Electrochemical polymerization of aniline investigated using on-line electrochemistry/electrospray mass spectrometry

Article abstract of DOI:10.1021/ac990527y

A thin-layer electrochemical flow cell coupled on-line with electrospray mass spectrometry (EC/ES-MS) was used to investigate the soluble products from the controlled-potential anodic polymerization of aniline in H₂O and H₂O/CH₃O H (1/1 v/v) with ammonium acetate and acetic acid or ammonium hydroxide as electrolytes (pH 4, 6.5, or 9). At a working electrode (glassy carbon) potential of 1.0 V versus Ag/AgCl, singly protonated aniline oligomers containing as many as 10 aniline units (10-mer) were observed in the ES mass spectra when the polymerization in H₂O/CH₃OH at pH 4 was carried out. The abundance of the higher n-mers decreased at higher solution pH and in 100% H₂O at pH 4. Most of the oligomers were observed in more than one redox state ranging from fully oxidized (all imine nitrogens) to fully reduced (all amine nitrogens). The number of different redox states observed for the n-mers increased with increasing n. The structures of the reduced (m/z 185) and oxidized (m/z 183) aniline dimer ions (head-to-tail, tail-to- tail, or head-to-head) produced from the polymerization of aniline at pH 4, 6.5, and 9 in H₂O/CH₃OH were revealed to vary as a function of pH by comparison of their tandem mass spectrometry product ion spectra with the product ion spectra of the dimer standards. EC/ES-MS potential scan experiments, in which the working electrode current and major n-mer ions for n = 2, 3, and 4 were monitored as a function of electrode potential, were used to probe the growth mechanism to higher aniline oligomers. Under the conditions used, the controlled-current electrolytic process inherent to the operation of the ES ion source did not significantly influence the formation or nature of the oligomers observed. Beyond the current application, the results presented here serve to demonstrate the utility of EC/ES-MS as a tool in identifying the initial products of electropolymerization and in studying the products of electrode reactions in general.

Full text of DOI:10.1021/ac990527y

Anal. Chem. 1999, 71, 4284-4293
Ele c t ro c h e m ic a l P o lym e riza t io n o f An ilin e
In ve s t ig a t e d Us in g On -Lin e Ele c t ro c h e m is t ry/
Ele c t ro s p ra y Ma s s S p e c t ro m e t ry
Ha ite ng De ng a nd Ga ry J . Va n Be rke l*
Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6365
aniline5 has been studied extensively and has found use in
batteries, ion exchange, and corrosion control among other areas.
This polymer is readily prepared electrolytically from aqueous
solution (anodic polymerization onto the electrode surface), is
A thin-layer electrochemical flow cell coupled on-line with
electrospray mass spectrometry (EC/ ES-MS) was used to
investigate the soluble products from the controlled-
potential anodic polymerization of aniline in H
2
O and
H
2
O/ CH OH (1/ 1 v/ v) with ammonium acetate and acetic
3
2
insoluble in H O, has a high conductivity and redox reversibility,
acid or ammonium hydroxide as electrolytes (pH 4 , 6 .5 ,
or 9 ). At a working electrode (glassy carbon) potential of
and is air stable. The base (i.e., neutral) polyaniline is thought to
be of the general structure and formula 1
1
.0 V versus Ag/ AgCl, singly protonated aniline oligomers
containing as many as 1 0 aniline units (1 0 -mer) were
observed in the ES mass spectra when the polymerization
in H
of the higher n -mers decreased at higher solution pH and
in 1 0 0 % H O at pH 4 . Most of the oligomers were
2 3
O/ CH OH at pH 4 was carried out. The abundance
1
2
observed in more than one redox state ranging from fully
oxidized (all imine nitrogens) to fully reduced (all amine
nitrogens). The number of different redox states observed
for the n -mers increased with increasing n . The structures
of the reduced (m / z 1 8 5 ) and oxidized (m / z 1 8 3 ) aniline
dimer ions (head-to-tail, tail-to-tail, or head-to-head)
produced from the polymerization of aniline at pH 4 , 6 .5 ,
which consists of alternating reduced (amine nitrogen) and
oxidized (imine nitrogen) head-to-tail linked aniline dimeric
subunits. The value of Y in each individual unit X may be Y ) 1,
5
the fully reduced form (leucoemeraldine base), Y ) 0.5, the form
that is half-oxidized (emeraldine base), and Y ) 0, the fully
6-8
oxidized form (pernigraniline base). The net oxidation state of
a given polymer has been described as arising from a mixture of
various amounts of these three different base forms.9-11 The value
and 9 in H
2
O/ CH
3
OH were revealed to vary as a function
of pH by comparison of their tandem mass spectrometry
product ion spectra with the product ion spectra of the
dimer standards. EC/ ES-MS potential scan experiments,
in which the working electrode current and major n -mer
ions for n ) 2 , 3 , and 4 were monitored as a function of
electrode potential, were used to probe the growth mech-
anism to higher aniline oligomers. Under the conditions
used, the controlled-current electrolytic process inherent
to the operation of the ES ion source did not significantly
influence the formation or nature of the oligomers ob-
served. Beyond the current application, the results pre-
sented here serve to demonstrate the utility of EC/ ES-
MS as a tool in identifying the initial products of
electropolymerization and in studying the products of
electrode reactions in general.
of X may be as small as that value which renders the polymer
H
2
O insoluble up to a value of ∼1000. For example, approximate
molecular weights of polyaniline fractions soluble in N-methylpyr-
rolidinone have been found using gel permeation chromatography
to range from about 4800 (light fraction) to 200 000-350 000
heavy fraction).2
(
The electrochemical polymerization mechanisms leading to the
insoluble polymeric state of aniline, particularly early in the
polymerization process, have been widely debated and extensively
studied over the last three decades using a variety of electro-
(
(
(
2) Kanatzidis, M. G. Chem. Eng. News 1 9 9 0 , 68 (49, Dec 3), 36-54.
3) Mirkin, C. A.; Ratner, M. A. Annu. Rev. Phys. Chem. 1 9 9 2 , 43, 719-754.
4) Heinze, J. Top. Curr. Chem. 1 9 9 0 , 152, 1-47.
(5) MacDiarmid, A. G. In Conjugated Polymers and Related Materials: The
Interconnection of Chemical and Electronic Structure; Salaneck, W. R.,
Lundstrom, I., Eds.; Oxford Scientific Press: Oxford, U.K., 1993; Chapter
6, pp 73-98.
Conducting polymers are of considerable interest given their
possible application in numerous areas including electronics,
energy storage, and medicine.1 Among such polymers, poly-
(6) Green, A. G.; Woodhead, A. E. J. Chem. Soc. 1 9 1 0 , 97, 2388-2403.
(
(
7) Green, A. G.; Woodhead, A. E. J. Chem. Soc. 1 9 1 2 , 101, 1117-1123.
8) Mohilner, D. M.; Adams, R. N.; Argersinger, W. A., Jr. J. Am. Chem. Soc.
-4
1
9 6 2 , 84, 3618-3622.
(
9) Masters, J. G.; Sun, Y.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1 9 9 1 ,
*
Corresponding author: (phone) 423-574-1922; (fax) 423-576-8559; (e-mail)
41-43, 715-718.
vanberkelgj@ornl.gov.
(10) Asturias, G. E.; MacDiarmid, A. G.; McCall, R. P.; Epstein, A. J. Synth. Met.
1 9 8 9 , 29, E157-E162.
(
1) Handbook of Conducting Polymers; Skotheim, T. A., Ed., Marcel Dekker: New
York, 1986; Vols. 1 and 2.
(11) Bacon, J.; Adams, R. N. J. Am. Chem. Soc. 1 9 6 8 , 90, 6596-6599.
4284 Analytical Chemistry, Vol. 71, No. 19, October 1, 1999
10.1021/ac990527y CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/01/1999

S c h e m e 1
chemical and spectroscopic techniques.8,12-21 The majority of these
studies have focused on the insoluble product deposited on the
electrode, although a few have looked at the soluble dimer species
formed.16,17,20 For illustration, Scheme 1 shows a proposal for the
reactions that take place in the early stages of the electrochemical
polymerization, which is based largely upon the mechanistic work
of Mohilner et al. and Yang and Bard.18
form the trimer (3-mer, 7 and 7 a) or addition of another dimer
molecule to form the tetramer (4-mer, 8 and 8 a). Increasingly
larger n-mers are thought to be formed by oxidation of the lower
n-mers and additions of aniline or another n-mer. Note that
polymer growth in Scheme 1 is shown proceeding through 4 as
this is most often reported as the major dimeric product of aniline
electropolymerization in acidic aqueous solution. However, it has
been shown that the proportion of the three dimers formed
depends on a number of experimental conditions including aniline
concentration and, in particular, pH.11,13
8
In this scheme, it is presumed that all of the aniline oligomers
(
n-mers) are formed via a series of oxidation/ addition reactions
starting with the oxidation of aniline to the radical cation followed
by radical-radical coupling to form the head-to-tail (4 , N-phenyl-
In reference to Scheme 1, it is apparent that direct information
regarding the mass, redox state, and structure of the soluble
products formed at the early stage of aniline polymerization would
provide corroboration of this or other polymer formation mech-
anisms. The combination of electrochemistry on-line with mass
spectrometry22 provides one means to acquire such information.
In fact, electrochemistry coupled with thermospray mass spec-
trometry23 (EC/ TS-MS) has already been applied by Stassen and
1,4-phenyldiamine), tail-to-tail (5 , benzidine or (1,1′-biphenyl)-4,4′-
diamine), and/ or the head-to-head (6 , hydrazobenzene) dimer (2-
mer). The dimers in turn are oxidized (4 a, N-phenyl-1,4-
phenyldiimine, 5a, (1,1′-biphenyl)-4,4′-diimine, and 6a, azobenzene,
respectively) and grow to larger n-mers via addition of aniline to
(
12) Matsuda, Y.; Shono, A.; Iwakura, C.; Ohshiro, Y.; Agawa, T.; Tamura, H.
Bull. Chem. Soc. Jpn. 1 9 7 1 , 44, 2960-2963.
Hambitzer24,25 to the analysis of aniline oxidation in H
(
(
(
(
(
(
(
(
(
13) Desideri, P. G.; Lepri, L.; Heimler, D. Electroanal. Chem. 1971, 32, 225-234.
14) Hand, R. L.; Nelson, R. F. J. Am. Chem. Soc. 1 9 7 4 , 96, 850-860.
15) Stilwell, D. E.; Park, S. M-. J Electrochem. Soc. 1 9 8 8 , 135, 2254-2262.
16) Gao, P.; Gosztola, D.; Weaver, M. J. J. Phys. Chem. 1 9 8 9 , 93, 3753-3760.
17) Shaolin, M.; Donghao, S.Synth. Met. 1 9 9 1 , 41-43, 3085-3088.
18) Yang, H.; Bard, A. J. J. Electroanal. Chem. 1 9 9 2 , 339, 423-449.
19) Mu, S.; Kan, J. Electrochim. Acta 1 9 9 6 , 41, 1593-1599.
20) Johnson, B. J.; Park, S. M. J. Electrochem. Soc. 1 9 9 6 , 143, 1277-1282.
21) Jannakoudakis, A. D.; Jannakoudakis, P. D.; Pagalos, N.; Theodoridou, E.
Electrochim. Acta 1 9 9 3 , 38, 1559-1566.
2
O containing
0
.1 M sulfuric acid. Upon oxidation of aniline they observed in
the mass spectrum, as the singly charged protonated molecules,
(22) Volk, K. J.; Yost, R. A.; Brajter-Toth, A. Anal. Chem. 1 9 9 2 , 64, 21A-33A.
(23) Willoughby, R.; Sheehan, E.; Mitrovich, S. A Global View of LC/ MS; Global
View Publishing: Pittsburgh, PA, 1998.
(24) Hambitzer, G.; Stassen, I. Synth. Met. 1 9 9 3 , 55-57, 1045-1050.
(25) Stassen, I.; Hambitzer, G. J. Electroanal. Chem. 1 9 9 7 , 440, 219-228.
Analytical Chemistry, Vol. 71, No. 19, October 1, 1999 4285

the reduced form of the dimer (m/ z 185) and, at a factor of 10
lower abundance, the reduced form of the trimer (m/ z 276). No
oxidized oligomers, nor oligomers beyond the trimer, were
observed. It was concluded that the higher n-mers, when formed,
were deposited onto the electrode because of their insolubility in
EXPERIMENTAL SECTION
The experimental setup used in this study has been described
in detail elsewhere.30,31 Briefly, a three-electrode, thin-layer
electrochemical cell combining commercially available and in-
house-built parts was coupled on-line with a PE Sciex API165
single-quadrupole or API365 triple-quadrupole instrument (Con-
cord, ON, Canada) using a “Turbo ion spray” ES ion source. The
three electrodes of the cell were a model RE-4 Ag/ AgCl reference
electrode (Bioanalytical Systems, Inc. (BAS), West Lafayette, IN),
an offset, 6.0-mm-diameter glassy carbon (GC) disk in a PEEK
block (BAS), and a Pt foil counter electrode centered in a PEEK
block. A 16-µm-thick Teflon spacing gasket (BAS) between the
working and counter electrode blocks provided a cell volume of
2
H O and, therefore, were not observed in the mass spectra.
Deuterium-exchange experiments indicated that the reduced
dimer ions observed in their experiments were exclusively the
head-to-tail structure 4 . This latter finding is in fact inconsistent
with electrochemical studies, which have found that both the head-
to-tail and tail-to-tail dimers are formed under these same elec-
tropolymerization conditions.11 Furthermore, their observation of
only the reduced dimer and trimer is intriguing as it has been
∼
1.0 µL. The reference electrode was mounted in the counter
demonstrated that the aniline dimers and higher polyaniline
n-mers are significantly easier to oxidize than aniline.8,11
electrode block and contacted the solution between the working
and counter electrodes through a small bore-through to the center
of the cell. A microprocessor-controlled gas displacement pump
One anticipates that it might be of advantage to reexamine
the aniline anodic polymerization process using electrochemistry
on-line with electrospray mass spectrometry (EC/ ES-MS). A
particular attribute of ES-MS26 is the ability to readily produce
abundant gas-phase ions from high-molecular-weight, nonvolatile
and polar analytes, including polymers, that are dissolved in
2
pressurized with N was used to pump a carrier/ electrolyte
solution through a six-port, two-position PEEK injection valve (1.0
mL loop) controlled by the same unit (Microneb 2000, Cetac,
Omaha, NE), through the cell, and on to the ES ion source. The
ES current at the curtain gas plate of the mass spectrometer was
measured by grounding this plate (normally held at 1.0 kV)
through a Keithley model 610C electrometer (Cleveland, OH).
The potential of the ES capillary was lowered by 1.0 kV to maintain
the same potential drop between these electrodes as in the normal
experiments (i.e., 3.2 kV), and the capillary was moved laterally
to a position at which none of the ES plume could travel through
the curtain plate. A PAR model 173 potentiostat with a PAR model
solution. Although 100% H
2
O can be sprayed at flow rates of 30-
5
0 µL/ min (the flow rates necessary for direct on-line coupling
of our EC cell) with pneumatic or heat-assisted ES, signals are
generally much better if a large fraction of the solvent system
(
∼50% (v/ v) or more) is organic (e.g., CH
OH).27 Use of an
3
organic solvent in the oxidation of aniline might provide additional
solubility for the higher n-mers allowing for their detection.
2 3
Therefore, ES-MS, particularly with a mixed H O/ CH OH solvent,
1
75 universal programmer (Princeton Applied Research Corp.,
should have advantage in the detection of higher molecular weight
polyaniline n-mers. One caveat, however, is the fact that the ES
ion source is by nature a controlled-current electrolytic (CCE)
flow cell.28 Therefore, care must be taken to ensure that the CCE
processes that take place in the ES emitter do not also produce
aniline oligomers or otherwise influence the nature of the aniline
oligomers generated in the upstream controlled-potential cell.
In this article, we report on the investigation of the soluble
products generated during the electrochemical polymerization of
Princeton, NJ) was used to step (20 V/ s) or scan among the
various potentials applied to the working electrode during the
course of an experiment. Potential and current readouts from the
potentiostat were directed to the mass spectrometer computer
system, and stored there, by means of a dual-channel PE Nelson
model 970A A/ D interface (Perkin-Elmer, San Jose, CA) and the
PE Sciex sample control software (Version 1.463). This synchro-
nized in time the acquisition of potential, current, and mass
spectral data. The time taken for the products generated in the
cell to reach the mass spectrometer and be detected (i.e., the
response time) was 5.1 s.
2 2 3
aniline in H O and H O/ CH OH (1/ 1 v/ v), at different solution
pH, using a thin-layer electrochemical flow cell coupled on-line
with ES-MS. Soluble polyaniline n-mers up to n ) 10 were
produced and detected and most of these oligomers were
observed in more than one redox state, ranging from fully oxidized
to fully reduced. We show that, under the operational parameters
used in this study, the CCE process in the ES emitter has little
or no influence on the aniline oligomers observed. In addition,
changes in the relative fraction of the electrochemically generated
aniline dimer ions of a given structure, as a function of solution
pH, are revealed by tandem mass spectrometry.29 Also discussed
are potential scanning experiments used to investigate the growth
mechanism of the polymer beyond the dimer.
All samples were prepared in deionized H
Millipore, Bedford, MA) or 1/ 1 (v/ v) H O/ CH
J. T. Baker, Phillipsburg, NJ) containing 5.0 mM ammonium
acetate (NH OAc, 99.999%, Aldrich, Milwaukee, WI, pH 6.5). To
2
O (Milli-RO 12 Plus,
2
3
OH (HPLC grade,
4
these solutions were added 0.75% (v/ v) acetic acid (HOAc, PPB/
Teflon grade, Aldrich, pH 4) or a sufficient quantity of ammonium
hydroxide (NH content 28.0-30.0%, Aldrich) to achieve pH 9.
3
Solution pH was estimated using ColorpHast indicator strips (EM
Science, Gibbstown, NJ). Samples were always injected into a
2 3 4
flowing stream of 1/ 1 (v/ v) H O/ CH OH containing 5.0 mM NH -
OAc and 0.75% (v/ v) HOAc (pH 4). Aniline (2 , 99%, ChemService,
West Chester, PA) was vacuum distilled prior to use. N-Phenyl-
(
(
(
(
26) Electrospray Ionization Mass Spectrometry; Cole, R. B., Ed.; Wiley: New York,
997.
27) Kebarle, P.; Ho, Y. Chapter 1 in Electrospray Ionization Mass Spectrometry;
Cole, R. B., Ed., Wiley: New York, 1997; pp. 3-63.
1
,4-phenyldiamine (4 , 98%, Aldrich), benzidine (5 , ∼95%, Sigma,
1
St. Louis, MO), and hydrazobenzene (6, contains varying amounts
of 6 a, Aldrich) were used as received. Solutions of each were
28) Van Berkel, G. J. In Electrospray Ionization Mass Spectrometry; Cole, R. B.,
Ed.; Wiley: New York, 1997; Chapter 2, pp 65-105.
29) Busch, K. L.; Glish, G. L.; McLuckey, S. A. Mass Spectrometry/ Mass
Spectrometry; VCH: New York, 1988.
(30) Pretty, J. R.; Van Berkel, G. J. Rapid Commun. Mass Spectrom. 1 9 9 8 , 12,
1644-1652.
(31) Deng, H.; Van Berkel, G. J. Electroanalysis, in press.
4286 Analytical Chemistry, Vol. 71, No. 19, October 1, 1999

Fig u re 1 . Full-scan EC/ES-MS mass spectrum of aniline obtained at a working electrode potential of 1.0 V. The spectrum shown is the sum
of six scans from which was subtracted the six scan summed spectrum obtained at 0.6 V (0.1 m/z step size and 10-ms dwell time per step for
each spectrum). At this latter potential, little or no oxidation of the aniline was observed and the subtraction therefore removes from the spectrum
ions of unreacted aniline and background ions unrelated to aniline. Multiplication factors applied to reach near 100% on the relative abundance
scale for the higher n-mers are shown in the spectra. Solvent/electrolyte: 100 µM aniline, 1/1 (v/v) H2O/CH3OH/5.0 mM NH4OAc/0.75% HOAc,
pH 4. Flow rate, 30 µL/min.
prepared immediately prior to analysis to avoid oxidation caused
by exposure of the sample to oxygen and light. Caution: 2 , 4 ,
and so on. The corrected abundance values were then normalized
to the largest peak in each cluster.
5
, and 6 , and their respective oxidation products are highly
toxic and care should be taken in the handling, analysis, and
disposal of these substances.
RESULTS AND DISCUSSION
Soluble Oligomers from Anodic P olymerization of Ani-
line. Off-line cyclic voltammetry (data not shown) and on-line EC/
ES-MS potential sweep experiments (see below) showed that both
aniline oxidation and the observation of polymerization products
in the ES mass spectrum began at a working electrode potential
of ∼0.7 V. A representative mass spectrum obtained from the on-
line EC/ ES-MS oxidation/ polymerization of aniline in 1/ 1 (v/ v)
Cyclic voltammetry experiments were controlled with a CH
Instruments model 660 electrochemical workstation (Cordova,
TN) and a C-2 cell stand (BAS). Experiments were carried out
with a freshly polished 3.0-mm GC disk working electrode (BAS)
in a 10-mL batch cell using a Pt wire counter electrode (BAS)
and Ag/ AgCl reference electrode (model RE-5B, BAS). Cyclic
voltammograms (CVs) were recorded for 80-100 µM solutions
2 3
H O/ CH OH at pH 4, at a constant working electrode potential
of 1.0 V, is shown in Figure 1. Multiplication factors have been
applied to various portions of the spectrum to make visible all
the major oxidation products. The major peaks may be assigned
to a series of aniline oligomers or n-mers, where n is 2-10 aniline
molecules, which are observed as the respective singly charged
of 2 , 4 , 5 , and 6 in 1/ 1 (v/ v) H
2 3
O/ CH OH containing 5.0 mM
NH OAc and 0.75% (v/ v) HOAc at a variety of scan rates.
4
Correction for isotopic overlaps within the individual n-mer ion
clusters in the mass spectrum of electropolymerized aniline was
carried out by first calculating the theoretical isotope pattern
+
32
protonated molecules ((n-mer + H) , see 1 and Scheme 1 for
(
using the Sheffield ChemPuter ) for each n-mer ion in a cluster.
the assumed basic structure of the oligomers). The base peak in
the spectrum is due to the trimer (m/ z 274) with the next most
abundant ions those of the dimer (m/ z 183). Successively lower
abundances of the tetramer and higher n-mers are noted. This is
the first on-line EC/ MS experiment in which discrete soluble
polyaniline n-mers larger than the 3-mer have been detected.
Two of the more minor peaks in the top panel of this spectrum
correspond to the sodiated trimer (m/ z 296) and the sodiated
pentamer (m/ z 476). Ions at m/ z 166 (not shown) and at m/ z 257
It was assumed that all ions observed were protonated molecules
and the different n-mer species in a cluster differed in mass by 2
Da (e.g., m/ z 183 and 185 for the dimer, m/ z 363 and 365 for the
tetramer, etc.). Starting with the lowest m/ z ion in each n-mer
cluster, its contribution to the abundance of each of the other
peaks in the cluster was calculated and subtracted from the
abundance value for those respective peaks in the original data.
These corrected intensities were then used for the calculation of
the contribution of the n-mer species 2 m/ z units higher in the
cluster to the abundance of the remaining peaks in the cluster,
3
are fragment ions (i.e., NH loss), formed by collision-induced
dissociation (CID) of the dimer and trimer ions, respectively, in
the atmospheric sampling interface. Two other minor ion clusters
of significance are observed at m/ z 227.5 and 318. On the basis
(
32) Winter, M. Sheffield ChemPuter. An isotope pattern calculation algorithm
accessible on the Internet at www.shef.ac.uk/ chemistry/ chemputer/ , 1998.
Analytical Chemistry, Vol. 71, No. 19, October 1, 1999 4287

Fig u re 2 . Replotted EC/ES-MS data from Figure 1 showing the abundances for each cluster of aniline n-mer ions corrected for isotopic
overlaps and renormalized to the most abundant peak within each of the respective n-mer clusters. The numbers in parentheses indicate the
calculated redox state of the n-mer (see text for details). Each tick mark on the x-axis represents 1 m/z unit.
of these m/ z values and the fact that the isotopic peak clusters
were not fully resolved, these ions can be assigned to the doubly-
charged pentamer, (5-mer + 2H)2+, and heptamer, (7-mer +
of the fully reduced n-mers is consistent with the results of
previous electrochemical experiments that have found that each
n-mer is generally easier to oxidize than aniline or the preceding
n-mer.8,11 A significant abundance of the fully oxidized n-mer is
observed only for the even-numbered n-mers. The fully oxidized
species is the base peak within the dimer and tetramer clusters,
and is observed at about 46 and 28% relative abundance in the 6-
and 8-mer cluster, respectively. The fully oxidized species was
not observed for the 9- or 10-mer. In reference to the general
formula 1 ascribed to polyaniline above, this observation is
consistent with the capacity to draw a stable, fully conjugated
structure for the fully oxidized species of the even-numbered
n-mers. In addition, the number of different redox states observed
for the n-mers is seen to increase with increasing n. To our
knowledge, this is the first discrete measurement of individual
electrochemically generated polyaniline oligomers in multiple
redox states.
2+
2H) , respectively. No other multiply charged n-mers are readily
apparent in this spectrum. However, multiply charged n-mers from
the even-numbered n-mers, if they occur, fall at m/ z values that
overlap the more abundant singly charged n-mer ions. Therefore,
their presence at abundance levels below that which might distort
the singly charged isotopic clusters (an indication of their
presence) cannot be ruled out.
Assuming a growth mechanism as in Scheme 1 and the
presence of mainly the singly charged protonated molecules of
each n-mer, the cluster of peaks for each n-mer and their m/ z
values reveals that many of the n-mers exist in several redox
5
states. These redox states are defined as the ratio of the number
of amine nitrogens to total (amine plus imine) nitrogens present
in the oligomer (see structure 1 ) and thus range in value from
1
.0 for a fully reduced n-mer (all amine nitrogen) to 0.0 for a fully
When the polymerization was attempted in 1/ 1 (v/ v) H
2
O/
oxidized n-mer (all imine nitrogen). To obtain a more accurate
picture of the distribution of redox states it was necessary to
correct the relative abundances of the peaks within each n-mer
cluster for isotopic overlaps (see Experimental Section). The
corrected data are presented in Figure 2 with the redox state for
the respective major ions at each n-mer listed in parentheses above
the peaks. One notices differences in redox states among the even-
and odd-numbered n-mers and among the lower n-mers and
higher n-mers. Except for the dimer, which shows a 30% relative
abundance of m/ z 185, a significant abundance of the fully reduced
n-mer is not observed for any of the n-mers. The low abundance
CH OH at pH 6.5 and pH 9, or in 100% H O at pH 4 (data not
shown), there was an overall reduction in the abundance of the
higher n-mers with the 7-mer being the largest n-mer observed.
3
2
2 3
At pH 9 in 1/ 1 (v/ v) H O/ CH OH, the dimer, rather than the
trimer, was the base peak in the spectrum. However, abundances
of the individual redox states for those n-mers for which we did
observe ions were similar to that shown in Figure 2.
Several reasons may be put forward to account for the lesser
abundance of the higher n-mers under these conditions. First, at
neutral or basic pH, there is some indication in the literature that
the extent to which aniline polymerizes to higher n-mers is less
4288 Analytical Chemistry, Vol. 71, No. 19, October 1, 1999

than at acidic pH.12 Second, at these higher pH values the solubility
of the high n-mers, if formed, might be less because of their lower
degree of protonation in solution.33 Reduced solubility of the
higher n-mers in 100% H
could also account for the lesser abundance of the higher n-mers
in 100% H O. Finally, the use of a solvent with neutral or basic
pH, or 100% H O as the solvent at any pH, may adversely affect
2
O versus the organic-containing solvent
2
2
ion formation in ES-MS. This relates both to the general need in
ES-MS for preformed ions in solution (promoted for amine/ imines
at acidic pH) and to the generally lower ion signals achieved when
2 2
using 100% H O versus mixed H
O/ organic solvents in ES-MS.27
It is very important to note that the nature of the electrochemi-
cally generated polyaniline spectra we observe here does not
appear to be influenced substantially by oxidative processes in
the metal ES capillary. The operational characteristics of the ES
ion source are known to be analogous to that of a CCE flow cell.28
Therefore, it is possible under some conditions for analytes to be
involved in electrolytic reactions in the ES capillary (the working
electrode of this system), which in positive ion mode, would be
oxidation reactions. The use of a low ES capillary voltage (3.2-kV
voltage drop), which minimizes the ES current (∼0.6 µA), a
stainless steel spray capillary, which is easily corroded and can,
therefore, provide some or all of the Faradaic current the system
requires, and the combination of a relatively high flow rate (30
µL/ min) and high concentration of aniline (100 µM) tend to
minimize the electrolysis efficiency for analyte species in the
solution. A Faraday’s law calculation indicates that a maximum
Fig u re 3 . ES-MS mass spectra showing the molecular ion region
of the three dimer standards (a) 4, (d) 5, and (g) 6, which were
obtained without the EC cell on-line and the spectra of these same
standards obtained with the EC cell on-line at a working electrode
potential of 1.0 V, viz., (b), (e), and (h), respectively. Solvent/
electrolyte: 100 µM 4, 5, or 6, 1/1 (v/v) H2O/CH3OH/5.0 mM NH4-
OAc/0.75% HOAc, pH 4. Flow rate, 30 µL/min. Spectra are the sum
of 10 scans obtained over the shown m/z range using a 0.1 m/z
step size and 10-ms dwell time per step. CVs shown are those
of (c) 4, (f) 5, and (i) 6, obtained in an off-line experiment in the same
solvent system. Arrows indicate the original scan direction. Scan rate,
-
of ∼5% of the aniline present (assuming a total of 2e transferred,
the occurrence of no other reactions, and the availability of all
the analyte for reaction at the metal/ solution interface within the
capillary) could be involved in the electrolysis and go on to form
the reduced dimer. (Note also that this small magnitude of
Faradaic current would little effect solution pH if oxidation of water
was the electrolytic reaction, because of both the high flow rate
34
4
and the buffering effect of the NH OH/ HOAc electrolyte. ) The
1
0 mV/s.
efficiency for the multielectron reactions required to produce the
oxidized dimer and the higher n-mers would be expected to be
even less. Examination of the ES mass spectrum of freshly distilled
aniline acquired under the same flow conditions as those used to
acquire the data in Figure 1, without the cell on-line, found that
ions corresponding to the fully oxidized dimer and trimer were
present. However, their absolute abundances were at least 30
times less than those abundances recorded for the same ions (m/ z
Structure of Dimers. The electrochemically generated poly-
aniline dimer ions were observed in two redox states, viz., fully
reduced at m/ z 185 and fully oxidized at m/ z 183 (Figure 2). Using
our EC/ MS setup and tandem mass spectrometry (MS/ MS)29 it
was possible to qualitatively determine whether different dimer
structures were formed from the anodic polymerization of aniline
183 and 274, respectively) in the spectrum in Figure 1. No higher
2 3
at pH 4.0, 6.5, and 9.0 in 1/ 1 (v/ v) H O/ CH OH. For this
n-mers were observed. To increase the abundance of these dimer
and trimer ions by just a factor of 2 required that the flow rate be
dropped to 10 µL/ min. Further proof that the ES electrolytic
process did not substantially affect the nature of the ions observed
is illustrated by the fact that standards of 4 , 5 , and 6 , when
sprayed without the EC cell on-line, were not oxidized to any
significant extent. However, near-quantitative conversion of 4 , 5 ,
and 6 to oxidation products was observed with the EC cell on-
line at a working electrode potential of 1.0 V (see Figure 3 and
related discussion below).
determination, we needed standards of all the possible reduced
and oxidized dimers. We assumed that the dimer ions observed
in the spectrum in Figure 1 at m/ z 185 were the protonated
molecules of 4 , 5 , and/ or 6 and that the dimer ions at m/ z 183
were the protonated molecules of 4 a, 5 a, and/ or 6 a. Standards
of 4 , 5 , and 6 were commercially available. Standards of 4 a, 5 a
and 6 a were prepared by on-line electrochemical oxidation of 4 ,
5
, and 6 , respectively.
Panels a, d, and g of Figure 3 show the ES mass spectrum of
standards 4 , 5 , and 6 , respectively, recorded without the thin-
layer EC cell on-line. Under these conditions, the most abundant
ions observed correspond to (4 + H) , (5 + H) , and (6 + H)
each appearing at m/ z 185. Only in the case of 6 was a substantial
signal owing to the oxidized form of the standard dimer observed
(
(
33) Ray, A.; Richter, A. F.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1 9 8 9 ,
9, E151-E156.
34) Van Berkel, G. J.; Zhou, F.; Aronson, J. T. Int. J. Mass Spectrom Ion Processes
9 9 7 , 162, 55-67.
+
+
+
2
1
Analytical Chemistry, Vol. 71, No. 19, October 1, 1999 4289

(
i.e., 6 a at m/ z 183, ∼35% relative abundance). As the CVs in
Figure 3 indicate, 6 is also the easiest to oxidize of the three
reduced dimers (Figure 3i, E ) 0.12 V). The source of 6 contains
p
an unspecified amount of 6 a, and when “aged” on the bench for
even short periods (a few hours), the ES mass spectrum of the
standard solution of 6 shows an increased abundance of 6a, which
is formed by exposure of the sample to oxygen and light. We
believe, therefore, that most of the signal owing to (6 a + H)+ in
Figure 3g arises from 6 a in the original solution. The amount of
6
a formed via oxidation in the ES capillary must be small for the
same reasons as already discussed above. The ES mass spectra
in Figure 3b and h show that with the EC cell on-line, at a working
electrode potential of 1.0 V, it was possible to anodically generate
4
a nearly quantitatively from 4 (m/ z 183, Figure 3b; E
Figure 3c) and to generate 6 a quantitatively from 6 (m/ z 183,
Figure 3h; E ) 0.12 V, Figure 3i). Surprisingly, generating a
p
) 0.31 V,
p
significant amount of the ion at m/ z 183 corresponding to (5 a +
+
H) via oxidation of 5 at 1.0 V proved much more difficult. Rather,
the major oxidation product was m/ z 184, presumably the radical
cation (5 )•+. A dimer of 5 (nominally a protonated aniline
tetramer) at m/ z 365 was also observed at a much lower
abundance (Figure 3e and inset; E
Nonetheless, enough (5 a + H)+ was produced to obtain an MS/
MS spectrum.
p
) 0.57 V, Figure 3f).
Figure 4 presents the EC/ ES-MS/ MS product ion spectra of
the ions at m/ z 183, corresponding to the oxidized dimeric species
generated from the anodic polymerization of aniline at pH 4.0,
Fig u re 4 . Product ion tandem mass spectra of m/z 183 formed
during the electrochemical polymerization of aniline (2) at pH (a) 4,
(b) 6.5, and (c) 9, and the product ion tandem mass spectra of m/z
6
.5, and 9.0, and the product ion spectra of the standard dimer
1
83 from (d) the protonated molecule of N-phenyl-1,4-phenylenedi-
+
+
+
ions (4 a + H) , (5 a + H) , and (6 a + H) , produced by on-line
oxidation of the respective reduced standards. Inspection of the
product ion spectra of each of the oxidized standards (Figure 4d-
f) reveals each to be distinct, and this fact can be used to
qualitatively determine whether particular dimeric species are
present in the polyaniline spectra. Simple visual inspection reveals
that the MS/ MS spectrum of the ions at m/ z 183 from aniline
oxidation at pH 4.0 is almost an exact match for the MS/ MS
+
imine ((4a + H) ), (e) the protonated molecule of (1,1′-biphenyl)-4,4′-
diimine) ((5a + H) , and (f) the protonated molecule of azobenzene
+
+
((6a + H) , Solvent/electrolyte: 100 µM 2, 4, 5, or 6, 1/1 (v/v) H2O/
CH3OH/5.0 mM NH4OAc. HOAc added for pH 4, NH4OH added for
pH 9; see Experimental Section. Flow rate, 30 µL/min. Center-of-
mass collision energy, ECM ) 3.3 eV with a N2 collision gas thickness
15
-2
of ∼1.8 × 10 cm . Oxidation of aniline and standard dimers 4, 5,
and 6 to 4a, 5a, and 6a, respectively, performed on-line at 1.0 V.
Spectra are the sum of 10 scans obtained over the shown m/z range
using a 0.1 m/z step size and 10-ms dwell time per step.
+
spectrum of (4 a + H) . Thus, the structure of these oxidized
dimer species is concluded to be exclusively the oxidized head-
to-tail dimer 4 a. At pH 6.5 and pH 9, the appearance of ions at
m/ z 77 and 95 in the product ion spectra indicates that ions
corresponding to the head-to-head dimeric structure are present.
Because the peak at m/ z 156 (a feature characteristic of (5 a +
Furthermore, inspection of the product ion spectrum of the ions
at m/ z 185 from aniline oxidation at pH 4.0 reveals, in contrast to
the dimeric species at m/ z 183, that a mixture of dimer structures
is present at m/ z 185. A simple addition of the normalized product
ion spectra of (4 + H)+ and (5 + H)+ and/ or (6 + H)+ would
+
H) , Figure 4e) shows no increase in abundance as pH is raised
(
Figure 4b and c), one can conclude that little if any of the tail-
generate much the same spectrum as that in Figure 5a. The
to-tail oxidized dimer 5 a is present. This is not unexpected. As
already discussed, 5 proved difficult to anodically oxidize to 5 a,
preferring to form the radical cation (m/ z 184) and to dimerize
prominence of the minor ions at m/ z 141, 151, and 158 (which
are a feature characteristic of (5 + H) , Figure 5e), however, leads
+
one to believe that the dimer ions at pH 4 are in fact a mixture of
(
m/ z 365). Thus, at pH 6.5 and 9, the oxidized dimer ions are a
(4 + H)
+
and (5 + H) . A significant presence of the reduced
+
mixture (6 a + H)+ and (4 a + H)+.
dimer structure 5 would be consistent with the fact that it is the
most difficult of the dimers to oxidize (Figure 3f). As the pH was
increased, there was an increase in the abundance of the ion at
m/ z 93 in the reduced dimer product ion spectra (Figure 5b and
c). Both that trend and the diminishing abundance of the minor
ions at m/ z 141, 151, and 158, are consistent with an increase in
Figure 5 presents the EC/ ES-MS/ MS product ion spectra of
m/ z 185 ions corresponding to the reduced dimeric species
generated from the anodic polymerization of aniline at pH 4.0,
6
.5, and 9.0, and the product ion spectra of the dimer ions (4 +
+
+
+
H) , (5 + H) , and (6 + H) . In this case, the product ion spectra
of each of the reduced dimer standards (Figure 5d-f) are not as
distinctive. The product ion spectra of (5 + H) and (6 + H)
+
the relative abundance of (6 + H) and a decrease in the
+
+
+
abundance of (5 + H) . Thus, we believe that at pH 9, the reduced
are quite similar, making it difficult to determine whether one or
dimer ions observed in the polyaniline spectrum are a mixture of
(4 + H)+ and (6 + H)+.
both of these dimers might be present in the polyaniline spectrum.
4290 Analytical Chemistry, Vol. 71, No. 19, October 1, 1999

Fig u re 5 . Product ion tandem mass spectra of m/z 185 formed
during the electrochemical polymerization of aniline (2) at pH (a) 4,
(
1
b) 6.5, and (c) 9, and the product ion tandem mass spectra of m/z
Fig u re 6 . Time-synchronized working electrode anodic current and
85 from (d) the protonated molecule of N-phenyl-1,4-phenylenedi-
+
+
selected ion current profiles for the protonated molecules of aniline
amine ((4 + H) , (e) the protonated molecule of benzidine ((5 + H) ,
and (f) the protonated molecule of hydrazobenzene ((6 + H) .
+
+
((2 + H) , m/z 94) and the dimer (2-mer, m/z 183 and 185), trimer
(3-mer, m/z 274), tetramer (4-mer, m/z 363 and 365), and pentamer
Solvent/electrolyte: 100 µM 2, 4, 5 or 6, 1/1 (v/v) H2O/CH3OH/5.0
mM NH4OAc. HOAc added for pH 4, NH4OH added for pH 9; see
Experimental Section. Flow rate, 30 µL/min. Center-of-mass collision
energy, ECM ) 3.3 eV with a N2 collision gas thickness of ∼1.8 ×
(5-mer, m/z 454 and 456) species formed during the EC/ES-MS
experiment as a function of applied potential. Each ion current profile
normalized relative to maximum intensity of that respective ion.
Freshly polished electrode. Potential scan rate, 2.0 mV/s. Solvent/
electrolyte: 100 µM aniline, 1/1 (v/v) H2O/CH3OH/5.0 mM NH4OAc/
15
-2
1
0
cm . Oxidation of aniline performed on-line at 1.0 V. Spectra
are the sum of 10 scans obtained over the shown m/z range using a
.1 m/z step size and 10-ms dwell time per step.
0.75% HOAc, pH 4. Flow rate, 30 µL/min. Multiple ion monitoring
0
mode, dwell time, 5.0 ms per m/z.
Ta b le 1 . Dis t rib u t io n o f Dim e r S t ru c t u re s P ro d u c e d b y
An ilin e P o lym e riza t io n
electrode potential was scanned at 2 mV/ s from 0.6 to 1.2 V and
back again while the working electrode current and the ions
corresponding to protonated aniline (m/ z 94), the major dimer
(m/ z 183, 185), trimer (m/ z 274), tetramer (m/ z 363 and 365),
and pentamer (m/ z 454 and 456) species were monitored. The
first of these experiments, the data from which are displayed in
Figure 6, was carried out with freshly polished and conditioned
electrodes.30 The anodic current was observed to increase above
background levels at 0.7 V and reached a maximum at 1.2 V, the
turnaround point in the potential scan. Note that the current profile
in the return potential sweep was not symmetric with the initial
potential sweep, indicating a change in the electrode surface as
expected for the deposition of polyaniline. For the most part,
however, the ion current profiles were symmetric. In the initial
potential sweep to 1.2 V, the increase in current measured at the
working electrode above baseline was mirrored by the appearance
in the ES mass spectrum of the oxidized dimer, reduced dimer,
mixed redox state trimer, reduced tetramer, and reduced pen-
tamer. Only the oxidized tetramer and oxidized pentamer were
first observed at higher potentials. Thus, one can conclude that
the higher order oxidized species follow formation of the higher
pHa
dimer
4
9
m/ z 183 (∼75% of total dimers)
m/ z 185 (∼25% of total dimers)
4 a
+
-
-
+
+
-
+
-
+
+
-
+
5
6
4
5
6
a
a
a
+, detected; -, not detected.
The structures of the electrochemically generated aniline
dimers formed at pH 4 and 9 are summarized in Table 1.
Determining the exact amount of each dimer present in all these
cases is difficult with the current analytical scenario. Combining
an on-line separation of the electrochemically generated dimers
with the ES-MS/ MS analysis for structure determination would
provide a more definitive measurement of the distribution of
structures.22
P olymer Growth Mechanism beyond the Dimer. Two EC/
ES-MS experiments were carried out in which the working
Analytical Chemistry, Vol. 71, No. 19, October 1, 1999 4291

order reduced species. Though these data are not definitive proof
of the polymer growth mechanism, they are consistent with the
reaction mechanism in Scheme 1. Because each n-mer is easier
to oxidize than aniline,8,11 one would expect to observe the dimer
and higher n-mers at or very near the same potential at which
the polymerization is initiated by aniline oxidation (i.e., ∼0.7 V).
Curious is the fact that, except for the trimer (m/ z 274) and
reduced pentamer (m/ z 456), n-mer ion intensities level off beyond
study are expected to provide further insight into the growth
mechanism of the aniline n-mers beyond the dimer stage.
The current study revealed the structure of the dimeric ions
produced from the polymerization of aniline at pH 4 and 9 as
summarized in Table 1. In the ES mass spectrum of oxidized
aniline, at all pH values, ∼75% of the total dimer ion current was
from the oxidized dimers and the remaining 25% of the ion current
that of reduced dimers. At pH 4, the oxidized species present were
found to be exclusively structure 4 a, the head-to-tail dimer, while
the reduced species were mixture of structures 4 and 5 (tail-to-
tail dimer). At pH 9, we found that the oxidized dimers were a
mixture of 4a and the head-to-head structure 6a while the reduced
dimers were a mixture of structures 4 and 6 . Classical electro-
chemical studies have previously shown that the distribution of
∼
0.85 V and then decrease as the potential is swept back to less
positive values. The intensities of m/ z 274 and 456 begin to
decrease beyond 0.85 V and then start to increase as the potential
is swept to less positive values, again maximizing at ∼0.85 V and
continuing to decrease with potential. This behavior may indicate
that these two species are consumed in further reactions to form
higher oligomers at potentials more positive than 0.85 V. On the
other hand, this decrease also coincides with the depletion of
aniline. Assuming these odd-numbered n-mers are formed from
the preceding oxidized even-numbered n-mer by aniline addition
different dimeric species generated from aniline oxidation changes
with pH.11,13 Specifically, in acidic solution, down to pH 4, formation
of 4 and 4 a is favored. Below pH 4, formation of 5 and 5 a
increases at the expense of 4 and 4 a. Under basic conditions,
(
e.g., formation of 7 and 7 a in Scheme 1), significant depletion
formation of the head-to-head dimers 6 and 6 a occurs at the
of aniline would reduce the amount of these species formed.
Another potential sweep experiment (data not shown) was
carried out 2 h and several additional potential scanning experi-
ments after the data in Figure 6 were acquired. Thus, it could be
assumed that polyaniline deposits covered at least some of the
working electrode surface. In this case, the working electrode
current and ion current profiles were much different from the data
in Figure 6. Rather than reaching a steady-state level or bimodal
maximum, the ion current profiles all showed a shallow increase
to a maximum at 1.2 V and a return to baseline level of the same
shape with the return scan to 0.6 V. The reduction in aniline
intensity again mirrored the working electrode current, but both
were significantly less than before. While the appearance of the
dimer and trimer ions still began at ∼0.7 V, the occurrence of
the pentamer ions in particular was shunted to significantly higher
potentials (∼0.85 V). It was apparent from these data that initial
formation of the lower n-mers was best studied using a “fresh”
electrode surface. The change in the electrode surface reaction
due to the deposition of polyaniline, as well as the contribution to
solution of n-mers already deposited onto the electrode, are just
two factors that complicated interpretation of the data obtained
with a polyaniline coated electrode.
expense of 4 and 4a.16 The EC/ ES-MS/ MS results presented here
are consistent with those earlier studies, with one possible
exception. No significant amount of (5 a + H)+ was found under
any conditions. However, this latter result was consistent with our
findings that 5 was preferentially oxidized to form 5 •+ rather than
5
a.
The fact that the head-to-tail, tail-to-tail, and head-to-head aniline
dimers were shown to be present in the ES mass spectrum re-
sulting from aniline oxidation indicates that the higher polyaniline
n-mers (depending on solution pH and assuming their possible
growth from all dimers) may be a mixture of linkage sequences,
not just the head-to-tail sequence shown in structure 1 and
Scheme 1. These conclusions are in line with the data of Bacon
and Adams11 and the reaction sequence proposed by Yang and
Bard.18 One must be aware, however, that the distribution of dimer
structures we detected may reflect both their relative rates of
production from aniline and their consumption leading to the growth
of larger n-mers. That is, the distribution we observe may be
skewed toward the species less likely to continue polymer growth.
The actual contribution of the individual dimers to the larger
n-mers is currently under study.
At this point it should be noted that the EC/ ES-MS spectrum
in Figure 1 is distinctly different from the EC/ TS-MS spectrum
obtained by Hambitzer and Stassen.24,25 Hambitzer and Stassen24,25
observed only the protonated molecules of the fully reduced dimer
CONCLUSIONS
The use of a thin-layer electrochemical flow cell on-line with
electrospray mass spectrometry provided important information
regarding the soluble products from the electrochemical polym-
erization of aniline. In contrast to the electrochemical or optical
spectroscopic approaches that have been applied to this issue,
the EC/ ES-MS approach provided directly, for the first time, both
the molecular weight and redox state of not only the soluble
dimeric species formed but also aniline oligomers ranging up to
n ) 10. Distinct differences in the redox-state distribution of the
smaller n-mers were apparent between even- and odd-numbered
n-mers. The number of different redox states observed for the
various n-mers increased with increasing n. Structural evaluation
of the aniline n-mers observed, using tandem mass spectrometry,
was limited to the dimers in this study. One could probe the
structure of the higher n-mers by a similar approach, and such
an EC/ ES-MS/ MS investigation is underway. The results of that
(
m/ z 185) and trimer (m/ z 276) of aniline whereas we observed
aniline oligomers up to the 10-mer and a distribution of oligomer
redox states. Several explanations for our observation of larger
n-mers are possible, but a major factor is probably the use of ES-
MS. ES-MS usually provides excellent signals for high molecular
weight, polar analytes, which probably benefits the detection of
the higher aniline n-mers.27 The work of Hambitzer and Stassen24,25
was carried out using H
was obtained in a mixed H
less acidic conditions (5.0 mM NH
2
O/ 0.1 M sulfuric acid while our spectrum
O/ CH OH solvent (1/ 1 v/ v) under
OAc/ 0.75% (v/ v) HOAc, pH
2
3
4
4). The higher n-mers would be expected to be more soluble in
the organic solvent system, and thus, it is possible that higher
n-mers produced at the electrode in our experiment remain in
solution rather than deposit at the electrode. However, we
4292 Analytical Chemistry, Vol. 71, No. 19, October 1, 1999

obtained a spectrum very similar to that shown in Figure 1 using
the same electrolyte and 100% H O. The major difference in 100%
O was an overall reduction in the abundance of the higher
demonstrate that EC/ ES-MS and EC/ ES-MS/ MS should prove
generally to be a highly informative tool in the study of the initial
stages of electropolymerization processes and in the study of the
products of any number of other electrode reactions.
2
H
2
n-mers with the 7-mer the largest n-mer observed. Flow dynamics
and the electrolytic characteristics of the electrochemical cell,
ACKNOWLEDGMENT
(
e.g., electrolysis efficiency and response time) might also influ-
H.D. acknowledges support through an appointment to the Oak
ence which oligomers are formed and remain in solution. For
example, the linear velocity in our thin-layer cell may be consider-
able because of the narrow spacing of electrodes (16 µm, ∼1 µL
volume) and high flow rate (30 µL/ min), which may develop
enough shear force at the electrode surface to somewhat inhibit
oligomer deposition. Flow dynamics and the electrolytic charac-
teristics of the electrochemical cell would also certainly affect the
redox state of the different oligomers. As has been shown, our
observation of polyaniline n-mers up to n ) 10 and the distribution
of n-mer redox states was not caused or influenced by electrolytic
processes in the ES ion source.
Ridge National Laboratory (ORNL) Postdoctoral Research As-
sociates Program administered jointly by the Oak Ridge Institute
for Science and Education and ORNL. ES-MS instrumentation was
provided through a Cooperative Research and Development
Agreement with Perkin-Elmer Sciex Instruments (CRADA No.
ORNL96-0458). This work was supported by the Division of
Chemical Sciences, Office of Basic Energy Sciences, United States
Department of Energy under Contract DE-AC05-96OR22464 with
ORNL. ORNL is managed by Lockheed Martin Energy Research
Corp.
In any event, the results from the EC/ ES-MS study presented
here, in addition to providing new information on anodic aniline
polymerization, are consistent with previous electrochemical
studies of anodic aniline polymerization.8,11,18 These results also
Received for review May 18, 1999. Accepted July 31,
1999.
AC990527Y
Analytical Chemistry, Vol. 71, No. 19, October 1, 1999 4293