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].
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, Ox, Sx, SOx, Nx, and NOx are ionized
with differing efficiencies.
Authors
Matthias Witt
Bruker Daltonik GmbH, Bremen, Germany
Keywords
Instrumentation and Software
Petroleomics
solariX XR
FTMS
APPI II source
APPI
Composer 1.0.6
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 and S1 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, 2nd injection from vial 2, 3rd injection from vial 3, 4th
injection from vial 4, 5th injection from vial 5, 6th injection
from vial 1, 7th 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 37H56) to align single scans to this mass. Spectra were
recalibrated internally with the N1 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 CnHhN3O3S 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 5th,
15th, and 25th 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 5th, 15th and 25th
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, N1, O1 and S1 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 N1O1 and O1S1. 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 N1, O1 and
S1 as radical cations for all injections are plotted in Figure
2. A small increase (< 3%) in the relative abundance of
compound class N1 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, N1 and S1 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
N1, O1 and S1 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, N1 and S1.
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, N1 and S1.
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, N1 and S1 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.
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Bruker Daltonics is continually improving its products and reserves the right
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For research use only. Not for use in diagnostic procedures.
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References