Final Version of LC-GC article to be published in 2011

FEBRUARY 2011 LCGC NORTH AMERICA VOLUME 29 NUMBER 2 1www.chromatographyonline.com
MS – THEPRACTICALART
In the days before
electrospray and the
rapid growth of mass
spectrometry as a premier
tool for life sciences thanks
to John Fenn’s efforts,
petroleum, polymers, and
environmental interests
were the focus and vacuum
gas-phase techniques
prevailed. Where is the
practitioners’ interest in
solving problems today
in the chemical industry?
What tools do they employ
with good success and
which ones do they look
forward to — or wish they
had?
Michael P. Balogh
MS—The Practical Art Editor
Mass spectrometry (MS) has
been a cornerstone technol-
ogy of the chemical indus-
tries including petroleum research and
related commercial product manufactur-
ing in polymers. The American Society
for Mass Spectrometry (ASMS) will
celebrate its 60th anniversary next year.
Although christened under its current
name in 1969 it began as “ASTM Com-
mittee E-14” in 1952. The work of John
Fenn and others that brought us elec-
trospray ionization (ESI) and heralded
the modern era where biological use of
MS predominates was not seen until the
1980s. Before the 1980s gas-phase inter-
ests predominated and techniques were
commonly performed in vacuum as
opposed to atmosphere today and many
times as a set experiment using a solids
probe rather than a flowing or chro-
matographic serial sample introduction.
Where is the interest among practi-
tioners today? What generates the most
problem solving interest today in the
chemical industry? What tools do they
employ with good success and which
ones do they look forward to — or wish
they had? I had an opportunity to speak
with Colin Moore, Fellow and Technol-
ogy Leader in Mass Spectrometry at
Chemtura Corporation (Middlebury,
Connecticut) about the types of ana-
lytical problems that his group is asked
to solve and what they learned in the
process of solving them; this discussion
evolved into a short tutorial on chemical
industry practice.
A UK native, Moore worked for
Shell Research for seven years doing
analytical work on agrochemicals and
simultaneously became a graduate of
the Royal Society of Chemistry. Post-
graduate studies at the University of
Problem Solving in the
Chemical Industry
In Memoriam: John B. Fenn
and Uwe D. Neue
As I was writing this month’s column I
learned of the passing of two important
people in the world of analytical science
within a week of each other. John Fenn,
who died on Dec 10, 2010, was a profes-
sor of chemistry first at Yale and then at
Virginia Commonwealth University. He
won a share of the Nobel Prize in 2002.
Uwe Neue, who received his Ph.D. at
Saarbrucken and went on to become
well recognized as an authority in chro-
matography, died December 3, 2010.
Fenn’s name has become almost syn-
onymous with the practice of electrospray
MS. Reference to his name and work
appeared in this column frequently over
the past eight years. Being able to say one
had at some time spoken with Fenn was
not an uncommon claim because he was
freely available it seemed to any and all.
When I had a chance years ago I com-
mented to him how, because he seemed
so freely available and generous with his
time, his Nobel-inspired fame must at
times be a burden. With genuine mod-
esty he replied that there are many who
deserve recognition for their work and he
was fortunate to have been singled out.
We have lost a true innovator and pioneer.
The work of my Waters colleague Uwe
in chromatography was well known and
widely respected. Aside from his scholarly
efforts, books, and numerous publications,
he also played a significant role in helping
to establish the Conference on Small Mol-
ecule Science. Since the first year Uwe at
times chaired or organized what is now a
most popular recurring session examining
the theoretical underpinnings of applied
chromatography. The session and discus-
sion workshops feature some of the great
names as participants thanks to Uwe.

2 LCGC NORTH AMERICA VOLUME 29 NUMBER 2 FEBRUARY 2011 www.chromatographyonline.com
Southampton were followed by three
years at Warwick University and a
Ph.D. with Professor Keith Jennings.
He joined Uniroyal (now Chemtura
Corporation) in 1994 as a member of
the MS group, becoming manager in
1997 and Research Fellow in 2002. He
has 20 publications in peer-reviewed
journals and 24 posters and presenta-
tions at conferences to his credit.
A Simplified Overview
Sample analysis in Moore’s laboratory gen-
erally falls into one of the following areas:
• Confirmation of the chemical struc-
ture of the main components and
identification of impurities to help
synthetic chemists improve the syn-
thesis.
• Identification of minor components in
a product that shouldn’t be there (for
example, color bodies).
• Identification of additives in a polymer
or oil (for example, antioxidants in
engine oil).
For highly pure samples analysis by
nuclear magnetic resonance (NMR)
spectroscopy is probably the best way
to obtain detailed structural informa-
tion and therefore most of the samples
that mass spectrometrists are asked
to analyze are mixtures. The analyst’s
first decision therefore is to determine
which separation technique will be
used: gas chromatography (GC), liquid
chromatography (LC), gel permeation
chromatography (GPC), solid-phase
microextraction (SPME) or some
type of liquid–liquid extraction. For
example, if an engine oil sample in
methylene chloride is shaken with
methanol then the phenolic and aminic
antioxidants will be concentrated in
the methanol. GC–MS analysis of the
methanol extract makes it easy to iden-
tify the major antioxidants in the oils
(Figures 1 and 2).
An interesting aspect of identifying
unknowns that, according to Moore,
“I’ve not seen in the literature,” stems
from making use of both electron
ionization (EI) and ESI spectra of an
unknown since the fragmentation pat-
terns can be “complementary” (see the
EI and ESI spectra in Figure 3).
In the EI spectrum the first major loss
is a C10 alkyl radical whereas in the ESI
spectrum it is loss of the C10 alcohol.
For those interested in a quick overview
on mass spectra a recent column was
devoted to that topic (1) that, in addi-
tion, highlights James Little’s insights
from his experiences solving problems at
Eastman Chemical in Tennessee.
A significant difference in analytical
practice between the pharmaceutical
and specialty chemical industries is
the level of dependence of the latter
on GC–MS. Today the pharmaceuti-
cal world favors LC–MS. For those
interested in the aspects of how LC–
MS became what we think of as open
access in the pharmaceutical world, I
chronicled the insights of a few prac-
titioners on how LC–MS transitioned
from a relatively obscure novelty to
become the beneficial tool we know
today (2). A recent article also exam-
ined the ongoing need for develop-
ment in MS ionization and related
technologies to support work in areas
where “electrospray just isn’t enough”
to do the job (3). According to a survey
among practitioners in various disci-
plines, those identified with the chemi-
cal materials industry arguably rely on
Figure 1: GC–MS response for an oil sample (upper is a methanolic extract; lower is
the oil sample dissolved in methylene chloride). (Courtesy of Colin Moore, Chemtura
Corporation.)
Figure 2: EI spectrum of peak at 13.82 min in the methanol extract shown in Figure 1.
(Courtesy of Colin Moore, Chemtura Corporation.)

GC three times more than LC. A quick
look around Moore’s laboratory sup-
ports the observation.
One issue that all laboratories need
to address is productivity. Two of
Moore’s GC–MS systems, an Agilent
5970 (Agilent Technologies, Palo Alto,
California) and a Waters GCT (Waters
Corporation, Milford, Massachusetts),
are fitted with two GC columns in the
injection port using a two-hole fer-
rule. One column is connected to the
mass spectrometer and the other to
an ancillary detection system such as
flame ionization detection (FID). As
Moore explained “ as soon as you tell
somebody what an unknown peak is in
a GC–MS trace, the next question is
usually, how much [of it] is there? Col-
lecting the FID data in parallel with
the GC–MS data allows us to give our
customers semiquantitative data as well
as MS identification in a single experi-
ment.” Such a practice is analogous to
LC–MS data acquisition with an in-
line UV detector.
Moore’s future interests include
installing a nitrogen–phosphorus
detection (NPD) system as well as FID,
on one GC system because many of
the antioxidants that Chemtura makes
are amines. Amines as a class are often
easily ionized by ESI. Yet in some cases
the analytes are just not amenable
to the technique. Insufficient polar-
ity, the inability to capture a proton,
or perhaps excessive volatility that
precludes transport and separation by
condensed phase means in an LC could
all contribute to ESI failure. Certainly
due to its popularity great strides have
occurred in recent years to increase
the sensitivity, resolution, and overall
utility of LC–ESI-MS instruments.
Today techniques such as atmospheric
pressure gas chromatography (APGC)
and atmospheric solids analysis probes
(ASAP), analogous to the vacuum sol-
ids probes used for years in GC–MS,
are viable without sacrificing perfor-
mance, which was not the case years
ago (3,4).
Data Handling
The ability of software-driven appli-
cations to amass increasingly refined
data streams has unleashed a data
handling problem that crosses into
all practices and disciplines. So much
so that handling complex data has
become a recurring workshop topic
at the Conference on Small Molecule
Science (CoSMoS) in recent years
(www.CoSMoScience.org).
Moore’s laboratory uses MassLynx
mass spectrometry software (Waters
Corporation) to process its MS data.
For GC–MS data, workers export
the files using the NetCDF converter
option in ChemStation software in the
three Agilent systems (Agilent Tech-
nologies) and then convert the files to
the MassLynx format using the Waters
DBridge program. NetCDF does not
produce a file for the FID system that
can be read by MassLynx, and therefore
processing the FID trace has to be done
using ChemStation. On the GCT sys-
tem the FID trace is recorded as analog
data by MassLynx that can be processed
Figure 3: The complementary diagnostic nature of EI and ESI spectra of the same
compound. (Courtesy of Colin Moore, Chemtura Corporation.)
Figure 4: Mass spectrum of the color body (inset) and its UV spectrum. (Courtesy of
Colin Moore, Chemtura Corporation.)

with the MS data. The retention time
differences between the two traces can
be time aligned in the chromatogram
plots (the peaks come out earlier in the
MS data because the vacuum system of
the MS increases the helium flow rate
in the column connected to the mass
spectrometer).
Understanding the Problem and
Designing the Experiment
As an analytical problem identifying
color bodies in polymers is neither
trivial nor obvious. In a 2005 paper
(5), Moore shows there is often more
than one way to solve a problem using
mass spectrometry. Analysis by LC–MS
with an inline photodiode-array (PDA)
detector, is probably the best way to
identify a new color body. Techniques
like time-of-flight secondary ionization
mass spectrometry (TOF SIMS) can be
used to quickly confirm that the color
body on fresh samples is the same as on
previously determined samples.
Moore’s 2005 work detailed the
analysis of a yellow discoloration on
the surface of a compounded eth-
ylene–propylene–diene monomer
(EPDM) rubber sample. Surface
discoloration of a polymer can result
from various phenomena, including
contamination, component migra-
tion, oxidation, and other chemical
reactions. Components rising to the
surface can give rise to “bloom,” a
process in which one component of a
polymer mixture (usually not a poly-
mer) undergoes phase separation and
migration to an external surface of
the mixture, according to the IUPAC
definition. Protective waxes can be
beneficial, but thiazoles (mercapto-
benzothiazole) leading to discoloration
of the product are undesirable. For
example, oxidation of antioxidants
can form color bodies (that is, phe-
nolic antioxidants can form quinone
methides) (6). The first step in iden-
tifying a color body is to separate it
from the polymer often by washing
the surface with a suitable solvent.
The washings result in a complex
mixture of the color bodies, additives,
and other surface contaminants. Iden-
tification of the colored components
requires further separation of the
mixture, analysis of the separated com-
ponents, and the ability to ascertain
which of the components are colored.
The combination of LC–MS-MS
with an inline PDA detector is able to
do the complete analysis in a single
experiment. Moore recalls in 1994
when he joined Uniroyal Chemical,
identifying a color body often involved
pooling fractions from multiple LC
runs to acquire enough material for
the particle beam LC–MS system (an
early 1990s rather short-lived tech-
nique that, although not very sensitive,
produced EI spectra from typical LC-
amenable analytes not volatile enough
for GC–MS). The much greater sen-
sitivity of ESI sources and TOF mass
spectrometers has made the process
much quicker and easier, illustrating a
central point in mixture analysis: the
importance of matching the separation
technique with the sensitivity of the
final analysis technique.
Matching aspects of the analytical
technique is not as simple as it sounds.
When light reflects off a colored
substance, the reflected light has the
complementary color to the wavelength
or wavelengths absorbed. Yellow light
covers the wavelength range 570–585
nm, but the complementary color to yel-
low is indigo over the range 420–430
nm. So when processing the LC–MS
data, Moore looked for a component
with strong absorbtion over that range
of wavelengths. He found one compo-
nent that yielded the UV and LC–MS
spectra shown in Figure 4. Note that
the color body is the neutral Cu(II)
dibutyl dithiocarbamate, but for it to be
detected by the LC–MS system it must
be oxidized in the electrospray process
to the positively charged Cu(III) com-
pound. A quick review of spectral color
properties can be found at www2.chem-
istry.msu.edu/faculty/reusch/VirtTxtJml/
Spectrpy/UV-Vis/spectrum.htm.
Color bodies are often polar mol-
ecules, which means that they are
easy to ionize in an ESI source. For
some time now, as the instruments
capabilities improved, accurate mass
MS has been recognized as an effi-
cient means of enhancing separation
of closely related chemical species. As
Moore points out, when “the electro-
spray spectrum contains few (if any)
fragment ions identifying unknowns
requires that either an LC–MS-MS
spectrum is acquired and/or exact
mass measurements [7] are performed
to get the elemental formula of the
Figure 5: Deuterated (ND3) CI and EI spectrum of 4,4′-methylenedianiline. (Courtesy
of Colin Moore, Chemtura Corporation.)
Figure 6: Potential alkyl group positions.
(Courtesy of Colin Moore, Chemtura
Corporation.)

pseudo-molecular ion.” An inline UV
detector is sometimes an overlooked
adjunct as well in LC–MS. Here the
color of the offending samples of
course indicates distinct chromophoric
benefits. Components separated by
the high performance liquid chroma-
tography (HPLC) column absorb at
the appropriate wavelength to give the
observed discoloration with the added
advantage of well-characterized algo-
rithms employed by PDA detectors to
distinguish differences in homologous
components on the samples that the
unaided eye cannot.
Moore noted another frequently
overlooked element in the analytical
chemist’s trade that bears mentioning:
Structure elucidation is made much
easier if the analyst has a thorough
knowledge of the sample chemistry and
its history.
He also points out that “imag-
ing mass spectrometry is a powerful
technique for mapping the concentra-
tion of a compound on the surface of
a matrix.” The technique applies in
many fields, including the analysis
of inorganic materials, polymers, and
biological materials. An early publica-
tion discusses TOF-SIMS analysis in
which a TOF system measures sec-
ondary ions produced by bombarding
a surface with high-energy particles
(8). TOF-SIMS has been used to
detect light stabilizers (9) and antioxi-
dants (10) on the surface of a polymer
as well as to characterize the bulk
polymer (11).
In the few years since Moore pub-
lished his work, a number of techniques
operating by various mechanisms
on or near the surface of a material
(as opposed to techniques requir-
ing analytes of interest be in solution
— desorption electrospray ionization
[DESI], direct analysis in real time
[DART], ASAP and a few others) have
been examined in some detail in this
column (4,12,13).
DESI can be used in combination
with chemical reactions to improve
the selectivity and sensitivity of the
analysis. Moore’s studies with Keith
Jennings and attending presentations
by Graham Cooks and John Beynon
that emphasized the utility of chemi-
cal reactions in MS encouraged him to
attempt using novel chemical ioniza-
tion (CI) reagent gases to help solve
problems. “Many antioxidants are
alkylated aromatic amines and there-
fore a paper by Buchanan [14] was of
great interest”, he says. As shown by
the spectra in Figure 5, the technique
not only gives the number of aminic
protons in the molecular ion, but it also
helps identify fragment ions. The m/z
106 ion in the EI spectrum becomes
m/z 108 in the CI data because of the
NH2 group.
Moore has also updated a method
first reported by Morgan and col-
leagues (15) for analyzing zinc dial-
kyldithiophosphate (ZDDP) in engine
oils. Engine oils are complex mixtures
of base oils and performance enhancing
multifunctional additives, like ZDDP.
They are excellent antiwear agents
and effective oxidation and corrosion
inhibitors (Figure 6).
The original work used negative ion
CI to produce chloride ion adducts
of the oil without any prior separa-
tion. Moore has used an atmospheric
pressure chemical ionization (APCI)
source and a mobile phase contain-
ing methylene chloride to give similar
results.
Note that the mass spectrum in Fig-
ure 7 yields two complementary pieces
of information about the ZDDP
sample and gives the molecular weight
of any phenolic antioxidants in the
Figure 7: Chloride ion APCI spectrum of an engine oil sample dissolved in methylene
chloride. (Courtesy of Colin Moore, Chemtura Corporation.)
Table I: Understanding the comparative diagnostic value of the
ZDDP spectrum*
L– [ZnL2Cl]–
Total
m/z R1 or R3 R2 or R4 m/z R1, R2 R1, R4
241 4 4
581 4, 4 4, 4 16
609 4, 4 4, 6 18
269 6 4
637 4, 6 4, 6 20
665 4, 6 6, 6 22
297 6 6 693 6, 6 6, 6 24
*Courtesy of Colin Moore, Chemtura Corporation.
Figure 8: Generic structure for PAMAs.
(Courtesy of Colin Moore, Chemtura
Corporation.)

oil. The phenolic antioxidant present
in the oil is evident by the response at
m/z 389 (M–H ion) and at m/z 425
(M+Cl ion).
The chloride adduct pseudomolecular
ion [ZnL2Cl]– permits calculating the
total number of carbon atoms in the
four alkyl groups (R1+R2+R3+R4). The
ligand ions, L–, tell us if the ZDDP
was prepared by blending individual
ZDDPs or was produced using a mix-
ture of alcohols. Table I illustrates how
we can deduce that a mixture of C4 and
C6 alcohols were used to prepare the
ZDDP.
Though effective for determining
carbon chain length of the R groups,
the chloride adduction technique does
not show whether the chains are linear
or branched. However, collision-induced
dissociation (CID)–ion mobility spec-
trometry (IMS)–CID data may hold the
key to solve that puzzle (16).
If the chromatographic conditions
are not ideal for interfacing with MS or
if other analytical techniques are going
to be used to assist in the identification
then LC fraction collection may be the
best methodology. Polyalkylmethac-
rylates (PAMAs) are used as viscosity
modifiers in oils (Figure 8). Moore
has used fraction collection from a
GPC system (Figure 9), then pyrolysis
GC–MS (Figure 10) and IR analysis
to identify PAMAs. If R1 and R2 are
likely either H or methyl and R3 is one
of a mixture of alkanes, pyrolysis of this
type of polymer gives two series of frag-
ment ions: alkenes and alkyl methac-
rylates. Thus if R3 is C12 then one gets
dodec-1-ene and dodecyl methacrylate
(Figure 11).
Future Developments
Moore visited Graham Cooks at
Purdue to try using a DESI source
to detect the color body (17). Simply
spraying the yellow polymer with
acetonitrile indeed gave a small signal
for the Cu dibutyl dithiocarbamate.
Nevertheless, the signal was enhanced
when the oxidizing agent I2 was added
to the DESI spray solvent.
An extension of the thermal inves-
tigations coming back into favor may
in the not-too-distant future provide
yet another chapter for these studies
by combining thermal MS capabilities
Figure 11: Acquired EI spectrum(top) and best library match for the peak
at 11.79 min found in the oil extract (Figure 10). (Courtesy of Colin Moore,
Chemtura Corporation.)
Figure 10: Pyrolysis GC–MS at 550 °C of a PAMA standard (top) and the high mass
component from the oil. (Courtesy of Colin Moore, Chemtura Corporation.)
Figure 9: GPC trace for oil sample. 99.5% of the sample yields an Mp of 564 and 0.5%
having an Mp of 8630 (Mp, molecular weight, as reported by WatersGPC). (Courtesy
of Colin Moore and John Mannello, Chemtura Corporation.)

with the surface information derived
by atomic force microscopy (AFM)
(Figure 12).
AFM uses a sharp-tipped probe,
often only 2 μm long and less than
100 Å in diameter, located on the free
end of a cantilever, which is brought
close to a sample surface. Forces
between the tip and the sample sur-
face cause the cantilever to deflect.
The deflection of the tip is measured
as it is scanned or changes position
relative to the sample generating a
surface topographic map. Most com-
monly tip deflection is a result of
interatomic (van der Waals) forces. A
good reference for those interested in
the topic can be found at http://inv-
see.asu.edu/nmodules/spmmod/.
Coupling AFM with MS has given
rise to an interest termed “molecular
cartography” by Gary Van Berkel (Oak
Ridge National Laboratory, Oak Ridge,
Tennessee). Their work was presented at
the 2010 Conference on Small Molecule
Science (Portland, Oregon) by Olga
Ovchinnikova and can be downloaded
at www.CoSMoScience.org (18).
Ovchinnikova points out currently
available techniques usually “face a
trade-off between spatial resolution and
chemical information.” Combining spa-
tial resolution, using a heated scanning
AFM probe to thermally desorb material
from a surface, they then draw the sam-
ple into either an ESI or APCI source
adding chemical information from the
sample surface. The authors refer to
the technique as atmospheric-pressure
hybrid proximal probe topography
chemical imaging. The AFM tip plays
a dual role being used for the thermal
desorption creating ions for MS analysis
while obtaining topographic images of
that same surface. Initial results dem-
onstrate the viability of this technique
for automated chemical interrogation
of caffeine thin films with ~250-nm
spatial resolution in the thermal desorp-
tion process. Lower resolution proximal
probe thermal desorption chemical
imaging results of different classes of
compounds amenable to this technique
including explosives, herbicides, phar-
maceuticals, and dyes. The authors
anticipate this analytical tool “will have
broad application for determining the
nanoscale spatial distribution of target
molecules in plant and animal tissue
and material junctions” (19).
Acknowledgments
The wisdom readers benefit from in
this column often is a distillation from
many years of endeavor by people like
Colin Moore, and as he recognizes “at
the end of the day it’s people that solve
problems and I’m very fortunate to work
with a very talented group of people in
the Analytical Services department at
Chemtura.”
References
(1) M.P. Balogh, LCGC North America 28(2),
122 (2010).
480 (2009).
440 (2010).
368 (2007).
(5) C. Moore and P. McKeown, J. Am. Soc.
Mass Spectrom. 16, 295–301 (2005).
(6) J. Pospíšil, W.-D. Habicher, J. Pilar, S.
Nešpurek, J. Kuthan, G.-O. Piringer, and
H. Zweifel, J. Polym. Degrad. Stab. 77, 531
(2002).
(7) M. Maizels and W.L. Budde, Anal. Chem.
73, 5436 (2001).
(8) M.L. Pacholski, and N. Winograd, Chem.
Rev. 99, 2977 (1999).
Figure 12: Nanoscale physical and chemical imaging of plant growth regulators using proximal probe thermal desorption MS.
(Courtesy of Olga Ovchinnikova and Gary Van Berkel, Organic and Biological Mass Spectrometry Group, Chemical Sciences
Division, Oak Ridge National Laboratory.)

For more information on
this topic, please visit
www.chromatographyonline.com/balogh
Michael P. Balogh
“MS — The Practical
Art” Editor Michael
P. Balogh is principal
scientist, MS technol-
ogy development,
at Waters Corp. (Mil-
ford, Massachusetts);
a former adjunct
professor and visiting scientist at Roger
Williams University (Bristol, Rhode Island);
cofounder and current president of the Soci-
ety for Small Molecule Science (CoSMoS) and
a member of LCGC’s editorial advisory board.
Visit ChromAcademy on LCGC’s Homepage
www.chromacademy.com
(9) F. Andrawes, T. Valcarcel, G. Haacke, and
J. Brinen, Anal. Chem. 70, 3762 (1998).
(10) M.J. Walzak, N.S. McIntyre, T. Prater, S.
Kaberline, and B.A. Graham, Anal. Chem.
71, 1428 (1999).
(11) D. Briggs, I.W. Fletcher, S. Reichlmaier, L.J.
Agulo-Sanchez, and R.D. Short, Surf. Inter-
face Anal. 24, 419 (1996).
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1184 (2007).
(14) M.V. Buchanan, Anal. Chem. 54(3), 570–
574 (1982).
(15) R.P. Morgan, C.A. Gilchrist, K.R. Jen-
nings, and I.K. Gregor., Int. J. Mass Spec-
trom. Ion Phys. 46, 309 (1983).
(16) C. Moore and A. Alexander, “The Iden-
tification of Engine Oil Additives Using
Chloride Ion Addition IMS-LCMS/MS,”
presented at the 58th ASMS Conference on
Mass Spectrometry and Allied Topics, Salt
Lake City, Utah, 2010.
(17) M. Nefliu, R.G. Cooks, and C. Moore,
J. Am. Soc. Mass Spectrom. 17, 1091–1095
(2006).
(18) O.S. Ovchinnikova and G.J. Van Berkel,
“Molecular Cartography: Moving Towards
Combined Topographical and Chemical
Imaging using AFM and Mass Spectrom-
etry,” presented at CoSMoS 2010, Portland,
Oregon, September 25, 2010.
(19) O.S. Ovchinnikova and G.J. Van Berkel,
“Molecular Surface Sampling and Chemi-
cal Imaging Using Proximal Probe Thermal
Desorption/Secondary Ionization Mass
Spectrometry,” https://external-portal.ornl.
gov/doi/abs/10.1021/ac102766w. Publica-
tion Date (Web): December 15, 2010 (Arti-
cle) DOI: 10.1021/ac102766w.

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