions,
followed
by
the
S
class.
1
2
both
cases the S1 class is the dominant class in both alkylation reactions, followed
by the S2 class.
Figure
Heteroatom
heavy crude
oil after
after deuterium labelled
labelled ethylation.
Figure 6.
6.
Heteroatom class
class distribution
distribution of
of
Figure
6. Heteroatom
class
distribution
of heavy
heavy crude
crude oil
oil after deuterium
deuterium labelled ethylation.
ethylation.
When comparing the methylation reactions, the deuterated S1 class in the ethylation extends a
methylation reactions,
reactions, the
the deuterated
deuterated SS11 class in the ethylation extends a
When comparing the methylation
slightly narrower DBE range of 4–24, and the S2 class has a slightly wider DBE range from 1 to 25 as
slightly narrower DBE
DBE range
range of
of 4–24,
4–24, and
and the
the SS22 class has a slightly wider DBE range from 1 to 25 as
demonstrated in Figure 7. The slight difference of the DBE range for S1 and S2 classes in methylation
in Figure
Figure 7.
7. The
The slight
slight difference
difference of
ofthe
theDBE
DBErange
rangefor
forSS11and
andSS22 classes in methylation
demonstrated in
and ethylation might lie in the slight imparity of heteroatom class distribution of the two cases.
slight imparity
imparity of
of heteroatom
heteroatom class
class distribution
distribution of
of the
the two
twocases.
cases.
and ethylation might lie in the slight
Figure 7. DBE versus carbon number plots for the D5S1 and D5S2 classes from the deuterated ethylated
7. DBE versus carbon
carbon number
number plots
plotsfor
forthe
theDD55SS11 and D
D55S22 classes
Figure 7.
classes from
from the deuterated ethylated
heavy crude oil.
heavy crude oil.
When considering the results from this study, the most interesting point is that sulfur
When considering
considering the
results
from
this study,
theinteresting
most interesting
point
is that
sulfur
When
from
thiscrude
study,
themixture
most
point is that
sulfur
containing
containing
compounds the
in aresults
very complex
oil
can be selectively
analyzed
by ESI-MS
containing compounds
in
a very crude
complex crude
oil mixture
can be selectively
analyzed
by ESI-MS
compounds
in a very
complex
mixture
can be
analyzed
by the
ESI-MS
after
after derivatization.
While
in theory, the oil
pyridine
nitrogen
is aselectively
better nucleophile
than
thiophene
after
derivatization.
While
in
theory,
the
pyridine
nitrogen
is
a
better
nucleophile
than
the
thiophene
derivatization.
While
in
theory,
the
pyridine
nitrogen
is
a
better
nucleophile
than
the
thiophene
sulfur
sulfur to react in an SN2 reaction with alkylating reagents in the polar aprotic solvent (DCE), things
sulfur
toinreact
in an SN2 reaction
with alkylating
reagents
in theaprotic
polar aprotic
solvent
(DCE),change
things
to
react when
an SAg
alkylating
thecrude
polar oil.
solvent
(DCE), things
N 2+ reaction
change
ions are with
involved
in the reagents
reaction in
with
Here,
the coordination
chemistry
+
change when
Ag
ions
are involved
in the reaction
with crude
oil. the
Here,
the coordination
chemistry
+ ions
when
areplays
involved
in the reaction
crude oil. favor
Here,the
coordination
the
of the Ag
silver
ions
an important
role with
to kinetically
sulfur
atoms chemistry
and form of
weak
of
the
silver
ions
plays
an
important
role
to
kinetically
favor
the
sulfur
atoms
and
form
weak
silver
ions
plays
an
important
role
to
kinetically
favor
the
sulfur
atoms
and
form
weak
silver-ligand
silver-ligand bonds. When we then add the alkyl iodide to the above system, the relatively weak
silver-ligand bonds. When we then add the alkyl iodide to the above system, the relatively weak
bonds.
When bond
we then
alkyl and
iodide
the above
system,
relatively
weak
metal-ligand
willadd
be the
broken
a to
much
stronger
S-C the
covalent
bond
willmetal-ligand
be formed
metal-ligand bond will be broken and a much stronger S-C covalent bond will be formed
bond
will
be
broken
and
a
much
stronger
S-C
covalent
bond
will
be
formed
accompanied
by the
accompanied by the strong thermodynamic drive of releasing solid silver iodide (AgI) from
the
accompanied by the strong thermodynamic drive of releasing solid silver iodide (AgI) from the
strong
thermodynamic
drive
of
releasing
solid
silver
iodide
(AgI)
from
the
solution.
The
generated
solution. The generated AgI could also further react with nitrogen compounds, to form insoluble
solution. The generated AgI could also further react with nitrogen compounds, to form insoluble
AgI
could alsocompounds
further react
nitrogen
compounds,
to form be
insoluble
coordination
compounds
of
coordination
ofwith
larger
size that
can subsequently
removed
from the system
[29]. An
coordination compounds of larger size that can subsequently be removed from the system [29]. An
important factor for the better selectivity toward sulfur compounds in crude oil is therefore, the right
important factor for the better selectivity toward sulfur compounds in crude oil is therefore, the right
30139
amount of AgBF4.
amount of AgBF4.
7
Int. J. Mol. Sci. 2015, 16, 30133–30143
larger size that can subsequently be removed from the system [29]. An important factor for the better
selectivity toward sulfur compounds in crude oil is therefore, the right amount of AgBF4 .
3. Materials and Methods
3.1. Alkylation
DBT, DCE, AgBF4 , anthracene (ANTH), dibenzofuran (DBF), acetic acid (AcOH) and dichloromethane
(DCM) were purchased from Sigma-Aldrich (high purity, St. Louis, MO, USA). C2 H5 I, acridine
(ACR), CD3 I and C2 D5 I were purchased from Sigma-Aldrich (high purity, Steinheim, Germany).
A heavy crude oil of North American origin was used.
10 mg of DBT were dissolved in 1 mL anhydrous DCE. During mixing, a solution of 2 molar eq.
AgBF4 (21.13 mg) in 0.5 mL DCE was added. After 2 min, 8 molar eqivalents (28 µL) of C2 H5 I was
then added and yellow silver iodide precipitated immediately. After 4 h, the solid was removed by
centrifugation and washed with 0.5 mL of DCE. A final concentration of 500 µg/mL of the ethylated
DBT was obtained through dilution with DCM; the procedure above was successively applied to a
mixture of standard compounds: 6.5 mg of ANTH, 2.75 mg of DBT, 0.25 mg of ACR, 0.5 mg of DBF.
The same procedure was further employed to 10 mg of the heavy crude oil by replacing C2 H5 I with
CD3 I and C2 D5 I. For a comparison, a heavy crude oil sample with a final concentration of 500 µg/mL
in DCE containing 0.2% AcOH was used for ESI analysis.
3.2. Nuclear Magnetic Resonance Spectroscopy
1 H-NMR
spectra were recorded on a Bruker Advance 300 NMR spectrometer. The chemical
shifts and the coupling constants were obtained through analysis of the spectra using Bruker TopSpin
NMR-Software version 2.1 (Karlsruhe, Germany). The chemical shifts of 1 H-NMR spectra is reported
as in units of parts per million (ppm). The 1 H-NMR spectrum is referenced through the solvent lock
(2 H) signal according to IUPAC (International Union of Pure and Applied Chemistry) recommended
secondary referencing method and the manufacturer’s protocols based on impurity of CHCl3 in
CDCl3 . Multiplicities were given as: s (singlet); br s (broad singlet); d (doublet); t (triplet); q (quartet);
m (multiplets), etc. The number of proton (n) for a given resonance is indicated by nH. The ethylated
DBT was evaporated to dryness under reduced pressure to remove DCE and the excess of C2 H5 I.
The dry ethylated DBT was then dissolved in 0.5 mL of CDCl3 was used for for 1 H-NMR analysis.
3.3. ESI Orbitrap FTMS Analysis
Mass analyses were performed on a hybrid mass spectrometer combining the dual linear ion
trap with a novel high field Orbitrap mass analyzer (LTQ Orbitrap Elite FTMS, Thermo Scientific,
Bremen, Germany) [9]. Up to 500 spectra were collected in positive mode using the ESI source
(Thermo Fisher, Bremen, Germany). A standard data acquisition and instrument control system was
employed (Thermo Scientific, Bremen, Germany). Acquisition mass range was 150 < m/z < 1200 and
the target value (AGC value) was set between 1E5 and 1E6; typical ESI conditions were as follows:
flow rate 5 µL/min; spray voltage 3.0 kV; a sheath gas flow of 5 (arbitrary unit), an auxiliary gas
flow of 2 (arbitrary unit). MS data were recorded with a resolving power of 480,000 at m/z 400
using a 1.5 s transient and up to 900,000 at m/z 400 using transient signals of 3 s for comparison.
These parameters allow separating all important mass splits throughout the detected scan range.
The instrument was calibrated with the Thermo Scientific Pierce LTQ Velos ESI positive ion calibration
solution. In addition, external calibration was performed using a mixture of the Agilent electrospray
calibration solution with masses at 300.04812, 622.02896, 922.00980, thus the whole mass range was
covered in the samples. The mass accuracy below 1 ppm and the resolving power up to 480,000 at
m/z 400 allows the analysis of the crude oil by the high-field Orbitrap.
30140
Int. J. Mol. Sci. 2015, 16, 30133–30143
3.4. ESI FT-ICR MS Analysis
Corresponding mass analysis was performed on a 7 T linear quadrupole ion-trap (LTQ) FT-ICR
MS (Thermo Fisher, Bremen, Germany). The same ESI source was employed. Mass acquisition,
ESI condition, AGC control and mass calibration follow the same setting implemented on ESI
Orbitrap FTMS.
3.5. Data Analysis
The obtained mass data were imported into Composer software V1.06 (Sierra Analytics,
Modesto, CA, USA). The following chemical constraints were applied: number of H 1000, 0 < D < 3
for reaction with CD3 I and 0 < D < 5 for reaction with C2 D5 I, 0 < C < 100, 0 < S < 3, 0 < O < 3,
0 < N < 3, and 0 < double bond equivalent (DBE) < 40, with a maximum mass error of 1.5 ppm.
The calculated molecular formulas were sorted into their heteroatom class (Nn Oo Ss ) according to
their denoted Kendrick mass defects, double bond equivalence (DBE = number of rings plus double
bonds involving carbon) distribution, and carbon number distribution [30].
4. Conclusions
Ultrahigh-resolution FT-ICR MS is an undoubtedly powerful analytical method for the analysis
of incredibly complex petroleum samples. Nowadays, the high-field Orbitrap FTMS with a
resolving power of up to 900,000 at m/z 400 is becoming an attractive alternative to FT-ICR MS
for petroleum analysis as shown in this work and others [9,10]. Different chemical methods, such
as protonation and alkylation, have certain selectivity toward some classes of compounds in crude
oil, and although MS has an unmatched advantage for the analysis of petroleum-type samples,
it is still not able to fully characterize such tremendously complex mixtures. The combination
of various selective methods [31], such as chemical derivatization, chromatographic methods, etc.,
might make a comprehensive characterization of a crude oil possible. Herein, by combining
the coordination chemistry and organic chemistry together with the ultrahigh resolution mass
spectrometry (FT-ICR MS and high-field Orbitrap FTMS) coupled with positive ESI, we demonstrate
a highly selective method toward sulfur-containing compounds analysis in a whole heavy crude
oil without any fractionation and chromatographic separation. Deuterium labeling is utilized to
introduce specific information of isotopically labeled atoms which are not present at high abundance
in crude oil and allow unambiguous differentiation of the nonpolar sulfur species formed from CD3
or C2 D5 from the original crude oil in positive ESI. Using the labeling procedure also allows to
fully assigning all reacted components within the very complex crude oil mixture, thus allowing
full characterization of the reaction.
Acknowledgments: The authors thank David Stranz (Sierra Analytics, Modesto, CA, USA) for assistance during
data interpretation. The authors also thank Jeffrey Garber (MPI) for additional proofreading of the manuscript.
Author Contributions: Wolfgang Schrader and Xuxiao Wang conceived the research idea and participated in
the design of the experiments; Xuxiao Wang carried out the experiments and interpreted the data; Xuxiao Wang
and Wolfgang Schrader wrote the manuscript. All authors read and approved the final manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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