Anal. Chem. 2003, 75, 1294-1299
Fluorinated Matrix Approach for the
Characterization of Hydrophobic
Perfluoropolyethers by Matrix-Assisted Laser
Desorption/Ionization Time-of-Flight MS
A. Marie, S. Alves, F. Fournier, and J. C. Tabet*
Laboratoire de Chimie Structurale Organique et Biologique, CNRS UMR 7613, Université Pierre et Marie Curie, Courier 45,
4 place Jussieu, 75252 Paris Cedex 05, France
Characterization of fluorinated polymers in MALDI is often
unsuccessful because commonly used matrixes, such as
2,5-dihydroxybenzoic acid, Indole acrylic acid, r-cyano4-hydroxycinnamic acid, etc., do not desorb/ionize fluorinated polymers efficiently. This could be in part attributed to the unfavorable interaction between the matrix
molecules and fluorinated oligomers due to differences
in their hydrophobicities. Moreover, the relative cation
affinity between the matrix molecules and the fluorinated
oligomers may not favor the gas-phase cationization
process of the fluorinated oligomers. To overcome these
limitations, fluorinated derivates of benzoic acid (pentafluorobenzoic acid) and cinnamic acid (Pentafluoro
cinnamic acid) were employed for the desorption/ionization of perfluoropolyethers. Presence of fluorine atoms
in the matrix might improve the interaction between the
matrix and perfluoroether during the crystallization or
ionization step. With a pentafluorobenzoic acid matrix,
intact silver cationized oligomers were desorbed, whereas
with a pentafluorocinnamic acid matrix, loss of end group
was observed. This loss could be rationalized by the
dissociation of the silver cationized oligomers via an iondipole mechanism. This work shows the possibility of
characterizing yet another important class of fluorinated
polymer by MALDI-TOFMS.
Fluorinated oligomers exhibit many useful properties, such as
high thermal stability, chemical inertness, low dielectric constants
and dissipation factors, good resistance to oxidation and aging,
low flammability, and very interesting surface properties.1 These
properties depend on the number and the position of the fluorine
atoms present in the oligomer chain. Fluorinated polymers are
used in oil, water, and soil repellency applications (monument and
textile protection), in antiadhesive coatings (utensils, walls), in
nonflammable clothing, in fluid handling components (linings),
gaskets and seals for chemical plants, etc. Global consumption of
fluorinated polymers is estimated to ∼$2200 million U.S. (1997),2
* Corresponding author. Phone: (33-1) 44-27-32-63. Fax: (33-1) 44-27-38-43.
E-mail: [email protected].
(1) Améduri, B.; Boutevin, B. Topics in Current Chemsitry; Springer: New York,
1997; Vol. 192, p 168.
(2) Holloway, J. H. J. Fluorine Chem. 2000, 104, 3.
1294 Analytical Chemistry, Vol. 75, No. 6, March 15, 2003
which indicates their commercial and industrial importance. Rapid
and reliable analytical techniques need to be developed for these
“speciality” polymers, which are important in our everyday lives.
Matrix assisted laser desorption/ionization (MALDI)3 and
electrospray ionization (ESI)4 are two “soft ionization” techniques
that are used to characterize synthetic polymers.5-28 Generally in
ESI, the oligomers are often desorbed as multiply charged species,
and the overlap of different charge-state distributions renders the
mass spectrum complex. Therefore, ESI is suitable for character(3) Karas, M.; Hillenkamp, F. Anal. Chem. 1988, 60, 2299.
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28, 1229.
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Am. Soc. Mass Spectrom. 1996, 7, 11.
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(11) Larsen, B. S.; Simonsick, W. J., Jr.; McEwen, C. J. Am. Soc. Mass Spectrom.
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Tabet, J. C. J. Mass Spectrom. 1996, 31, 1409.
(13) Jackson, A. T.; Yates, H. T.; MacDonald, W. A.; Scrivens, J. H. J. Am. Soc.
Mass Spectrom. 1997, 8, 132.
(14) Whittal, R. M.; Schriemer, D. C.; Li, L. Anal. Chem. 1997, 69, 2734.
(15) Montaudo, M. S.; Puglisi, C.; Samperi, F.; Montaudo, G. Macromolecules
1998, 31, 8666.
(16) Rashidzadeh, H.; Guo, B. Anal. Chem. 1998, 70, 131.
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Chem. 2000, 72, 2490.
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11, 731.
(19) Chen, R.; Tseng, A. M.; Uhing, M.; Li, L. J. Am. Soc. Mass Spectrom. 2001,
12, 55.
(20) Polce, M. J.; Klein, D. J.; Harris, F. W.; Modarelli, D. A.; Wesdemiotis, C.
Anal. Chem. 2001, 73, 1948.
(21) Fournier, I.; Marie, A.; Lesage, D.; Bolbach, G.; Fournier, F.; Tabet, J. C.
Rapid Commun. Mass Spectrom. 2002, 16, 696.
(22) Nohmi, T.; Fenn, J. B. J. Am. Chem. Soc. 1992, 114, 3241.
(23) O’Connor, P. B.; McLafferty, F. W. J. Am. Chem. Soc. 1995, 117, 12826.
(24) Jasieczek, C. B.; Buzy, A.; Haddleton, D. M.; Jennings, K. R. Rapid Commun.
Mass Spectrom. 1996, 10, 509.
(25) Latourte, L.; Blais, J. C.; Tabet, J. C.; Cole, R. B. Anal. Chem. 1997, 69,
2742.
(26) Scrivens, J. H.; Jackson, A. T. Int. J. Mass Spectrom. 2000, 200, 419.
(27) Hanton, S. D. Chem. Rev. 2001, 101, 527.
(28) Adamus, G.; Sikorska, W.; Kowalczuk, M.; Montaudo, M.; Scandola, M.
Macromolecules 2000, 33, 5797.
10.1021/ac0260802 CCC: $25.00
© 2003 American Chemical Society
Published on Web 02/07/2003
izing synthetic polymers characterized by low molecular mass
distribution. On the other hand, in MALDI, the oligomers are
desorbed/ionized as monocharged species, and the obtained mass
spectrum is relatively easy to interpret. A successful characterization of synthetic polymers by MALDI depends on not only the
polymer properties, such as molecular mass distribution, chemical
composition, polydispersivity, solubility, etc., but also on its
interaction with matrix molecules during sample target preparation
and desorption/ionization steps. New sample target preparation
protocols, such as prefractionation of highly polydisperse polymers
(by size-exclusion chromatography),29 chemical derivatization,30
and solvent-free methodologies31-33 for the analysis of different
synthetic polymers by MALDI technique have been reported in
the literature. It should be noted that MALDI is often applied to
characterize polar and nonpolar polymers using matrixes such
as 2,5-dihydroxybenzoic acid (DHB), R-cyano-4-hydroxycinnamic
acid (HCCA), indole acrylic acid (IAA), etc. However, desorption/
ionization of fluorinated polymers is very unlikely by the abovementioned matrixes. One explanation for this observation could
be the difference in hydrophobicity between the fluorinated
oligomers and the matrix molecules. It should be pointed out that
the fluorinated oligomers are relatively hydrophobic compared
to commonly used MALDI matrixes.
A strategy for the analysis of hydrophobic, fluorinated oligomers by MALDI would be to facilitate the interaction between
the fluorinated oligomers and the matrix molecules. For this
purpose, fluorinated derivatives of MALDI matrixes might be
efficient for the desorption/ionization of fluorinated oligomers. The
model fluorinated polymer system used in this work is a perfluoropolyether. This class of polymers exhibit high thermal
stability and are used in applications where high temperature
performance is critical (e.g., lubrication). To our knowledge, no
study has been reported in the literature on the characterization
of hydrophobic perfluoropolyethers by MALDI. In this work, we
report the results obtained on the desorption/ionization of
perfluoropolyether by using fluorinated matrixes under MALDITOFMS conditions.
EXPERIMENTAL SECTION
MALDI mass spectra were acquired by using a Voyager Elite
time-of-flight (TOF) mass spectrometer (PerSeptive Biosystems,
Boston, MA), equipped with a nitrogen pulsed laser (λ ) 337 nm,
pulse duration 3 ns, repetition rate 2 Hz). All of the MALDI mass
spectra were recorded in positive ion mode under delayed
extraction conditions (225 ns) and reflectron mode. The accelerating voltage was 20 kV. The spectral data were treated using
GRAMS/386 software. Each spectrum was the average of 256 laser
shots.
Polyperfluoroether, Poly(tetrafluoroethylene oxide-co-difluoromethyleneoxide) R,ω-diol (Mn ) 2000, ethylene-to-methylene ratio
) 1:1), pentafluorobenzoic acid (PFBA), pentafluorocinnamic acid
(PFCA) and hexafluoro-2-propanol used in this work were all
(29) Montaudo, M. S.; Puglisi, C.; Samperi, F.; Montaudo, G. Rapid Commun.
Mass Spectrom. 1998, 12, 519.
(30) Barry, J. P.; Carton, W. J.; Pesci, K. M.; Anselmo, R. T.; Radek, D. R.; Evans,
J. Rapid Commun. Mass Spectrom. 1997, 11, 437.
(31) Skelton, R.; Dubois, F.; Zenobi, R. Anal. Chem. 2000, 72, 1707.
(32) Marie, A.; Fournier, F.; Tabet, J. C. Anal. Chem. 2000, 72, 5106.
(33) Trimpin, S.; Rouhanipour, A.; Az, R.; Räder, H. J. Müllen, K. Rapid Commun.
Mass Spectrom. 2001, 15, 1364.
Chart 1. Structure of the Fluorinated Matrixes
Used in This Work
obtained from Sigma (Saint Quentin Fallavier, France). All of the
materials and reagents were used as received without further
purification.
Sample Target Preparation. About 2 µL of the polyperfluoroether sample was dissolved in 100 µL of hexafluoro-2-propanol.
Saturated matrix solutions were prepared in tetrahydrofuran, and
salt solutions of AgNO3, LiCl, NaCl, Cu (II) acetate, and Fe2(SO4)3
of concentration of ∼10 ppm were prepared separately in water.
To the sample target, ∼1 µL of the salt solution was applied first
and dried, and ∼1 µL of the diluted polyfluoroethers solution was
then added. After the evaporation of the solvent, ∼2 µL of the
matrix solution was placed over the polymer layer and dried. The
sample target thus obtained was submitted to MALDI experiments.
RESULTS AND DISCUSSION:
Fluorinated polymers are often insoluble in a suitable solvent
system and, therefore, are difficult to analyze by MALDI. But
characterization of perfluoropolyether presents a different challenge: it is fairly soluble in hexafluoro-2-propanol but gives no
signal in MALDI experiments. We observed that DHB is the only
matrix that functions well for the desorption/ionization of many
classes of fluorinated oligomers (e.g., poly(vinylidiene)fluoride,
perfluoroacrylate, poly(vinylidene)fluoride xanthate, etc.).25,32,34
Guarini et al.35 reported the characterization and the fragmentation
chemistry of perfluoropolyethers by desorption chemical ionization
in negative ion mode. However, desorption/ionization of perfluoropolyethers was not successful by DHB or by any other
matrix (e.g., IAA, HCCA, etc.). This could be in part attributed to
the unfavorable interaction between the fluorinated oligomers and
the matrix molecules. To overcome this limitation, we chose pentafluorobenzoic acid (PFBA) and pentafluorocinnamic acid (PFCA)
as matrixes for the desorption/ionization of perfluoroether by
(34) Marie, A. Ph.D. Thesis, University Pierre et Marie Curie, Paris, 2002.
(35) Guarini, A.; Guglielmetti, G.; Vincenti, M.; Guarda, P.; Marchionni, G. Anal.
Chem. 1993, 65, 970.
Analytical Chemistry, Vol. 75, No. 6, March 15, 2003
1295
Figure 1. MALDI-TOF mass spectrum of perfluoropolyether obtained with PFBA matrix (a) in the presence of AgCl, (b) in the presence of
LiCl, and (c) in the presence of NaCl.
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Analytical Chemistry, Vol. 75, No. 6, March 15, 2003
Figure 2. MALDI-TOF mass spectrum of perfluoropolyether obtained by using PFCA matrix. AgCl was used as the cationization salt.
MALDI (Chart 1). The fluorine atoms present in the matrix might
improve the hydrophobic interaction with the fluorinated oligomers during the sample target preparation. Moreover, we assumed
that the relative cation affinity between the fluorinated matrix and
perfluoropolyether oligomers might facilitate the efficient cationization of the fluorinated oligomers. The chemical structure of
the perfluoropolyether used in this work is represented as
HO-CH2CF2-(OCF2CF2)x-(OCF2)y-OCF2CH2OH
Desorption/Ionization of Perfluoropolyether by Pentafluorobenzoic Acid Matrix. The MALDI-TOF mass spectrum
of perfluoropolyether obtained by using a PFBA matrix is shown
in Figure 1. To obtain a good MALDI mass spectrum, the salt
solution was deposited and dried on the sample target, followed
by the deposition of the oligomer solution. After evaporation of
the solvent, the matrix solution was applied and dried at room
temperature. No MALDI mass spectrum could be obtained when
the order of the deposition was changed. To facilitate the
cationization of the oligomers, cations such as Na+, Li+, Fe2+, Cu2+,
and Ag+ were added, but only Ag+ resulted in the observation of
intense oligomer signals. However, weak oligomer signals were
obtained in the presence of LiCl salt (Figure 1b).
In the MALDI mass spectrum, the oligomeric distribution is
centered approximately at 1700 Th, and two series of peaks
separated by 66 and 116 Th that correspond to the repeating units
-OCF2- and -OCF2CF2-, respectively, are observed. The natural
isotopic distribution (Figure 1a) of the oligomers (P) in the
MALDI mass spectrum confirms that the oligomers are cationized
by Ag+. Examination of the MALDI mass spectrum reveals that
the m/z ratio of the cationized oligomers (P + 107Ag)+ are lower
by ∼1 Th than the calculated m/z values. This difference could
be attributed to the loss of calibration of the instrument, although
the precision of the measured m/z appears to be stable, as
indicated by the mass defects of the oligomers. It should be noted
that the calibrants normally used (e.g., peptides) could not be
desorbed by employing fluorinated matrixes. Hence, external
calibration was carried out using DHB matrix. To verify the loss
of calibration, the desorption/ionization of poly(vinylidene)fluoride
telomers36 was carried out using DHB and PFBA matrixes. As
expected, PFBA matrix gave lower m/z values for the telomers,
as compared to the m/z values obtained from DHB matrix (data
not shown). In addition, the m/z values of the oligomers obtained
by using an ESI-ion trap mass spectrometer confirmed that the
(36) Marie, A.; Fournier, F.; Tabet, J. C.; Améduri, B.; Walker, J. Anal. Chem.
2002, 74, 3213.
Analytical Chemistry, Vol. 75, No. 6, March 15, 2003
1297
Scheme 1. Dissociation Mechanism of Perfluoropolyether Cationized by Ag+ via Ion-Dipole Complex
mass shift observed in the MALDI mass spectrum is due to a
loss of calibration and not due to the difference in the chemical
structure of the oligomers (see discussion on PFCA matrix).
One interesting observation is the cationization of the perfluoropolyether oligomers by the silver cation. Gas-phase stabilization of Ag+ (considered as Lewis acid) is observed with oligomers
containing electron-rich groups, such as phenyl (e.g., polystyrene)
or unsaturated groups (e.g., polybutadiene). However, it is
surprising to note the solvation of Ag+ by perfluoropolyether,
which does not contain any electron-rich groups to promote
cation-π-electron interaction. This led us to conclude that the
fluorine atoms present in the oligomers play an important role in
the stabilization of the silver cations in the gas phase. For example,
the repeating motif of poly(ethylene glycol) (-OCH2CH2-) and
perfluoropolyether (-OCF2CF2-) are similar; however, poly(ethylene glycol) is not cationized by silver or could not be
desorbed/ionized using fluorinated matrixes. This suggests that
oxygen atoms present in the oligomer do not play any role in the
cationization. Taking into account the number of fluorine atoms
presents in the perfluoropolyether backbone, we can assume the
presence of electron clouds along the oligomer chain that could
stabilize the Ag+ by simple electrostatic solvation. Moreover, both
the oligomer chain length and its capacity to adopt a favorable
confirmation around the cation37,38 as well as the size of the cation
have to be considered to explain the cationization processes. The
less intense signal of the lithiated perfluoro oligomers observed
in Figure 1b could be explained by its smaller, unfavorable atomic
size as compared to silver cations.
Desorption/Ionization of Perfluoropolyether by Pentafluorocinnamic Acid Matrix. Figure 2 shows the MALDI mass
spectrum of perfluoropolyethers obtained by employing a PFCA
matrix. Compared to the PFBA matrix (Figure 1a), desorption/
ionization of high-mass oligomers is favored by the PFCA matrix;
(37) Wyttenbach, T.; Helden, G. V.; Bowers, M. T. Int. J. Mass Spectrom. Ion
Processes 1997, 165/166, 377.
(38) Gidden, J.; Wyttenbach, T.; Batka, J. T.; Weis, P.; Jackson, A. T.; Scrivens,
J. H.; Bowers, M. T. J. Am. Chem. Soc. 1999, 10, 833.
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Analytical Chemistry, Vol. 75, No. 6, March 15, 2003
we notice that the oligomer distribution is centered approximately
at 1950 Th (manufacturer’s data Mn ) 2000). As expected, two
series of oligomers separated by 66 and 116 Th are observed in
the mass spectrum. Surprisingly, the isotopic distribution of the
oligomers indicates that the ionized oligomers are not silvercationized. However, a MALDI mass spectrum could be obtained
only in the presence of the silver cation and not in the presence
of other cations. Considering the oligomer x ) y ) 9 (calculated
mass ) 1816 u) and when it is cationized by 107Ag+, we expect a
peak at m/z 1923, which is not the case here. A peak is observed,
however, in the mass spectrum at m/z 1815 Th.
To explain the unexpected m/z ratio of the oligomer ions in
the MALDI mass spectrum, we might postulate that the peak
observed at m/z 1815 might correspond to the oligomer ion (P H)+. However, this is very unlikely, since it is difficult to rationalize
why the MALDI mass spectrum could be specifically obtained in
the presence of silver cations and not in the presence of other
salts. Moreover, the mass spectrum obtained in Figure 1a shows
that perfluoropolyethers can be cationized by Ag+. To explain the
oligomer peaks observed in the MALDI mass spectrum, we
consider that the oligomers are cationized by Ag+ during the
desorption/ionization processes, but it is possible that the residual
internal energy acquired by the oligomers might be sufficient to
cause the dissociation of the (P + Ag+) adduct ions (in-source
decay39 or post-source decay40). It should be recalled that the
quantity of internal energy acquired by the analytes under MALDI
conditions depends on the physiochemical properties of the
matrix.41
The assumed dissociation mechanism of the silver cationized
oligomer is represented in Scheme 1. The initial step is the
(39) Brown, R. S.; Carr, B. L.; Lennon, J. J. J. Am. Soc. Mass Spectrom. 1996, 7,
225.
(40) Kaufmann, R.; Kirsch, D.; Spengler, B. Int. J. Mass Spectrom. Ion Processes
1997, 165/166, 405.
(41) Gormann, J. J.; Ferguson, B. L.; Nguyen, T. B. Rapid Commun. Mass
Spectrom. 1996, 10, 529.
formation of an ion-dipole complex, and its dissociation leads to
the formation of a cationic cyclic perfluoropolyether that has lost
a terminal group. The silver cation can polarize the C-F bond
that is relatively weak in the oligomer, and this concerns the CF2
group vicinal to an oxygen atom42 which can stabilize the
carbocation. For example, the oligomer x ) y ) 9 cationized by
107Ag+ (calculated m/z ) 1923 Th), can dissociate via an iondipole complex intermediate (Scheme 1) to lose HF and AgOCF2CH2OH. Thus, the oligomer ion (not cationized by Ag+) is
observed at m/z 1699. The ions observed at m/z 1815 and 1765
correspond to the oligomers x ) 10, y ) 9 and x ) 9, y ) 10,
respectively. Similarly, the ion at m/z 1881 corresponds to the
oligomer ion x ) y ) 10. In our recent work, formation of an iondipole complex between a lithium cation and the fluorinated
oligomer in the gas phase was invoked to explain the fragmentation chemistry of poly(vinylidiene fluoride) telomers.36 In ESI, the
perfluoroethers were desorbed as (P+NH4)+ adduct ions in
positive ion mode and the mass of the oligomers calculated from
MALDI mass spectrum agrees well with that obtained from ESI
mass spectrum (data not shown). From this result it can be
inferred that the instrumental calibration is not lost when PFCA
is used. The commercial perfluoroether sample used in this work
could have been synthesized by the photooxidation of different
perfluoroolefins such as tetrafluoroethylene, hexafluoropropene,
perfluorobutadiene, etc.,43 and therefore a precise knowledge of
(42) Smart, B. E. Molecular Structure and Energetics; VCH Publishers: New York,
1986; Vol. 3, Page 149.
(43) Améduri, B.; Boutevin, B. J. Fluorine Chem. 1999, 100, 97.
the synthetic route is required to attribute the end groups to the
other series (apart from the principal series given by the structural
formula) present in the MALDI mass spectrum.
CONCLUSION
This study shows that MALDI-TOFMS is an efficient analytical
tool for characterizing hydrophobic perfluoropolyethers, provided
an appropriate matrix and cation are employed. In this study, we
have shown that PFBA and PFCA are two fluorinated matrixes
that can be used to characterize perfluoropolyethers. With PFBA,
the oligomers are desorbed as silver adduct ions, whereas with
PFCA, a modification of the terminal groups was observed. At
this point, it should be required that calibrants compatible with
the fluorinated matrixes should be researched in order to interpret
the MALDI mass spectrum without any ambiguity. The potential
of fluorinated organic compounds as efficient matrixes for the
desorption/ionization of fluorinated polymers, which are very
delicate to analyze by MALDI, should be further investigated.
ACKNOWLEDGMENT
Financial support from Atofina (France), CNRS, and Université
Pierre et Marie Curie is kindly acknowledged. We thank Dr. J.
Walker (CRRA, Atofina, Lyon) for encouragement and suggestions.
Received for review August 27, 2002. Accepted December
10, 2002.
AC0260802
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