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Voltammetric method for sensitive determination of herbicide picloram in environmental and biological samples using boron-doped diamond film electrode
Electrochimica Acta 111 (2013) 242–249
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Voltammetric method for sensitive determination of herbicide picloram in environmental and biological samples using boron-doped diamond film electrode b a ˇ ´ Lenka Bandˇzuchová a,∗ , L’ubomír Svorc , Jozef Sochr b , Jana Svítková b , Jaromíra Chylková a Institute of Environmental and Chemical Engineering, Faculty of Chemical Technology, University of Pardubice, Studentská 573, Pardubice CZ-532 10, Czech Republic b Institute of Analytical Chemistry, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, Bratislava SK-812 37, Slovak Republic
a r t i c l e
i n f o
Article history: Received 12 June 2013 Received in revised form 29 July 2013 Accepted 15 August 2013 Available online 23 August 2013 Keywords: Picloram Boron-doped diamond film electrode Differential pulse voltammetry Environmental sample Human urine
a b s t r a c t The voltammetric behavior and determination of picloram, a member of a pyridine herbicide family, was for the first time investigated on a boron doped diamond film electrode using cyclic and differential pulse voltammetry. The influence of supporting electrolyte and scan rate on the current response of picloram was examined to select the optimum experimental conditions. It was found that picloram provided one well-shaped oxidation peak at very positive potential (+1.5 V vs. Ag/AgCl electrode) in strong acidic medium. At optimized differential pulse voltammetric parameters, the current response of picloram was proportionally linear in the concentration range from 0.5 to 48.07 mol L−1 and the low limit of detection of 70 nmol L−1 as well as good repeatability (relative standard deviation of 2.6% at 10 mol L−1 for n = 11) were obtained on unmodified boron-doped diamond film electrode. The proposed method was successfully applied in analysis of environmental (tap and natural water) and biological (human urine) samples spiked with picloram with good accuracy (relative standard deviations less than 5% for all samples, n = 5). By this way, the boron-doped diamond could introduce a green (environmentally acceptable) alternative to mercury electrodes for the monitoring of herbicides. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Pesticides became an important part of the protection of agricultural production and of the improvements in rangeland during the last century. Application of pesticides solves the problems with unwanted pests or weed, but it unfortunately constitutes a risk of the environment contamination. Pesticides are generally toxic for living organisms even in low concentrations. Moreover, they are oftentimes persistent and can be accumulated in the human body through the food chain. Although, many of them have been forbidden or restricted to use in many countries, many pesticides are still frequently applied for pest or weed control [1]. Picloram (4-amino-3,5,6-trichloropyridine-2-carboxylic acid; PCR) is a systematic herbicide, which belongs to a group of pyridine herbicides [2] and was firstly introduced to the marketplace in 1963 by Dow Chemical Company [3,4]. It is the most persistent member of this herbicide family and its half-life in soils may vary
∗ Corresponding author. Tel.: +420 466 038 080. E-mail addresses: [email protected], [email protected], [email protected] (L. Bandˇzuchová). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.08.071
from one month to three years and depends on application rate, soil type, temperature or moisture [2,5–13]. PCR is not expected to be adsorbed on soils but it could be leached to the groundwater due to its high mobility in soils and the good water solubility (430 mg L−1 ), which could lead to contamination of natural waters. This problem was described more in detail in various papers [3,6,8,12–15]. Concerning the major degradation pathway of PCR in soils, the relatively slow microbial decomposition was observed. PCR can also be degraded by sunlight, when it is directly exposed in water or on the surface of plants or soil. The hydrolysis of PCR is negligible [2,5,12,13]. Determination of PCR in environmental samples is highly current because of its persistence and high possibility of bioaccumulation. Moreover, PCR is still wide-spread used for protection of the crop or management of the unwanted vegetation in a rangeland, forestry, railways, roads and airports [3]. Many analytical, especially chromatographic methods have been developed for the determination of PCR in environmental samples, e.g. gas chromatography (GC) with various detection techniques for determination in soils [16,17], water [15], fishes [18] or different aqueous matrices [19], liquid chromatography (LC) or high performance liquid chromatography (HPLC) with various detectors for analysis of
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PCR content in soils [20,21], fruit juices [22], rape plant and soil [23] or for the monitoring of PCR biosorption from water to biomasses [24]. Recently, fluorescence [25] and derivative spectrophotometric [26] determinations of this herbicide have been introduced. Electrochemical methods represent an appropriate alternative to the above mentioned analytical methods, particularly due to the relatively low costs of instrumentation, possibility of miniaturization, time saving and sensitive performance of analysis [27]. Regarding the determination of PCR, Gilbert and Mann used dropping mercury electrode (DME) and differential pulse polarography (DPP) based on catalytic hydrogen electrochemical process for analysis of natural waters containing PCR with limit of detection (LD) equaled to 83 nmol L−1 [28]. The sensitive polarographic method for determination of PCR (LD = 60 nmol L−1 ) and another herbicide Dowco 229 was described by Whittaker and Osteryoung [29]. The static mercury drop electrode (SMDE) in the connection with square-wave voltammetric (SWV) mode was utilized as a sensitive tool (LD = 44–156 nmol L−1 ) for analysis of natural waters containing PCR [30]. Two reduction signals of PCR were described about potentials −0.90 and −0.95 (vs. Ag/AgCl) [30]. The determination of PCR in natural waters performed by sequential injection (SI) SWV using hanging mercury drop electrode (HMDE) was described by Santos and Masini [31]. The two reduction peaks at −0.81 V and −0.85 V vs. Ag/AgCl were observed and the LD value of 149 nmol L−1 was obtained [31]. A mechanism of electrochemical reduction of PCR and clopyralid (other member of pyridine herbicide family) on mercury electrodes was studied by Mellado et al. using mercury pool electrode and the products of electrode reaction were identified by GC/MS [32]. The same authors also dealt with adsorption–desorption processes of PCR on mercury electrodes [33]. An immunosensor based on three-dimensional gold nanoclusters was utilized for the determination of PCR with good sensitivity (LD = 2.1 nmol L−1 ) and proposed amperometric method was applied for the analysis of peach extract and excess sludge supernatant [34]. Development of new electrode materials which could replace the liquid mercury due to its alleged toxicity is one of the current trends of electrochemistry. Boron-doped diamond is a one of the novel carbon-based material, which has been studied more in detail in last 20 years [35,36]. The most relevant feature of this material is high overpotential for the oxygen and hydrogen evolution causing the fact that boron-doped diamond electrodes have absolutely the widest working potential range of currently using working electrodes. The other important properties are high thermal conductivity, high hardness and chemical inertness, extreme electrochemical stability in an alkaline and an acidic media, very low and stable background current [35–39]. Boron-doped diamond film (BDDF) electrodes have successfully been applied in voltammetric analysis of various biological active compounds, e.g. this research group has recently proposed the voltammetric methods using BDDF electrode for the determination of caffeine [40], penicillin V [41] and simultaneous determination of penicillin V and paracetamol [42], codeine [43] and atrazine [44]. This electrode was also applied as an anode in electrochemical incineration of various pesticides [45]. The voltammetric behavior and determination of PCR on unmodified BDDF electrode is described in present paper. In respect of other voltammetric methods based on electrochemical reduction of PCR, our approach solely consists in electrochemical oxidation of PCR at very positive potentials confirming the advantages and feasibility of using BDDF electrode. Generally, to the best of our knowledge, a detection and determination of PCR at this perspective and nontoxic type of electrochemical sensor has not been previously investigated in literature until now. Optimum working conditions for differential pulse voltammetric determination of PCR were found and proposed sensitive method was employed
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in the analysis of environmental and biological samples spiked with PCR. 2. Experimental 2.1. Chemicals All chemicals used for the preparation of supporting electrolytes, standard and other solutions were of p.a. purity. All solutions were prepared using with double-distilled deionized water with resistivity greater than 18 M cm. Picloram (CAS No. 1918-02-1, Sigma–Aldrich, Czech Republic) was dissolved in 50% acetonitrile (Lach-ner, Czech Republic) and the standard solution was stored in a glass flask in a refrigerator. PCR working solutions were prepared daily by dilution of the stock solution with the supporting electrolyte. Various supporting electrolytes such as nitric acid, perchloric acid and sulfuric acid were purchased from Lachema (Brno, Czech Republic). Britton–Robinson buffer solution (BR) was prepared by mixing of the same concentrations (0.04 mol L−1 ) of orthophosphoric acid, boric acid and acetic acid (all three purchased from Lachema, Brno, Czech Republic) in deionized water and adjusting to the desired pH value with 0.2 mol L−1 sodium hydroxide (Lachema, Czech Republic). The stock solutions of compounds used as interfering agents (ascorbic acid, urea, uric acid, sucrose, barbituric acid and creatinine (all Sigma–Aldrich, Czech Republic)) were prepared by dissolving of calculated amount in deionized water. Stock solution of folic acid (Sigma–Aldrich, Czech Republic) was prepared by dissolving of an appropriate amount in 0.01 mol L−1 NaOH (Lachema, Brno, Czech Republic). 2.2. Instrumentation The voltammetric measurements were performed with AUTOLAB PGSTAT 302N (Metrohm Autolab B.V., The Netherlands) potentiostat/galvanostat controlled by NOVA 1.7 electrochemical software. All measurements were provided in three-electrode set up, where Ag/AgCl/3 mol L−1 KCl and platinum wire served as a reference and an auxiliary electrode, respectively. BDDF electrode inserted in polyether ether ketone body with inner diameter of 3 mm, resistivity of 0.075 cm and boron doping level of 1000 ppm (declared by Windsor Scientific Ltd., United Kingdom as a producer) was used as a working electrode. All the pH values of solutions were measured using pH meter Model 215 (Denver Instrument, USA) with a combined electrode (glass-reference electrode), which was daily calibrated with standard buffer solutions. Standard solution of PCR was prepared using ultrasonic bath Bandelin Sonorex (Schalltec GmbH, Germany). All the potentials reported in this paper were given against Ag/AgCl/3 mol L−1 KCl at a laboratory temperature of 25 ± 1 ◦ C. 2.3. Measurement procedures A known volume of standard solution of PCR was pipetted into a 20 mL volumetric flask and then filled up with the supporting electrolyte. This solution was subsequently transferred quantitatively into voltammetric cell. Cyclic voltammetry (CV) was used for investigation of dependence between voltammetric response of PCR and pH of the supporting electrolyte and for study of the effect of scan rate on voltammetric behavior of PCR. Differential pulse voltammetry (DPV) was used for the purpose of quantification. Five cyclic voltammograms were obtained for each measurement, and the last scan was always considered for evaluation and making the figures reported in this paper. DP voltammograms were recorded after optimization of instrumental parameters (modulation amplitude, modulation time, scan rate and initial potential). The calibration
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curve was constructed from the average of five replicate measurements for each calibration solution of PCR. The peak currents (Ip ) recorded using CV and DPV were evaluated from the straight lines connecting the minima before and after the peak maximum. In order to clean BDDF electrode surface prior to start the first voltammetric measurement, we have performed CV from −2 V to +2 V for 10 min after immersion of electrode into 1 mol L−1 HNO3 , then rinsed with deionized water and polished with a piece of damp silk cloth until a mirror-like character of surface was obtained. To renew a hydrogen-terminated surface of BDDF electrode once a week, it was exposed to microwave-inducted hydrogen plasma for 5 min. The linear least-square regression in OriginPro 7.5 (OriginLab Corporation, USA) was used for the evaluation of calibration curve and the relevant results (slope and intercept) were reported with confidence interval for 95% probability. The limit of detection was calculated as three times the standard deviation for the blank solution (supporting electrolyte) divided by the slope of the calibration curve [46]. 2.4. Preparation of environmental and biological samples The tap water was sampled from the water supply in Bratislava. The samples of natural waters were collected from the rivers Elbe (Pardubice, Czech Republic) and Danube (Bratislava, Slovak Republic) and from the nameless brook which is closed to the agricultural area (Kameniˇcany, Northwest part of the Slovak Republic). The water samples were analyzed with no further pretreatment or purification. None of the water samples contained the measurable amount of PCR, thus, they were spiked with stock solution of PCR (200 L of 1 mmol L−1 in 100 mL of water sample) to concentration level of 2 mol L−1 (0.5 mg L−1 ), which is declared by EPA as a Maximum Contaminant Level for PCR [47]. 10 mL of spiked water was diluted with the supporting electrolyte (1 mol L−1 H2 SO4 ) to 20 mL. This solution was subsequently transferred quantitatively into the voltammetric cell and analyzed. The human urine sample was obtained from non-smoker female volunteer of the age of 28. The sample was stored in the refrigerator after sampling and analyzed without any further pretreatment about 15 h after sampling. 1 mL of the urine was pipetted into a volumetric flask, then spiked with stock solution of PCR (50 L of 1 mmol L−1 ) a filled up with the supporting electrolyte (1 mol L−1 H2 SO4 ) to 20 mL. The prepared solution was quantitatively transferred into the voltammetric cell and analyzed. The content of PCR in all samples was determined by standard addition method, when at least two standard additions were added. Each determination was repeated five times and relative standard deviation of 5 repeated determinations (RSDD (5)) was calculated [46].
Fig. 1. The cyclic voltammogram of PCR recorded in H2 SO4 on BDDF electrode. Experimental conditions: method: CV; parameters: Ein = −1.5 V, Efin = 2 V, v = 100 mV s−1 ; supporting electrolyte: 1 mol L−1 H2 SO4 ; c(PCR) = 0.9 mmol L−1 .
electrode is totally irreversible electrochemical process. Concerning BR buffer solution, no voltammetric peak of PCR was observed at pH 4 and higher (results not shown). The best results from all studied supporting electrolytes on the basis of well developed and repeatable peak and the highest current response of PCR were obtained in sulfuric acid. The cyclic voltammogram of absence (dotted line) and presence (continuous line) of PCR (0.9 mmol L−1 ) in 1 mol L−1 H2 SO4 is shown in Fig. 1, where the arrows indicate a direction of the scan. It is apparent that the residual current appeared to be low at very positive potentials on BDDF electrode confirming its excellent properties for determination of electrochemically highly oxidizable analytes. The influence of concentration of sulfuric acid in range from 0.01 to 2 mol L−1 was also examined. The peak height increased with ascending concentration of H2 SO4 as evidenced in Fig. 2. The highest current response of PCR was observed in the most concentrated H2 SO4 . The difference of the PCR peak height provided in 2 mol L−1 and 1 mol L−1 H2 SO4 was not considered to be significant. The peak potential of PCR did not change significantly with variation of concentration of H2 SO4 and this dependence is also shown in Fig. 2. Following this fact, 1 mol L−1 H2 SO4 was chosen as an
3. Results and discussion 3.1. Effect of supporting electrolyte on voltammetric behavior of PCR Generally, the supporting electrolyte plays an important role in an electrode reaction of studied analyte because it can modify the thermodynamics and kinetics of electrochemical processes and charge transfer within the cell. In our case, the effect of various supporting electrolytes such as nitric acid, sulfuric acid, perchloric acid and Britton–Robinson (BR) buffer solution on voltammetric behavior of 0.9 mmol L−1 PCR was investigated in whole working potential range on BDDF electrode. It was found that PCR provided one well-shaped oxidation (anodic) peak at very positive potential of about +1.5 V in strong acidic medium. The absence of reduction peak indicated, that the electrode reaction of PCR on BDDF
Fig. 2. Effect of concentration of sulfuric acid on the peak height (, Ip ) and peak position (, Ep ) of PCR recorded on BDDF electrode. Experimental conditions: method: CV; parameters: Ein = −1.5 V, Efin = 2 V, v = 100 mV s−1 ; supporting electrolyte: 0.01, 0.025, 0.05, 0.1, 0.5, 1 and 2 mol L−1 H2 SO4 ; c(PCR) = 0.9 mmol L−1 .
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Fig. 3. Voltammetric response of PCR depending on scan rate recorded on BDDF electrode. Experimental conditions: method: CV; parameters: Ein = −1.5 V, Efin = 2 V, v: (a) 10, (b) 25, (c) 50, (d) 75, (e) 100, (f) 150, (g) 200 and (h) 250 mV s−1 ; supporting electrolyte: 1 mol L−1 H2 SO4 ; c(PCR) = 0.9 mmol L−1 . Inset: Dependence of the peak current of PCR peak (Ip ) on square root of scan rate (v1/2 ).
optimal and from our point of view not so concentrated supporting electrolyte for all subsequent measurements due to the sufficient current response and well-shaped oxidation peak of PCR. The electrochemical behavior of PCR on BDDF electrode is not consistent with those reported for mercury electrodes [28–32]. Two reduction signals provided by PCR at potentials about −0.90 V and −0.95 V vs. Ag/AgCl [30] and at −0.81 V and −0.85 V vs. Ag/AgCl [31], respectively, in the acidic medium on the mercury electrodes were described, e.g. in the papers [30,31]. The anodic signal of PCR on BDDF electrode was observed at very positive potential, which could not be applied for mercury electrodes. Even, when the potential range from −1.5 V to +2 V was applied, no reduction peaks of PCR were recorded on BDDF electrode.
3.2. The effect of scan rate The useful information including the electrochemical mechanism (rate-limiting step) may be observed from the dependence of the peak current on the scan rate. The influence of scan rate (v) on the current response of 0.9 mmol L−1 PCR in 1 mol L−1 H2 SO4 was studied in the range from 10 to 250 mV s−1 using CV. Obtained voltammetric curves are shown in Fig. 3. It was found that the peak current of PCR peak increased linearly with the square root of the scan rate (inset of Fig. 3) and this dependence may be described by Eq. (1) Ip (A) = (26.181 ± 0.355)v1/2 (V s−1 ) (R = 0.998)
1/2
+ (0.195 ± 0.011) (1)
This result suggested a diffusion-control process in electrode reaction of PCR on unmodified BDDF electrode. This fact is oftentimes typical for this electrode surface in determination of organic compounds due to its low adsorption properties. Moreover, the peak potential of the registered current response slightly shifts to the more positive potential values with increasing scan rate thus confirming the irreversible character of the electrode reaction of PCR.
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Fig. 4. DP voltammograms of PCR recorded on BDDF electrode. Experimental conditions: parameters: Ein = 0.6 V, Efin = 2 V, v = 20 mV s−1 , modulation amplitude: 75 mV, modulation time: 25 ms; supporting electrolyte: 1 mol L−1 H2 SO4 ; c(PCR): (a) 0.50, (b) 1.00, (c) 1.50, (d) 2.00, (e) 2.49, (f) 2.99, (g) 3.49, (h) 4.48, (i) 5.47, (j) 6.46, (k) 7.44, (l) 8.43, (m) 9.41, (n) 10.39, (o) 20.09, (p) 29.60, (q) 38.92 and (r) 4807 mol L−1 . Curves are after baseline correction. Insets: Dependence of height of PCR peak (Ip ) on its concentration (c).
3.3. Analytical performance 3.3.1. Optimization of DPV operating parameters DPV is considered to be an effective electrochemical technique, which has already been applied for analysis of numerous biologically and electrochemically active compounds. The optimization of DPV operating parameters influencing the current response of analyte is an important step in the development of electroanalytical methodology. Accordingly, the instrumental parameters such as an initial potential (Ein ), modulation amplitude, modulation time and scan rate (v) were investigated in order to optimize the experimental set-up for determination of PCR. All experiments were carried out at 10 mol L−1 PCR in 1 mol L−1 H2 SO4 . Firstly, the influence of initial potential to the peak current was studied in the range from 0 to +1.4 V. The peak current changed negligibly applying Ein = 0–0.8 V but when the higher Ein was set up, the oxidation peak rapidly started to decrease. Hence, Ein = 0.6 V was chosen for all subsequent measurements because of sufficient intensity of peak current and reasonable time of analysis. Consequently, the modulation amplitude was changed from 5 to 150 mV at fixed modulation time of 50 ms. It was found that the oxidation peak of PCR slightly shifted to the less positive potential values together with grow and expansion of peak. The modulation amplitude of 75 mV was selected as an optimal for all subsequent analysis thank to convenient peak current and potential. In the case of modulation time, the peak current decreased with the increase of modulation time in the range of 10–100 ms. The most stable peak current was observed at 25 ms at fixed value of modulation amplitude at 75 mV. Concerning the scan rate, the peak current increased with rising scan rate from 10 to 100 mV s−1 , however, when the higher scan rates were applied, the peak started to expand and deform. As a suitable value of scan rate from the view of shape and intensity of peak as well as appropriate time of analysis, 20 mV s−1 was chosen. DPV with optimized operating parameters was applied for a construction of the calibration curve. 3.3.2. Determination of PCR Calibration curve was constructed by plotting of the peak current against PCR concentration in the range from 0.5 to 48.07 mol L−1 as depicted in Fig. 4. The oxidation peak increased
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Table 1 Influence of potential interfering agents (IA) on the voltammetric response of 5 mol L−1 PCR. Concentration ratio of PCR and IA
1:1 1:10 1:100
Signal change of PCR in presence of interference agent (%)
UA
FA
U
S
AA
BA
C
+1 −1.8 −9
−3.2 −65 −
<0.5 <0.5 <0.5
<0.5 −2.2 −0.6
<0.5 −2.1 −17
+1.3 +2.1 +4.2
−1.7 −1.6 −1.8
Abbreviations: UA, uric acid; FA, folic acid; U, urea; S, sucrose; AA, ascorbic acid; BA, barbituric acid; C, creatinine.
of PCR was mostly influenced by presence of excesses of FA. The 10 times excess of FA affected the significant decrease of PCR signal, while in the case of 100 times excess the current response of PCR completely disappeared. UA and AA also caused slightly decrease of the signal but only when the concentration of UA and AA, respectively, was 100 times higher than concentration of PCR. Other substances did not influence the response significantly under applied conditions. According to these results, it can be concluded that the satisfactory selectivity of proposed method in the voltammetric determination of PCR was attained. 3.4. Application of proposed method to analysis of environmental and biological samples
Fig. 5. Comparison of DP voltammograms of PCR and particular interfering agents (IA) in the concentration ratio 1:10. Experimental conditions: method: DPV, parameters: Ein = 0.6 V, Efin = 2 V, v = 20 mV s−1 , modulation amplitude: 75 mV, modulation time: 25 ms; supporting electrolyte: 1 mol L−1 H2 SO4 ; c(PCR) = 5 mol L−1 ; c(IA) = 50 mol L−1 .
linearly in this concentration range (inset in Fig. 4) and could be described by Eq. (2) Ip (A) = (97547 ± 397)c (mol L−1 ) + (0.0369 ± 0.0023)
(2)
70 nmol L−1
The limit of detection was considered to be and this low value confirms the high sensitivity of proposed method. The repeatability of measurement was tested by application of 11 repeated scans at 10 mol L−1 concentration level of PCR under the proposed operating parameters and the relative standard deviation of 11 repeated measurements (RSDM (11)) was calculated. Its low value (2.6%) verified an excellent repeatability of the procedure and confirming the minimal adsorption of PCR oxidation product on the BDDF electrode surface without need of any regeneration of the surface. The proposed procedure proved to be suitable for precise detection and quantification of PCR. 3.3.3. Interference studies A selectivity of proposed procedure and the effect of possible interfering agents were examined under found operating conditions at fixed concentration of PCR (5 mol L−1 ). Some biomolecules potentially present in biological samples (human urine) such as uric acid (UA), folic acid (FA), urea (U), sucrose (S), ascorbic acid (AA), barbituric acid (BA) and creatinine (C) were chosen as possible interfering electroactive compounds. The results obtained for three various concentration ratios (1:1, 1:10 and 1:100) of studied biomolecules are summarized in Table 1. The comparison of voltammetric curves of PCR and particular interferent in concentration ratio 1:10 (PCR:biomolecule) is shown in Fig. 5. The substance was considered to interfere seriously when it gave a PCR signal change more than 5%. The presence of more tested substances at the same concentration level as PCR (5 mol L−1 ) did not affect followed signal significantly. However, the peak current
As it was described in the previous chapters, our proposed voltammetric method is very sensitive and sufficiently selective. Therefore, it could be applied for the determination of PCR in various environmental and biological samples. Firstly, the tap water and the water from the rivers Elbe and Danube as well as from the brook were spiked (as it was described in Section 2.4) to a concentration level 2 mol L−1 (0.5 mg L−1 ). Standard addition method was applied for the determination of PCR and each sample was analyzed 5 times. The obtained results expressed as confidence interval with 95% probability with particular relative standard deviations from 5 repeated determinations (RSDD (5)) are summarized in Table 2. It is obvious, that determined values well corresponded with the added contents and the proposed method could be found as reliable for the analysis of tap and natural water samples. The particular relative standard deviations showed the high reproducibility of determinations. The illustrative examples of DP voltammograms for PCR determination in the water sample from Danube (A) and the brook (B) are shown in Fig. 6. Concerning the biological samples, it is generally known that PCR is rapidly absorbed through the gastrointestinal tract and unchanged form could be passed into urine [2]. Therefore, the applicability of proposed method for the analysis of PCR in human urine is justified and also was tested. The sample of spiked human urine was analyzed without any complex pretreatment only after simple dilution of the urine. The obtained DP voltammograms are depicted in Fig. 7. The curve a (dotted line) presented the diluted urine in the absence of PCR. It is clearly shown that the observed peak at about +1.55 V is assigned to the PCR oxidation since this peak increases after each PCR standard addition. No attempts have been carried out to identify the other peaks at about +0.6 and +0.7 V in DP voltammograms which are likely to be due to the oxidation of common urinary compounds. It is also obvious, that especially due to the very positive peak potential, there is no significant signal, which could interfere with PCR determination in human urine. The results are summarized in Table 2. It is evident, that obtained results are consistent with added amount of PCR and the method is accurate according to low RSDD (5). 3.4.1. Comparison with other methods The comparison between the analytical performance of our proposed method and other electrochemical methods for
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Table 2 Results of repeated determinations for PCR in various model samples (n = 5). Sample
Added (mol L−1 )
Determined* (mol L−1 )
Tap water Danube Elbe Brook water Urine
2 2 2 2 50
1.97 2.03 1.98 1.97 49.04
*
± ± ± ± ±
RSDD (5) (%)
0.06 0.04 0.04 0.06 1.33
4.4 2.5 3.1 4.4 4.1
Confidence interval calculated according [¯x ± tn−1,˛ SD/sqt(n)]; t4;0.05 = 2.1318
Table 3 Comparison of the proposed voltammetric method with previously reported electrochemical methods for determination of PCR. Electrode
Supporting electrolyte
Technique
Peak potential (V)
Linear range (mol L−1 )
DME DME SMDE HMDE BDDFE
ABS, pH 4.35 Na2 SO4 + 2EE, pH 1.9 0.1 mol L−1 H2 SO4 0.1 mol L−1 H2 SO4 1 mol L−1 H2 SO4
DPP DPP SWV SI-SWV DPV
−1.28 (vs. SCE) −0.95 (vs. SCE) −0.90 (vs. Ag/AgCl) −0.82 (vs. Ag/AgCl) +1.55 (vs. Ag/AgCl)
0.083–9.90 0.10–50 0.16–2.1 0.41–10.5 0.5–48.1
LD (nmol L−1 ) 83 60 44–156 149 70
Sample analyzed
References
Natural water – Natural water Natural water Tap and natural water, human urine
[28] [29] [30] [31] Present paper
Abbreviations: DME, dropping mercury electrode; ABS, acetate buffer solution; DPP, differential pulse polarography; SCE, saturated calomel electrode; 2EE; 2-ethoxyethanol; HMDE, hanging mercury drop electrode; SWV, square-wave voltammetry; SMDE, static mercury drop electrode; SI, sequential injection; BDDFE, boron-doped diamond film electrode; DPV, differential pulse voltammetry; LD, limit of detection.
Fig. 6. Determination of PCR in water from Danube (A) and the brook (B). Experimental conditions: method: DPV, parameters: Ein = 0.6 V, Efin = 2 V, v = 20 mV s−1 , modulation amplitude: 75 mV, modulation time: 25 ms; supporting electrolyte: 1 mol L−1 H2 SO4 ; (a) sample spiked with PCR, (b–e) standard additions of PCR. Curves are after baseline correction.
Fig. 7. Determination of PCR in human urine. Experimental conditions: method: DPV, parameters: Ein = 0.6 V, Efin = 2 V, v = 20 mV s−1 , modulation amplitude: 75 mV, modulation time: 25 ms; supporting electrolyte: 1 mol L−1 H2 SO4 ; (a) diluted human urine in absence of PCR, (b) diluted human urine in presence of PCR, (c–e) standard additions.
determination of PCR from last decade is summarized in Table 3. On the basis of literature survey, the mercury electrodes in various constructions (DME, SMDE and HMDE) have mostly been applied for the voltammetric analysis of PCR in connection with differential pulse polarography and square-wave voltammetry, respectively [28–31]. In this case, no solid or paste working electrodes have been utilized according to accessible literature. It is obvious from Table 3, that the proposed method reached a comparable LD with other voltammetric methods [28–30] with the exception of the use of sequential analysis with LD about once higher [31]. Moreover, two reduction signals recorded in the presence of PCR on mercury electrodes were located closed to each other, e.g. at −0.90 and −0.95 vs. Ag/AgCl [30] or at −0.81 V and −0.85 V vs. Ag/AgCl [31], affecting their possible overlapping, which could worsen possibilities of its determination due to lower ability of quantification of PCR. On the other hand, the oxidation signal, registered on unmodified BDDF electrode and described in the present paper, was well developed and had satisfactory position from the selectivity point of view in spite of high positive peak potential of PCR. However, the use of conventional carbon-based electrodes (graphite, glassy carbon, carbon paste) in determination of PCR would be
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inconvenient due to oxygen generation on these surfaces at higher positive potentials. Thus, BDDF electrode with DPV technique possesses the benefits of modern electroanalytical chemistry (rapidity, simplicity, sensitivity, repeatability and low cost). It could also represent the nontoxic (green) alternative for the environmental monitoring of trace amounts of pyridine herbicides and other harmful organic compounds. 4. Conclusion Unmodified BDDF electrode was applied for the first time in combination with DPV technique to elaborate the sensitive, simple and cheap analytical method for determination of PCR. The method was based on the highly irreversible electrochemical oxidation of PCR at very positive potentials in strong acidic medium instead of reduction process typical for mercury electrodes. The fully validated analytical procedure was exploited for the analysis of environmental (tap and natural water) and biological (human urine) samples with good accuracy. The method enables the determination of PCR with sensitivity below the maximum contaminant level for PCR declared by EPA and could be applied directly without any difficult sample preparation (just simple dilution). The selectivity of method appears to be sufficient as there were no substantial interferences present from matrix. It can be concluded that the proposed voltammetric method can undoubtedly be considered as an effective, sensitive and green (environmentally acceptable) tool in analysis of PCR and other herbicides as well as may represent the electrochemical alternative to mercury electrodes. Acknowledgements This work was supported by The Ministry of Education, Youth and Sports of the Czech Republic (project No. CZ.1.07/2.3.00/30.0021 “Strengthening of Research and Development Teams at the University of Pardubice”), the Grant Agency VEGA of the Slovak Republic (grant No. 1/0051/13) and the Slovak Research and Development Agency under the Contract No. APVV0797-11. References [1] S.N. Rekha, R. Naik, Prasad, Pesticide residue in organic and conventional foodrisk analysis, J. Chem. Health Saf. 13 (2006) 12. [2] M. Tu, C. Hurd, J.M. Randall, Weed Control Methods Handook, The Nature Conservancy, 2001, http://www.invasive.org/gist/handbook.html (30.4.2013). [3] FAO, Specification and Evaluations for Picloram (Evaluation Report 174/2004), 2004. [4] Dow Chemical Corporate, http://www.dow.com/ (30.4.2013). [5] M.G. Merkle, R.W. Bovey, F.S. Davis, Factors affecting the persistence of picloram in soil, Agron. J. 59 (1967) 413. [6] C.J. Scifres, R.R. Hahn, J. Diaz-Colon, M.G. Merkle, Picloram persistence in semiarid rangeland soils and water, Weed Sci. 19 (1971) 381. [7] J.D. Fryer, P.D. Smith, J.W. Ludwig, Long-term persistence of picloram in a sandy loam soil, J. Environ. Qual. 8 (1979) 83. [8] T.N. Johnsen, Picloram in water and soil from semiarid pinyon-juniper watershed, J. Environ. Qual. 9 (1980) 601. [9] D.G. Neary, P.B. Bush, J.E. Douglass, R.L. Todd, Picloram movement in an appalachian hardwood forest, J. Environ. Qual. 14 (1985) 585. [10] V.J. Watson, P.M. Rice, E.C. Monnig, Environmental fate of picloram used for roadside weed control, J. Environ. Qual. 18 (1989) 198. [11] M.E. Close, L. Pang, J.P.C. Watt, K.W. Vincent, Leaching of picloram, atrazine and simazine through two New Zealand Soils, Geoderma 84 (1998) 45. [12] Canadian Council of Ministers of the Environment, Canadian Water Quality Guidelines for Protection of Aquatic Life: Picloram, Winnipeg, 1999, ISBN 1896997-34-1. [13] C. Britt, A. Mole, F. Kirkham, A. Terry, D. Arnold, J. Clarke, R. McLaren, A. Gundrey, S. McMillan, The Herbicide Handbook, English Nature in Assoc. with FACT, Wetherby, 2003, ISBN 1-85716-7465. [14] D.W. Kolpin, J.E. Barbash, R.J. Gillion, Pesticides in Ground Water of the United States, Groundwater 38 (2000) 858, 1992-1996. [15] J.R. Baur, R.W. Bovey, M.G. Merkle, Concentration of picloram in runoff water, Weed Sci. 20 (1972) 309.
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Huang, Highly sensitive fluorescence quantification of picloram using immunoirecognition liposome, Talanta 83 (2010) 210. ˇ c, ´ V.B. Anderluh, F.F. Gaál, D.V. Soji ´ Derivative spectro[26] B.F. Abramovic, photometric determination of the herbicides Picloram and Triclopyr in mixtures, J. Serb. Chem. Soc. 72 (2007) 809. [27] E.M. Garrido, C. Delerue-Matos, J.L.F.C. Lima, A.M.O. Brett, Electrochemical methods in pesticides control, Anal. Lett. 37 (2004) 1755. [28] D.D. Gilbert, J.M. Mann, Herbicide analysis by pulse polarography-picloram, Int. J. Environ. Anal. Chem. 2 (1973) 221. [29] J.W. Whittaker, J. Osteryoung, Determination of picloram and Dowco 290 by pulse polarography, J. Agric. Food Chem. 28 (1980) 89. [30] M.R.C. Massaropi, S.A.S. Machado, L.A. Avaca, Electroanalytical determination of the herbicide picloram in natural waters by square wave voltammetry, J. Braz. Chem. Soc. 14 (2003) 113. [31] L.B.O. dos Santos, J.C. Masini, Determination of picloram in natural waters employing sequential injection square wave voltammetry using hanging mercury drop electrode, Talanta 72 (2007) 1023. [32] J.M.R. Mellado, M.C. Corredor, L. Pospíˇsil, M. Hromadová, Electrochemical reduction of pyridine herbicides picloram and clopyralid on a mercury pool electrode, Electroanalysis 17 (2005) 979. [33] J.M.R. Mellado, M.C. Corredor, M. Ruiz Montoya, L. Pospíˇsil, M. Hromadová, A Voltammetric study of the adsorption–desorption processes in the reduction of the herbicide picloram on mercury electrodes, J. Electrochem. Soc. 152 (2005) E379. [34] L. Chen, G. Zeng, Y. Zhang, L. Tang, D. Huang, C. Liu, Y. Pang, J. Luo, Trace detection of picloram using an electrochemical immunosensor based on threedimensional gold nanoclusters, Anal. Biochem. 407 (2010) 172. [35] K. Pecková, J. Musilová, J. Barek, Boron-doped diamond film electrodes-new tool for voltammetric determination of organic substances, Cr. Rev. Anal. 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Bustin, Voltammetric determination of caffeine in beverage samples on bare boron-doped diamond electrode, Food Chem. 135 (2012) 1198. ˇ [41] L’. Svorc, J. Sochr, M. Rievaj, P. Tomˇcík, D. Bustin, Voltammetric determination of penicillin V in pharmaceutical formulations and human urine using a borondoped diamond electrode, Bioelectrochemistry 88 (2012) 36. ˇ [42] L’. Svorc, J. Sochr, P. Tomˇcík, M. Rievaj, D. Bustin, Simultaneous determination of paracetamol and penicillin V by square-wave voltammetry at a bare borondoped diamond electrode, Electrochim. Acta 68 (2012) 227. ˇ [43] L’. Svorc, J. Sochr, J. Svítková, M. Rievaj, D. Bustin, Rapid and sensitive electrochemical determination of codeine in pharmaceutical formulations and human urine using a boron-doped diamond film electrode, Electrochim. Acta 87 (2013) 503.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Voltammetric method for sensitive determination of herbicide picloram in environmental and biological samples using boron-doped diamond film electrode b a ˇ ´ Lenka Bandˇzuchová a,∗ , L’ubomír Svorc , Jozef Sochr b , Jana Svítková b , Jaromíra Chylková a Institute of Environmental and Chemical Engineering, Faculty of Chemical Technology, University of Pardubice, Studentská 573, Pardubice CZ-532 10, Czech Republic b Institute of Analytical Chemistry, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, Bratislava SK-812 37, Slovak Republic
a r t i c l e
i n f o
Article history: Received 12 June 2013 Received in revised form 29 July 2013 Accepted 15 August 2013 Available online 23 August 2013 Keywords: Picloram Boron-doped diamond film electrode Differential pulse voltammetry Environmental sample Human urine
a b s t r a c t The voltammetric behavior and determination of picloram, a member of a pyridine herbicide family, was for the first time investigated on a boron doped diamond film electrode using cyclic and differential pulse voltammetry. The influence of supporting electrolyte and scan rate on the current response of picloram was examined to select the optimum experimental conditions. It was found that picloram provided one well-shaped oxidation peak at very positive potential (+1.5 V vs. Ag/AgCl electrode) in strong acidic medium. At optimized differential pulse voltammetric parameters, the current response of picloram was proportionally linear in the concentration range from 0.5 to 48.07 mol L−1 and the low limit of detection of 70 nmol L−1 as well as good repeatability (relative standard deviation of 2.6% at 10 mol L−1 for n = 11) were obtained on unmodified boron-doped diamond film electrode. The proposed method was successfully applied in analysis of environmental (tap and natural water) and biological (human urine) samples spiked with picloram with good accuracy (relative standard deviations less than 5% for all samples, n = 5). By this way, the boron-doped diamond could introduce a green (environmentally acceptable) alternative to mercury electrodes for the monitoring of herbicides. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Pesticides became an important part of the protection of agricultural production and of the improvements in rangeland during the last century. Application of pesticides solves the problems with unwanted pests or weed, but it unfortunately constitutes a risk of the environment contamination. Pesticides are generally toxic for living organisms even in low concentrations. Moreover, they are oftentimes persistent and can be accumulated in the human body through the food chain. Although, many of them have been forbidden or restricted to use in many countries, many pesticides are still frequently applied for pest or weed control [1]. Picloram (4-amino-3,5,6-trichloropyridine-2-carboxylic acid; PCR) is a systematic herbicide, which belongs to a group of pyridine herbicides [2] and was firstly introduced to the marketplace in 1963 by Dow Chemical Company [3,4]. It is the most persistent member of this herbicide family and its half-life in soils may vary
∗ Corresponding author. Tel.: +420 466 038 080. E-mail addresses: [email protected], [email protected], [email protected] (L. Bandˇzuchová). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.08.071
from one month to three years and depends on application rate, soil type, temperature or moisture [2,5–13]. PCR is not expected to be adsorbed on soils but it could be leached to the groundwater due to its high mobility in soils and the good water solubility (430 mg L−1 ), which could lead to contamination of natural waters. This problem was described more in detail in various papers [3,6,8,12–15]. Concerning the major degradation pathway of PCR in soils, the relatively slow microbial decomposition was observed. PCR can also be degraded by sunlight, when it is directly exposed in water or on the surface of plants or soil. The hydrolysis of PCR is negligible [2,5,12,13]. Determination of PCR in environmental samples is highly current because of its persistence and high possibility of bioaccumulation. Moreover, PCR is still wide-spread used for protection of the crop or management of the unwanted vegetation in a rangeland, forestry, railways, roads and airports [3]. Many analytical, especially chromatographic methods have been developed for the determination of PCR in environmental samples, e.g. gas chromatography (GC) with various detection techniques for determination in soils [16,17], water [15], fishes [18] or different aqueous matrices [19], liquid chromatography (LC) or high performance liquid chromatography (HPLC) with various detectors for analysis of
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PCR content in soils [20,21], fruit juices [22], rape plant and soil [23] or for the monitoring of PCR biosorption from water to biomasses [24]. Recently, fluorescence [25] and derivative spectrophotometric [26] determinations of this herbicide have been introduced. Electrochemical methods represent an appropriate alternative to the above mentioned analytical methods, particularly due to the relatively low costs of instrumentation, possibility of miniaturization, time saving and sensitive performance of analysis [27]. Regarding the determination of PCR, Gilbert and Mann used dropping mercury electrode (DME) and differential pulse polarography (DPP) based on catalytic hydrogen electrochemical process for analysis of natural waters containing PCR with limit of detection (LD) equaled to 83 nmol L−1 [28]. The sensitive polarographic method for determination of PCR (LD = 60 nmol L−1 ) and another herbicide Dowco 229 was described by Whittaker and Osteryoung [29]. The static mercury drop electrode (SMDE) in the connection with square-wave voltammetric (SWV) mode was utilized as a sensitive tool (LD = 44–156 nmol L−1 ) for analysis of natural waters containing PCR [30]. Two reduction signals of PCR were described about potentials −0.90 and −0.95 (vs. Ag/AgCl) [30]. The determination of PCR in natural waters performed by sequential injection (SI) SWV using hanging mercury drop electrode (HMDE) was described by Santos and Masini [31]. The two reduction peaks at −0.81 V and −0.85 V vs. Ag/AgCl were observed and the LD value of 149 nmol L−1 was obtained [31]. A mechanism of electrochemical reduction of PCR and clopyralid (other member of pyridine herbicide family) on mercury electrodes was studied by Mellado et al. using mercury pool electrode and the products of electrode reaction were identified by GC/MS [32]. The same authors also dealt with adsorption–desorption processes of PCR on mercury electrodes [33]. An immunosensor based on three-dimensional gold nanoclusters was utilized for the determination of PCR with good sensitivity (LD = 2.1 nmol L−1 ) and proposed amperometric method was applied for the analysis of peach extract and excess sludge supernatant [34]. Development of new electrode materials which could replace the liquid mercury due to its alleged toxicity is one of the current trends of electrochemistry. Boron-doped diamond is a one of the novel carbon-based material, which has been studied more in detail in last 20 years [35,36]. The most relevant feature of this material is high overpotential for the oxygen and hydrogen evolution causing the fact that boron-doped diamond electrodes have absolutely the widest working potential range of currently using working electrodes. The other important properties are high thermal conductivity, high hardness and chemical inertness, extreme electrochemical stability in an alkaline and an acidic media, very low and stable background current [35–39]. Boron-doped diamond film (BDDF) electrodes have successfully been applied in voltammetric analysis of various biological active compounds, e.g. this research group has recently proposed the voltammetric methods using BDDF electrode for the determination of caffeine [40], penicillin V [41] and simultaneous determination of penicillin V and paracetamol [42], codeine [43] and atrazine [44]. This electrode was also applied as an anode in electrochemical incineration of various pesticides [45]. The voltammetric behavior and determination of PCR on unmodified BDDF electrode is described in present paper. In respect of other voltammetric methods based on electrochemical reduction of PCR, our approach solely consists in electrochemical oxidation of PCR at very positive potentials confirming the advantages and feasibility of using BDDF electrode. Generally, to the best of our knowledge, a detection and determination of PCR at this perspective and nontoxic type of electrochemical sensor has not been previously investigated in literature until now. Optimum working conditions for differential pulse voltammetric determination of PCR were found and proposed sensitive method was employed
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in the analysis of environmental and biological samples spiked with PCR. 2. Experimental 2.1. Chemicals All chemicals used for the preparation of supporting electrolytes, standard and other solutions were of p.a. purity. All solutions were prepared using with double-distilled deionized water with resistivity greater than 18 M cm. Picloram (CAS No. 1918-02-1, Sigma–Aldrich, Czech Republic) was dissolved in 50% acetonitrile (Lach-ner, Czech Republic) and the standard solution was stored in a glass flask in a refrigerator. PCR working solutions were prepared daily by dilution of the stock solution with the supporting electrolyte. Various supporting electrolytes such as nitric acid, perchloric acid and sulfuric acid were purchased from Lachema (Brno, Czech Republic). Britton–Robinson buffer solution (BR) was prepared by mixing of the same concentrations (0.04 mol L−1 ) of orthophosphoric acid, boric acid and acetic acid (all three purchased from Lachema, Brno, Czech Republic) in deionized water and adjusting to the desired pH value with 0.2 mol L−1 sodium hydroxide (Lachema, Czech Republic). The stock solutions of compounds used as interfering agents (ascorbic acid, urea, uric acid, sucrose, barbituric acid and creatinine (all Sigma–Aldrich, Czech Republic)) were prepared by dissolving of calculated amount in deionized water. Stock solution of folic acid (Sigma–Aldrich, Czech Republic) was prepared by dissolving of an appropriate amount in 0.01 mol L−1 NaOH (Lachema, Brno, Czech Republic). 2.2. Instrumentation The voltammetric measurements were performed with AUTOLAB PGSTAT 302N (Metrohm Autolab B.V., The Netherlands) potentiostat/galvanostat controlled by NOVA 1.7 electrochemical software. All measurements were provided in three-electrode set up, where Ag/AgCl/3 mol L−1 KCl and platinum wire served as a reference and an auxiliary electrode, respectively. BDDF electrode inserted in polyether ether ketone body with inner diameter of 3 mm, resistivity of 0.075 cm and boron doping level of 1000 ppm (declared by Windsor Scientific Ltd., United Kingdom as a producer) was used as a working electrode. All the pH values of solutions were measured using pH meter Model 215 (Denver Instrument, USA) with a combined electrode (glass-reference electrode), which was daily calibrated with standard buffer solutions. Standard solution of PCR was prepared using ultrasonic bath Bandelin Sonorex (Schalltec GmbH, Germany). All the potentials reported in this paper were given against Ag/AgCl/3 mol L−1 KCl at a laboratory temperature of 25 ± 1 ◦ C. 2.3. Measurement procedures A known volume of standard solution of PCR was pipetted into a 20 mL volumetric flask and then filled up with the supporting electrolyte. This solution was subsequently transferred quantitatively into voltammetric cell. Cyclic voltammetry (CV) was used for investigation of dependence between voltammetric response of PCR and pH of the supporting electrolyte and for study of the effect of scan rate on voltammetric behavior of PCR. Differential pulse voltammetry (DPV) was used for the purpose of quantification. Five cyclic voltammograms were obtained for each measurement, and the last scan was always considered for evaluation and making the figures reported in this paper. DP voltammograms were recorded after optimization of instrumental parameters (modulation amplitude, modulation time, scan rate and initial potential). The calibration
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curve was constructed from the average of five replicate measurements for each calibration solution of PCR. The peak currents (Ip ) recorded using CV and DPV were evaluated from the straight lines connecting the minima before and after the peak maximum. In order to clean BDDF electrode surface prior to start the first voltammetric measurement, we have performed CV from −2 V to +2 V for 10 min after immersion of electrode into 1 mol L−1 HNO3 , then rinsed with deionized water and polished with a piece of damp silk cloth until a mirror-like character of surface was obtained. To renew a hydrogen-terminated surface of BDDF electrode once a week, it was exposed to microwave-inducted hydrogen plasma for 5 min. The linear least-square regression in OriginPro 7.5 (OriginLab Corporation, USA) was used for the evaluation of calibration curve and the relevant results (slope and intercept) were reported with confidence interval for 95% probability. The limit of detection was calculated as three times the standard deviation for the blank solution (supporting electrolyte) divided by the slope of the calibration curve [46]. 2.4. Preparation of environmental and biological samples The tap water was sampled from the water supply in Bratislava. The samples of natural waters were collected from the rivers Elbe (Pardubice, Czech Republic) and Danube (Bratislava, Slovak Republic) and from the nameless brook which is closed to the agricultural area (Kameniˇcany, Northwest part of the Slovak Republic). The water samples were analyzed with no further pretreatment or purification. None of the water samples contained the measurable amount of PCR, thus, they were spiked with stock solution of PCR (200 L of 1 mmol L−1 in 100 mL of water sample) to concentration level of 2 mol L−1 (0.5 mg L−1 ), which is declared by EPA as a Maximum Contaminant Level for PCR [47]. 10 mL of spiked water was diluted with the supporting electrolyte (1 mol L−1 H2 SO4 ) to 20 mL. This solution was subsequently transferred quantitatively into the voltammetric cell and analyzed. The human urine sample was obtained from non-smoker female volunteer of the age of 28. The sample was stored in the refrigerator after sampling and analyzed without any further pretreatment about 15 h after sampling. 1 mL of the urine was pipetted into a volumetric flask, then spiked with stock solution of PCR (50 L of 1 mmol L−1 ) a filled up with the supporting electrolyte (1 mol L−1 H2 SO4 ) to 20 mL. The prepared solution was quantitatively transferred into the voltammetric cell and analyzed. The content of PCR in all samples was determined by standard addition method, when at least two standard additions were added. Each determination was repeated five times and relative standard deviation of 5 repeated determinations (RSDD (5)) was calculated [46].
Fig. 1. The cyclic voltammogram of PCR recorded in H2 SO4 on BDDF electrode. Experimental conditions: method: CV; parameters: Ein = −1.5 V, Efin = 2 V, v = 100 mV s−1 ; supporting electrolyte: 1 mol L−1 H2 SO4 ; c(PCR) = 0.9 mmol L−1 .
electrode is totally irreversible electrochemical process. Concerning BR buffer solution, no voltammetric peak of PCR was observed at pH 4 and higher (results not shown). The best results from all studied supporting electrolytes on the basis of well developed and repeatable peak and the highest current response of PCR were obtained in sulfuric acid. The cyclic voltammogram of absence (dotted line) and presence (continuous line) of PCR (0.9 mmol L−1 ) in 1 mol L−1 H2 SO4 is shown in Fig. 1, where the arrows indicate a direction of the scan. It is apparent that the residual current appeared to be low at very positive potentials on BDDF electrode confirming its excellent properties for determination of electrochemically highly oxidizable analytes. The influence of concentration of sulfuric acid in range from 0.01 to 2 mol L−1 was also examined. The peak height increased with ascending concentration of H2 SO4 as evidenced in Fig. 2. The highest current response of PCR was observed in the most concentrated H2 SO4 . The difference of the PCR peak height provided in 2 mol L−1 and 1 mol L−1 H2 SO4 was not considered to be significant. The peak potential of PCR did not change significantly with variation of concentration of H2 SO4 and this dependence is also shown in Fig. 2. Following this fact, 1 mol L−1 H2 SO4 was chosen as an
3. Results and discussion 3.1. Effect of supporting electrolyte on voltammetric behavior of PCR Generally, the supporting electrolyte plays an important role in an electrode reaction of studied analyte because it can modify the thermodynamics and kinetics of electrochemical processes and charge transfer within the cell. In our case, the effect of various supporting electrolytes such as nitric acid, sulfuric acid, perchloric acid and Britton–Robinson (BR) buffer solution on voltammetric behavior of 0.9 mmol L−1 PCR was investigated in whole working potential range on BDDF electrode. It was found that PCR provided one well-shaped oxidation (anodic) peak at very positive potential of about +1.5 V in strong acidic medium. The absence of reduction peak indicated, that the electrode reaction of PCR on BDDF
Fig. 2. Effect of concentration of sulfuric acid on the peak height (, Ip ) and peak position (, Ep ) of PCR recorded on BDDF electrode. Experimental conditions: method: CV; parameters: Ein = −1.5 V, Efin = 2 V, v = 100 mV s−1 ; supporting electrolyte: 0.01, 0.025, 0.05, 0.1, 0.5, 1 and 2 mol L−1 H2 SO4 ; c(PCR) = 0.9 mmol L−1 .
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Fig. 3. Voltammetric response of PCR depending on scan rate recorded on BDDF electrode. Experimental conditions: method: CV; parameters: Ein = −1.5 V, Efin = 2 V, v: (a) 10, (b) 25, (c) 50, (d) 75, (e) 100, (f) 150, (g) 200 and (h) 250 mV s−1 ; supporting electrolyte: 1 mol L−1 H2 SO4 ; c(PCR) = 0.9 mmol L−1 . Inset: Dependence of the peak current of PCR peak (Ip ) on square root of scan rate (v1/2 ).
optimal and from our point of view not so concentrated supporting electrolyte for all subsequent measurements due to the sufficient current response and well-shaped oxidation peak of PCR. The electrochemical behavior of PCR on BDDF electrode is not consistent with those reported for mercury electrodes [28–32]. Two reduction signals provided by PCR at potentials about −0.90 V and −0.95 V vs. Ag/AgCl [30] and at −0.81 V and −0.85 V vs. Ag/AgCl [31], respectively, in the acidic medium on the mercury electrodes were described, e.g. in the papers [30,31]. The anodic signal of PCR on BDDF electrode was observed at very positive potential, which could not be applied for mercury electrodes. Even, when the potential range from −1.5 V to +2 V was applied, no reduction peaks of PCR were recorded on BDDF electrode.
3.2. The effect of scan rate The useful information including the electrochemical mechanism (rate-limiting step) may be observed from the dependence of the peak current on the scan rate. The influence of scan rate (v) on the current response of 0.9 mmol L−1 PCR in 1 mol L−1 H2 SO4 was studied in the range from 10 to 250 mV s−1 using CV. Obtained voltammetric curves are shown in Fig. 3. It was found that the peak current of PCR peak increased linearly with the square root of the scan rate (inset of Fig. 3) and this dependence may be described by Eq. (1) Ip (A) = (26.181 ± 0.355)v1/2 (V s−1 ) (R = 0.998)
1/2
+ (0.195 ± 0.011) (1)
This result suggested a diffusion-control process in electrode reaction of PCR on unmodified BDDF electrode. This fact is oftentimes typical for this electrode surface in determination of organic compounds due to its low adsorption properties. Moreover, the peak potential of the registered current response slightly shifts to the more positive potential values with increasing scan rate thus confirming the irreversible character of the electrode reaction of PCR.
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Fig. 4. DP voltammograms of PCR recorded on BDDF electrode. Experimental conditions: parameters: Ein = 0.6 V, Efin = 2 V, v = 20 mV s−1 , modulation amplitude: 75 mV, modulation time: 25 ms; supporting electrolyte: 1 mol L−1 H2 SO4 ; c(PCR): (a) 0.50, (b) 1.00, (c) 1.50, (d) 2.00, (e) 2.49, (f) 2.99, (g) 3.49, (h) 4.48, (i) 5.47, (j) 6.46, (k) 7.44, (l) 8.43, (m) 9.41, (n) 10.39, (o) 20.09, (p) 29.60, (q) 38.92 and (r) 4807 mol L−1 . Curves are after baseline correction. Insets: Dependence of height of PCR peak (Ip ) on its concentration (c).
3.3. Analytical performance 3.3.1. Optimization of DPV operating parameters DPV is considered to be an effective electrochemical technique, which has already been applied for analysis of numerous biologically and electrochemically active compounds. The optimization of DPV operating parameters influencing the current response of analyte is an important step in the development of electroanalytical methodology. Accordingly, the instrumental parameters such as an initial potential (Ein ), modulation amplitude, modulation time and scan rate (v) were investigated in order to optimize the experimental set-up for determination of PCR. All experiments were carried out at 10 mol L−1 PCR in 1 mol L−1 H2 SO4 . Firstly, the influence of initial potential to the peak current was studied in the range from 0 to +1.4 V. The peak current changed negligibly applying Ein = 0–0.8 V but when the higher Ein was set up, the oxidation peak rapidly started to decrease. Hence, Ein = 0.6 V was chosen for all subsequent measurements because of sufficient intensity of peak current and reasonable time of analysis. Consequently, the modulation amplitude was changed from 5 to 150 mV at fixed modulation time of 50 ms. It was found that the oxidation peak of PCR slightly shifted to the less positive potential values together with grow and expansion of peak. The modulation amplitude of 75 mV was selected as an optimal for all subsequent analysis thank to convenient peak current and potential. In the case of modulation time, the peak current decreased with the increase of modulation time in the range of 10–100 ms. The most stable peak current was observed at 25 ms at fixed value of modulation amplitude at 75 mV. Concerning the scan rate, the peak current increased with rising scan rate from 10 to 100 mV s−1 , however, when the higher scan rates were applied, the peak started to expand and deform. As a suitable value of scan rate from the view of shape and intensity of peak as well as appropriate time of analysis, 20 mV s−1 was chosen. DPV with optimized operating parameters was applied for a construction of the calibration curve. 3.3.2. Determination of PCR Calibration curve was constructed by plotting of the peak current against PCR concentration in the range from 0.5 to 48.07 mol L−1 as depicted in Fig. 4. The oxidation peak increased
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Table 1 Influence of potential interfering agents (IA) on the voltammetric response of 5 mol L−1 PCR. Concentration ratio of PCR and IA
1:1 1:10 1:100
Signal change of PCR in presence of interference agent (%)
UA
FA
U
S
AA
BA
C
+1 −1.8 −9
−3.2 −65 −
<0.5 <0.5 <0.5
<0.5 −2.2 −0.6
<0.5 −2.1 −17
+1.3 +2.1 +4.2
−1.7 −1.6 −1.8
Abbreviations: UA, uric acid; FA, folic acid; U, urea; S, sucrose; AA, ascorbic acid; BA, barbituric acid; C, creatinine.
of PCR was mostly influenced by presence of excesses of FA. The 10 times excess of FA affected the significant decrease of PCR signal, while in the case of 100 times excess the current response of PCR completely disappeared. UA and AA also caused slightly decrease of the signal but only when the concentration of UA and AA, respectively, was 100 times higher than concentration of PCR. Other substances did not influence the response significantly under applied conditions. According to these results, it can be concluded that the satisfactory selectivity of proposed method in the voltammetric determination of PCR was attained. 3.4. Application of proposed method to analysis of environmental and biological samples
Fig. 5. Comparison of DP voltammograms of PCR and particular interfering agents (IA) in the concentration ratio 1:10. Experimental conditions: method: DPV, parameters: Ein = 0.6 V, Efin = 2 V, v = 20 mV s−1 , modulation amplitude: 75 mV, modulation time: 25 ms; supporting electrolyte: 1 mol L−1 H2 SO4 ; c(PCR) = 5 mol L−1 ; c(IA) = 50 mol L−1 .
linearly in this concentration range (inset in Fig. 4) and could be described by Eq. (2) Ip (A) = (97547 ± 397)c (mol L−1 ) + (0.0369 ± 0.0023)
(2)
70 nmol L−1
The limit of detection was considered to be and this low value confirms the high sensitivity of proposed method. The repeatability of measurement was tested by application of 11 repeated scans at 10 mol L−1 concentration level of PCR under the proposed operating parameters and the relative standard deviation of 11 repeated measurements (RSDM (11)) was calculated. Its low value (2.6%) verified an excellent repeatability of the procedure and confirming the minimal adsorption of PCR oxidation product on the BDDF electrode surface without need of any regeneration of the surface. The proposed procedure proved to be suitable for precise detection and quantification of PCR. 3.3.3. Interference studies A selectivity of proposed procedure and the effect of possible interfering agents were examined under found operating conditions at fixed concentration of PCR (5 mol L−1 ). Some biomolecules potentially present in biological samples (human urine) such as uric acid (UA), folic acid (FA), urea (U), sucrose (S), ascorbic acid (AA), barbituric acid (BA) and creatinine (C) were chosen as possible interfering electroactive compounds. The results obtained for three various concentration ratios (1:1, 1:10 and 1:100) of studied biomolecules are summarized in Table 1. The comparison of voltammetric curves of PCR and particular interferent in concentration ratio 1:10 (PCR:biomolecule) is shown in Fig. 5. The substance was considered to interfere seriously when it gave a PCR signal change more than 5%. The presence of more tested substances at the same concentration level as PCR (5 mol L−1 ) did not affect followed signal significantly. However, the peak current
As it was described in the previous chapters, our proposed voltammetric method is very sensitive and sufficiently selective. Therefore, it could be applied for the determination of PCR in various environmental and biological samples. Firstly, the tap water and the water from the rivers Elbe and Danube as well as from the brook were spiked (as it was described in Section 2.4) to a concentration level 2 mol L−1 (0.5 mg L−1 ). Standard addition method was applied for the determination of PCR and each sample was analyzed 5 times. The obtained results expressed as confidence interval with 95% probability with particular relative standard deviations from 5 repeated determinations (RSDD (5)) are summarized in Table 2. It is obvious, that determined values well corresponded with the added contents and the proposed method could be found as reliable for the analysis of tap and natural water samples. The particular relative standard deviations showed the high reproducibility of determinations. The illustrative examples of DP voltammograms for PCR determination in the water sample from Danube (A) and the brook (B) are shown in Fig. 6. Concerning the biological samples, it is generally known that PCR is rapidly absorbed through the gastrointestinal tract and unchanged form could be passed into urine [2]. Therefore, the applicability of proposed method for the analysis of PCR in human urine is justified and also was tested. The sample of spiked human urine was analyzed without any complex pretreatment only after simple dilution of the urine. The obtained DP voltammograms are depicted in Fig. 7. The curve a (dotted line) presented the diluted urine in the absence of PCR. It is clearly shown that the observed peak at about +1.55 V is assigned to the PCR oxidation since this peak increases after each PCR standard addition. No attempts have been carried out to identify the other peaks at about +0.6 and +0.7 V in DP voltammograms which are likely to be due to the oxidation of common urinary compounds. It is also obvious, that especially due to the very positive peak potential, there is no significant signal, which could interfere with PCR determination in human urine. The results are summarized in Table 2. It is evident, that obtained results are consistent with added amount of PCR and the method is accurate according to low RSDD (5). 3.4.1. Comparison with other methods The comparison between the analytical performance of our proposed method and other electrochemical methods for
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Table 2 Results of repeated determinations for PCR in various model samples (n = 5). Sample
Added (mol L−1 )
Determined* (mol L−1 )
Tap water Danube Elbe Brook water Urine
2 2 2 2 50
1.97 2.03 1.98 1.97 49.04
*
± ± ± ± ±
RSDD (5) (%)
0.06 0.04 0.04 0.06 1.33
4.4 2.5 3.1 4.4 4.1
Confidence interval calculated according [¯x ± tn−1,˛ SD/sqt(n)]; t4;0.05 = 2.1318
Table 3 Comparison of the proposed voltammetric method with previously reported electrochemical methods for determination of PCR. Electrode
Supporting electrolyte
Technique
Peak potential (V)
Linear range (mol L−1 )
DME DME SMDE HMDE BDDFE
ABS, pH 4.35 Na2 SO4 + 2EE, pH 1.9 0.1 mol L−1 H2 SO4 0.1 mol L−1 H2 SO4 1 mol L−1 H2 SO4
DPP DPP SWV SI-SWV DPV
−1.28 (vs. SCE) −0.95 (vs. SCE) −0.90 (vs. Ag/AgCl) −0.82 (vs. Ag/AgCl) +1.55 (vs. Ag/AgCl)
0.083–9.90 0.10–50 0.16–2.1 0.41–10.5 0.5–48.1
LD (nmol L−1 ) 83 60 44–156 149 70
Sample analyzed
References
Natural water – Natural water Natural water Tap and natural water, human urine
[28] [29] [30] [31] Present paper
Abbreviations: DME, dropping mercury electrode; ABS, acetate buffer solution; DPP, differential pulse polarography; SCE, saturated calomel electrode; 2EE; 2-ethoxyethanol; HMDE, hanging mercury drop electrode; SWV, square-wave voltammetry; SMDE, static mercury drop electrode; SI, sequential injection; BDDFE, boron-doped diamond film electrode; DPV, differential pulse voltammetry; LD, limit of detection.
Fig. 6. Determination of PCR in water from Danube (A) and the brook (B). Experimental conditions: method: DPV, parameters: Ein = 0.6 V, Efin = 2 V, v = 20 mV s−1 , modulation amplitude: 75 mV, modulation time: 25 ms; supporting electrolyte: 1 mol L−1 H2 SO4 ; (a) sample spiked with PCR, (b–e) standard additions of PCR. Curves are after baseline correction.
Fig. 7. Determination of PCR in human urine. Experimental conditions: method: DPV, parameters: Ein = 0.6 V, Efin = 2 V, v = 20 mV s−1 , modulation amplitude: 75 mV, modulation time: 25 ms; supporting electrolyte: 1 mol L−1 H2 SO4 ; (a) diluted human urine in absence of PCR, (b) diluted human urine in presence of PCR, (c–e) standard additions.
determination of PCR from last decade is summarized in Table 3. On the basis of literature survey, the mercury electrodes in various constructions (DME, SMDE and HMDE) have mostly been applied for the voltammetric analysis of PCR in connection with differential pulse polarography and square-wave voltammetry, respectively [28–31]. In this case, no solid or paste working electrodes have been utilized according to accessible literature. It is obvious from Table 3, that the proposed method reached a comparable LD with other voltammetric methods [28–30] with the exception of the use of sequential analysis with LD about once higher [31]. Moreover, two reduction signals recorded in the presence of PCR on mercury electrodes were located closed to each other, e.g. at −0.90 and −0.95 vs. Ag/AgCl [30] or at −0.81 V and −0.85 V vs. Ag/AgCl [31], affecting their possible overlapping, which could worsen possibilities of its determination due to lower ability of quantification of PCR. On the other hand, the oxidation signal, registered on unmodified BDDF electrode and described in the present paper, was well developed and had satisfactory position from the selectivity point of view in spite of high positive peak potential of PCR. However, the use of conventional carbon-based electrodes (graphite, glassy carbon, carbon paste) in determination of PCR would be
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inconvenient due to oxygen generation on these surfaces at higher positive potentials. Thus, BDDF electrode with DPV technique possesses the benefits of modern electroanalytical chemistry (rapidity, simplicity, sensitivity, repeatability and low cost). It could also represent the nontoxic (green) alternative for the environmental monitoring of trace amounts of pyridine herbicides and other harmful organic compounds. 4. Conclusion Unmodified BDDF electrode was applied for the first time in combination with DPV technique to elaborate the sensitive, simple and cheap analytical method for determination of PCR. The method was based on the highly irreversible electrochemical oxidation of PCR at very positive potentials in strong acidic medium instead of reduction process typical for mercury electrodes. The fully validated analytical procedure was exploited for the analysis of environmental (tap and natural water) and biological (human urine) samples with good accuracy. The method enables the determination of PCR with sensitivity below the maximum contaminant level for PCR declared by EPA and could be applied directly without any difficult sample preparation (just simple dilution). The selectivity of method appears to be sufficient as there were no substantial interferences present from matrix. It can be concluded that the proposed voltammetric method can undoubtedly be considered as an effective, sensitive and green (environmentally acceptable) tool in analysis of PCR and other herbicides as well as may represent the electrochemical alternative to mercury electrodes. Acknowledgements This work was supported by The Ministry of Education, Youth and Sports of the Czech Republic (project No. CZ.1.07/2.3.00/30.0021 “Strengthening of Research and Development Teams at the University of Pardubice”), the Grant Agency VEGA of the Slovak Republic (grant No. 1/0051/13) and the Slovak Research and Development Agency under the Contract No. APVV0797-11. References [1] S.N. Rekha, R. Naik, Prasad, Pesticide residue in organic and conventional foodrisk analysis, J. Chem. Health Saf. 13 (2006) 12. [2] M. Tu, C. Hurd, J.M. Randall, Weed Control Methods Handook, The Nature Conservancy, 2001, http://www.invasive.org/gist/handbook.html (30.4.2013). [3] FAO, Specification and Evaluations for Picloram (Evaluation Report 174/2004), 2004. [4] Dow Chemical Corporate, http://www.dow.com/ (30.4.2013). [5] M.G. Merkle, R.W. Bovey, F.S. Davis, Factors affecting the persistence of picloram in soil, Agron. J. 59 (1967) 413. [6] C.J. Scifres, R.R. Hahn, J. Diaz-Colon, M.G. Merkle, Picloram persistence in semiarid rangeland soils and water, Weed Sci. 19 (1971) 381. [7] J.D. Fryer, P.D. Smith, J.W. Ludwig, Long-term persistence of picloram in a sandy loam soil, J. Environ. Qual. 8 (1979) 83. [8] T.N. Johnsen, Picloram in water and soil from semiarid pinyon-juniper watershed, J. Environ. Qual. 9 (1980) 601. [9] D.G. Neary, P.B. Bush, J.E. Douglass, R.L. Todd, Picloram movement in an appalachian hardwood forest, J. Environ. Qual. 14 (1985) 585. [10] V.J. Watson, P.M. Rice, E.C. Monnig, Environmental fate of picloram used for roadside weed control, J. Environ. Qual. 18 (1989) 198. [11] M.E. Close, L. Pang, J.P.C. Watt, K.W. Vincent, Leaching of picloram, atrazine and simazine through two New Zealand Soils, Geoderma 84 (1998) 45. [12] Canadian Council of Ministers of the Environment, Canadian Water Quality Guidelines for Protection of Aquatic Life: Picloram, Winnipeg, 1999, ISBN 1896997-34-1. [13] C. Britt, A. Mole, F. Kirkham, A. Terry, D. Arnold, J. Clarke, R. McLaren, A. Gundrey, S. McMillan, The Herbicide Handbook, English Nature in Assoc. with FACT, Wetherby, 2003, ISBN 1-85716-7465. [14] D.W. Kolpin, J.E. Barbash, R.J. Gillion, Pesticides in Ground Water of the United States, Groundwater 38 (2000) 858, 1992-1996. [15] J.R. Baur, R.W. Bovey, M.G. Merkle, Concentration of picloram in runoff water, Weed Sci. 20 (1972) 309.
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