Anal Bioanal Chem (2002) 373 : 277–283 DOI 10.1007/s00216-002-1345-4
O R I G I N A L PA P E R
Manuel Chicharro · Antonio Zapardiel · Esperanza Bermejo · Mónica Moreno · Elena Madrid
Electrocatalytic amperometric determination of amitrole using a cobalt-phthalocyanine-modified carbon paste electrode
Received: 28 September 2001 / Revised: 15 February 2002 / Accepted: 22 April 2002 / Published online: 6 June 2002 © Springer-Verlag 2002
Abstract Cobalt-phthalocyanine-modified carbon paste electrodes are shown to be excellent indicators for electrocatalytic amperometric measurements of triazolic herbicides such as amitrole, at low oxidation potentials (+0.40 V). The detection and determination of amitrole in flow injection analysis with a modified carbon paste electrode with Co-phthalocyanine is described. The concentrations of amitrole in 0.1 M NaOH solutions were determined using the electrocatalytic oxidation signal corresponding to the Co(II)/Co(III) redox process. A detection limit of 0.04 µg mL–1 (4 ng amitrole) was obtained for a sample loop of 100 µL at a fixed potential of +0.55 V (vs. Ag/AgCl) in 0.1 M NaOH and a flow rate of 4.0 mL min–1. Furthermore, the modified carbon paste electrodes offers reproducible responses in such a system, and the relative standard deviation was 3.3% using the same surface, 5.1% using different surface, and 6.9% using different pastes. The performance of the cobalt-phthalocyanine-modified carbon paste electrodes is illustrated here for the determination of amitrole in commercial formulations. The response of the electrodes is stable, with more than 80% of the initial retained activity after 50 min of continuous use. Keywords Modified carbon paste electrodes · Cobalt-phthalocyanine · Amitrole · Aminotriazole · Flow injection analysis
Introduction Amitrole (3-amino-1,2,4-triazole) is a non-selective herbicide. It is frequently used in mixed formulations con-
M. Chicharro (✉) · A. Zapardiel · E. Bermejo · M. Moreno · E. Madrid Departamento de Química Analítica y Análisis Instrumental, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain e-mail:
[email protected]
taining other herbicides such as atrazine and simazine. Leaching of amitrole residues from top soils may pollute ground water and subsequently drinking water [1, 2, 3]. The lack of data in environmental water is easily explained by the difficult determination of this compound at trace levels in water, because there is no efficient analytical procedure that allows its extraction from aqueous samples. It is very important to note its excellent water solubility (280 g L–1), high polarity, and poor solubility in organic solvents [4]. The UV detection of amitrole suffers from a fairly low absorption coefficient [5]. Different HPLC methods have been described in literature [6, 7, 8] to determine amitrole derivatives. Due to the high polarity of this compound, the determination by reversed-phase HPLC with the usual aqueous phases is not possible, since the resolution between the amitrole and solvent peaks is poor. The determination by gas chromatography is also difficult owing to its high polarity and low volatility. The acetylation of amitrole using acetic anhydride has been reported [2, 3]. Fluorescence detection is possible only with a pre-column derivatization process [9]. Paper chromatography and TLC have also been used for a quantitative estimation of amitrole [4, 10]. The electrochemical detection has successful by been applied in pesticide analysis and has proved to be more sensitive and selective than UV detection [11, 12, 13, 14]. Previously, amitrole has been determined by electrochemical detection after chromatographic separation [15, 16]. In all cases the working electrode was a glassy carbon electrode and the applied voltage was higher than +1.10 V. Other kind of electrodes such as micro-array gold electrodes [17] and nafion/lead-ruthenium oxide pyrochlore chemically-modified electrodes [18] were also used for the detection or determination of amitrole. In the former, the electrochemical reduction of amitrole was carried out at –0.14 V in acetate buffer. In the latter, the electrode exhibited a good electrocatalytic response towards the amitrole oxidation at +1.10 V. Other types of electrodes such as carbon paste electrodes or modified carbon paste electrodes were used.
278
One promising approach for minimizing overpotential is the use of electrocatalytic chemically-modified electrodes. Electrocatalytic-modified electrodes have thus been widely used for improving the selectivity and sensitivity of numerous analytes [19, 20]. In particular, after their introduction by Baldwin et al. [21], carbon paste electrodes (CPEs) modified with cobalt-phthalocyanine (CoPC) have been used for the amperometric detection of compounds that are otherwise non-electroactive or only oxidizable at extreme potentials. Some applications of CoPC-modified CPEs (CoPC/CPEs) including peroxides [22], ketoacids [23], thiols [24], hydrazines [25], carbohydrates [26], aliphatic amines [27], phenols [28], and inorganic anions [29]. Mainly, the enhancement derived from the ability of electrochemically generated higher oxidation states of cobalt at modest potentials. The electrochemical behavior of triazolic compounds at CoPC/CPEs has not been studied. The electrochemical oxidation of amitrole at CoPC/CPEs was examined and the electrode was used as a detector in flow injection amperometric determinations. The method was successfully applied to the determination of amitrole in commercial formulations in the presence of other herbicides or compounds. Other more complex samples could be determined subsequent to a previous separation process such as HPLC or capillary electrophoresis.
CPE and modified CPE into a PTFE tube compatible with the Metrohm electrochemical detection system described below. Apparatus The voltammetric experiments were performed with a voltammetric controller BAS CV-27 (West Lafayette, USA); the output was displayed on a BAS X-Y recorder. The working electrode, reference electrode (Ag/AgCl, model RE-1-BAS), and platinum auxiliary electrode were connected to the cell through holes in the PTFE cover. The amperometric experiments were performed with a Metrohm 641 VA-Detector (Metrohm, Switzerland); the output was displayed on a Perkin-Elmer Strip-Chart 56 recorder. The voltammetric and batch measurements were carried out in a 2.0 mL cell. A magnetic stirrer and stirring bar provided the convective transport during amperometric measurements. A stirring rate of 500 rpm was employed in the batch experiments. The flow injection systems consisted of a carrier reservoir, a Watson-Marlow 202U (maximum speed 50 rpm) peristaltic pump (Falmouth, UK), interconnecting PTFE tubing (0.8 mm i.d.), a rheodyne type 50, 4-way low pressure valve (Alltech, Spain) with a 100 µL sample loop and a Metrohm 656 electrochemical detection system composed of a wall-jet detection cell EA-1096 (Metrohm, Switzerland), with a reference Ag/AgCl electrode and a gold auxiliary electrode. A carrier flow of 4.0 mL min–1 was used in all flow injection measurements. All experiments were carried out at room temperature applying the desired operating potential and allowing the transient current to decay prior to the amperometric monitoring.
Results and discussion Experimental Reagents Cobalt phthalocyanine was purchased from Aldrich Chemical Co. (Madrid, Spain) and was used as received. Standards of herbicides: amitrole, asulam, benzthyazuron, diuron, atrazine, 2-hydroxyatrazin, atrazin-desethyl, simazine, aziprotryne, cyanazine, ametryne and hexazinone (Riedel-de-Haën, Spain), and urazole (Aldrich Chemical Co, Spain) were prepared using methanol Pestanal® (Riedelde-Haën, Spain) as solvent. Amitrole was prepared in the same way, but with water as solvent. In the case of 2-hydroxyatrazin, methanol was used with the addition of a small amount of concentrated HCl. All 0.01 M stock solutions were kept in the dark and under refrigeration. Other standards solutions were obtained by suitable dilution in water. All chemicals were of analytical reagent grade. The water used was obtained from a Millipore (Milli-Q/ Milli-RO plus) system (Waters, Spain). For sample preparations, 5 mg of each formulation (5 mL for Untro®) was dissolved in 10 mL of methanol Pestanal® and then adjusted to a final volume of 1000 mL in a brown flask with water.
In practice, a very useful technique for monitoring easily oxidizable species is the flow through amperometric detection. However, electrochemical determination of amitrole is difficult using an ordinary carbon electrode due to a large oxidation overpotential (see Table 1). One promising approach for minimizing overpotential is the use of electrocatalytic chemically-modified electrodes; in particular the ability of phthalocyanines to catalyze both cathodic and anodic reactions suggests that these metal chelates are capable of exhibiting both donor and acceptor properties. This implies that the electronic properties of the metal in the phthalocyanine play an important role in determining the catalytic activity. We have considered the application of cobalt phthalocyanine electrodes for the electrocatalytic determination of amitrole. Cyclic voltammetry
Electrodes The carbon paste for unmodified electrodes was handmade by mixing graphite powder (Acheson 38#, Fisher Scientific. Code no. G/0900/60) and mineral oil (Aldrich Chemical Co. no. 16/140–3). The ratio of graphite powder to mineral oil was 70:30. The carbon paste for modified electrodes was prepared in a similar way, except that the appropriate amount of cobalt phthalocyanine was first mixed with the graphite powder to give a 2% w/w paste mixture or other percentage of cobalt phthalocyanine. The bodies of the CPE and modified CPE were a PTFE tube (3.0 mm i.d.) filled with the carbon paste. Electrical contact was established with a stainless steel screw. Special activation of the paste was not necessary. The electrodes used for injection experiments were constructed in the same way as the batch working electrodes, by sealing the
It has been reported that an electrochemical response for amino compounds could be generated at CoPC/CPEs in strongly alkaline solutions [27]. In all cases the electrooxidation of the compounds happened at potentials substantially lower than those required for conventional carbon electrodes. At the modified carbon paste electrodes, each of these oxidations occurred at an applied potential that closely matched that of the Co(II)/Co(III) oxidation of the phthalocyanine and thus have been explained by a mechanism involving mediation or catalysis by the Co(III) state of the modifier. A similar electrocatalysis was observed for amitrole oxidation.
279
Table 1 Different methods found in the literature for electrochemical determination of amitrole Working electrode
Electrolyte
Working potential (V)
Lineal range
Detection limit
Literature
Glassy carbon
Acetonitrile/water (30:70) + 2 mM SDS + 1 g L–1 lithiumperchloratetrihydrate. Adjusted with 6% perchloric acid to pH 3.2 Acetonitrile/water (1:4) + 10 mM potassium dihydrogen phosphate, 3.0 mM sodium chloride + 4.0 mM SDS, adjusted to pH 3.0 with phosphoric acid Hexane/2-propanol/water (62:36:2) + 2.5 g L –1 lithium perchlorate + 2.0 g L –1 trichloroacetic acid 65% 50 mM SDS at pH 3.0 with perchloric acid and 35% acetonitrile Methanol/water (55:45) + 1.4 g sodium dihydrogenphosphate monohydrate + 1.2 g SDS, adjusted to pH 3.6 with ortho phosphoric acid 0.1 M perchlorate buffer at pH 3.6
+1.20
0.05–0.50 mg g –1
0.06 mg g –1
[30]
+1.10
1–100 mg L –1
0.4 mg L –1
[14]
+1.65
0.1–100 mg L –1
0.04 mg L –1
[16]
+0.85
0.2–100 mg L –1
0.2 mg L–1
[31]
+0.73
0.1–5.0 mg L –1
0.05 mg L –1
[15]
+1.1
0–8.5 mg L –1
7.5 mg L –1
[18]
Glassy carbon
Glassy carbon
Highly porous carbon Array electrode (sixteen porous graphite electrodes)
Nafion/lead-ruthenium oxide pyrochlore CME
Before considering the operation of the CoPC/CPEs in a flow environment, it was first useful to study the cyclic voltammogram behavior. For this purpose, a few arbitrary experimental choices had to be made. In particular, the medium selected for the experiments described below was carbonate-free sodium hydroxide, and the cyclic voltammograms were always initiated from the most negative potential (–0.5 V vs. Ag/AgCl) and then scanned at 20 mV s–1 in the positive direction. The cyclic voltammograms for the background (0.1 M NaOH) and 1 mM (84.1 µg mL–1) amitrole are shown in Fig. 1 for CPE and CoPC/CPEs. The amitrole oxidation occurs at both types of electrodes; on CPE, a well-defined anodic peak at +0.65 V, and on CoPC/CPE, a well-defined anodic peak at +0.55 V, correspond to the amitrole electrocatalytic oxidation. The enhancement in the anodic peak current clearly showed the catalytic behavior of the CoPC/CPE. The fact that the corresponding cathodic peak is totally absent indicated the irreversible nature of the oxidation process on the CoPC/CPEs. In Fig. 1, we could also observe the presence of two anodic peaks in the CoPC/ CPEs for amitrole oxidation. The first peak appeared at +0.15 V and the second one is similar to the peak observed for the other amines on cobalt phthalocyaninemodified electrodes [25, 27]. Although the mechanism of the redox process had to be examined, the first peak could be assigned to the Co(I)/Co(II) transition [25, 30, 31, 32] occurring in the phthalocyanine. Both peaks showed increased current levels at the CoPC/CPEs when the amitrole concentration was increased. The studies at different pH values showed that both processes shifted to more positive potentials when the pH was decreased; this behavior has been reported previously for the hydrazine electrocatalysis at this kind of electrodes [34]. The effect of the voltage scanning rate until 250 mV s–1, was studied on the cyclic voltammograms in 0.1 M so-
Fig. 1A,B Cyclic voltammograms for amitrole (dashed lines) and background (solid line) at a plain CPE (A) and CoPC/CPE (B). Amitrole concentration 1 mM; electrolyte 0.1 M NaOH; scan rate 20 mV s–1
dium hydroxide with two amitrole concentrations (0.2 and 1.0 mM; 16.8 and 84.1 µg mL–1). The results showed a straight line starting from the origin at two different concentrations of amitrole indicating a perfect diffusion-controlled charge-transfer mechanism on the CoPC/CPEs. The CPEs signal at +0.65 V suffered from a severe decrease during repetitive scans and it was virtually eliminated after 5 or 6 scans. This phenomenon did not appear in CoPC/CPEs; in this type of electrode the signal remains constant after 50 scans and then the signal suffered from a slight decrease. The CoPC/CPEs were allowed to stand in a stirred solution or cycled over the –0.5 to +0.7 V range
280
two oxidation steps could be observed; this profile was similar to the cyclic voltammograms of amitrole at 20 mV s–1. The significant lowering of the potential was also accompanied by a dramatic enhancement of the current signals compared to the plain CPE. Such catalytic currents were associated primarily with the Co(II)/Co(III) redox reaction, although the Co(I)/Co(II) process may account for the smaller anodic current observed at lower potentials. Previous studies described in literature [25] have shown that CoPC and other substituted cobalt phthalocyanines promote this reaction through a redox-catalysis type of mechanism, which involves the Co(I)/Co(II) couple, namely: → Co(II)PC + e− [Co(I)PC]− ← Co(II)PC + Amitrole → [Co(I)Amitrole + ]
Fig. 2A,B Hydrodynamic voltammograms for amitrole at plain CPE and CoPC/CPE. (A) Batch experiments in 0.1 M NaOH; 500 rpm stirring rate and 0.1 mM of amitrole. (B) Flow injection analysis of 1.0 µg mL–1 amitrole; flow rate 4.0 mL min–1 and a 0.1 M NaOH carrier electrolyte
for at least 2 h without any noticeable loss of catalytic activity. Because of their stability and ability oxidize amitrole at low potentials, compared to the plain carbon paste electrode and other types of electrodes previously described in literature (see Table 1), the CoPC/CPEs were evaluated for their possible use for detection in flow systems. Hydrodynamic behavior and flow injection studies In order to select an optimal detection potential for amitrole, the dependence of the current on the potential was studied in the range from 0.0 to +0.8 V. Hydrodynamic voltammograms obtained with the amperometric monitoring system described in the experimental are shown in Fig. 2 for plain CPE and CoPC/CPEs (each data point represents the average of three experiments with the same surface). The hydrodynamic voltammograms were obtained in batch (500 rpm) for a 0.1 mM (8.5 µg mL–1) amitrole concentration (Fig. 2A) and in flow injection analysis at 4.0 mL min–1 for 1.0 µg mL–1 amitrole (Fig. 2B). The hydrodynamic voltammograms showed similar behavior in both studies. The oxidation of amitrole starts at +0.3 V in CoPC/CPEs and +0.4 V for plain CPE. Furthermore, in flow injection analysis and batch experiments
This reaction scheme is supported by the previous studies described in literature and by the observation of Tafel slopes very close to 0.060 V decade–1 for CoPC/CPEs (not shown). The maximum response is centered around +0.6 V for amitrole oxidation at CoPC/CPEs, and decreases at higher potentials as a result of the electrode deactivation process occurring at such values. Further studies of the potential dependence of the CoPC/CPEs response for amitrole were performed with repetitive sample injections in the constant potential detection mode. The analytical signal increased up to +0.65 V and then it decreased; the overall pattern was similar to the hydrodynamic voltammograms. When potentials higher than +0.60 V were applied, a gradual loss of the analytical signal was evident. The deactivation process of CoPC-modified electrodes has been attributed to further oxidation and decomposition of the phthalocyanine ring, to irreversible complexation of the Co(III) center [23], and the gradual leaching of CoPC from the electrode surface [32, 33]. The stability of the CoPC/CPEs response was markedly improved by using only a potential lower than +0.60 V. As +0.55 V was sufficient to maintain the electrocatalytic activity of CoPC, an operating potential of +0.55 V (vs. Ag/AgCl) was selected for all subsequent studies. The influence of the optimal quantity of CoPC in the electrode was also investigated. Nine electrodes with different contents of CoPC (0–10% in graphite powder) were prepared, and the currents of 2.0 µg mL–1 amitrole oxidation in 0.1 M NaOH were recorded at +0.40 V and +0.55 V. The evolution of diffusion currents at these potentials was similar. Figure 3 shows the evolution of the signal at +0.40 V as a function of CoPC content of the electrode. The evolution of the obtained signal went through a maximum at 2–3% modifier. At higher CoPC concentrations, significant increases of background current masked the amitrole oxidation current. This behavior agrees with other studies presented in the literature [24, 33]. Two percent of CoPC content was selected for all subsequent studies. Figure 4 compares the dynamic amperometric response (at +0.40 V) of the plain CPE and CoPC/CPEs at successive additions of 0.4 µg mL–1 of amitrole in batch experiments. As expected from the previous studies in cyclic voltammetry, the plain carbon paste electrodes did not re-
281
Fig. 3 Dependence of current for amitrole on concentration of modifier CoPC at +0.40 V. Amitrole concentration 20 µg mL–1; electrolyte 0.1 M NaOH; stirring rate 500 rpm
Fig. 4A,B Amperometric response of amitrole (A) at plain CPE (a) and CoPC/CPE (b) to successive concentration increments of 0.4 µg mL–1 of herbicide. Calibration plots (B) resulting from (a) and (b). Working potential +0.40 V; sample loop 100 µL; flow rate 4.0 mL min–1; carrier electrolyte 0.1 M NaOH
spond to these additions of amitrole until the concentration in solution was 2.0 µg mL–1 and even under these conditions the sensitivity of the plain CPEs was really poor. In contrast, the CoPC/CPE showed a well-defined response to these additions. The steady-state currents were produced within short times. The resulting calibration plots shown in Fig. 4B, display the dynamic amperometric response for amitrole at +0.40 V in CoPC/CPEs. The calibration plots were linear for amitrole concentrations in the range 0.2–2.0 µg mL–1, with a slope of 52.4 nA mL µg–1 (r=0.9993, n=7). The detection limit was 0.09 µg mL–1. It was calculated as the concentration giving a signal equal to a+3sa, where a is the intercept of the calibration plot and sa its standard deviation. The relative standard deviation was lower than 4.3% over the useful analytical range (0.2–2.0 µg mL–1). Amperometric monitoring of amitrole in flowing streams could greatly benefit from their easy oxidation at CoPC/ CPEs. The effect of the flow rate upon the response peak was studied over the 0.1–6.0 mL min–1 range. The catalytic signal increased rapidly upon increasing the flow rate between 0.5 and 4.0 mL min–1, and then decreased very slowly at higher rates. Such a profile reflected the influence of both the sample dispersion and the convective transport towards the electrode surface. As expected, the peak width at the baseline decreased sharply from 115 to 25 s on raising the flow rate between 1.0 and 4.0 mL min–1. All subsequent work employed a flow rate of 4.0 mL min–1. In order to obtain the maximum sensitivity, the working potential was adjusted at +0.55 V (vs. Ag/AgCl). Under these conditions, the flow injection peaks for increasing levels of the amitrole concentrations were shown in Fig. 5A. The CoPC/CPEs detector responded favorably to the concentration increments over the range of 0.1– 2.5 µg mL–1. The resulting calibration plot (Fig. 5B) was linear for amitrole concentrations in the range 0.05– 2.00 µg mL–1, with a slope of 79.98 nA mL µg–1 (r=0.9995, n=10). The detection limit was 0.04 µg mL–1 and was calculated as described for the batch experiments. The relative standard deviation was lower than 3.6% in the useful analytical range (0.05–2.00 µg mL–1). The stability of the CoPC/CPEs was also evaluated in combination with the flow injection system. A fresh surface of CoPC/CPEs containing 2% of cobalt phthalocyanine was exposed to the carrier solution of 0.1 M NaOH at a flow rate of 4.0 mL min–1 and repetitive injection of 0.5 µg mL–1 amitrole solution. Peak currents obtained for repeated injections usually decreased gradually after 30 min and then remained roughly constant for several hours of use (80% of the initial signal). This time (30 min) was sufficient to carry out the calibration and determination of amitrole in the flow injection system. The relative standard deviation of the peak current was 3.7% in the first 30 min for the same CoPC/CPE surface. Apparently, no deactivation process of the CoPC/CPE surface or graduate leaching of cobalt phthalocyanine from the electrode occurred during this time. These experiments showed that flow injection analysis measurements with the CoPC/CPE gave a sensitive re-
282 Fig. 5A,B Flow injection response peaks (A) for increasing levels of amitrole: 0.2 (a), 0.6 (b), 1.0 (c), 1.4 (d), and 1.8 (e) µg mL–1 at CoPC/CPE. Resulting calibration plot (B). Working potential +0.55 V. Other conditions as in Fig. 4
sponse for amitrole. However, other electro-active and nonelectro-active compounds participating in the electrode reactions or present in the same media of amitrole (residual waters, natural waters, commercial formulations, etc.) at the detection potentials were potential interferents. The results of an interference study are summarized in Table 2. The electrocatalytic action of the cobalt phthalocyanine present in the electrode could be extended to diuron and urazole. For both compounds, the electrochemical oxidation was irreversible at CoPC/CPE. Other compound such as NH4SCN and s-triazines were entirely non-electro-active at the electrode in the pH range investigated, and no interferences were observed. This fact is very important because amitrole usually is commercially available in different formulations. Most of the products contain amitrole and additionally some s-triazines (simazine, generally) or NH4SCN. The application and the accuracy of the developed method using a CoPC/CPE in flow injection analysis were
studied by analyzing commercial samples containing amitrole. The samples used were “Untro®” from Etisa (composition: amitrole 20%, simazine 20% and NH4SCN 18% w/v, suspension concentrated) and “Herbicruz® Duat Simple” from KenoGard (composition: amitrole 25%, diuron 25% wetly powder). The results obtained by determination of amitrole are summarized in Table 3. The values for amitrole are in good agreement with the certified values. In commercial samples containing diuron, it was necessary to correct the signal in accordance with the interference; these studies are shown in Table 2. The technique developed was helpful for the selective control of amitrole in industrial products. Measurements of traces of this herbicide in different kinds of water (natural, tap, ground, etc.) were complicated because of the difficulties concerned with its isolation and enrichment. More complex samples, such as environmental waters, may be addressed by coupling the CoPC/CPE detection with liquid chromatographic systems or an analogous detector scheme.
Table 2 Effect of various interferents on the flow injection electrocatalytic amperometric response of amitrolea Analytical signal ratio P(I+A)/P(A)
NH4SCN Asulam Benzthiazuron Diuron s-Triazinesc Urazole aElectrode:
[I]/[A]=10
[I]/[A]=1
–b –b –b
–b –b –b 2.1 –b 1.9
–b
2% CoPC/CPE, flow rate: 4.0 mL min–1, detection potential: +0.55 V, carrier electrolyte 0.1 M NaOH. P(I+A); P(A) peak current obtained with solutions containing both the interferent (I) and amitrole (A); and the amitrole alone. [A]=1.0 µg mL–1. bRatio
[I]/[A]=0.5
[I]/[A]=0.1
[I]/[A]=0.05
1.6
1.3
1.1
1.5
1.2
–b
within the reproducibility range of the measurements. cAtrazine, atrazine-2-hydroxy, atrazine-desethyl, simazine, azyprotryne, cyanazine, ametrine, and hexazinone.
283 Table 3 Amitrole determination in commercial formulations by flow injection analysis with electrocatalytic amperometric detectiona Content of amitrole (%) UNTRO® (Certified value 20% amitrole)
20.2 20.0 19.7 19.4 20.6
Mean value
20.0±0.5
HERBICRUZ® Duat Simple (Certified value 25% amitrole)
25.2 24.8 25.0 24.3 24.2
Mean value
24.7±0.4
aElectrode:
2% CoPC/CPE, flow rate: 4.0 mL min–1, detection potential: +0.55 V, carrier electrolyte 0.1 M NaOH.
Conclusions It has been shown that the electrocatalytic-mediated oxidation of amitrole commences at a potential where a catalytic Co(II)/Co(III) species is electrogenerated at the carbon paste electrode modified with cobalt phthalocyanine. The CoPC/CPE can be used for measurements of amitrole at levels higher than 4 ng. These CoPC/CPEs show great stability over a wide potential window in alkaline medium. Upon cycling or under a flowing stream they have stable electrochemical responses for more than 50 minutes. The procedure is susceptible to interferences from other herbicides such as diuron and urazole. This study has demonstrated that FIA can be used to apply the CoPC/ CPE to the detection and determination of amitrole with good sensitivity. The reliability and stability of the electrode offers a good possibility for extending, in the future, the detector to routine analysis of amitrole in more complex samples by coupling the CoPC/CPE system detection with liquid chromatographic systems or for monitoring capillary electrophoretic separation of amitrole in environmental or clinical samples. Acknowledgements The authors wish to thank Ministerio de Ciencia y Tecnología de España, Fondo Europeo de Desarrollo Regional de la Unión Europea and Comunidad Autónoma de Madrid for financial support for this project (BQU2001–0949 and 07 M/0040/2001). M. Moreno acknowledges a fellowship from Universidad Autónoma de Madrid.
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