The quantitative surface analysis of an antioxidant additive in a lubricant oil matrix by desorption electrospray ionization mass spectrometry

Article abstract of DOI:10.1002/rcm.6690

Rationale Chemical additives are incorporated into commercial lubricant oils to modify the physical and chemical properties of the lubricant. The quantitative analysis of additives in oil-based lubricants deposited on a surface without extraction of the sample from the surface presents a challenge. The potential of desorption electrospray ionization mass spectrometry (DESI-MS) for the quantitative surface analysis of an oil additive in a complex oil lubricant matrix without sample extraction has been evaluated. Methods The quantitative surface analysis of the antioxidant additive octyl (4-hydroxy-3,5-di-tert-butylphenyl)propionate in an oil lubricant matrix was carried out by DESI-MS in the presence of 2-(pentyloxy)ethyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate as an internal standard. A quadrupole/time-of-flight mass spectrometer fitted with an in-house modified ion source enabling non-proximal DESI-MS was used for the analyses. RESULTS An eight-point calibration curve ranging from 1 to 80 μg/spot of octyl (4-hydroxy-3,5-di-tert-butylphenyl)propionate in an oil lubricant matrix and in the presence of the internal standard was used to determine the quantitative response of the DESI-MS method. The sensitivity and repeatability of the technique were assessed by conducting replicate analyses at each concentration. The limit of detection was determined to be 11 ng/mm² additive on spot with relative standard deviations in the range 3-14%. CONCLUSIONS The application of DESI-MS to the direct, quantitative surface analysis of a commercial lubricant additive in a native oil lubricant matrix is demonstrated. 2013 The Authors. Rapid Communications in Mass Spectrometry published by John Wiley & Sons, Ltd.

Full text of DOI:10.1002/rcm.6690

Research Article
Received: 3 June 2013
Revised: 16 July 2013
Accepted: 18 July 2013
Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2013, 2420–2424
(wileyonlinelibrary.com) DOI: 10.1002/rcm.6690
The quantitative surface analysis of an antioxidant additive in a
lubricant oil matrix by desorption electrospray ionization mass
spectrometry
Caitlyn Da Costa1, James C. Reynolds1, Samuel Whitmarsh2, Tom Lynch2
and Colin S. Creaser1
*
1Centre for Analytical Science, Department of Chemistry, Loughborough University, Loughborough LE11 3TU, UK
2Castrol, Technology Centre, Whitchurch Hill, Pangbourne, Reading RG8 7QR, UK
RATIONALE: Chemical additives are incorporated into commercial lubricant oils to modify the physical and chemical
properties of the lubricant. The quantitative analysis of additives in oil-based lubricants deposited on a surface without
extraction of the sample from the surface presents a challenge. The potential of desorption electrospray ionization mass
spectrometry (DESI-MS) for the quantitative surface analysis of an oil additive in a complex oil lubricant matrix without
sample extraction has been evaluated.
METHODS: The quantitative surface analysis of the antioxidant additive octyl (4-hydroxy-3,5-di-tert-butylphenyl)
propionate in an oil lubricant matrix was carried out by DESI-MS in the presence of 2-(pentyloxy)ethyl 3-(3,5-di-tert-butyl-
4-hydroxyphenyl)propionate as an internal standard. A quadrupole/time-of-flight mass spectrometer fitted with an
in-house modified ion source enabling non-proximal DESI-MS was used for the analyses.
RESULTS: An eight-point calibration curve ranging from 1 to 80 μg/spot of octyl (4-hydroxy-3,5-di-tert-butylphenyl)
propionate in an oil lubricant matrix and in the presence of the internal standard was used to determine the quantitative
response of the DESI-MS method. The sensitivity and repeatability of the technique were assessed by conducting
replicate analyses at each concentration. The limit of detection was determined to be 11 ng/mm2 additive on spot with
relative standard deviations in the range 3–14%.
CONCLUSIONS: The application of DESI-MS to the direct, quantitative surface analysis of a commercial lubricant
additive in a native oil lubricant matrix is demonstrated. © 2013 The Authors. Rapid Communications in Mass Spectrometry
published by John Wiley & Sons, Ltd.
Desorption electrospray ionization (DESI) is an ambient
ionization technique that enables the direct analysis of
molecular analytes from surfaces with minimal sample
preparation. DESI utilizes an electrospray directed at the
surface to desorb and ionize analytes at atmospheric pressure
resulting in electrospray ionization (ESI)-like mass spectra.
Since the development of DESI in 2004,[1] it has become one
of the most commonly used techniques for the qualitative
surface analysis of target analytes, and has been applied to
the detection of a wide range of compounds desorbed from
a variety of surface materials.[1–7] The use of DESI mass
spectrometry (DESI-MS) in quantitative analyses remains
relatively unexplored, but has been reported for the analysis of
complex matrices such as biological fluids,[8,9] foodstuffs,[10,11]
polymers,[12] and cosmetic formulations,[13] showing the
potential of the technique.
Lubricants form an essential component within tribological
systems, i.e. systems where interacting surfaces move in relative
motion, such as engines. Common commercial lubricants are
formulated from a range of base fluids, either mineral or
synthetic oils, in which chemical additives are dissolved. The
base oil formulation and nature of the chemical additives will
influence the physical and chemical properties of the lubricant
and alter its potential applications.[14] The use of atmospheric
pressure ionization techniques for the qualitative mass
spectrometric analysis of base oils and oil distillates has been
demonstrated.[15] Such techniques include atmospheric pressure
photoionization,[16] matrix-assisted laser desorption/ionization
(MALDI),[17] direct analysis in real time,[18] ESI[19] and DESI.[20,21]
Chemical additives are used in the development of lubricants
for specific functions and can increase the life span of the
product. The analysis of additives provides information
regarding additive age, composition and degradation of the
lubricant and many commercial lubricant additives have been
studied by gas chromatography (GC) and ESI.[22–25] One group
of additives is antioxidants, which are commonly sterically
hindered phenols or aromatic amines.[14] The presence of oxygen
and elevated temperatures within a tribological environment can
* Correspondence to: C. S. Creaser, Centre for Analytical Science,
Department of Chemistry, Loughborough University,
Loughborough, LE11 3TU, UK.
E-mail: C.S.Creaser@lboro.ac.uk
This is an open access article under the terms of the
Creative Commons Attribution License, which permits
use, distribution and reproduction in any medium,
provided the original work is properly cited.
Rapid Commun. Mass Spectrom. 2013, 2420–2424
© 2013 The Authors. Rapid Communications in Mass Spectrometry published by John Wiley & Sons, Ltd.

Surface analysis of an antioxidant additive in a lubricant oil matrix
cause rapid oxidation of lubricants, resulting in variation of the
viscosity and acidity, beyond acceptable ranges, and in the
generation of undesired breakdown products such as sludge.
The analysis of lubricant antioxidant additives has been
demonstrated using ESI-MS and MALDI-MS,[26–28] but there
have been no reports of the application of DESI-MS to the
quantitative analysis of additives present in a native oil
lubricant matrix. In this paper, we report the application of
DESI-MS for the direct, quantitative analysis of a commercial
lubricant antioxidant additive in a complex and native base
oil matrix using a custom-built DESI source designed to
enable non-proximal DESI-MS surface analysis.
The electrospray capillary was positioned at an angle of
approximately 45° relative to the sample surface with an ESI
tip to sample distance of ~3 mm. The sample was positioned
horizontally to the mass spectrometer inlet with an inlet to
sample distance of ~1 mm. Each sample was analysed in
negative ion mode using an electrospray solvent of 95:5
MeOH/H2O at a flow rate of 20 μL/min. The instrumental
parameters were: capillary voltage, –3 kV; sampling cone
voltage, 20 V; source temperature, 120°C; desolvation gas
(N2) flow rate, 100 L/h; and trap collision energy, 6 eV.
Structural confirmation of I in the oil matrix was conducted
using DESI-MS/MS. Isolation of the precursor ion was
achieved in the quadrupole and fragmentation was induced
in the trap region using a trap collision energy of 36 eV. The
observed product ion spectrum was compared with one
obtained with a standard sample of I spotted in methanol
onto a filter paper surface.
EXPERIMENTAL
Reagents and chemicals
Methanol and water were purchased from Fisher Scientific
(Loughborough, UK) and hexane was purchased from Sigma
Aldrich (Gillingham, UK). All solvents were HPLC grade. The
base oil matrix (group one treated base oil) and an antioxidant
additive octyl (4-hydroxy-3,5-di-tert-butylphenyl)propionate (I)
were supplied by Castrol (Pangbourne, UK) for the analysis.
Ethylene glycol monopentyl ether and concentrated sulphuric
acid were purchased from Sigma Aldrich and 3,5-di-tert-butyl-4-
hydroxyphenylpropionic acid was purchased from Alfa Aesar
(Heysham, UK) for the in-house synthesis of the internal standard.
Synthesis of internal standard (II)
The internal standard 2-(pentyloxy)ethyl 3-(3,5-di-tert-butyl-4-
hydroxyphenyl)propionate (II) was synthesised via a Fischer
esterification reaction. Ethylene glycol monopentyl ether (100 μL)
and 3,5-di-tert-butyl-4-hydroxyphenylpropionic acid (47 mg) were
mixed in a HPLC vial and concentrated H2SO4 (1 μL) was added
as a catalyst. A pierced lid was fixed onto the vial to enable water
to escape from the reaction mixture as steam, and the sample
vortexed. The reaction vial was then heated to 105°C for 16 h.
DESI-MS equipment and experimental conditions
Sample preparation
The DESI-MS analysis was conducted on a Waters Synapt HDMS
quadrupole-time-of-flight (Q-TOF) mass spectrometer (Waters,
Manchester, UK) which consists of a quadrupole, trap, T-wave
ion mobility and transfer stacked-ring ion guide regions and a
time-of-flight mass analyser. The instrument was equipped with
an in-house modified DESI source equipped with a custom-built
outer cone and x,y,z -manipulated sample stage. A schematic
diagram of the DESI-MS source is shown in Fig. 1. The
custom-built outer stainless steel cone fits over the standard
inner cone of the inlet to the z-spray interface of the
spectrometer, replacing the standard outer cone. A stainless
steel Swagelok tube fitting (part number: SS-100-R-2,
Swagelok, Manchester, UK) was welded to the cone enabling
attachment of stainless steel extender tubing (1/16" (0.16 cm)
o.d., 0.05 cm i.d., length 5–20 cm), which acts as an ion
transfer tube for non-proximal DESI analyses.
Stock solutions were prepared by dissolving known weights
of I (0.5–40 mg) in 1 mL hexane and spiking in 10 μL of II
to give a nominal concentration of 13 mg/mL II. An aliquot
of each standard solution (100 μL) was mixed with the base
oil (400 μL). The resulting oil (10 μL) was spotted onto a filter
paper surface to give deposited amounts of additive in the
range of 1–80 μg of I per spot. The filter paper was secured
onto a glass slide for support and positioned under the
electrospray capillary on the sample stage. The sample was
traversed under the electrospray in a horizontal motion
perpendicular to the mass spectrometer inlet using the x,y,
z-manipulator and data was acquired for a total of 1.5 min.
A blank analysis was conducted before and after each sample
to check for carryover. Six replicate analyses were conducted
at each concentration of I. The sensitivity of the method was
determined by calculating the limit of detection (LOD) from
the absolute response of I.
RESULTS AND DISCUSSION
The quantitative surface analysis of a commercial lubricant
antioxidant additive, octyl (4-hydroxy-3,5-di-tert-butylphenyl)-
propionate (I), in a complex base oil matrix by DESI-MS was
carried out in negative ion mode using 2-(pentyloxy)ethyl
3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (II) as an
internal standard, to generate the deprotonated molecules
([M–H]–) of the target analytes. The structures of I and II are
shown in Fig. 2. The modified DESI ion source (Fig. 1) was
found to have improved sensitivity compared with the
standard spectrometer configuration, because the custom-built
Figure 1. Schematic diagram of the modified inlet of the
quadrupole/time-of-flight mass spectrometer for non-proximal
DESI-MS analysis.
Rapid Commun. Mass Spectrom. 2013, 2420–2424
wileyonlinelibrary.com/journal/rcm
© 2013 The Authors. Rapid Communications in Mass Spectrometry published by John Wiley & Sons, Ltd.

C. Da Costa et al.
Figure 2. Structures of octyl (4-hydroxy-3,5-di-tert-butylphenyl)propionate (I), a
commercial antioxidant additive, and 2-(pentyloxy)ethyl 3-(3,5-di-tert-butyl-4-
hydroxyphenyl)propionate (II) internal standard.
outer cone enables a closer proximity of the mass spectrometer
inlet to the sample surface. The cone assembly also allows mass
spectrometer sampling to be carried out away from the front of
the mass spectrometer, through the use of stainless steel ion
transfer tubes, which can be used for the non-proximal
DESI-MS analysis of larger objects.
surface were measured before and after the sample analysis,
demonstrating the absence of background interference and
sample-to-sample carry over. The analyte and internal
standard responses began to decrease as a result of depletion
of the sample from the surface and the sample spot was
moved under the electrospray so that a new area of the spot
could be analyzed. This process was repeated for the 1.5-min
acquisition time, covering a cross-section of the spot, which
was a representation of the whole sample deposited.
DESI-MS/MS was used to confirm the identity of I at
m/z 389.3 in the lubricant oil matrix. Analyses were conducted
for a standard sample of I in MeOH deposited on a filter
paper surface and for a sample of I spiked into the base oil
matrix and spotted onto the surface. The [M–H]– ion of I
generated from the oil matrix was isolated by the quadrupole
and fragmentation induced in the trap ion guide section of
the T-wave region using collision-induced dissociation (CID).
The product ion spectrum of the m/z 389.3 ion of I (Fig. 4)
observed when I was spiked into the base oil matrix closely
matched that obtained from the CID of the standard sample
of I, confirming the absence of interfering ions from the
oil matrix. The proposed fragmentation pattern for the
deprotonated molecule of I is shown in Fig. 4.
The characterization of base oils and the detection of
additives have been reported using both polar and non-polar
electrospray phases with differing detection capabilities.[21]
The use of a polar electrospray phase, 95:5 MeOH/H2O,
and negative mode analysis for the detection of the additive
I in a base oil matrix using DESI-MS was found to generate
a mass spectrum that had little chemical noise resulting from
the base oil matrix. Figure 3 shows the DESI-MS spectrum
obtained from the desorption of a spot containing I (10 μg)
and the internal standard II (27 μg) in a base oil matrix
deposited on a filter paper surface. Desorption from glass,
PTFE and metal surfaces is also possible (data not shown).
The deprotonated molecule of the additive at m/z 389
(observed 389.3062, calculated 389.3056, 1.5 ppm error) and
the internal standard (m/z 391) can be clearly distinguished
from the chemical noise resulting from the oil matrix. The
insert in Fig. 3 shows the selected ion responses for the
deprotonated molecules of I, m/z 389.3, and II, m/z 391.3,
with the lubricant spot positioned under the electrospray to
generate a DESI response for the sample. Blank areas of the
The quantitative determination of I spiked into the oil matrix
was carried out by DESI-MS using the relative mass spectral
response of I, at concentrations in the range 1–80 μg/spot, to
Figure 3. Mass spectrum showing the deprotonated molecules of I (10 μg) and
II spiked into an oil matrix, spotted onto filter paper and analyzed by DESI-MS
in negative ion mode. Insert: selected ion responses of the [M–H]– ions of I
(m/z 389.3) and II (m/z 391.3) for the DESI-MS analysis of a single spot.
wileyonlinelibrary.com/journal/rcm
Rapid Commun. Mass Spectrom. 2013, 2420–2424
© 2013 The Authors. Rapid Communications in Mass Spectrometry published by John Wiley & Sons, Ltd.

Surface analysis of an antioxidant additive in a lubricant oil matrix
Figure 4. DESI-MS/MS of the [M–H]– ion of I in an oil matrix from a filter
paper surface.
the internal standard II (27 μg/spot). The calibration plot was
produced by accumulating 1-min of DESI-MS data for each
sample and extracting the mass spectrum. The intensities of
the [M–H]– ions of I and II at each concentration of I in
the extracted mass spectra were used to calculate a ratio of
I/II, which corrected for variation in the DESI ion profile
(Fig. 3, insert). This generated a calibration plot (see
Supplementary Fig. S1, Supporting Information) with an R2
value >0.993, indicating a good linear response for the
DESI-MS analysis. The use of an internal standard in the
analysis of I overcomes many inherent problems relating
to the use of surface analysis techniques, such as DESI,
in quantitative measurements. Inhomogeneous sample
deposition on the surface and variations in the positioning
and rate of movement of the sample under the electrospray
during the analysis can affect the absolute response of an
analyte. This is especially problematic when spotting and
traversing of the sample surface are carried out manually as
was the case in this experiment. Investigations into the
suitability of different internal standards for quantitative
surface analysis using DESI have been reported,[29] and key
properties include similarity in the solubility of the internal
standard and target analyte in the spotting solutions and
the electrospray solvent composition. In addition, similar
proton affinities for the analyte and internal standard will
reduce any potential ion suppression effects. In this study
II, an analogue of I, was synthesized and used as the internal
standard to minimize structural and chemical differences
between the analyte and internal standard. The internal
standard retains the functionality around the aromatic ring
present in the target analyte, with the substitution of CH2
for an oxygen atom in the hydrocarbon chain (Fig. 2). This
structural modification is considered to have minimal impact
on the physical and chemical properties of the molecule, such
as the proton affinity, making II a suitable internal standard.
This is supported by the good precision for the DESI-MS
analysis achieved by addition of the internal standard (II)
to the spotting solution. Replicate analyses (n = 6) were
conducted at each concentration to determine the precision
of the technique, generating relative standard deviations
(RSDs) in the range of 3–14%, which is consistent with other
reported data for quantitative DESI-MS.[12,13,29] The RSD for
the additive I in the absence of the internal standard was in
the range of 15–44%.
The limit of detection (LOD) was calculated as the blank
response of I plus three standard deviations of the blank using
the absolute mass spectral response of I, obtained from a section
of the spot on the surface which was then related to the total spot
size. The LOD was determined to be 11 ng/mm2 additive on
spot, which relates to less than 0.7 μg ablated from the surface
during the acquisition. This corresponds to <0.03% w/v, which
is below the reported concentration of additives in commercial
lubricant products that are typically in the range of 0.1–5% w/v.
The DESI-MS method is therefore sensitive enough to detect
and quantify commercial additives in a native oil matrix.
CONCLUSIONS
The application of DESI-MS to the quantitative surface analysis
of a lubricant antioxidant additive in a complex oil lubricant
matrix has been demonstrated with good linearity and
repeatability (R2 >0.99, RSD = 3–14%). Modification of the
electrospray source on the Waters Synapt HDMS instrument
enabled the development of a robust DESI-MS system capable
of non-proximal DESI-MS analysis of surfaces. The use of an
internal standard minimized variations between DESI-MS runs
caused by inhomogeneous sample distribution on the surface
and other factors affecting the use of DESI-MS in quantitative
measurements. The LOD for the additive in the oil lubricant
was below the typical levels of additive concentration found
in commercial lubricant products. The reported DESI-MS
procedure has the potential for the quantitative determination
of sub-microgram quantities of compounds deposited on a
surface in the presence of a complex oil lubricant matrix with
the appropriate choice of internal standard.
Rapid Commun. Mass Spectrom. 2013, 2420–2424
wileyonlinelibrary.com/journal/rcm
© 2013 The Authors. Rapid Communications in Mass Spectrometry published by John Wiley & Sons, Ltd.

C. Da Costa et al.
[15] C. Cheng, J. Lai, M. Huang, J. Oung, J. Shiea, in Analysis of
Polar Components in Crude Oil by Ambient Mass Spectrometry,
Crude Oil Emulsions-Composition Stability and Characterisation,
(Ed: M. El-Sayed Abdul-Raouf), 2012. ISBN: 978-953-51-0220-
5, InTech. Available: http://www.intechopen.com/books/
crude-oil-emulsions-composition-stability-and-characterisation-/
differentiation-of-crude-oils-by-ambient-ionisation-mass-
spectrometry.
SUPPORTING INFORMATION
Additional supporting information may be found in the
online version of this article.
Acknowledgements
[16] J. M. Purcell, C. L. Hendrickson, R. P. Rodgers, A. G. Marshall.
Atmospheric pressure photoionization Fourier transform ion
cyclotron resonance mass spectrometry for complex mixture
analysis. Anal. Chem. 2006, 78, 5016.
We would like to thank the EPSRC for providing an industrial
CASE studentship (to C. Da Costa) and Castrol for their
financial and other support.
[17] E. Lorente, C. Berrueco, A. A. Herod, M. Millian, R. Kandiyoti.
The detection of high-mass aliphatics in petroleum by matrix-
assisted laser desorption/ionisation mass spectrometry. Rapid
Commun. Mass Spectrom. 2012, 26, 1581.
[18] J. L. Rummel, A. M. McKenna, A. G. Marshall, J. R. Eyler,
D. H. Powell. The coupling of direct analysis in real time
ionization to Fourier transform ion cyclotron resonance
mass spectrometry for ultrahigh-resolution mass analysis.
Rapid Commun. Mass Spectrom. 2010, 24, 784.
[19] X. Zhou, Q. Shi, Y. Zhang, S. Zhao, R. Zhang, K. H. Chung,
C. Xu. Analysis of saturated hydrocarbons by redox reaction
with negative-ion electrospray Fourier transform ion cyclotron
resonance mass spectrometry. Anal. Chem. 2012, 84, 3192.
[20] C. Wu, K. Qian, M. Nefliu, R. G. Cooks. Ambient analysis of
saturated hydrocarbons using discharge-induced oxidation
in desorption electrospray ionization. J. Am. Soc. Mass
Spectrom. 2010, 21, 261.
[21] P. A. Eckert, P. J. Roach, A. Laskin, J. Laskin. Chemical
characterisation of crude petroleum using nanospray
desorption electrospray ionization coupled with high-
resolution mass spectrometry. Anal. Chem. 2012, 84, 1517.
[22] R. Becker, W. Hoffmann, A. Knorr, W. Walther, A. Lehmann.
Determination of zinc O,O’-dialkyl dithiophosphate lubricant
additives via the corresponding methyl and p-nitrobenzylic
ester derivatives using GC-MS, GC-NPD and HPLC-MS.
Fresenius J. Anal. Chem. 1997, 357, 688.
REFERENCES
[1] Z. Takats, J. M. Wiseman, B. Gologan, R. G. Cooks. Mass
spectrometry sampling under ambient conditions with
desorption electrospray ionisation. Science 2004, 306, 471.
[2] D. J. Weston, R. Bateman, I. D. Wilson, T. R. Wood,
C. S. Creaser. Direct analysis of pharmaceutical drug
formulations using ion mobility spectrometry/quadrupole-
time-of-flight mass spectrometry combined with desorption
electrospray ionization. Anal. Chem. 2005, 77, 7572.
[3] R. G. Cooks, Z. Ouyang, Z. Takats, J. M. Wiseman. Ambient
mass spectrometry. Science 2006, 311, 1566.
[4] G. Atwal, D. J. Weston, P. S. Green, S. Crosland, P. L. Bonner,
C. S. Creaser. Analysis of tryptic peptides using desorption
electrospray ionisation combined with ion mobility
spectrometry/mass spectrometry. Rapid Commun. Mass
Spectrom. 2007, 1131.
[5] E. L. Harry, J. C. Reynolds, A. W. Bristow, I. D. Wilson,
C. S. Creaser. Direct analysis of pharmaceutical formulations
from non-bonded reversed-phase thin-layer chromatography
plates by desorption electrospray ionisation ion mobility mass
spectrometry. Rapid Commun. Mass Spectrom. 2009, 23, 2597.
[6] D. R. Ifa, C. Wu, Z. Ouyangbc, R. G. Cooks. Desorption
electrospray ionisation and other ambient ionisation methods:
current progress and preview. Analyst 2010, 135, 669.
[7] D. J. Weston. Ambient ionization mass spectrometry:
current understanding of mechanics theory; analytical
performance and application areas. Analyst 2010, 135, 661.
[8] J. H. Kennedy, J. M. Wiseman. Evaluation and performance of
desorption electrospray ionisation using a triple quadrupole
mass spectrometer for quantitation of pharmaceuticals in
plasma. Rapid Commun. Mass Spectrom. 2010, 24, 309.
[23] M. Bernabei, R. Seclì, G. Bocchinfuso. Determination of
additives in synthetic base oils for gas turbine engines.
J. Microcolumn. Sep. 2000, 12, 585.
[24] M. Becchi, F. Perret, B. Carraze, J. F. Beziau, J. P. Michel.
Structural determination of zinc dithiophosphates in
lubricating oils by gas chromatography–mass spectrometry
with electron impact and electron-capture negative ion
chemical ionization. J. Chromatogr. A 2001, 905, 207.
[25] I. Eide, K. Zahlsen, H. Kummernes, G. Neverdal.
Identification and quantification of surfactants in oil using
the novel method for chemical firgerprinting based on
electrospray mass spectrometry and chemometrics. Energy
Fuels 2006, 20, 1161.
[9] J. Thunig, L. Flø, S. Pedersen-Bjergaard, S. Honoré Hansen,
C. Janfelt. Liquid-phase microextraction and desorption
electrospray ionization mass spectrometry for identification
and quantification of basic drugs in human urine. Rapid
Commun. Mass Spectrom. 2012, 26, 133.
[10] C. Berchtold, V. Muller, L. Meier, S. Schmid, R. Zenobi. Direct
detection of chlorpropham on potato skin using desorption
electrospray ionization. J. Mass Spectrom. 2013, 48, 587.
[11] P. D’Aloise, H. Chen. Rapid determination of flunitrazepam
in alcoholic beverages by desorption electrospray ionization-
mass spectrometry. Sci. Justice 2012, 52, 2.
[12] S. M. Reiter, W. Buchberger, C. W. Klampfl. Rapid
identification and semi-quantitative determination of polymer
additives by desorption electrospray ionization/time-of-flight
mass spectrometry. Anal. Bioanal. Chem. 2011, 400, 2317.
[13] J. L. Nizzia, A. E. O’Leary, A. T. Ton, C. C. Mulligan. Screening
of cosmetic ingredients from authentic formulations
and environmental samples with desorption electrospray
ionization mass spectrometry. Anal. Methods 2013, 5, 394.
[14] R. M. Mortier, S. T. Orszulik. Chemistry and Technology of
Lubricants, (2nd edn.), Blackie Academic and Professional,
1997, chaps. 3–6.
[26] R. M. Alberici, R. C. Simas, P. V. Abdelnur, M. N. Eberlin,
V. de Souza, G. F. de Sa, R. J. Daroda. A highly effective
antioxidant and artificial marker for biodiesel. Energy Fuels
2010, 24, 6522.
[27] A. Kassler, E. Pittenauer, N. Doerrb, G. Allmaiera. CID of
singly charged antioxidants applied in lubricants by means
of a 3D ion trap and a linear ion trap-Orbitrap mass
spectrometer. J. Mass Spectrom. 2011, 46, 517.
[28] J. Schnoller, E. Pittenauer, H. Hutter, G. Allmaier. Analysis of
antioxidants in insulation cladding of copper wire:
a
comparison of different mass spectrometric techniques
(ESI-IT, MALDI-RTOF and RTOF-SIMS). J. Mass Spectrom.
2009, 44, 1724.
[29] D. R. Ifa, N. E. Manicke, A. L. Rusine, R. G. Cooks. Quantitative
analysis of small molecules by desorption electrospray
ionization mass spectrometry from polytetrafluoroethylene
surfaces. Rapid Commun. Mass Spectrom. 2008, 22, 503.
wileyonlinelibrary.com/journal/rcm
Rapid Commun. Mass Spectrom. 2013, 2420–2424
© 2013 The Authors. Rapid Communications in Mass Spectrometry published by John Wiley & Sons, Ltd.