RAPID COMMUNICATIONS IN MASS SPECTROMETRY
Rapid Commun. Mass Spectrom. 2009; 23: 3151–3157
Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rcm.4226
Mass spectrometry of intact neutral macromolecules
using intense non-resonant femtosecond laser
vaporization with electrospray post-ionization
John J. Brady, Elizabeth J. Judge and Robert J. Levis*
Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, USA
Received 10 June 2009; Revised 22 July 2009; Accepted 23 July 2009
Intact, nonvolatile, biological macromolecules can be transferred directly from the solid state into the
gas phase, in ambient air, for subsequent mass spectral analysis using non-resonant femtosecond (fs)
laser desorption combined with electrospray ionization (ESI). Mass spectral measurements for neat
samples, including a dipeptide, protoporphyrin IX and vitamin B12 adsorbed on a glass insulating
surface, were obtained using an 800 nm, 70 fs laser with an intensity of 1013 W cm2. No appreciable
signal was detected when atmospheric matrix-assisted or neat (matrix-free) fs laser desorption was
performed without ESI, indicating neutral desorption. Copyright # 2009 John Wiley & Sons, Ltd.
The detection of large, nonvolatile molecules with minimal
sample preparation is important for biochemistry assays and
forensic investigation. Matrix-assisted laser desorption/
ionization (MALDI)1,2 and electrospray ionization (ESI)3,4
are two common soft ionization techniques for vaporizing
and ionizing large nonvolatile molecules.5 Both methods
require the sample to be soluble in polar solvents. A series of
ambient ionization techniques has been developed to reduce
the complexity of sample preparation protocols prior to mass
spectral analysis. Desorption electrospray ionization (DESI),6
for instance, uses an electrospray plume to impinge on a
sample to desorb analytes. Samples analyzed by DESI
include explosives,7,8 carbohydrates,9 and biological tissues.10 Electrospray-assisted laser desorption ionization
(ELDI)11 combines laser desorption with electrospray postdesorption ionization for transfer of analyte into the mass
spectrometer at atmospheric pressure, thus avoiding the
need for transferring analyte samples into vacuum for
subsequent laser desorption. Proteins,12 peptides,13 caffeine
and dyes14 have been detected using ELDI. When ELDI is
utilized to investigate a sample mixed with matrix the
technique is called MALDESI.15
Both ELDI and MALDESI employ lasers wherein a firstorder resonant absorption excites molecules to a low lying
state in the analyte or matrix16 to enable vaporization. For
instance, laser ablation electrospray ionization (LAESI) uses
a 100 ns 2.94 mm laser pulse to resonantly excite the OH
vibration in water,17 while ELDI uses a 4 ns 337 nm laser. The
absorption cross section of a molecule, matrix or substrate
increases on the order of six orders of magnitude when there
is a resonant transition in comparison with non-resonant
excitation. This allows for more energy to be absorbed in the
*Correspondence to: R. J. Levis, Department of Chemistry,
Temple University, Philadelphia, Pennsylvania 19122, USA.
E-mail: [email protected]
resonant case when laser power densities are of the order
of 106 W cm2. The absorbed energy desorbs molecules via
thermal,18 phase explosion,19,20 impulsive,21,22 or electronicinduced23–25 mechanisms. For example, the LAESI desorption mechanism for water-rich samples proceeds via a
phase explosion17 while the desorption mechanism for dry or
wet samples using ELDI is still under investigation. In
general, the vast majority of molecules will not have a
resonant excitation for a given laser frequency in the optical
region. Hence, a specific matrix is used to absorb a specific
excitation frequency. The use of non-resonant laser excitation
for the desorption of macromolecules at atmospheric
pressure would further reduce sample compatibility restrictions and allow a variety of molecules to be studied without
the need for resonant excitation in the analyte or matrix.
While nanosecond lasers are typically used for resonant
excitation and vaporization of molecules in atmospheric
laser desorption, the coupling mechanisms of femtosecond
duration lasers into molecules suggests that ultrafast excitation may be useful for such desorption experiments, as well.
For the case of laser ionization in vacuum, more fragmentation is measured in the mass spectra of a biomolecule after
interaction with a resonant nanosecond laser than with a
resonant femtosecond laser (for conditions providing similar
ion intensities).26 The additional fragmentation is due to a
ladder-switching mechanism during nanosecond excitation/
ionization. This mechanism allows the molecule to absorb
energy and fragment during the excitation process and the
fragments subsequently absorb additional energy along the
way to ionization, leading to extensive fragmentation.
Ladder switching occurs because the molecular dissociation
time is shorter than the laser pulse duration. During femtosecond (fs) excitation, the ladder-climbing mechanism occurs
because the pulse duration is shorter than the molecular
rearrangement time, so the molecule cannot fragment during
the pulse. For instance, the polypeptide gramicidin D in
Copyright # 2009 John Wiley & Sons, Ltd.
3152 J. J. Brady, E. J. Judge and R. J. Levis
vacuum undergoes more fragmentation after irradiation by a
resonant ns pulse (260 nm) at an intensity of 107 W cm2 than
by a fs pulse (245–265 nm) that is two orders of magnitude
higher in intensity (109 W cm2).26 For the case of laserinduced vaporization, non-resonant fs laser desorption of
condensed cryogenic benzene from a Pt (111) surface into
vacuum has been demonstrated without fragmentation.27–29
Desorption of condensed cryogenic macromolecules into
vacuum, such as bradykinin in an ice matrix, has been
performed via resonant absorption of fs duration laser pulses
by the gold substrate.30 These measurements suggest that a
fs duration laser pulse would enable desorption of intact
molecules from a surface even in the case of non-resonant
excitation.
A means to desorb macromolecules without matrix at
atmospheric pressure is important for analyzing biologically
relevant molecules, particularly those with limited solubility
in polar solvents. We explore here the possibility of desorbing
intact macromolecules from a surface under ambient
conditions after interaction with an intense, non-resonant
fs pulse. We show that intact nonvolatile molecules can be
transferred from the solid state into air using non-resonant fs
laser desorption combined with ESI for subsequent ionization before mass spectral analysis. This is shown using mass
spectral measurements for neat samples including a dipeptide, protoporphyrin IX and vitamin B12 adsorbed on a glass
insulating surface using an 800 nm, 70 fs laser.
EXPERIMENTAL
Sample preparation
Samples of pseudoproline dipeptide (Fmoc-Tyr(tBu)-Ser
(CMe,Mepro)-OH) (Novabiochem, Gibbstown, NJ, USA),
protoporphyrin IX (MP Biomedicals, Solon, OH, USA) and
vitamin B12 (MP Biomedicals) were prepared by drying a
250 mL aliquot of 104 M (103 M for vitamin B12) solution in
methanol (Mallinckrodt Chemicals, Phillipsburg, NJ, USA)
on a glass slide. The applied solution dried into an even film
of analyte covering approximately 8 cm2. To test the effect of
matrix, a 1000:1 mixture of 2,3-dihydroxybenzoic acid (DHB,
Aldrich Chemical Co., St. Louis, MO, USA) was added to the
prepared solutions and then dried on a glass slide. The glass
slide was placed on a metal plate in the ESI source chamber.
The metal plate was supported by a three-dimensional stage
which permits the sample to be manually translated to allow
fresh sample to be analyzed on each laser shot.
Mass spectrometry
The mass spectrometry apparatus combines an ESI source for
ionizing and transferring desorbed sample into a vacuum
chamber where pulsed deflection orthogonal time-of-flight
(o-TOF) is performed. The ESI source is run in trapping mode
where the ions are collected in a hexapole before injection
into the o-TOF analyzer at 10 Hz, as seen in Fig. 1. For these
experiments, the ESI source (Analytica of Branford Inc.,
Branford, CT, USA) was operated in positive ion mode and
consisted of a needle, dielectric capillary, skimmer and
hexapole. The needle was maintained at ground while the
voltage on the capillary was adjusted until a stable ion
current/signal was observed, typically 5000 V. The electrospray solvent flow rate was 3 mL/min as set by a syringe
pump (Harvard Apparatus, Holliston, MA, USA). The ESI
plume was dried by the counter-current nitrogen gas at
1808C before the sample entered the dielectric capillary. After
the dielectric capillary the electrosprayed ions passed through
a skimmer to a linear hexapole. The ions were trapped in the
linear hexapole for 250 ms before being pulsed out of the
source and transferred to the extraction region by a radiofrequency (RF)-only hexapole (Ardara Technologies, North
Huntingdon, PA, USA). The timing pulse for the trap was
created using a digital delay generator (DDG, Stanford Research
Systems, Sunnyvale, CA, USA). Figure 2 shows the timing
diagram for the sequence of trapping and extraction pulses.
Figure 1. Schematic view of the API-pulsed deflection TOF mass spectrometer.
Copyright # 2009 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2009; 23: 3151–3157
DOI: 10.1002/rcm
Mass spectrometry of intact neutral macromolecules
3153
Figure 2. Timing diagram of the laser pulse, trapping, extraction and acceleration pulses.
The ions were transferred from the RF-only hexapole to
the extraction region of the o-TOF analyzer using a set of
ion lenses (Ardara Technologies). The ions were then
pulsed orthogonally into the TOF analyzer (Jordan TOF
Products Inc., Grass Valley, CA, USA) using two highvoltage pulsers (Directed Energy Inc., Fort Collins, CO,
USA and Quantum Technology Inc., Lake Mary, FL, USA)
triggered by the DDG. The extraction region and field free
flight tube were maintained at a pressure of 106 Torr. The
ions were detected using microchannel plates (Burle, Inc.,
Lancaster, PA, USA) in a Chevron configuration. The
resulting mass spectrum was collected for 600 laser shots
using a digital oscilloscope (LeCroy Corporation, Chestnut
Ridge, NY, USA) and averaged to obtain the signal to noise
displayed.
The ESI o-TOF system was mass calibrated using an
electrospray tuning solution (Agilent Technologies Inc.,
Santa Clara, CA, USA) that produced ions with known
m/z values to ensure mass accuracy. The mass spectra were
obtained from the flight time of the ions using a quadratic
calibration performed in our software (Labview 8.5).
Figure 3. A schematic representation of the desorption and
ionization apparatus. The needle to capillary distance, d1, is
7 mm. The distance from the bottom of the capillary to the
sample, d2, was maintained at 3 mm. The sample plate holder
was mounted to a three-dimensional stage to allow for easy
translation. The laser had an angle of incidence of 458 with
respect to the sample plate. The distance from the ESI needle
to the laser ablation spot, d3, was 1 mm.
Desorption and ionization apparatus
A Ti:Sapphire oscillator (KM Labs Inc., Boulder, CO, USA)
was used to seed a regenerative amplifier (Coherent Inc.,
Santa Clara, CA, USA) to create 70 fs laser pulses centered
at 800 nm with pulse energy of 1.5 mJ. The laser repetition
rate of the laser and electrospray is 1 kHz but it was set to
10 Hz to couple to the ESI-MS instrument. The synchronous
pulse of the laser was used to trigger the digital delay
generator to set the timing of the trap and the extraction
plates in the mass spectrometer. The laser pulse was
directed at the sample to induce desorption from the dried
film. The laser was focused to a spot size of 300 mm in
diameter using a 17.5 cm focal length lens, with an incident
angle of 458 with respect to the sample (Fig. 3). The intensity
of the beam hitting the sample was approximately 1013 W/
cm2. The metal sample holder plate was biased to 1.5 kV
to correct for the distortion in the electric field between the
needle and capillary inlet caused by the sample plate. This
bias optimizes the entrance current of electrosprayed ions
into the dielectric capillary. The ablated sample was
captured and ionized by electrospraying methanol with 1%
Copyright # 2009 John Wiley & Sons, Ltd.
acetic acid for the pseudoproline dipeptide and protoporphyrin IX and 80:20 methanol/water with 1% acetic acid
for vitamin B12. The solvent was chosen based on the
solubility of the sample. The spray direction is perpendicular to the laser-vaporized plume trajectory. After
vaporization and ionization, the sample subsequently
enters the inlet capillary. An ESI solvent background mass
spectrum (no laser present) was acquired before each
experiment and subtracted from the laser vaporization
measurement to produce the spectra shown. Significant
differences in the intensity of the solvent mass spectrum
were caused by the presence of the laser-vaporized
molecules. The molecules compete for the charge in the
electrosprayed solvent cluster, causing a change in the
observed solvent signal. This leads to the formation of
negative and positive solvent features seen in the subtracted
mass spectra. The solvent-related features are the unlabeled
peaks in the figures.
Rapid Commun. Mass Spectrom. 2009; 23: 3151–3157
DOI: 10.1002/rcm
3154 J. J. Brady, E. J. Judge and R. J. Levis
Figure 4. (a) Mass spectrum corresponding to the non-resonant laser vaporization of neat
(matrix-free) dipeptide sample spotted onto a glass slide reveals the protonated molecule.
The inset displays a 6 magnification of the [MþH]þ and [M–tBu–H]þ peaks. (b) Mass
spectrum corresponding to the non-resonant laser vaporization of a 1000:1 molar solution of
DHB and dipeptide spotted on a glass slide. All unlabeled peaks are solvent-related.
RESULTS AND DISCUSSION
The biological macromolecules investigated in this paper
include pseudoproline dipeptide, protoporphyrin IX and
vitamin B12. These biomolecules were chosen based on their
size, solubility and their ability to form multiply charged
ions. Representative ESI mass spectra were obtained for each
of the samples investigated and compared with the laservaporized sample collected in either the matrix-free (fsELDI) or matrix-included (fs-MALDESI) modes of operation.
The positive ion mode mass spectrum for the neat,
pseudoproline dipeptide sample is shown in Fig. 4(a). The
spectrum reveals the protonated molecule at m/z 588,
demonstrating the ability to transfer molecules into the
gas phase using an intense, non-resonant fs duration pulse at
800 nm. The electrospray solvent produces a series of peaks
in the low-mass region of the mass spectrum that must be
subtracted to clearly see the dipeptide features. Without the
electrospray present, no ions were observed in the spectrum
(data not shown), suggesting that ionization occurs when the
vaporized molecules are captured by the ESI droplets.11 Note
that the bias voltages remained on for the measurements
without the electrospray. The amount of sample consumed
for this analysis was approximately 1 nmol as estimated from
the amount of sample deposited, the concentration of the
sample, the area covered, and the surface area vaporized.
MALDI matrices are chosen, in part, to enable desorption
of macromolecules. To determine whether non-resonant
desorption was possible with a MALDI matrix, the pseudoproline dipeptide was mixed with DHB and deposited on a
glass slide. The non-resonant fs-MALDESI spectrum
Copyright # 2009 John Wiley & Sons, Ltd.
(Fig. 4(b)) reveals an increase of an order of magnitude in
the signal for the [MþH]þ ion in comparison with the neat
sample. The increase in signal reveals that the matrix does
assist in vaporizing more molecules from the film. Once
again, no ions were detected without the electrospray present
suggesting desorption of molecules even with MALDI matrix
under the conditions of non-resonant fs vaporization.
The pseudoproline dipeptide mass spectrum reveals
fragmentation with and without matrix. The fragment ion at
m/z 530 is consistent with [M–tB-H]þ and was observed in
the fs-MALDESI and fs-ELDI spectra (Fig. 4). The spectra
shown in Fig. 4 have the same ratios of [M–tBu-H]þ to
[MþH]þ as observed in a control experiment of the
conventional ESI-MS of pseudoproline dipeptide (data
not shown). Therefore, the [M–tBu-H]þ fragment ion was
probably formed by collision-induced dissociation (CID),
which occurs between the capillary and the skimmer in the
ESI ion optics, and is not a result of laser interaction with
the molecule.
Many biological macromolecules have low solubility in
common polar solvents and analysis is difficult using MALDI
and ESI. Protoporphyrin IX is an example of a biologically
relevant molecule that has low solubility in common solvents,
such as methanol and water. When protoporphyrin IX is
placed in methanol a turbid solution is formed indicating the
formation of a heterogeneous mixture. An aliquot of this
solution was spotted onto a glass slide and dried. The thin
film of protoporphyrin was then analyzed using non-resonant
fs laser desorption with ESI. A conventional ESI mass spectrum
is acquired using a filtered portion of the protoporphyrin IX
solution.
Rapid Commun. Mass Spectrom. 2009; 23: 3151–3157
DOI: 10.1002/rcm
Mass spectrometry of intact neutral macromolecules
3155
Figure 5. (a) Mass spectrum corresponding to the non-resonant laser vaporization of neat
(matrix free) 104 M protoporphyrin IX sample spotted on a glass slide. (b) Mass spectrum
corresponding to the non-resonant laser vaporization of a 1000:1 molar solution of DHB and
protoporphyrin IX spotted on a glass slide. All unlabeled peaks are solvent-related.
The fs-ELDI mass spectrum (Fig. 5(a)) shows a strong
protonated molecule of m/z 564. When protoporphyrin IX is
mixed with matrix and vaporized using the non-resonant fs
laser pulse, little fragmentation occurs and a factor of twofold enhancement is observed in the [MþH]þ ion, as seen in
Fig. 5(b). The morphology of the MALDI matrix after
deposition reveals two distinct domains, one contains large
crystalline structures and the other is an area of microcrystalline film. The majority of the useful signal was obtained from
the microcrystalline film and no signal was observed when
the large crystals were probed. No dimer was present in
either the fs-ELDI or the fs-MALDESI spectrum for
protoporphyrin IX; however, the dimer was clearly present
in the ESI-MS measurement. This suggests that the laservaporization process does indeed transfer intact single
molecules into the gas phase, while the ESI process can result
in dimers and aggregates. When the electrospray was not
present during the ablation of protoporphyrin IX, with or
without matrix, no ions were observed in the spectrum.
The protoporphyrin IX mass spectrum reveals two
fragment ions at m/z 407 and 433. The low-abundance
fragment ions in the fs-MALDESI spectrum are also present
in a conventional ESI spectrum of protoporphyrin IX but in
different ratios to the [MþH]þ ion. The fragment ions are
presumed to be caused by interaction with the laser, and are
not attributed to CID between the capillary and the skimmer
in the ESI ion optics. The ions are consistent with the [M–
CHCH2–CH2COOH–CH2COOHþH]þ and [M–CH2COOH–
CH2COOHþH]þ fragments, respectively. Such fragmentation
channels are accessible at laser intensities 1013 W cm2 for
unsaturated hydrocarbons. For instance, extensive fragmentation of beta-carotene has been demonstrated at an intensity of
1012.5 W cm2 for an 800 nm 40 fs pulse yielding a low
Copyright # 2009 John Wiley & Sons, Ltd.
abundance of the [MþH]þ ion in comparison with the observed
fragment ions.31 In the case of protoporphyrin IX, a higher
abundance of the [MþH]þ ion is observed in relation to the
observed fragment ions. The protoporphyrin IX example
demonstrates that molecules with low solubility in polar
solvents can still be detected using non-resonant laser
desorption from matrix-free samples without extensive
fragmentation due to laser excitation.
The non-resonant fs desorption of vitamin B12 was explored
to further investigate the ionization mechanism and determine
the ability to desorb larger macromolecules, to produce an ion
of m/z 1355 in this case. Previous investigations have
demonstrated that multiple charging is prevalent in the ESIMS of vitamin B12. Our ESI mass spectrum of vitamin B12
revealed singly and doubly protonated molecules (data not
shown), similar to what was obtained for the fs laser vaporization mass spectrum shown in Fig. 6(a). This again suggests
that neutrals are desorbed in the vaporization process and
subsequently ionized in the electrospray process occurring
during the transfer to the capillary inlet orifice. Other biologically relevant macromolecules with higher charge states
should also be amenable to investigation using this technique.
Vitamin B12 is a complex macromolecule with a
propensity to fragment after irradiation with UV and IR
lasers.32–34 The low-mass region of the matrix-free vitamin
B12 mass spectrum reveals ions at m/z 132 (fs-ELDI only), 147
(fs-ELDI only), 666 (fs-ELDI and fs-MALDESI), 914 (fs-ELDI
only) and 1331(fs-ELDI only). These fragment ions are not
present in the conventional ESI mass spectrum of vitamin
B12; therefore, they are presumed to be caused by interaction
with the laser. The ions are consistent with the pentose
fragment, the dimethylbenzimidazole (base) fragment,
[M–CNþ2H]2þ and [M–Co–CN–base–sugar–PO4]þ, respectively.
Rapid Commun. Mass Spectrom. 2009; 23: 3151–3157
DOI: 10.1002/rcm
3156 J. J. Brady, E. J. Judge and R. J. Levis
Figure 6. (a) Mass spectrum corresponding to the non-resonant laser-vaporized, neat
(matrix-free) sample of vitamin B12 spotted on a glass slide shows the singly and doubly
protonated molecules. The inset is a 2 magnification of the doubly protonated molecule (left)
and the protonated molecule (right). (b) Mass spectrum corresponding to the non-resonant
laser vaporization of a 1000:1 molar solution of DHB and vitamin B12 spotted on a glass slide
shows the singly and doubly protonated molecules. The inset displays a 2 magnification of
the doubly charged molecules. All unlabeled peaks are solvent-related.
When vitamin B12 is mixed with matrix and vaporized
using the non-resonant fs laser pulse, little fragmentation
occurs and a factor of three enhancement is observed for both
the doubly and the singly protonated molecules, as seen in
Fig. 6(b). When the ESI plume was not present during the
ablation of vitamin B12, with or without matrix, no ions were
observed in the spectrum.
CONCLUSIONS
Intact desorption of neutral, biological macromolecules has
been demonstrated using non-resonant fs lasers with and
without matrix. Signals are observed for neat samples of
pseudoproline dipeptide, protoporphyrin IX and vitamin
B12. When the analyte under investigation was mixed with
DHB matrix, an increase in mass spectral signal was
observed, with a decrease in fragmentation being observed
for the dipeptide and vitamin B12. The use of non-resonant fs
lasers overcomes the need for resonant absorption occurring
in analyte, matrix or substrate, thus making the desorption
more universal and sample independent. The detection of
matrix-free samples relaxes sample preparation requirements and reduces preparation time, making fs-ELDI a
possible tool for biological assays and forensics.
Acknowledgements
This work was supported by grants from the National
Science Foundation CHE0518497 and the Army Research
Office W911NF0810020.
Copyright # 2009 John Wiley & Sons, Ltd.
REFERENCES
1. Hillenkamp F, Karas M, Beavis RC, Chait BT. Anal. Chem.
1991; 63: A1193.
2. Nordhoff E, Ingendoh A, Cramer R, Overberg A, Stahl B,
Karas M, Hillenkamp F, Crain PF. Rapid Commun. Mass
Spectrom. 1992; 6: 771.
3. Fenn JB, Mann M, Meng CK, Wong SF, Whitehouse CM.
Mass Spectrom. Rev. 1990; 9: 37.
4. Fenn JB, Mann M, Meng CK, Wong SF, Whitehouse CM.
Science 1989; 246: 64.
5. Aebersold R, Mann M. Nature 2003; 422: 198.
6. Takats Z, Wiseman JM, Gologan B, Cooks RG. Science 2004;
306: 471.
7. Cotte-Rodriguez I, Hernandez-Soto H, Chen H, Cooks RG.
Anal. Chem. 2008; 80: 1512.
8. Popov IA, Chen H, Kharybin ON, Nikolaev EN, Cooks RG.
Chem. Commun. 2005; 1953.
9. Bereman MS, Williams TI, Muddiman DC. Anal. Chem. 2007;
79: 8812.
10. Cooks RG, Ouyang Z, Takats Z, Wiseman JM. Science 2006;
311: 1566.
11. Shiea J, Huang MZ, Hsu HJ, Lee CY, Yuan CH, Beech I,
Sunner J. Rapid Commun. Mass Spectrom. 2005; 19: 3701.
12. Huang MZ, Hsu HJ, Lee LY, Jeng JY, Shiea LT. J. Proteome
Res. 2006; 5: 1107.
13. Peng IX, Shiea J, Loo RRO, Loo JA. Rapid Commun. Mass
Spectrom. 2007; 21: 2541.
14. Lin SY, Huang MZ, Chang HC, Shiea J. Anal. Chem. 2007; 79:
8789.
15. Sampson JS, Hawkridge AM, Muddiman DC. J. Am. Soc.
Mass Spectrom. 2006; 17: 1712.
16. Cheng CY, Yuan CH, Cheng SC, Huang MZ, Chang HC,
Cheng TL, Yeh CS, Shiea J. Anal. Chem. 2008; 80: 7699.
17. Nemes P, Vertes A. Anal. Chem. 2007; 79: 8098.
18. Calvano CD, Palmisano F, Zambonin CG. Rapid Commun.
Mass Spectrom. 2005; 19: 1315.
19. Peterlongo A, Miotello A, Kelly R. Phys. Rev. E 1994; 50: 4716.
Rapid Commun. Mass Spectrom. 2009; 23: 3151–3157
DOI: 10.1002/rcm
Mass spectrometry of intact neutral macromolecules
20. Yoo JH, Jeong SH, Greif R, Russo RE. J. Appl. Phys. 2000; 88:
1638.
21. Levine RD, Bernstein RB. Molecular Reaction Dynamics and
Chemical Reactivity. Oxford University Press: New York,
1987.
22. Zhu XY. Annu. Rev. Phys. Chem. 1994; 45: 113.
23. Misewich JA, Heinz TF, Newns DM. Phys. Rev. Lett. 1992; 68:
3737.
24. Prybyla JA, Heinz TF, Misewich JA, Loy MMT, Glownia JH.
Phys. Rev. Lett. 1990; 64: 1537.
25. Zimmermann FM, Ho W. J. Chem. Phys. 1994; 100: 7700.
26. Weinkauf R, Aicher P, Wesley G, Grotemeyer J, Schlag EW.
J. Phys. Chem. 1994; 98: 8381.
27. Arnolds H, Levis RJ, King DA. Chem. Phys. Lett. 2003; 380: 444.
Copyright # 2009 John Wiley & Sons, Ltd.
3157
28. Arnolds H, Rehbein C, Roberts G, Levis RJ, King DA. J. Phys.
Chem. B 2000; 104: 3375.
29. Arnolds H, Rehbein CEM, Roberts G, Levis RJ, King DA.
Chem. Phys. Lett. 1999; 314: 389.
30. Berry JI, Sun SX, Dou YS, Wucher A, Winograd N. Anal.
Chem. 2003; 75: 5146.
31. Lezius M, Blanchet V, Rayner DM, Villeneuve DM, Stolow
A, Ivanov MY. Phys. Rev. Lett. 2001; 86: 51.
32. Kinsel GR, Preston LM, Russell DH. Biol. Mass Spectrom.
1994; 23: 205.
33. Blankenship JF, VanStipdonk MJ, Schweikert EA. Rapid
Commun. Mass Spectrom. 1997; 11: 143.
34. Rousell DJ, Dutta SM, Little MW, Murray KK. J. Mass Spectrom. 2004; 39: 1182.
Rapid Commun. Mass Spectrom. 2009; 23: 3151–3157
DOI: 10.1002/rcm