Anal. Chem. 2004, 76, 859-862
Voltammetric Sensor for General Purpose Organohalide Detection at Picogram per Liter Concentrations Based on a Simple Collector-Generator Method Wisitsree Wiyaratn,† Mithran Somasundrum,*,‡ and Werasak Surareungchai*,§
Joint Graduate School of Energy and Environment, PDTI, and School of Bioresources and Technology, King Mongkut’s University of Technology Thonburi, Bangkhuntien-Chaitalay Road, Thakam, Bangkok 10150, Thailand
With the aim of producing a general purpose sensor for environmental analysis, we describe a simple and sensitive method for organohalide detection, based on an electrochemical collector-generator process. The sensor consists of four coplanar electrodes contacting a solution volume of 300 µL, containing organohalide. At the first working electrode (a Zn/PTFE composite), the analyte is electrolyzed to liberate halide ions. At the second working electrode (Ag), the halide ions are detected by cathodic stripping voltammetry. Using a preconcentration time of 600 s, with differential pulse voltammetry for stripping, the responses to 1-chloropropane, chloroform, carbon tetrachloride, iodoethane, and bromoethane can be plotted on a common calibration curve, with a detection limit of 0.1 nM (1.3 pg L-1 or less depending on the organohalide). To the best of our knowledge, this is the lowest reported organohalide detection limit by an electrochemical method and is so far the only general purpose electrochemical method sensitive enough for regulatory requirements. The sensor response was invariant for ∼40 measurements. Analysis of tap water, spiked with chloroform or carbon tetrachloride, gave recoveries within 1.0-2.6% of the recoveries by the standard GC method. Organohalides are used as solvents in a number of industries and are also often used as biocides in agriculture. This has led to them being a major source of water pollution. Hence, there is a need for organohalide analysis of environmental samples. The traditional form of measurement has been either liquid1 or gas chromatography.2 A drawback to these techniques is that they require expensive equipment operated by trained personnel and are not suitable for field use. In the event of suspected contamination of a water source, a simple, hand-held sensor capable of measuring total organohalide concentration would be desirable. For this reason, there has been interest in the detection of * To whom correspondence should be addressed: (tel.) 66 2 4709732; (fax) 66 2 4523455; (e-mail)
[email protected] or
[email protected]. † Joint Graduate School of Energy and Environment. ‡ PDTI. § School of Bioresources and Technology. (1) Hosoya, K.; Sawada, E.; Kimata, K.; Araki, T.; Tanaka, N. J. Chromatogr., A 1994, 662, 37-47. (2) Fuji, T. J. Chromatogr., A 1977, 139, 297-302. 10.1021/ac0350918 CCC: $27.50 Published on Web 12/23/2003
© 2004 American Chemical Society
organohalides by electrochemistry. So far, most work in this field has focused on redox catalysis using electrodes modified by metal complexes.3-5 Unfortunately, the limits of detection reported so far have not been low enough for regulatory requirements, e.g., 0.5-30 µM for a range of organohalides using cobalt(II) tetraphenylporphine,3 from 0.06 to 1.2 ppm for four different organochlorides and bromides using electropolymerized cobalt porphyrin and salen films,4 and 80 µM for 3,4-dichloropropylanilide using iron(III) porphyrin,5 whereas EPA guidelines for many organohalides are at the ppb level.6 Alternative electrochemical methods include the following: impedance changes due to the sorption of dichloromethane in a conducting polymer (LOD ) 500 ppm);7 ISE-based detection of Cl- or Br- ions liberated by reaction with Rhodococcus sp. cells8 (LOD ) 50 µg L-1 and 10 µg L-1 for 1,3dibromopropane and 1-chlorobutane respectively) or with Xanthobacter autotrophicus cells9 (LOD ) 500 mg L-1 for 1,2dichloroethane); oxidation at a pencil graphite electrode following accumulation in a clay/sol-gel film (LOD ) 19.32 nM for 2,4dichlorophenol).10 Of these, only the last method is sensitive enough for regulatory requirements. However, since the currentproducing oxidation is of the phenol OH group,11 this method cannot form the basis of a general purpose organohalide sensor. Composite-plated electrodes are produced by electrodepositing a metal in the presence hydrophobic polymer particles, which are co-deposited to form a mechanically stable, hydrophobic film. These have been used for either synthesis or pollutant degradation, for a number of organic compounds.12-15 The concept is that, (3) Dobson, D. J.; Saini, S. Anal. Chem. 1997, 69, 3532-3528. (4) Ordaz, A. A.; Rocha, J. M.; Aguilar, F. J. A.; Grandos, S. G.; Bedioui, F. Analusis 2000, 28, 238-244. (5) Priyantha, N.; Weerabahu, D. Anal. Chim. Acta 1996, 320, 263-268. (6) U.S. Environmental Protection Agency. List of Contaminants & Their MCLs, accessible at http://www.epa.gov/safewater/mcl.html (7) Salzer, C. A.; Elliot, C. M. J. Electroanal. Chem. 2002, 538-539, 223230. (8) Hutter, W.; Peter, J.; Swoboda, H.; Hampel, W.; Rosenberg, E.; Kramer, D.; Kellner, R. Anal. Chim. Acta 1995, 306, 237-241. (9) Peter, J.; Hutter, W.; Stollnberger, W.; Karner, F.; Hampel, W. Anal. Chem. 1997, 69, 2077-2079. (10) Ozsoz, M.; Erdem, A.; Ozkan, D.; Kerman, K.; Pinnavaia, T. J. Langmuir 2003, 19, 4728-4732. (11) Ureta-Zanarta, M. S.; Bustos, P.; Berrios, C.; Diez, M. C.; Mora, M. L.; Gutierrez, C. Electrochim. Acta 2002, 47, 2399-2406. (12) Kunugi, Y.; Nonaka, T.; Chong, Y.-B.; Watanabe, N. J. Electroanal. Chem. 1993, 356, 163-169.
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because of the hydrophobic nature of the coating, water molecules are excluded from the electrode surface, and therefore, hydrogen evolution is suppressed. Hence, the current efficiency is increased for reactions occurring at highly negative potentials. We have previously used reduction at a Zn/PTFE composite-plated electrode for the chronocoulometric determination of some organohalides.16 Unfortunately, this method only provided detection limits of 50 µM and might suffer interference from other organic reductions. However, as we report here, we have since found that we can lower the detection limits to subnanomolar levels, and improve selectivity, by making the Zn/PTFE reaction the first stage of a collector-generator process. The principle of the technique is that the composite electrode is first used to fully electrolyze the organohalide sample, according to the reaction
R-X + H+ + 2e- f R-H + X-
(1)
The resulting halide ions are then detected at a Ag electrode by cathodic stripping voltammetry (CSV). The sensitivity of the technique comes from the inherent sensitivity of CSV, due to the preconcentration step. The selectivity of the technique is based on the fact that a CSV run can be performed prior to electrolysis, to provide an anion background signal. EXPERIMENTAL SECTION Materials. All organohalides were HPLC grade and were purchased from Fluka. Acetonitrile (HPLC grade) was purchased from Aldrich. Zinc sulfate and tetrabutylammonium toluene-4sulfonate (TBATS, electrochemical grade) were from Sigma. Humic acid was from ICN Biomedicals Inc. All other chemicals were provided by Aldrich. All electrochemical experiments were performed in a 3:2 CH3CN/H2O mixture containing TBATS as electrolyte at 0.1 M unless otherwise stated. Apparatus. Electrochemical experiments were performed with an Autolab PGSTAT 10 (Eco Chemie) using GPES software version 4.3. The sensor assembly consisted of four electrodes fitted into holes in a 1-cm-diameter Teflon disk, such that they were coplanar. The electrodes were as follows: a Ag reference (2-mm diameter), a Pt counter (5-mm diameter), a Zn working electrode (2-mm diameter), and a Ag working electrode (2-mm diameter). The Teflon disk was sealed into one end of a glass tube, which served as the sensor housing. This tube was then pushed into a glass test tube of slightly larger diameter to form the electrochemical cell. The solution volume in the cell was 300 µL. All potentials are quoted versus the Ag reference () -52 mV vs SCE in 0.1 M KCl). The Zn working electrode was converted to a Zn/PTFE composite based on a reported proceedure12 as follows: the fourelectrode assembly was placed in a stirred cell containing 5 mL of distilled water, 350 g dm-3 ZnSO4, 60 g dm-3 PTFE particles (0.05-0.5-µm particle size), 0.6 g dm-3 didodecyldimethylammo(13) Kunugi, Y.; Nonaka, T.; Chong, Y.-B.; Watanabe, N. J. Electrochim. Acta 1992, 37, 353-355. (14) Kunugi, Y.; Nonaka, T.; Chong, Y.-B.; Watanabe, N. J. Electroanal. Chem. 1991, 318, 321-326. (15) Kunugi, Y.; Ono, Y.; Nonaka, T. J. Electroanal. Chem. 1992, 333, 325329. (16) Wiyaratn, W.; Somasundrum, M.; Surareungchai, W. Electroanalysis 2003, 15, 1719-1722.
860 Analytical Chemistry, Vol. 76, No. 3, February 1, 2004
Figure 1. Charge passed during the electrolysis of 1.0 µM 1-chloropropane solutions at a Zn/PTFE composite electrode.
nium bromide, 15.0 g dm-3 ammonium chloride, and 30.0 g dm-3 aluminum sulfate. The Zn electrode of the sensor assembly was set as cathode, a separate Zn electrode (2-mm diameter) was set as anode, and a separate Ag/AgCl electrode (BAS) was used as reference. A current of 50 mA was passed through the cell for 260 s. All electrochemical experiments were performed at room temperature (22 ( 3 °C) under a nitrogen atmosphere. GC analysis of organohalides was performed with a HewlettPackard gas chromatograph (model HP-6890) equipped with an HP-5 capillary column (flow rate 2.0 mL/min) and electron capture detector. Tap water was collected in polyethylene bottles and spiked with chloroform or carbon tetrachloride at concentrations from 1.5 to 5.0 ppb. A 5.0-mL aliquot of each sample together with 0.5 g of Na2SO4 was placed in the headspace sampler (model HP7694) and heated at 70 °C for 10 min. Each concentration was measured in triplicate. RESULTS AND DISCUSSION The Zn/PTFE electrode was used for electrolysis at the same potential as it was used in the previous analytical determinations,16 i.e., -1.8 V. Electrolysis was performed for different times on 1.0 µM solutions of 1-chloropropane. The charge passed during each electrolysis was calculated from the area under the resulting I-t curve, after subtracting the area representing capacitive discharge, as determined from application of potential for the same length of time, to a 1-chloropropane-free solution. The result is shown in Figure 1. As can be seen, electrolysis appeared virtually complete after 600 s. From Faraday’s law, the quantity of charge passed by this time (5.4 × 10-5 C) corresponded to 93.3% conversion of the total organohalide. From a cyclic voltammogram of the Ag electrode in the CH3CN/H2O mixture (not shown), oxidation of the Ag began at ∼0.2 V, and the resulting Ag+ ions were reduced at ∼-0.2 V. Hence, preconcentration of the silver halide film was performed at 0.6 V, and cathodic stripping was carried out by scanning from 0.6 to -0.3 V. Stripping was initially performed by linear sweep voltammetry (LSV), but it was subsequently found that the limits of detection could be further lowered 1 order of magnitude, by using differential pulse voltammetry (DPV). Using 10 nM NaCl solutions, the preconcentration time was varied from 100 to 1100 s (not
Figure 2. Differential pulse voltammograms of CCl4 solutions at Ag electrode, following 600-s electrolysis at Zn/PTFE electrode (-1.8 V). Preconcentration: 600 s at 0.6 V. Differential pulse: step potential 5 mV, pulse interval 0.5 s, and modulation potential 20 mV. Electrolyte is 3:2 CH3CN/H2O containing 0.1 M TBATS. Inset: Expansion of main figure with same axes. Numbers refer to molar concentration of each corresponding CCl4 solution.
shown), and it was found that, beyond 600 s, there was no further increase in the DPV peak. In Figure 2, a series of differential pulse voltammograms at the Ag electrode is presented, each one following the 600-s electrolysis of a different concentration of carbon tetrachloride (pulse conditions given in caption). The peak at 0.4 V was also observed in organohalide-free solution. The peak at 0.1 V was not observed in organohalide-free solution and was consistent with the potential for stripping of the silver halide film, as predicted from cyclic voltammetry. In this manner, calibration curves were plotted for 1-chloropropane, chloroform, carbon tetrachloride, iodoethane, and bromoethane. The result is shown in Figure 3. Because of the wide concentration range examined, the concentrations are plotted in log format. It can be seen that these five organohalides can be determined from a common calibration curve, which indicates the potential of this technique to provide a general purpose organohalide sensor. The detection limit was 0.1 nM. To the best of our knowledge, this is the lowest reported organohalide detection limit by an electrochemical method. It corresponds to 1.3 pg L-1 or less, for the organohalides examined, and is therefore low enough for regulatory requirements. The stability of the sensor was examined by performing electrolysis and then quantification, of 60 10 nM solutions of 1-chloropropane using a single Zn/PTFE film. This process was then repeated with a second Zn/PTFE film, and the results for both films are shown in Figure 4. The response was invariant for ∼40 measurements and then decreased. It can be seen that the sensor had good within-batch precision. It should be noted that this set of experiments was performed at an earlier stage of the work, where quantification was by LSV (scan rate, 30 mV s-1) rather than by DPV. However, the result should be a fair estimate of overall stability, since we expect the main stability limitation to be the condition of the Zn/PTFE film rather than the bare Ag electrode, which is anyway returned to the initial reduced state by each cathodic scan.
Figure 3. Calibration of 1-chloropropane (]), chloroform (0), carbon tetrachloride (3), iodoethane (4), and bromoethane (O) based on DPV procedure given in Figure 2, following 600-s electrolysis at Zn/PTFE composite electrode (-1.8 V). Inset a: Response to 10 nM CCl4 as a function of TBATS concentration. All other conditions as in Figure 2. Inset b: Response to 10 nM CCl4 as a function of water content, using 0.12 M TBATS. All other conditions as in Figure 2.
Figure 4. Peak current at Ag electrode from LSV of a 10 nM solution of 1-chloropropane (scan rate 30 mV s-1) following 600-s electrolysis at Zn/PTFE composite. Data points are mean of duplicate determinations made using two different Zn/PTFE films. Error bars show value of each determination. Where error bars are not shown they are within the data point.
The sensor accuracy was examined by spiking tap water with known quantities of chloroform or carbon tetrachloride (from 1.5 to 5.0 ppb) and then adding a 10-µL aliquot of this sample to the CH3CN/H2O mixture in the cell, to give 300 µL in total. Concentrations were determined from a seven-point calibration curve. Each concentration was analyzed in triplicate. The same Zn/PTFE film was used to electrolyze each chloroform solution (calibrations and samples), and then a fresh Zn/PTFE film was deposited for the carbon tetrachloride experiments. For comparison, the same tap water samples were analyzed by GC. Since adding a 10-µL tap water aliquot to the CH3CN/H2O mixture would change the solution composition (i.e., H2O content to increase Analytical Chemistry, Vol. 76, No. 3, February 1, 2004
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Table 1. Recoveries from Tap Water Spiked with Either CCl4 or CHCl3a G-D
GC
concn (ppb)
% recovery
SD
% recovery
SD
1.5 CCl4 3.0 CCl4 5.0 CCl4 1.5 CHCl3 3.0 CHCl3 5.0 CHCl3
84.5 90.2 93.5 91.3 95.0 90.4
0.06 0.10 0.20 0.10 0.20 0.20
85.6 92.7 95.7 93.5 97.5 93.0
0.14 0.25 0.37 0.81 1.52 1.70
a G-D, generator-detector (i.e., Zn/PTFE composite-Ag). GC, gas chromatography. SD, Std Dev. Each recovery is the mean of three determinations.
from 40 to 43.3% and TBATS concentration to decrease from 0.1 to 0.097 M), we examined the effect of these parameters on the current to 10 nM CCl4, using the same conditions as in Figure 2. As shown in Figure 3 inset a, the CCl4 response increased with TBATS concentration from 0.08 to 0.1 M and then became constant. As shown in inset b of Figure 3, using a TBATS concentration of 0.12 M, the CCl4 response was sensitive to water content, with a maximum at ∼40%. Therefore, in determining recoveries, TBATS was used at 0.12 M, and the CH3CN/H2O ratio was adjusted so that, after adding the tap water aliquot to the cell, the final water content was the same as in the calibration samples (i.e., 40%). As shown in Table 1, the sensor percentage recoveries were within 1.0-2.6% of those determined by GC. River water will contain varying concentrations of “humic substances”, a generic term covering a range of organic compounds, generally attributed to the breakdown of biogenic organic matter through largely unknown chemical mechanisms.17 Humics are subdivided into humic acid (MWs of 1000-2000, acid insoluble) and fulvic acid (MWs of 600-1000, acid soluble).17 As a preliminary investigation of the possible interference of humic substances, the response to 10 nM CCl4 was measured in the absence and presence of increasing concentrations of a commercial humic acid, using the conditions given in Figure 2. In each case, a DPV was run before and after electrolysis of the organohalide, and the peak height of the organohalide response was plotted after background subtraction. As seen in Figure 5, beyond 0.06 mg L-1 the presence of humic acid caused an increase in the CCl4 response. As shown in the inset to Figure 5, DPV of (17) Fox, L. E. In Organic Substances and Sediments in Water. Humics and Soils; Baker, R. A., Ed.; Lewis Publications: Chelsea, MI, 1991; Vol. 1, Chapter 9, pp 129-162. (18) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 77th ed.; CRC Press: Boca Raton, FL, 1996-1997; pp 4-84. (19) Yamada, E.; Doi, K.; Okamo, K.; Fuse, Y. Anal. Sci. 2000, 16, 125-129. (20) Sholkovitz, E. R.; Boyle, E. A.; Price, N. B. Earth Planet. Sci. Lett. 1978, 40, 130-136.
862 Analytical Chemistry, Vol. 76, No. 3, February 1, 2004
Figure 5. Response to 10 nM CCl4 in the presence of humic acid using DPV procedure given in Figure 2. Inset: Differential pulse voltammogram of 0.07 mg L-1 humic acid solution in the absence of CCl4 using the DPV procedure given in Figure 2, before (-) and after (- -) 600 s electrolysis at Zn/PTFE electrode (-1.8 V).
a halide-free humic acid solution (0.07 mg L-1), following electrolysis, gave rise to a cathodic peak ∼100 mV positive of the peak observed for organohalides. DPV without electrolysis did not produce a peak. It seems likely, therefore, that this peak represents stripping of an insoluble silver salt other than a halide. Other H2O-insoluble silver compounds include the citrate, cyanide, and oxalate salts of Ag(I).18 These organic functional groups were presumably liberated by reaction of humics at the Zn/PTFE composite. The level of expected humics in the environment is difficult to judge. As an example, sampling of four Japanese rivers produced humic acid in the range 0.023-0.052 mg L-1,19 but concentrations elesewhere may be much higher. The removal of humic substances by membrane filtration has been reported,20 and this may be a sample pretreatment method suitable to field use. Further work will also be to widen the range of organohalides examined, to see whether aromatic organohalides can also be determined from the same calibration curve. ACKNOWLEDGMENT This project was funded by MTEC (grant code MT-B-44-POL20-157-G). W.W. gratefully acknowledges a Ph.D. scholarship from the Royal Golden Jubilee Project of the Thailand Research Fund. M.S. is an employee of BIOTEC. Received for review September 17, 2003. Accepted November 24, 2003. AC0350918
