ELSEVIER
Bioelectrochemistry and Bioenergetics 42 (1997) 11 I-116
Sonoelectrochemical studies of guanine and guanosine l
Ana Mafia Oliveira Brett ', Frank-Michael Matysik
Deparramentode Qu[mica,Unirersi&ldede Coirabra.3049Coimbra,Portugal
Abstract
The voltammetric behaviour of the purine base guanine and the corresponding n,.,c!eosideguanosine at a glaxgycarbon ek~rode was
investigated with the help of ultrasound. The adsorption of guanine and guanosiae as well as the adsorption of tJieir ox~Kion pf~:gh~
affects the overall voltammetric characteristics dramatically. In particular, the effect of ultrasound on the simultaneous adsoq~aa of
guanine and guanosine was studied in detail. Quantitative sonovdtammetric experiments show the number of electrons invdved ia the
electro-oxidation of guanosine to be two. Sonovoltammetry, i.e. the combination of voltammetry arid ultrasonic irrad~t/on, is
demonstrated to be a useful approach to control the extent of adsorption of tie relevant species and to avoid electrode fouling. T~s alkrws
the development of reliable analytical pr~edures for the determination of guanine and guanosine, which are described in deudL © 1997
Elsevier Science S.A.
Keywords: Adsorption: Guanine; Guar, J.sine; Ultrasound; Voltamn~tty; Sonovoltammeu'y
1. Introduction
Purine derivatives have great significance in various
biological processes. For example, the nucleotides of adenine and guanine can be regained as monomer units of the
nucleic acids. In order to gain deeper understanding of
complex biological systems it is important to investigate
the building blocks in detail.
The electrochemical hehaviour of purine derivatives has
been addressed in several previous reports. It was found
that many of them can be reduced at mercury electrodes
[i-3] and oxidized at carbon-based electrodes [4,5]. The
present paper deals specifically with the voltammetric
properties of guanine and the corresponding nucleoside
guanosine which have the following chemical structures:
0
Guanine
0
Guanostne
The electrochemical oxidation of guanine has been
reported to proceed as a two-step mechanism with 8-
' Corresponding author.
J PJ~sented at the 13th International Symposium on Bioelectrochemistry and Bioenergetics, Ein Gedi, Israel, 7-12 January 1996,
0302-4598/97/$17.00 © 1997 Elsevier Science S.A, All rights reserved.
PII S0302-4598(96)05123-9
oxyguanine as intermediate and involves the t o ~ loss of
four electrons and four protons [6]. However, ne~ much is
known about the reaction mechanism of the electrochemical oxidation of guanosine [7]. Regarding the electroanalytical determination of guanine and guanosine, there is still
some controversy in the IReramre as to whether or not
adsoq~ion plays a role in the a~lytical proced~. D~hurst found that adsorption has an er~'n'~es influence on
the determination of guanine and guanosine when using a
pyrolytic graphite electrode [7]. Similar behaviour was
found for glassy carbon electrodes [8]. ha contrast to this.
Gilmmin and Hart [9] reported guanine determinations at
carbon paste and glassy carbon electrodes wher~ no effect
of adsorption was observed.
The present work is concerned with mechanistic
analytical investigations of guanine and guanosine with L~
help of ultrasonic irradiation. Ulaasound-assisted elect~
chemistry is attracting growing interest of various research
groups [10-13]. In addition to fundamental studies [14],
some analytical applications have also been reported [15].
In a recent paper [16] we described a small-vohme sonovolt~mmewic cell which is well suited for analytical sonovoltama~try. From the analytical point of view there are
two important main effects of ultrasonic irradiation on
voltammetric experiments. Firstly, the mass m~aSlX~t is
dramatically enhanced and secondly the electroce surface
can be activated in site, acting as a strategy aga/nst
electrode fouling. Both effects were utilized in the present
112
A.M. OliceiraBrett. F.-M.Ma~.'sil~/ Bioelectrocheraist~'aM 8iaenergetics42 (I997) 1Ii-16
study to investigate the voltammetric characteristics of
guanine and guanosine and to develop reliable procedures
for their determination.
2. Experimental
2.1. Apparatus and equipment
The cell configuration used for the sonovoltammetric
experiments is illustrated in Fig. I. The jacketed cell was
thermostatted by circulating water from a constant temperature bath (25 °C). The volume of the cell electrolyte was
always 20 ml. A platinum coil was used as counter electrade and a laboratory-made silverlsilvcr chloridej3 M KCI
electrode served as reference elec~ode. The glassy carbon
working electrode (d = 6 mm) was positioned so as to face
the tip of the sonic horn. The horn was connected with a
tapered microfip (d = 3 ram) which was fabricated from
high grade titanium alloy. The ultrasonic processor was a
model VCSOI (Sonics and Materials Inc., USA) capable of
delivering up to 500 W at 20 kHz frequency. The ultrasonic processor is designed to deliver constant amplitude
which can be selected via the amplitude control setting
ranging from '0' to '100"; however, in conjunction with
the microtip the amplitude control setting must not be
higher than '40'. The actual power intensity entering the
system was calibrated caloriraetrically according to the
procedure of Mason et aL [17]. For relevant amplitude
control settings of '10', '!5' and '20' the corresponding
power intensities were 16 + 3 Wcm-:, 30 + 3 Wcm -2
and 72 + 5 Wcm -2 respectively. The sonovoitammetric
cell and the sonic horn were housed in a sound proofed
cage in order to protect the operator from high-intensity
acoustic noise.
All voitammetric experiments were done using an Autolab PGSTATI0 potentiostat (Eco Chemic, Utrecht, Netherlands) equipped with an ECD low currer~t module. The
current signal was filtered through a third-order Sallen-Key
filter with a ti;o.c constant of 0.1 s in order to remove high
frequency a.c. components.
The glassy carbon electrode (GCE) was prepared for
measurement by polishing using plastic foils (Hirschmann,
Germany) with adherent alumina of decreasi;~g particle
size ranging from 9 Ixm to 0.3 p,m, followed by thorough
rinsing with Milli-Q wa~,er. Prior to recording voltammograms of electroactivc species, several cyclic voltammograms were recorded in the background solution until a
stable vohammetric response was obtained.
2.2. Chemicals and solutions
Guanine and guanosine were obtained from Sigma
Chemical Co. and were used as received. The complex
K4[W(CN)s]. 2H~O was prepared according to the literature [18]. All solutions were made up using high-purity
water from a Millipore Milli-Q system (resistivity greater
than or equal to 18 MII cm) and analytical reagent grade
chemicals,
An acetate buffer containing 0.1 M sodium acetate +
acetic acid with a pH of 4,50 and a 0.1 M phosphate buffer
(pH 7.00) served as supporting electrolytes. Solutions of
the purine derivatives were prepared directly in the buffer
solutions except in the case of guanine. Stock solutions of
guanine of I mM were made either in O.i M NaOH or in
0.1 M HCIO4. Working solutions of guanine were prepared by adding small volumes of stock solution to the
acetate buffer solutions. The solutions were then sonicated
to ensure homogenization.
3. Results and discussion
3.1. Voltammetric and sonovoltammetric characterizatioa
of guanine
h
i
Fig. 1. Sooovolhammetriccell:(a) sonichornwithmic~ip; (b) AgIAgCI
(3 M KCI)referenceelectrode:(c) platinumcoil counterelectrode:(d)
coolantoutlet;(e) coolantinlet;(O cavitationalplume;(g) glassycarbon
surface;(h) O-ringseal,(i) workingelectrodelead.
One problem of studying the electrochemical behaviour
of guanine is its low solubility at pH values where it is a
neutral molecule. Dryhurst [7] reported a concentration of
about 5 × 10-4 M for saturated guanine solutions in the
pH range between 4 and 7. As will be specified later, our
results indicate an even lower concentration for saturated
solutions of guanine in acetate buffer. However, owing to
the protolytic properties of guanine, in acids and bases it is
possible to dissolve appreciable amounts of guanine because it is transformed into an ionic form (pK~ = 3.0,
pg 2 = 9.3, pK 3 = 12.6 [19]). This was utilized for preparing ! mM guanine stock solutions of accurately known
concentration.
The adsorption properties of guanine can be deduced
from cyclic voltammograms recorded in the presence and
A.M. OliveiraBreu.F.-M.Matysik/ BiodearochemistryandBioenergetics42 (1997)Ii I-116
~
gA
....
i
o.,
.
l
o~
,
o'~
,
1D
2a
;.o
,
~.
EIV
Fig.2. Singlesweep voltammograms(backgroundsubca~) of 2X I0-5
M guanine afterI0 s conditioningat startingpotential,0.I M aceta~
buffer,pH 4.66 in the absence(curvesI) and in lhe presence(craves2)
.-----
of ultrasound:(a) firstvoltammognzmand(b) subsequentv o ~
Ultrasoundconditions:8 mmhorntip-electrodeseparation;powerintensity 72 Wcm-2. Scanrate 50 mVs-z .
02 0.4 0.6 08 1.0
12
~.4
E/V
Fig.3. Cyclics o ~ o ~
(backO'ee~ ud~,c~) of 2.4x I0-s
M geame in acetatetufter(0.IM. pX 4.50).~
cmdi6ecs: S
nun h~n tip-electnxle~ ;
Ix,~ imnsity30 W m -2. St'n n ~
absence of ultrasound. In the absence of ultrasound the
cur:~,~nt response decrea,vcs progressively when recording
successive voltammofran~.~(see Fig. 2 traces la and lb).
This is probably fo'r ,wo reasons. There is a d ~ v e
accumulation of guanine at the electrode surface which
leads to a higher signal in the first scan, and the l ~ u c t s
of oxidation remain partially adsorbed resulting in some
surface blockage. The adsorption of guanine was further
studied through transfer experiments where the C.,CE was
left for a certain time at open circuit potential in an acetate
buffer solution containing 2 X 10-s M guanine and was
then transferred to a pure buffer solution after careful
cleaning with a jet of deionized water. In the first scan a
signal corresponding to guanine was obtained, the s~x~ of
which was clearly dependent on the accumulation time.
This behaviour was qualitatively the same either in pH 4.5
acetate t~,"nH 7.4 phosphate buffer solution.
In the presence of ultrasound, the voitammetric response was independent of previously n ~ k d
scans as
illustrated in Fig. 2, traces 2a and 2b. Reproducible measurements could be taken without the effects of electrode
fouling. However, it seems that, even with sonication,
adsorption has some effect on the v o l ~ g ~ r i c response
as demonstrat~l by cyclic sonovoltanunognms of guanine
which have the peak-shaped response of the forward scan
illustrated in the cyclic sonovoltaJmnogam shown in Fig.
3. These show a slight difference between the half-wave
potentials of the forward and reverse scans respectively.
Under these conditions the wave height of the reverse scan
was used for quantitative determinations because in that
case pure mass transport control caa be assumed.
The surface state of the glassy carbon elecuode resulting from potential scans between 0.1 V and 1.4 V Womotes guanine a d s ~ o n in comparison with ~e potential
range scanned for the results in Fig. 2 (0.5-1.1 V). I~
should be added that various experiments without sonication were conducted in order to find alternative possibili-
50 mVs-I.
ties for activating the elecuede ,surface between successive
measurements, lbe~e attempts included t ~ application of
sevor',d conditioning puen~a]s prior to n~onliag
sweep, differential pulse or squm wave v o ~
However, no pmceduze gave results as good as those
obtained by performing sonovoltammet~ ngmurengn~
3.Z Voham~aic and ~ : ; o l a ~ e u ~ c
of guanosine
clmm~rim~
The solubility of guanosine is much better Oaa that of
guanine wl~:h allows studies a~ a ~ m m ~ m s up te ~e
millimole range. Fig. 4 shows successive cyclic voltamuograins of guanostne in the absence of ultzaseeed. Sin,~hr
to the behav~our ob~ined for guanine, there is a progressive decrease in the ¢ment response for repetitive scaus.
In contrast to this, repetitive cyclic sonovo~tanmognns
show no signal degndation as illusua~ in Fig. 5. F r m
/~ f / V O)
0.0
02
0.4
0.6
0.8
1.0
12
1,4
1.6
E/V
Fig. 4. C~lic vu~anunosra=sof !mM guenesieeie acetate.~'Tgr(O.l
M. pX ~_50)z ~ Scans(1)-(4) ~
the 1~. : ~ ~ ~1
cyclicv o ~
respectively:scanrate 50 mVs"~.
I 14
A.M. Olil'eira Brett. F. -M. Matysik / BioelectrochemistO" and t]iaenergetics 42 (1997) I 1i - 16
f
~
lCO~,A
(c)
//~(b)
F /] ~
r(,)
(a)
0.0
0.3
0.6
0.9
1.2
o'o'o:io',-o'~,o'~io ,'i;,
o:o-o'io',-o',,-o'a;o-,:f ;4
E/V
E/V
1.5
E/V
Hg. 5. Cyclicsonovoltammom"ams(backgroundsubtracted)of 0.1 mM
gua,,msinein acetate buffer(0.1 M. pH 4,50) at a GCE. Scans (a)-(c)
represent successivesonovoltammograms.Uhrasoundconditions: 8 mm
horn tip-electrodeseparation:power intensity30 Wcm--'. Scan rate 50
mVs-L
measurements at various concentrations of guanosine it
was found that only the main signal at 1,15 V depends
clearly on hhe bui!x concentration of guanosine.
The most probable explanation for the above results is
that tlc guanosine used contained some guanine impurity
(less than 1%), Such an observai:ion was also reported by
Dryharst [7] for man?/commercial samples. Indeed, in Fig.
5, the small wave (0.75-0.95 V) that appears in the cyclic
soaovoltammograms of 0.1 mM guanosine solutions can
be attributed to the oxidation of guanine.
The adsorption of guanosine was studied in solutions
containing guanosine at very low concentration, where
signal contributions related to mass transport from the bulk
solution during the recording can be neglected. After an
accumulation period of 10 min at 0 V in a solution
containing 2.5 × 10 -6 M guanosine, the first cyclic
voltammogram shows two signals at 0.88 V and I.I V,
Fig. 6(A,a). The cyclic voltammograms shown in Fig,
6(A,b) were recorded under identical conditions except
that immediately before starting the recording a potential
of 0.95 V was applied for 10 s. This procedure results in
selective removal of adsorbed guanine which gives an
oxidation signal at 0.88 V, while the second signal is not
affected.
In addition, Fig, 6(B) shows that the accumulation of
guanosine in the presence of ultrasound results in higher
signals at 0.88 V than at I.i V in comparison with
accumulation in silent solution. This suggests that the
species oxidized at 0.88 V, guanine, is the more strongly
adsorbed, since the other, guanosine, can be partially
removed by ultrasonic irradiation. Concerning the very
small amount of guanine impurity, the high sensitivity of
the adsorptive sonovoltammogram for the guanine ad-
Fig. 6. Cyclicvoltammogramsof guanosinein acetate buffer(0.1 M. pH
4.50) containing2.5x 10 -6 M guanosine,scan rate 200 mVs~ I after
differentaccumulationperiods:(A) 10 min at 0 V in quiescentsolution
(lst and 2nd scans shown), the differencebetween(a) and (b) is that
immediately before starting recording (b) a potential of 0.95 V was
applied for 10 s; (B) (a) 10 rain at 0 V in quiescentsolution,(b) 5 rainat
0 V in the presence of ultrasoundof power intensity30 Wcm-: (c) 5
min at 0 V in the presenceof ultrasoundof power intensity72 Wcm--'.
The distancebetweenthe sonichorn tip and GCE is 8 mm.
sorbed on the electrode surface is remarkable -- its concentration can be estimated to be in the nanomolar range.
Fig, 7 shows the response obtained when the electrode
was immersed in 0,1 mM guanosine solution for 5 rain and
then transferred to a pure buffer solution for recording the
cyclic voltammograms. The second peak of guanosine at
1.1 V is substantially larger than that of guanine at 0.88 V.
This shows that the s~cond signal is not due to further
oxidation of products formed in the first oxidation step at
0.88 V, because it corresponds to a much higher concentration of guanosine adsorbed on the electrode surface than
the small peak from oxidation of adsorbed guanine.
Experiments were undertaken in order to determine the
number of electrons transferred during oxidation of guano,.
7-
~(1)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
EIV
Fig. 7+ Cyclic voltammograms (scan rate 50 mV s- ~) in acetate buffer
(0.1 M, pH 4,50). The GCE was placedin 0.1 mMguanosineso]utionfor
5 rain hefore transferringto the acetatebuffer. Voltammograms(I) and
(2) show first and secondcyclesrespectively.
A.M. Olireira Bren, F..M. Ma~.sik / 8ioelectrocheraistq"and Bioenergetics42 (1997) 11I- tit
115
of 30 Wcm -2 was cbosen. A lineal" dependence on concenlration of the limiting current was obtained fo¢ guanine
concentrations between 4 × 10-7 M and 2 x I0 ~ M. The
results of linear regression of the calibration dam are
!
ll+m[~.A] = 0.79S[v.A/~.M] × c[~.M] + 0.766[gA]
0~
' ~2
' o!,
' 0.~
' 0'e ' ;0
'
¢~'~,
EIV
Fig. g. Cyclic ~novoltammogram (background subtracted) of 2A x !0 -4
M guanosine and I.I × 10"a M K.~[W<CN)s] in acelat¢ buffer (0.1 M.
pH 4.50) containing 0.3 M KCI. UIwasound conditions: 7 mm horn
tip-electrode separa:ion: power intensity 21 Wcm--', Scan rote 50
mVs- ~.
sine. For this a sonovoltammogram of a mixture of
K4[W(CN) s] and guanosine was recorded, Fig. 8. The
limiting current Is~m derived from a sonovoltammogram
can be described by the following equation
llim
=
knFADc
where k is an empirical coefficient related to the experimental parameters, n is the number of electrons transferred, F is the Faraday constant, A is the electrode area,
D is the diffusion coefficient of the electroactive species
and c its bulk concentration. The octacyanotungstate complex shows a simple one-electron oxidation and is used as
an internal standard in order to eliminate the empirical
coefficient. The number of electrons transferred during
guanosine oxidation multiplied by the ratio of the diffusion
coefficients of guanosine and octacyanotungstate amounts
to 2. I + 0.3. In a similar experiment performed with guanine instead of guanosine, a value of 4.8 :l: 0.3 wa~ obtained. Assuming that the diffusion coefficients of guanine
and guanosine are similar, it can be concluded that tho.
number of elecu'ons transferred in the oxidation of gumbosine is half that transferred for guanine. Guanine has been
reported to be oxidized in a two-electron step, forming
8-oxyguanine which can be further oxidized in a second
two-electron step resulting in a quinonoid-diimine species
[6]. According to the structure of guanosine, an analogous
oxidative electrode reaction leading to 8.-oxyguanosine
would be possible, however, further oxidation leading to a
diimine species is not possible. Thus, a two-electron process can [~e assumed for the electro-oxidation of guanosine.
3.3. Analytical detenninations of guanine and guanosine
Cyclic sonovolmmmograms allow reliable determinations of the guanine concentration bas~.d on the measurement of the wave height of the reverse scan. The horn
tip-electrode separation was 8 mm and a power intensity
with n = 7, and regression coefficient 0.9993.
Under these experimental conditions a detection limit of
2 X 10 -7 M guanine was determinod basod on a signalto-noise ratio of 3 which compares favom-ably wi~h pevions d,c. voltammemc determinations at pyroly~c gr,~ite
electrodes (e.g. the concentration range studied in [7] is
4 X 10-s M to 5 X 10-4 M). Tbe l ' C ~ y
bw Hn~t of
detection oblained for linear sweep or cyclic sonovol~amme~c measumn~nts reflects the high mass transJJort efficiency due to ultrasonic irradiation. C o m p ~ e Io~ limits
of detection for guanine have only been reported mcemly
[9] based on differential pulse volr~nmeuy using c'Moon
paste electrodes (I × lO-7 M) and glassy cafoon dectrodes (7.5 X 10-7 M).
Using the analytical procedure described above, concentrations in saturated guanine solutim~s were de~nnined.
Saturated guanine solutions were ixcpa~d by sonicalion of
an acetate buffer con~ning excess solid guanine for 15
min and subsequent filtration. A p p ~ l y
dilutedsamples were measured using dm method of muttiph: s~ndm~
additions.
•
The guanine concena'ation in a freshly p ~
satura~l solution was 2 × 10-s M. Somewhat higher
concentrated guanine solutions (3--4 x 10-5 M) can be
pmpaml by adding small volumes of the alkaline or acid
stock solutions of guanine to an aceta~ barfer. However,
the guanine concentntion in these solutions tends to decrease progressively accompanied by s e c f i ~
of
guanine. According to our results the solubiliv/of guanine
in its neuwal form is approximately an order of magnitude
lower than reported previously [7,20]. However, an older
report by Albert and Brown [21[ in which the mass rato
between water and the soluble am~ant of guanine was
determined to be 200000 corresponding to a c o n c e m n ~
of about 3 x 10-s M, confirms ou~ result.
Guanosine was determined in the same way as described for guanine. However, background subtraclion is
necessary because the sonovoltammelric wave is close to
the positive potential limit of the acetate buffer system.
Differential pulse voltamn~try was used for guanine
and guanosine determinations as an al~em~ive to cyclic
sonovoltammeu'y. Ulo~sound was applied only during the
initial part of the voltammogram until a chosen polecat of
0.65 V, in order to control the extent of ~ l s o ~ o n during
the determination. The final part of the vottammog~zn was
recorded in silent solution because this leads to n~+e
precise signals than in the presence of ultrasound.
Fig. 9(A) illustrates the typical response obwined when
recording differential pulse volmmmograms of guanosine
with ultrasonic irradiation during the initial part of ~he
recoiling. Again the smaller peak appearing at 0.8 V
~M. Oli~'eiraBrett, F,-M. Mab'sik/ Bioelectrochemistr)'#.,ul Bioenergetics42 (/997) I/1-16
116
A
st~
~
/
:: ' /
i",/ i(1)
'i
",, "~L' :', i
E/v
I
B
.[
",,
:,,
(dl
,:(o
/:i
i*'!~)
:' /::
:"//:
';
(i'
E/v
l~g. 9. Differemial pulse (DP) voltammogrm~ of guanine (A) and
guanine and guanosi~ mixtures (B). Arrows indicate the potential at
which the ultrasound is switched off. UIwasoundconditions: 8 nun horn
tip-electrode separation; power intensity 30 Wcm-2. DP conditions:
scan r~e 5 mVs -), pulse amplitu~ 50 mV, Supporting elec~lyte
~:eta~e buffet (0,1 M, pH 4.50). (A) Guanosine. 5xl0 -5 M, scans
(1)-(3) are successive recordings: (B) mixture of l a x 10-s M guanine
plus(a)5× 10-6 M.(b) 1.4x10 -5 M,(c) 2.4× 10-5 M,(d)3.3X l0 -5
M guanosine.
con~sponds to adsorbed species of guanine traces and was
of constant size independent of varying guanosine concen~'afions (10-s-10 -4 M), whereas the signal at 1.05 V can
be used to e,~aluate the concentration of guanosine.
Both guanine and guanosine can be reliably determined
by this method. Linear calibration plots were obtained for
both compounds. The detection limits were 8 X 10 -7 M
and 3 X 10 -6 M for guanine and guanosine respectively.
However, one must have in mind that during differential
poise experiments the ultrasound is switched off before the
peaks. Consequently, the limit of detection for guanine is
higher using differential pulse voltammetry instead of
cyclic sonovoltammetry (2 x 10 -7 M) and this method
should therefore be pnferred for determination of guanine
at very low concentrations.
The voltammetric response was also studied for mixtures of guanine and guanosine. Fig. 9(B) shows differential pulse voltammograms for various guanosine concentrations in the presence of ~. constant concentration of guanine of 1.4 x 10 -s M. In the presence of guanosine of
I0 -5 M concentration or higher the guanine signal decreases by about 8% compared with a pure guanine solulion. The reason is probably that guanosine adsorption
displaces some adsorbed guanine which leads to a reduction in the guanine signal due to the reduced electrode area
available to the reaction. However, both components could
easily be determined in mixed solutions and gave linear
calibration plots. The procedure described was found to be
a reliable approach to determine guanine and guanosine as
single compounds or in a mixture.
4. Conclusions
The combination of sonochemical and electrochemical
techniques permits a detailed study of the adsorption behaviour of guanine and guanosine at glassy carbon e!ectrodes. Owing to the high mass transport efficiency in the
presence of ultrasound, even traces of guanine lead to
easily detectable oxidation signals of adsorbed guanine
which was shown to adsorb more strongly than guanosine,
Sonovoltammetric determinations of guanine and guanosine, either separately or in a mixture, are characterized by
high sensitivity and good reproducibility even for extended
measuring periods. The latter criterion is a particular advantage over conventional voltammemc determination of
these compounds and results from a continuous in situ
activation of the electrode surface and a well defined
control of adsorption of the analytes by ultrasonic irradiation.
Acknowledgements
We thank the European Union for financial support
under the Human Capital and Mobility Scheme (contract
no. CHRX CT94 0475).
References
[I] G. Dryhurs!and PJ. Eiving, Taianta, 16 (1969) 855.
[2] P.J. EMng. SJ. Pace and J.E. O'Reilly, J. Am. Chem. Soc., 95
(1973) 647.
[3] E. Palecek. Electroanalysis, 8 (1996) l.
[4] T. Yao and S. Musha, Bull. Chem, Soc. Jpn,, 52 0979) 2307.
[5] G. Dryhurst, Talanta. 19 (1972) 769.
[6] R.N. Goyal and G. Dryhurst, J. Electroana]. Chem., 135 0982) 75.
[7] G. Dryhurst, Anal. Chim. Acta, 57 (1971) 137.
[8] T, Yao, Y. Taniguchi, T. Wasa and S. Musha, Bull, Chem. Soc.
Jpn., 51 (1978) 2937,
[9] M.A.T. Gilmattin and J.P. Har~, Analyst, 117 (1992) 1613.
[10] TJ. Mason, J,P. Lorimer, DJ. Walton, Ultrasonics, 28 (1990) 333.
[II] H. Zhang and L.A. Coury, Jr., Anal. Chem., 65 (1993) 1552.
[12] R.G. Compton, J.C. Eklund, S.D. Page, G.H.W, Sanders and J.
Booth, J. Phys, Chem,, 98 (1994) 12410.
[13] I. Kl[ma, C. Bernard and C. Degrand, J, Electro~,nal, Chem., 367
(1994) 297.
[14] F. Marken, J. Eklond and R,G. Complon.J. Electroanal. Chem., 395
(1995)335.
[15] R.G. Compton and F.-M. Matysik, Electroanalysis, 8 (1996) 218.
[16] A.M. Oii.:eim Brett and F.-M. Matysik, Electroohim. Acta, 42
(1997) 945.
[17] TJ. Mason, J,E Lorimerand D.M. Bates. Ultrasonics, 30 (1992) 40.
[18] O. OIsson. Z, Anorg. AIIg. Chem., 88 (1914) 49.
[19] W. Pfleiderer, Liebigs Ann. Chem.. 647 0961) 167.
[20] T. Yao, T. Wasa and S. Musha, Bull, Chem. Soe. Jpn., 50 0977)
2917.
[21] A. Albert and DJ. Brown, J. Chem. Soc,, Part I, (1954) 2060.