Full Paper
Voltammetric Determination of Acibenzolar-S-Methyl Using
a Renewable Silver Amalgam Film Electrode
Dariusz Guziejewski,*a Mariola Brycht,a Sławomira Skrzypek,a Agnieszka Nosal-Wiercińska,b Witold Ciesielskia
a
Department of Instrumental Analysis, University of Lodz, Pomorska 163, 90-236 Lodz, Poland
Department of Analytical Chemistry and Instrumental Analysis, Maria Curie-Sklodowska University, M. Curie-Sklodowska 3 Sq.,
20-031 Lublin, Poland
*e-mail: dguziejewski@uni.lodz.pl
b
Received: August 8, 2012;&
Accepted: October 10, 2012
Abstract
Acibenzolar-S-methyl (ASM) is a novel fungicide applied for crop protection. A renewable silver amalgam film
electrode was used for the determination of ASM in pH 3.4 Britton Robinson buffer using square wave adsorptive
stripping voltammetry (SW AdSV). The parameters of the method were optimized. The electroanalytical procedure
made possible to determine ASM in the concentration range of 5 10 8–3 10 7 mol L 1 (LOD = 4.86 10 9, LOQ =
1.62 10 8 mol L 1). The effect of common interfering pesticides and heavy metal ions was checked. The validated
method was applied in ASM determination in spiked water samples.
Keywords: Acibenzolar-S-methyl, Fungicide, Voltammetry, Determination, Silver amalgam film electrode
Hg(Ag)FE
DOI: 10.1002/elan.201200435
1 Introduction
Acibenzolar-S-methyl (ASM; CAS 126448-41-7, Figure 1)
is a novel synthetic pesticide used in the protection of
several crops against miscellaneous bacterial, fungal and
viral diseases [1–7]. This mechanism, acting systemically
and/or locally, is biologically or chemically activated in response to pathogens or bacteria [8, 9].
Many studies have been reported concerning the activation of SAR (Systemic Activated Resistance) by ASM
for the control of plant diseases. On the other hand, only
a few analytical methods for its determination are available in the literature (maximum residue level for most
food samples equals to 0.02 mg kg 1). HPLC systems have
been employed for the determination of ASM in tomato
and pepper plants [10]. The determination of ASM in
soils by HPLC-diode array detection has been recently
reported [11]. An application of HPLC-UV and LC/MS
has been developed for grains [12]. Also, several multiresidue pesticide analysis methods, which include ASM de-
termination, have been developed and reported [13, 14].
The use of voltammetric techniques has been often exploited in the characterization [15–17] or determination
[18–22] of various organic compounds [23–25]. Unfortunately so far, no voltammetric studies have been presented regarding acibenzolar-S-methyl. Due to fears of mercury toxicity there is a driving force to minimize the use
of metallic mercury.
In the last decade a novel type of silver amalgam film
electrode (Hg(Ag)FE) [26,27] as well as a silver amalgam
annular band electrode [28] invented by Cracow group
are applied. The latter has already been used for the determination of some popular vitamins: C, B1 and B2.
Also other research groups are interested in replacing
liquid mercury electrodes with more environmentally
friendly types of solid electrodes [29–34].
Only few papers have concerned the determination of
biologically active compounds using Hg(Ag)FE (moroxydine [35], an antiviral agent; blasticidin S [36], a plant antibiotic; dinotefuran [37], an insecticide; and proguanil
[38], antimalaria drug) and of similar type mercury meniscus modified silver solid amalgam electrode [33, 34].
Our aim in this paper is to investigate the possible application of a renewable silver amalgam film electrode for
the determination of the pesticide acibenzolar-S-methyl.
The standard addition method was used to determine
ASM in spiked water samples.
Fig. 1. Chemical structure of ASM.
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D. Guziejewski et al.
2 Experimental
2.1 Instrumentation
All experiments were performed using a microAutolab/
GPES (General Purpose Electrochemical System version
4.9, Eco Chemie, Netherlands) computer-controlled electrochemical system. The cell stand included a three-electrode system with a renewable silver amalgam film electrode [27] (Hg(Ag)FE; mtm anko, Poland) as the working
electrode (electrode area 0.12 cm2, renewed before each
measurement), a silver/silver chloride electrode (Ag/
AgCl, 3 mol L 1 KCl) as the reference electrode, and
a platinum wire as the counter electrode. All potentials
were referred to the Ag/AgCl reference electrode. All
pH measurements were performed with the aid of a pH
meter (type CP-315, Elmetron, Poland) using a conjugated
glass membrane electrode.
2.2 Reagents
All chemicals used were of analytical reagent grade and
the solutions were prepared in deionized water. The analytical standard of acibenzolar-S-methyl (Dr. Ehrenstorfer, Germany) was of 99.5 % purity. ASM stock solution
was prepared at a concentration of 1 10 3 mol L 1 by dissolving 5.3 mg of the pesticide in 25 mL of water-ethanol
mixture (1 : 1, v:v). All dilute solutions were prepared
from the stock solution. Britton Robinson (BR) buffer
solutions for voltammetric measurements were prepared
from a stock solution consisting of 0.04 mol L 1 phosphoric acid (85 %, POCh, Poland), 0.04 mol L 1 boric acid
(POCh, Poland) and 0.04 mol L 1 acetic acid (99.5 %,
POCh, Poland); sodium hydroxide solution (0.20 mol L 1,
POCh, Poland) was added to obtain the required pH
value. Analytical grade ethanol was purchased from
POCh (Poland). Argon (5N) was obtained from Linde
Gas (Poland) and was used without further purification.
2.3 General Voltammetric Procedure
The general procedure used to obtain cathodic stripping
voltammograms was as follows: 10 mL of the supporting
electrolyte (5.0 mL of the buffer mixed with 5.0 mL of
water) was placed in the voltammetric cell and the solution was purged with argon for 300 s with stirring.
The procedure of renovating the Hg(Ag)FE surface
was carried out before each measurement [26]. After the
formation of a new layer, a conditioning step was performed by the application of an adequate negative potential for a certain period of time. Subsequently, the accumulation step at a constant potential was applied with
stirring of the solution, followed by an equilibration time
of 3 s. After the equilibrium step, a negative ongoing potential scan was applied. If some reagents were subsequently added, the solution was purged with argon for
a further 30 s. The reported signals were measured after
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subtracting the blank signal and using the smoothing procedure available in GPES software.
In the present study, the best response was obtained in
BR buffer at pH 3.4 with the following parameters: conditioning potential 2.0 V for 5 s, accumulation potential
0.05 V for 30 s, amplitude 90 mV; frequency 200 Hz and
step potential 8 mV. All measurements were performed
at room temperature.
Voltammograms of pesticide solutions were recorded at
the same parameters as for pure supporting electrolyte
analysis. The recovery of the compound was calculated in
sextuplicate experiments.
2.4 Analysis of Water Samples
2.4.1 Tap Water
ASM stock solution (5 mL) was diluted in a 100 mL calibrated flask (final concentration 5 10 5 mol L 1). Next
10 mL of this diluted ASM solution was transferred again
to the 100 mL calibrated flask and filled up to the mark
with tap water. Later 100 mL of the spiked tap water was
moved to the voltammetric cell containing 5 mL of BR
buffer (pH 3.4) and 5 mL of tap water. The concentration
of the pesticide in the voltammetric cell was 4.95 10 8 mol L 1.
2.4.2 River Water
River water samples for analysis were obtained from
Warta River. 10 mL of the diluted ASM stock solution
was transferred to the 100 mL calibrated flask and filled
up to the mark with river water. Later 100 mL of the
spiked river water was moved to the voltammetric cell
containing 5 mL of BR buffer (pH 3.4) and 5 mL of distilled water. The concentration of the pesticide in the voltammetric cell was 4.95 10 8 mol L 1.
The voltammograms of spiked samples were recorded
with the same parameters as for pure pesticide solutions.
The recovery of the ASM was calculated in six runs.
Quantifications were performed by means of the standard
addition method. Voltammograms were recorded after
each addition.
3 Results and Discussion
3.1 Experimental and Instrumental Conditions
The typical SW voltammetric response of ASM at the
Hg(Ag)FE consists of a single peak reduction. Britton
Robinson (BR) buffers (pH 2 to 10) were selected as supporting electrolytes. At first with increasing pH (from 2.2
to 3.5), the peak height increased and next the signal decreased significantly, indicating the involvement of protons in the reaction mechanism. The highest analytical
signals of ASM (with respect to peak height and halfpeak width) were observed in the acidic medium of BR
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Electroanalysis 2012, 24, No. 12, 2303 – 2308
Voltammetric Determination of Acibenzolar-S-Methyl
Fig. 2. Effect of pH on (a) SW AdSV peak current and (b)
peak potential of 2 10 6 mol L 1 ASM recorded in 0.04 M Britton Robinson buffer, accumulation time 50 s at 0.1 V. The parameters of the potential modulation were frequency f = 150 Hz,
amplitude Esw = 130 mV, and step potential DE = 8 mV.
buffer at pH 3.4 (Figure 2). Stability of the ASM voltammetric response was observed over 1 day.
The observed ASM response is highly sensitive to the
accumulation factor. The influence of the accumulation
potential (Eacc) in the potential range from 0.15 to 0.4 V
was studied at 50 s accumulation time (tacc). As Eacc decreased from 0.15 to 0.0 V, an increase of ASM signal was
observed (Figure 3). For consecutive Eacc values, the current decreased markedly and remained nearly constant
for Eacc values lower than 0.25 V. Such phenomena are
related with specific adsorption of ASM molecules on the
working electrode surface. Due to the defined electron
density of the compound and applied potential to the
electrode surface the maximum adsorption and close
packing of the molecules can be achieved. When the accumulation potential approaches the ASM peak potential
the reduction process already can be involved in the accumulation step. Then the further decrease of the measured
Fig. 3. Effect of accumulation potential on (a) SW AdSV peak
current and (b) peak potential of 2.5 10 7 mol L 1 ASM recorded in 0.04 M Britton-Robinson buffer pH 3.4, accumulation time
50 s. The parameters of the potential modulation were the same
as in Figure 2.
Electroanalysis 2012, 24, No. 12, 2303 – 2308
Fig. 4. Effect of amplitude Esw on (a) SW AdSV peak current
and (b) the ratio Ip/DEp/2 of 2.5 10 7 mol L 1 ASM recorded in
0.04 M Britton-Robinson buffer pH 3.4, accumulation time 30 s
at 0.05 V. The parameters of the potential modulation were frequency f = 150 Hz, and step potential DE = 8 mV.
current is noticed. The same situation was observed for
other compounds [19, 38]. The best results were obtained
for the accumulation potential of 0.05 V with respect
also to its peak shape and half-peak width. The ASM
peak potential shifted nonlinearly towards more negative
values with decreasing accumulation potential. The accumulation time was changed in the range from 1 s to 250 s.
The peak current of acibenzolar-S-methyl rose with increasing accumulation time. The maximum response was
achieved at tacc = 30 s. A further increase of tacc caused
a significant decrease of the ASM signal. Such a behavior
suggests a complete saturation of the electrode surface
with the adsorbed molecules [20].
The influence of the amplitude Esw was recorded in the
range from 10 to 170 mV (Figure 4). A consequent increase in the ASM voltammetric response was observed.
The acibenzolar-S-methyl peak was studied also with respect to its half-peak width DEp/2, which remained constant up to Esw = 80 mV, and after that started to increase
proportionally with amplitude. Analysis of the ratio Ip/
DEp/2 showed a maximum for an amplitude of 90 mV and
this value was used in further studies.
The influence of step height was analyzed in the range
from 1 to 20 mV. A step potential of 8 mV was applied.
Additionally, a further increase in the parameter value
caused a distortion in the ASM peak shape.
The influence of frequency was studied in the range
from 8 to 501 Hz (Figure 5). As f increased, a rise in the
ASM signal was observed with a maximum between 150
and 251 Hz. A further increase of f values caused a decrease in the peak current. Above the frequency of
251 Hz, an ill-defined ASM signal was observed. The
highest peak current and the best-defined peak shape
(with respect to DEp/2 and the ratio Ip/DEp/2) were obtained for the frequency of 200 Hz, and this value was selected for further work. The peak potential shifted towards more negative values with increased frequency. At
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D. Guziejewski et al.
Fig. 5. Effect of frequency f on (a) SW AdSV peak current and
(b) peak potential of 2.5 10 7 mol L 1 ASM recorded in 0.04 M
Britton Robinson buffer pH 3.4, accumulation time 30 s at
0.05 V. The parameters of the potential modulation were amplitude Esw = 90 mV, and step potential DE = 8 mV.
higher frequencies, a linear dependence could be seen,
which is typical of irreversible mechanisms [17].
3.2 Electrochemical Behavior of ASM
The peak potential shifted linearly towards more negative
values according to the equation Ep(V) = 0.0568 pH–
0.3804 (r = 0.985). The slope being close to the expected
theoretical value of 59 mV/pH indicates that the number
of protons and electrons involved in the electrode mechanism is equal [40, 41]. Constant potential electrolysis revealed number of electrons exchanged in the electrode
process equal to two. Cyclic voltammograms showed only
one peak in the cathodic part. To explain the nature of
the process the influence of the scan rate (v) was investigated. The relationship between the scan rate and the
peak current was nonlinear. Also the dependence between the peak current and square root of scan rate gave
non-ideal linear function. The regression of log Ip vs. log
v gave a slope with a value of 0.684 (the correlation coefficient of the straight line is 0.989), indicating that the reduction current is of mixed adsorption and diffusion controlled nature [42]. Based on above results we believe
that this peak arises from the irreversible reduction of the
N=N bond, which could be the site of attachment of
electrons and protons, of the adsorbed ASM.
3.3 Analytical Characteristics of Acibenzolar-S-Methyl
The analytical response to the voltammetric determination of ASM was studied under the optimal conditions
described in the previous section. The quantitative determination of ASM at Hg(Ag)FE is based on a linear relationship between the peak current intensity Ip and the
ASM concentration c. The calibration curve and voltammograms for acibenzolar-S-methyl are presented in
Figure 6. A linear relationship between peak height and
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Fig. 6. SW AdSV voltammograms of ASM recorded in BR
buffer solution at pH 3.4. c(ASM10 7): (1) 0, (2) 0.5, (3) 1.0, (4)
2.0, (5) 2.5, and (6) 3.0 mol L 1. The conditions were the same as
in Figure 5 and frequency f = 200 Hz. Inset: Corresponding calibration line.
ASM concentration was obtained over the range of 5 10 8–3 10 7 mol L 1.
The mathematical relation between the analytical
signal (amperes) and ASM concentration (mol L 1) was
Ip = (86.4 0.2) c + [(2.13 1.4) 10 7] (for a confidence
limit of 95 %). The linear responses evaluated by the correlation coefficient and average relative standard deviation were 0.999 and 1.6 %, respectively. The lowest detectable concentration (LOD) and the lowest quantifiable
concentration (LOQ) of ASM (4.86 10 9 and 1.62 10 8 mol L 1, respectively) were estimated based on the
following equations: LOD = 3 s/m and LOQ = 10 s/m.
The abbreviation s represents the standard deviation of
the peak current (six runs) and m stands for the slope of
the related calibration curve [43].
The repeatability (1 day) of the voltammetric procedure was assessed by comparing the peak heights of six
replicate measurements at a single ASM concentration.
Relative standard deviations (RSD; in percent) for the
lowest and highest ASM concentration were 2.50 and
0.63, respectively. In order to check the correctness of the
method (Table 1), the precision and recovery of the
method were also calculated for different concentrations
in the linear range.
3.4 Effect of Interferences
We have checked commonly used pesticides like metam,
cyromazine, clothianidin, dodine, thiophanate and also
heavy metal ions (cadmium, zinc and lead). The presence
of these substances (in BR buffer pH 3.4 and under optimized potential modulation parameters for ASM) was investigated with respect to the peak current and potential
of the pesticide being under study. The ASM concentration was equal to 5 10 8 mol L 1 and was fixed during
the study. Other pesticides and ions in the concentration
range from 1 10 8 mol L 1 to 1 10 5 mol L 1 were added
to the voltammetric cell. These are an equivalent of the
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Electroanalysis 2012, 24, No. 12, 2303 – 2308
Voltammetric Determination of Acibenzolar-S-Methyl
Table 1. Recovery and precision obtained by SW AdSV using
Hg(Ag)FE in ASM determination at its various concentrations.
Table 2. Results of the ASM determination in spiked water samples by SW AdSV technique, n = 6.
Added
(mmol L 1)
Found [a]
(mmol L 1)
Precision RSD
(%)
Accuracy [b]
(%)
Sample
water
Found
Precision
Added
(10 8 mol L 1) (10 8 mol L 1) RSD (%)
0.050
0.075
0.100
0.150
0.200
0.250
0.300
0.050 0.001
0.075 0.001
0.099 0.002
0.149 0.002
0.202 0.002
0.251 0.002
0.299 0.002
2.6
2.4
2.3
1.7
1.0
1.2
0.6
101.0
99.8
98.9
99.5
100.9
100.4
99.5
Tap
River
(Warta)
4.95
4.95
[a] t(S/n1/2), p = 95 %, n = 6; [b] Recovery = 100 % + [(Found –
Added)/Added] 100 %.
compound/ASM ratios: 0.2, 2, 10, 20, 100 and 200. The
presence of metam had major effect on the recorded
peak current (only 0.2 and 2 fold concentration didnt decrease the signal). Clothianidin and dodine caused major
decrease only when 100- and 200-fold concentration was
applied. The presence of cyromazine and thiophanate had
no effect. The influence of common heavy metal ions
generally has no effect on the measured ASM peak current. Only cadmium ions at 20-fold and higher concentration ratios caused distortion in the pesticide analysis.
3.5 Analysis of ASM in Spiked Water Samples
The optimized voltammetric procedure was successfully
applied for ASM determination in spiked water samples.
The applicability of the procedure for the pesticide determination was tested with the standard addition method
by running 6 replicate analyses using SW AdSV
(Figure 7). All the experiments were performed as described in the Experimental section. The recovery results
of ASM in spiked water are given in Table 2. The method
is sufficiently accurate and precise to be applied for the
determination of ASM in spiked water samples.
4.93 0.07 [a] 1.46
4.81 0.04 [a] 0.81
Recovery
[b]
99.5
97.1
[a] t(S/n1/2), p = 95 %, n = 6; [b] Recovery = 100 % + [(Found –
Added)/Added] 100 %.
4 Conclusions
Acibenzolar-S-methyl is a novel synthetic pesticide, being
one of the most effective insecticides used for the protection of several crops against various bacterial, fungal and
viral diseases. The electrochemical behavior of ASM was
studied for the first time using a renewable silver amalgam
film electrode. This work shows that the pesticide can be
determined using voltammetric techniques on the basis of
its reduction process. This behavior provides a useful tool
for the detection and quantification of the compound at
low concentration levels. The limitation of the presented
method is its narrow linear concentration range what can
limit practical usefulness of the application.
The procedure showed clear advantages, such as no pretreatment or time-consuming extraction steps, and could
be adopted for further kinetic and dynamic studies as well
as for quality control studies. The presented method of
analysis can be applied for ASM determination in matrices
such as natural water samples. We are aware that more
complex environmental matrices would require separation
techniques such as GC or HPLC with MS detection for
precise qualification and accurate quantification. Due to
the high investment and operating costs, in large scale
monitoring applications they could be substituted with less
selective, but much cheaper, electroanalytical methods
based on SW AdSV at Hg(Ag)FE. Such a method can correctly detect the presence of ASM at concentrations
higher than the limit of determination with the aid of the
standard addition method. However, if the matrix SW
peak is found in the potential region where the acibenzolar-S-methyl peak is situated, more powerful separation
techniques should be used for definitive identification.
Acknowledgements
This work was supported by Grant No. 545/098 from the
University of Lodz, Poland.
Fig. 7. SW AdS voltammograms of ASM determination in
spiked river water sample (after subtracting of the blank, spiked
amount: 0.5 nmol) using standard addition method; additions indicated by each line. The conditions were the same as in Figure 5
and frequency f = 200 Hz.
Electroanalysis 2012, 24, No. 12, 2303 – 2308
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