Chlorinated Phenol Analysis Using Off-LineSolid-Phase Extraction and CapillaryElectrophoresis Coupled with AmperometricDetection and a Boron-Doped DiamondMicroelectrode

Grace W. Muna, Veronika Quaiserova-Mocko, and Greg M. Swain*

Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322

The analysis of chlorinated phenols (2-chlorophenol,3-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, pentachlorophenol) in river water wasaccomplished using off-line solid-phase extraction (SPE)and capillary electrophoresis coupled with electrochemi-cal detection. A key to the sensitive, reproducible, andstable detection of these pollutants was the use of a boron-doped diamond microelectrode in the amperometricdetection mode. An off-line SPE procedure was utilizedto extract and preconcentrate the pollutants prior toseparation and detection, with ENVI-Chrom P, a highlycross-linked styrene-divinylbenzene copolymer, beingemployed as the sorbent. Pollutant recoveries in the 95-100% range with relative standard deviations of 1-4%were achieved. The diamond microelectrode provided alow and stable background current with low peak-to-peaknoise. The oxidative detection of the pollutants wasaccomplished at +1.05 V vs Ag/AgCl without the needfor electrode pretreatment. The method was evaluated interms of the linear dynamic range, sensitivity, limit ofquantitation, response precision, and response stability.A reproducible electrode response was observed duringmultiple injections of the chlorinated phenol solutionswith a relative standard deviation of e5.4%. Good elec-trode response stability was observed over many days ofcontinuous use with no significant electrode deactivationor fouling. The separation efficiencies for all six pollutantswere greater than 170 000 plates/m. The minimumconcentration detectable for all six ranged from 0.02 to0.2 ppb (S/N g 3) using a 250:1 preconcentration factor.

Chlorinated phenols are pollutants present in several types ofindustrial wastewater effluent, such as that from the manufactureof dyes, plastics, pesticides, antioxidants, paper, and petroleumproducts.1-3 Leaching of these pollutants from point sources, suchas landfills, is another source of ground and surface water

contamination. Their presence in the environment is of particularconcern because of the toxicity to humans and most aquaticorganisms and their tendency to bioaccumulate in the foodchain.4,5 The toxicity depends on the total number of chlorineatoms present in the molecule with pentachlorophenol (PCP)being the most toxic. Phenols can also impart unpleasant tasteand odor to drinking water and food products, even at lowconcentrations.5,6 The EPA has classified chlorinated phenols aspriority pollutants due to their toxicity and carcinogenicity.7-9 Forexample, PCP is regulated in drinking water with a maximumallowable contaminant level (MCL) of 1 µg/L (1 ppb).10 Healthadvisories would be issued to the general public if the concentra-tions of 2-chlorophenol, 2,4-dichlorphenol, and phenol in drinkingwater exceed 40, 20, and 4000 µg/L (ppb), respectively. Accordingto published research, chlorinated phenol toxicity and organolepticproperties are manifested in the ppb range.11,12

There is a need for sensitive, reproducible, stable, easy-to-use,and low-cost analytical methods for monitoring phenols andchlorinated phenols in water supplies. Capillary electrophoresis(CE) is a separation technique useful for environmental analysisbecause of highly efficient separations, short analysis times, lowanalysis costs, and low sample and reagent consumption.13

Electrochemical detection (EC) is often coupled with CE andgenerally provides excellent detection figures of merit for manysolutes. EC provides high sensitivity, a wide dynamic range, andutilizes rather simple and inexpensive instrumentation.14 There

* To whom correspondence should be addressed. E-mail: swain@chemistry.msu.edu.(1) Masque, N.; Pocurull, E.; Marce, R.; Borull, F. Chromatographia 1998, 47,

176.(2) Galceran, M.; Jauregui, O. Anal. Chim. Acta 1995, 304, 75.

(3) Veningerova, M.; Prachar, V.; Uhnak, J.; Lucacsova, M.; Trnovec, T. J.Chromatogr., B 1994, 657, 103.

(4) Vuorinen, P. J. Chemosphere 1985, 14, 1729.(5) Sarkka, J. Chemosphere 1985, 14, 469.(6) Realini, P. A. J. Chromatogr. Sci. 1981, 19, 124.(7) Keith, L. H. Advances in the Identification and Analysis of Organic Pollutants

in Water; Ann Arbor Science: Ann Arbor, MI, 1981; Vol. 1.(8) EPA Method 604. Phenols; In Federal Register, Environmental Protection

Agency, Part VIII, 40 CFR Part 136, Washington, DC, 1984; p 58.(9) EPA Method 625. Semivolatile Compounds; In Federal Register Environmental

Protection Agency, Part VIII, 40 CFR Part 136, Washington, DC, 1984; p153.

(10) National Primary Drinking Water Regulations, Technical Fact sheet onPentachlorophenol, Office of Ground and Drinking Water, U.S. EPA, 1998.

(11) Bosto, O.; Olucha, J. C.; Borull, F. Chromatographia 1991, 32, 423.(12) Czuczwa, J.; Leuenberger, C.; Tremp, J.; Giger, W.; Ahel, M. J. Chromatogr.

1987, 403, 233.(13) Jorgenson, J. W.; Lukacs, K. D. Anal. Chem. 1981, 53, 12.

Anal. Chem. 2005, 77, 6542-6548

6542 Analytical Chemistry, Vol. 77, No. 20, October 15, 2005 10.1021/ac050473u CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 09/20/2005

are challenges, however, with mating this detection method withCE, including the decoupling of the separation voltage from theelectrode signal, and the reproducible positioning of the workingelectrode when end-column detection is used as a means fordecoupling. Electrochemical electrodes can be isolated from thehigh CE separation voltage by either the use of a decoupler orend-column detection.15-22 The high voltage used for separationwill cause an increase in the detection electrode’s backgroundcurrent and electrical noise, as well as a shift in the detectionpotential if there is inadequate decoupling.

Another issue with EC detection, and a potential drawback, iselectrode deactivation and the need for frequent electrode removal,reconditioning, and realignment. Chlorinated phenols are elec-troactive and can be oxidized at moderately positive potentials(∼ +1.0 V vs Ag/AgCl). However, sensitive and stable electro-chemical detection (i.e., amperometric) of these pollutants isusually not possible with most electrode materials because ofirreversible adsorption of reaction intermediates and products.This adsorption causes rapid electrode response deactivation andeventually complete fouling.23-25 The boron-doped diamond thin-film electrode possesses electrochemical properties that distin-guish it from commonly used sp2-bonded carbon electrodes, suchas glassy carbon, pyrolytic graphite, and carbon paste.26-32 Unlikemost other electrodes, diamond shows less tendency to bedeactivated and fouled by phenol and chlorophenol oxidationreaction products. This is, in part, due to the hydrogen surfacetermination and the absence of an extended π-electron system.In previous work by our group,33,34 and by others,35,36 it has beenshown that the diamond thin-film electrode provides a sensitive,reproducible, and stable oxidation response for chlorinated phe-nols.

Many of the detection methods employed with CE forchlorinated phenol analysis do not achieve the low detection limits

(e.g., 1 ppb for PCP) required for water quality monitoring.Therefore, new approaches for the sensitive and stable detectionof these pollutants is an important area of research. Our grouphas demonstrated that CE-EC, with a diamond microelectrode,can be used to analyze for chlorinated phenols in standard aqueoussolutions down to the mid to low ppb level.34 The linear dynamicrange can be extended and the detection limit lowered if anextraction and preconcentration step is utilized. Liquid-liquidextraction (LLE) is one technique utilized in several approvedmethods for phenol analysis (EPA methods 604, 625, and8041).8,9,37 LLE, however, has the drawbacks of large organicsolvent volumes, foam formation, lengthy analysis times, anddifficulty with automation. Solid-phase extraction (SPE) is a sampleenrichment procedure that does not suffer from these limitations.SPE requires low eluent volumes, involves short analysis times,and is easy to automate and even integrate on-line. Even in theoff-line mode, it is a less time-consuming and more practicalextraction and preconcentration method than is LLE. SPE can beused not only to extract trace levels of organic compounds fromaqueous solutions but also to reduce the effects of interferingcomponents in a sample matrix.

This paper describes results from the analysis of chlorinatedphenols in water using off-line SPE and CE-EC. Electrochemicaldetection was performed amperometrically with a diamond mi-croelectrode. Details of the microelectrode preparation, theextraction and preconcentration procedure, and the instrumenta-tion are given. Electropherograms and detection figure of meritdata are presented for six different pollutants in both laboratory-prepared and natural water samples. A brief conclusion follows,summarizing the results and the main contributions of the work.We sought to demonstrate that this new electrode provides ahighly sensitive, reproducible, and stable response for the oxida-tive detection of these pollutants in a real sample, and a methodcould be developed that meets EPA requirements for monitoringchlorinated phenols in drinking water.

EXPERIMENTAL SECTIONDiamond Deposition and Microelectrode Preparation.

Boron-doped diamond thin film was deposited on a sharpened,76-µm-diameter Pt wire using microwave-assisted chemical vapordeposition (1.5 kW, 2.54 GHz, ASTeX, Woburn, MA), as describedpreviously.38 Deposition was accomplished using a 0.5% CH4/H2

source gas mixture containing 10 ppm diborane (B2H6) dilutedin H2 for doping. All source gases were ultrahigh-purity (99.999%)grade. For growth, the system pressure was 45 Torr, the substratetemperature was ∼700 °C (as estimated with an optical pyrom-eter), the microwave power was 400 W, and the total gas flowwas 200 sccm. The deposition time was 10 h. The nominal growthrate under these conditions was estimated to be ∼0.4 µm/h asthe final film thickness was in the 3-5-µm range.

After deposition, the diamond-coated Pt wire was attached toa copper wire (∼7 cm in length, 0.2 mm in diameter, GoodfellowCambridge Ltd., Huntingdon, England) using conductive silver

(14) Ewing, A. G.; Wallingford, R. A.; Olefirowicz, T. M. Anal. Chem. 1989, 61,292A.

(15) Wallingford, R. A.; Ewing, A. G. Anal. Chem. 1987, 59, 1762.(16) Park, S.; Lunte, C. E. Anal. Chem. 1995, 67, 4366.(17) Wallen, S. R.; Nyholm, L. Anal. Chem. 1999, 71, 544.(18) Huang, X.; Zare, R. N.; Ewing, A. G. Anal. Chem. 1991, 63, 189.(19) Lu, W.; Cassidy, R. M.; Baranski, A. J. J. Chromatogr. 1993, 64, 433.(20) Matysik, F.-M. J. Chromatogr. 1996, 742, 229.(21) Lu, W.; Cassidy, R. M. Anal. Chem. 1994, 66, 200.(22) Ratzlaff, K. L.; Martin, R. S.; Huynh, B. H.; Lunte, S. M. Anal. Chem. 2002,

74, 1136.(23) Gattrell, M.; Kirk, D. W. J. Electrochem. Soc. 1993, 140, 903.(24) Gattrell, M.; Kirk, D. W. J. Electrochem. Soc. 1993, 140, 1534.(25) Wang, J.; Ruiliang, L. Anal. Chem. 1989, 61, 2809.(26) Xu, J.; Chen, Q.; Swain, G. M. Anal. Chem. 1998, 70, 3146.(27) Xu, J.; Swain, G. M. Anal. Chem. 1998, 70, 1502.(28) Koppang, M. D.; Witek, M.; Blau, J.; Swain, G. M. Anal. Chem. 1999, 71,

1188.(29) Show, Y.; Witek, W. A.; Sonthalia, P.; Swain, G. M. Chem. Mater. 2003, 15,

879.(30) Hupert, M.; Muck, A.; Wang, J.; Stotter, J.; Cvackova, Z.; Haymond, S.; Show,

Y.; Swain, G. M. Diamonds Relat. Mater. 2003, 12, 1940.(31) Swain, G. M. Electroanal. Chem. 2004, 22, 181.(32) Fischer, A. E, Show, Y.; Swain, G. M. Anal. Chem. 2004, 76, 2553.(33) Muna, G. W.; Tasheva, N.; Swain, G. M. Environ. Sci. Technol. 2004, 38,

3674.(34) Muna, G. W.; Quaiserova-Mocko, V.; Swain, G. M. Electroanalysis 2005,

17, 1160.(35) Terashima, C. Rao, T. N.; Sarada, B. V.; Tryk, D. A.; Fujishima, A. Anal.

Chem. 2002, 74, 895.(36) Shin, D.; Sarada, B. V.; Tryk, D. A.; Fujishima, A. Chemical Sensors 2002,

18 (Suppl. B), 124. Shin, D.; Sarada, B. V.; Tryk, D. A.; Fujishima, A.; Wang,J. Anal. Chem. 2003, 75, 530.

(37) EPA Method 8041. Phenols by Gas Chromatography: Capillary ColumnTechnique, Environmental Protection Agency, Washington, DC, 1995; p 1.List of Drinking Water Contaminants and MCLs, EPA, http://www.epa.gov/safewater/mcl.html.

(38) Cvacka, J.; Quaiserova, V.; Park, J. W.; Show, Y.; Muck, A., Jr.; Swain, G.M. Anal. Chem. 2003, 75, 2678.

Analytical Chemistry, Vol. 77, No. 20, October 15, 2005 6543

epoxy (CW2400 Chemtronics, Kennesaw, GA). The epoxy wascured overnight at room temperature. The microelectrode wasthen sealed (i.e., insulated) in a polypropylene pipet tip (Daigger,Vernon Hills, IL). After inserting the diamond-coated Pt wire intothe tip, a heat gun was used to first melt the polypropylene at thetop of the microelectrode assembly thereby securing the copperwire. The tip end was then carefully heated to soften thepolypropylene, which formed a conformal coating around themorphologically rough microelectrode. The insulated microelec-trode had both a conical and cyclindrical geometry exposed.38

Electrochemical Measurements. Cyclic voltammetry wasperformed in a ∼5-mL, single-compartment glass cell. A three-electrode system consisting of a diamond working electrode, aAg/AgCl (3 M KCl) reference electrode, and a carbon rodauxiliary electrode was connected to a CS-2000 potentiostat(Cypress Systems, Inc., Lawrence, KS). The cell was housed inan electrically grounded Faraday cage in order to reduce theelectrical noise. All measurements were performed at roomtemperature in solutions first deoxygenated with nitrogen gas forat least 5 min and then blanketed with the gas during ameasurement.

Capillary Electrophoresis. CE was performed using ahomemade system consisting of a 30-kV variable-power supply(model CZE1000R Spellman, Hauppauge, NY), a home-builtelectronic timer and electrical relay for the electrokinetic injection,and a Plexiglas box with a safety interlock that housed the 76-cm-long, 30-µm-i.d. fused-silica capillary (Polymicro Technologies,Phoenix, AZ), buffer reservoir, sample vessel, and detectioncell.34,38 The separations were conducted by applying the desiredvoltage (relative to earth ground) between the run buffer reservoirand the electrically grounded detection reservoir. The injectionswere made electrokinetically by applying 10 kV for 3 s betweenthe sample and detection (grounded) reservoirs. The end-columndetection cell was fabricated from glass and consisted of (i) fourTeflon rods with Kel-F bolts for adjusting the capillary position inthe x and y directions, (ii) two Pt wires with one serving as aground for the high voltage and the other as an auxiliary electrodefor the electrochemical cell, (iii) a bolt for adjusting the separationcapillary-electrode distance in the z direction, and two openingsfor mounting the working and the reference electrodes.34,38 Athree-electrode system was employed along with an analogpotentiostat (EI-400 Bipotentiostat, Cypress Systems, Inc.). Thecurrent flowing through the detection electrode (analog) wasdigitized and recorded as a function of time with a Powerlab Chart5 (model ML 826 ADInstruments, Colorado Springs, CO). Eachnew capillary was activated by sequentially flowing (pressure-driven) 0.5 M NaOH, deionized water, and run buffer for 20 mineach. The capillary was then equilibrated in the run buffer underan electric field of 263 V/cm (20 kV) for ∼20 min prior to sampleinjection.

SPE Procedure. Local river water (Red Cedar River, MSUcampus) was used as the sample matrix in which to analyze forthe chlorinated phenols. The water was collected with a 500-mLNalgene bottle affixed to a 3-m sampling rod. The bottle was rinsedthree times with the river water before collecting a sample. Thesample was sealed in the bottle and transported back to thelaboratory for analysis. Prior to use, solid particulates wereremoved from the river water by passage through a nylon

membrane with a 0.4-µm pore size (Sigma-Aldrich, St. Louis, MO)mounted in a filtration assembly. After filtration, the water wasacidified to pH 2-3 by adding a few drops of concentrated HCl.The acidified water was then spiked with the chlorinated phenols.Pollutant extraction was carried out using ENVI-Chrom P SPEcartridges (Supelco, Bellefonte, PA). Liquid passage through thesorbent was facilitated using an Alltech vacuum manifold consist-ing of a glass chamber, a vacuum gauge and valve, manifold lid,and stopcocks. The sorbent had a particle size between 80 and160 µm, a surface area between 800 and 950 m2/g, and a meanpore size of 110-175 Å. The SPE tube had a volume of 3 mL anda stationary phase mass loading of 0.25 g. Prior to extraction, thesorbent was conditioned by sequentially passing 3.0 mL of ethylacetate, 3.0 mL of methanol, and 3.0 mL of ultrapure wateradjusted to pH 2.5 using a few drops of concentrated HCl. Thefiltered and acidified river water sample was then passed throughthe moist sorbent at a flow rate of <5 mL/min. After passage,the solid phase was dried under vacuum for 5 min prior to analyteelution, which was accomplished by adding 2.0 mL of ethyl acetateand allowing it to interact with the solid phase for 5 min beforeelution. This elution was performed (vacuum assisted) at a flowrate of ∼2 mL/min. The resulting extract had two phasespresentsan aqueous (more dense) and a nonaqueous one (lessdense). The chlorinated phenols were found to be concentratedin the nonaqueous ethyl acetate phase as the analysis of theaqueous phase revealed no detectable phenols. The nonaqueousphase was then carefully removed (using a pipet) and the volumeadjusted to using 1.0 mL of ethyl acetate. Ethyl acetate has somemiscibility with water, and therefore, the volume of the nonaque-ous phase was sometimes around 1 mL or less. If the nonaqueousphase had a volume greater than 1 mL, then the solution volumewas reduced to less than 1 mL by blowing a stream of nitrogengas across the liquid surface and then adjusting the final volumeto 1.0 mL with ethyl acetate. The preconcentration factor waseither 50:1 or 250:1.

Materials and Chemicals. The platinum wire was obtainedfrom Aldrich (76-µm diameter, 99.99%, St. Louis, MO). Allchemicals were reagent-grade quality, or better, and used withoutadditional purification. Potassium hydroxide (Columbus ChemicalIndustries, Inc., Columbus, WI), hydrochloric acid (Sigma-Ald-rich), and sodium hydroxide (Spectrum Chemical, Gardena, CA)were used to prepare the basic solutions. Ethyl acetate (Mallinck-rodt) and methanol (Sigma-Aldrich) were used in the SPEprocedure. The phenols tested were 2-chlorophenol (8.52), 3-chlo-rophenol (8.97), 4-chlorophenol (9.37), 2,4-dichlorophenol (7.90),2,4,6-trichlorophenol (6.00), and pentachlorophenol (4.74) (Sigma-Aldrich). The pKa values are given in parentheses. Chlorinatedphenols are toxic, volatile, and can be readily absorbed through theskin. Decomposition liberates toxic fumes and products that arecorrosive. These chemicals should only be handled in a ventilatedhood using protective eyewear and impervious gloves. The chemicalsused for the preparation of the run buffer and the supportingelectrolyte were sodium phosphate monobasic, sodium phosphatedibasic, and potassium chloride (Spectrum Chemical). The runbuffer was filtered through a 0.4-µm nylon membrane filter (Sigma-Aldrich) using the same filtration apparatus before use. Thesodium tetraborate (Na2B4O7‚10H2O) and potassium ferrocyanidewere obtained from Sigma-Aldrich. Ultrahigh purity perchloric acid

6544 Analytical Chemistry, Vol. 77, No. 20, October 15, 2005

(99.999%) was also obtained from Sigma-Aldrich. The ultrapurewater used for the solution preparations was distilled, deionized,and passed through activated carbon (>17 MΩ, BarnesteadE-Pure System).

RESULTS AND DISCUSSIONMicroelectrode Electrochemical Response. The electro-

chemical properties of the diamond microelectrode were evaluatedby cyclic voltammetry. A typical background voltammetric i-Ecurve in 1 M KCl is shown in Figure 1A. A low and featurelessbackground current is seen with a large overpotential for thehydrogen evolution reaction (HER). The cathodic current for HERcommences at potentials negative of -1000 mV. This largeoverpotential for HER indicates that there were no defects presentin the diamond coating. If the diamond film incompletely coveredthe substrate, then solution could penetrate the film and reachthe platinum. This would be manifested in HER occurring at muchmore positive potentials. The increase in anodic current at ∼1500mV is due to onset of chlorine evolution, which occurs with alower overpotential at diamond, as compared to oxygen evolution.

Figure 1B shows a typical cyclic voltammetric i-E curve for1 mM Fe(CN)6

3-/4- in 1 M KCl at 10 mV/s. A sigmoidally shapedi-E curve is seen at this low scan rate with an E1/2 of 330 mVand a limiting current, ilim, of ∼35 nA. This curve shape andmagnitude were seen after the first scan and remained unchangedduring multiple cycles for a given electrode. The reproducibilityof the response from electrode to electrode was also very good(RSD <5%). The architecture and size of the electrode were suchthat a steady-state flux of reactant was achieved only at low scanrates (ν e 50 mV/s). The diamond microelectrodes used in thiswork had a calculated surface area (cone + cylinder) of 2.2(( 0.7) × 10-4 cm2.

Separation and Amperometric Detection of ChlorinatedPhenols. Decoupling the separation voltage from the electrodesignal is a critical issue in CE-EC. In the present work,decoupling was achieved through the use of end-column detection.In this arrangement the microelectrode was positioned outsidethe capillary (i.e., outside the electric field) at a distance ofapproximately 20-30 µm. This was kept as short as possible inorder to minimize dilution effects and band broadening. In thisarrangement, the microelectrode response was relatively unaf-fected by the separation voltage with acceptable signal-to-noiseratios and negligible potential shifts. The average peak-to peaknoise at the detection potential (+1.05 V vide infra) ranged from0.5 to 1 pA.

Electrophoretic separation of the chlorinated phenols wasaccomplished using a 10/20 mM borate/phosphate run buffer atpH 8.4, a 76-cm-long fused-silica capillary (30-µm i.d.), and aseparation voltage of 20 kV.34 This run buffer was selected basedupon results from numerous optimization runs in which the bufferpH and composition were varied. The run buffer pH influencesan electrophoretic separation by affecting the analyte charge and,thus, its electrophoretic mobility, as well as the electroosmoticflow (EOF) due to changes in the charge density on the wall ofthe silica capillary (i.e., ú potential). The electrophoretic mobilityof a solute is governed by its charge/size ratio. The pKa of eachchlorinated phenol is dependent on the position and number ofchlorine atoms present. The values range from 4.74 for pentachlo-rophenol to 9.37 for 4-chlorophenol. At pH 8.4, all the chloro-phenols possess some negative charge, which means that thedirection of their electrophoretic movement is in opposition tothe electroosmotic flow. This results in a migration order thatexhibits a correspondence with the pKa as the compound withthe highest pKa (least negative charge) migrates first and the onewith the lowest pKa (greatest negative charge) migrates last. Thesample was injected electrokinetically at 10 kV for 3 s. The injectedvolume of a neutral electroosmotic flow marker, methanol, wascalculated to be 1.4 nL. Based on its migration time, the separationvoltage, and the column length, the EOF was calculated to be 6.2× 10-4 cm2/V‚s. After applying the detection potential, theseparation voltage was turned on and the detection electrode’sbackground current allowed to stabilize prior to an injection. Thisusually occurred within 3-4 min after separation voltage turn-on.

Figure 2 displays a typical electropherogram for a standardsolution of 2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and pentachlorophenol, allat the 40 µM level. The pollutants were detected amperometricallyat 1.05 V. Separation of all six pollutants occurs in less than 14min with all migration times being very reproducible over an 8-hperiod of use (RSD e0.5%). The peaks are asymmetric in shape(i.e., tailing), and this has previously been attributed to the natureof the solute mass transfer off the column and across the conicaland cylindrical diamond microelectrode, and not to on-columneffects.34 Detection figures of merit for each of the pollutants withdiamond electrodes (both planar and microelectrode architec-tures) in FIA-EC and LC-EC and CE-EC have been re-ported.33,34 For example, in CE-EC, response precisions of 5%(RSD), or less, and limits of detection (S/N ) 3-4) from 13 (2-chlorophenol) to 133 ppb (pentachlorophenol) were observed for

Figure 1. (A) Background cyclic voltammetric i-E curve for a boron-doped diamond microelectrode in 1 M KCl. Scan rate ) 100 mV/s.(B) Cyclic voltammetric i-E curve for 1 mM K4[Fe(CN)6] in 1 M KClat a boron-doped diamond microelectrode. Scan rate ) 10 mV/s.

Analytical Chemistry, Vol. 77, No. 20, October 15, 2005 6545

an 8-kV, 3-s electrokinetic injection.34 Importantly, the responsestability was excellent over many weeks of use with no evidenceof electrode deactivation and fouling.

Off-Line SPE Coupled with CE-EC. The previously re-ported CE-EC method, using a diamond microelectrode, achieveddetection limits that were below the health advisory levels of 40,20, and 4000 ppb for 2-chlorophenol, 2,4-dichlorphenol, and phenol,respectively. However, the method did not yield the 1 ppbdetection limit required for pentachlorophenol. Therefore, an SPEprocedure was implemented to extract the chlorinated phenolsfrom the aqueous solution for preconcentration, thereby extendingthe linear dynamic range and lowering the limit of detection. Themost widely used solids for the extraction and enrichment ofphenolic compounds from aqueous media are C8 and C18 modifiedsilica, carbon black, or polymeric resins.39-43 The effectiveness ofan extraction is reflected by the recovery, a term that refers tothe ratio of the number of molecules collected in the extractrelative to the number of molecules loaded onto the sorbent.43

Another performance parameter is the breakthrough volume. Thisis the maximum sample volume that can be passed through thesorbent while still maintaining, at least theoretically, 100% retentionof the solute molecules.43 The breakthrough volume depends onthe sorbent surface area available for solute retention as well ofthe free energy of the interaction between solute and the sorbent.For the above listed sorbents, polar solutes have the lowestbreakthrough volumes due to their high solubility in the aqueousphase. Highly cross-linked packing materials, such as LiChrolutEN, Styrosorb and Macronet Hypersol, Isolute ENV, HYSphere-1, and ENVI-Chrom P, have some of the highest breakthroughvolumes for polar analytes.42-48 These stationary phases arechemically stable over a broad range of pH, a property that

extends the applicability of the method. The cross-linking not onlyimproves the chemical stability but also increases the specificsurface area and allows for greater π-π interaction between theanalytes and the solid phase.43,48

The ENVI-Chrom P columns used in this work were developedfor the extraction of polar compounds from aqueous media. Thesolid phase consists of small, nonionic, styrene-divinylbenzenecopolymer beads that offer a much greater surface area than doesa typical silica-based packing.44,48 The aromatic sites promotestrong interaction with the aromatic phenols, thereby improvingthe recoveries. Retention of the phenols occurs by a reversed-phase partitioning mechanism as well as through π-π interaction.These cartridges have breakthrough volumes of greater than 200mL and yield chlorinated phenol recoveries of greater than 90%.44,48

In the present work, the extraction was evaluated usingdifferent volumes of spiked river water. By performing the analysison a natural water sample, we were able to assess to what extentthe humic and fulvic acids present in the river water affect theefficiency of the chlorinated phenol extraction. Humic matter canbind organic pollutants and cause both a decrease in thebreakthrough volume and a lowering of the recovery.43,46,49 Theinteraction of pollutants with humic material is pH dependent;therefore, the extraction of the chlorinated phenols at both acidicand neutral pH was evaluated. Preparation of the neutral solutionsimply involved spiking the river water (pH 7-8 normally) withthe chlorinated phenols. Preparation of the acidic solution involvedspiking the river water with the chlorinated phenol standards andadjusting the pH to 2.5 with concentrated HCl. Acidification is anormal practice in water analysis and is often necessary topreserve a sample and to minimize deprotonation of low-pKa

analytes. In the present work, the recovery was determined foreach chlorinated phenol using a 50 mL of the river water spikedat the 0.1-0.2 ppm level. The recoveries for 2-chlorophenol,3-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, and pentachlo-rophenol remained constant near 100% at both acidic and neutralpH, while the recovery for 2,4,6-trichlorophenol decreased fromnear 100% at low pH to 64% at high pH. This decrease is attributedto strong analyte interaction with the humic acid material, whichreduces the number of molecules interacting with the sorbent andextracted from the river water.46,49 Adsorption of phenols with lowpKa values on fulvic or humic acid material occurs at neutral pH.49

Based on these results, the river water pH was adjusted to 2.5 inorder to maximize the recovery of all the pollutants. Calibrationcurves were prepared using standard solutions of each chlorinatedphenol. Plots were constructed for concentrations ranging from0.1 to 13 ppm for 2-chlorophenol, 3-chlorophenol, and 4-chlo-rophenol, 0.2-16 ppm for 2,4-dichlorophenol; 0.2-20 ppm for2,4.6-trichlorophenol, and 0.3-27 ppm for the pentachlorophenol.Least-squares linear regression correlation coefficients for all thecurves were greater than 0.99. The spiked concentrations of thechlorinated phenols (0.1-0.2 ppm) fell within the linear dynamicrange of the calibration curve after preconcentration. The percentrecoveries ranged from 95 to 100% for all six pollutants, as shownin Table 1. Since the phenols are usually present in natural watersamples at trace levels, relatively large volumes of a sample areoften needed with SPE in order to achieve low limits of detection.Therefore, the percent recoveries were also determined for a 250-

(39) Pocurull, E.; Marce, R. M.; Borull, F. Chromatographia 1995, 41, 521.(40) Achilli, G.; Cellerino, G. P.; Melzi d’Eril, G.; Bird, S. J. Chromatogr., A 1995,

697, 357.(41) Somsen, G. W.; Jagt, I.; Gooijer, C.; Velthorst, N. H.; Brinkman, U. A.Th,

Visser, T. J. Chromatogr., A 1996, 756, 145.(42) Pichon, V.; Hennion, M. C. J. Chromatogr., A 1994, 665, 269.(43) Hennion, M.-C. J. Chromatogr., A 1999, 856, 3.(44) Pocurull, E.; Marce, R. M.; Borull, F. J. Chromatogr., A 1996, 738, 1.(45) Fiehn, O.; Jekel, M. Anal. Chem. 1996, 68, 3083.(46) Puig, D.; Barcelo, D. J. Chromatogr., A 1996, 733, 371.(47) Rodriguez, I.; Llompart, M. P.; Cela, R. J. Chromatogr., A 2000, 885, 291.(48) Pocurull, E.; Calull, M.; Marce, R. M.; Borull, F. J. Chromatogr., A 1996,

719, 105. (49) Porchmann, J.; Stotlmaster, U. Chromatographia 1993, 36, 207.

Figure 2. Electropherogram for a standard solution of chlorinatedphenols obtained using direct amperometric detection. The separationwas performed in a 10/20 mM borate/phosphate run buffer, pH 8.4.Separation voltage 20 kV. Capillary 30 µm i.d. × 76 cm long.Detection potential +1.05 V (vs Ag/AgCl). Injection 10 kV for 3 s.Solute concentration ∼40 µM.

6546 Analytical Chemistry, Vol. 77, No. 20, October 15, 2005

mL sample of river water spiked with chlorinated phenols atconcentrations ranging from 0.7 to 15 ppb. Good recoveries wereobserved for all the chlorinated phenols with values greater than95%. The relative standard deviation of the recovery from mea-surement to measurement was 4%, or less.

Figure 3 shows a typical electropherogram obtained for theextract from a 50-mL river water sample. The peaks for all sixpollutants are sharp and well resolved with some minor asym-metry. Good separation efficiencies were obtained with platenumbers greater than 170 000 plates/m. A system peak ofunknown origin is present with a migration time of ∼500 s. Thispeak became more pronounced in samples of lower chlorinatedphenol concentration but did not interfere with the separation or

quantitation of the analytes. Table 2 presents a summary of theseparation and detection figures of merit for a higher preconcen-tration factor of 250:1. The detection figures of merit weredetermined from measurements of six different chlorinated phenolconcentrations ranging from 0.02 to 200 ppb. A wide lineardynamic range is seen for each pollutant with linear regressioncorrelation coefficients greater than 0.997. The linear dynamicrange is from 0.02 to 100 ppb (i.e., 5 orders of magnitude) for2-chlorophenol, 3-chlorophenol, and 4-chlorophenol, 0.04-130 ppbfor 2,4-dichlorophenol, 0.2-150 ppb for 2,4,6-trichlorophenol, and0.05-200 ppb for pentachlorophenol. The good response sensitiv-ity of the diamond microelectrode, as well as low peak-to-peakbackground noise (0.5-1 pA), enabled minimum detectableconcentrations in the mid to high ppt range to be achieved for allthe pollutants. Good peak height response reproducibility is seenwith a nominal value of 5.4% or less. Importantly, the diamondelectrode response was stable for days (even weeks) of continuoususe, with no deactivation or fouling and no need for pretreatment.These data are in agreement with previous results from our group,and others, indicating that diamond thin-film electrodes are usefulfor the stable oxidative detection of chlorinated phenols, exhibitinggood resistance to surface deactivation and fouling.33-36 Electro-chemical detection of phenols and chlorinated phenols is usuallycomplicated by rapid surface deactivation and fouling caused bypolymeric film formation on the electrode surface.23-25 Themechanistic work by Gattrell and Kirk reveals that the phenoloxidation reaction proceeds initially through the formation of aphenoxy radical species, which can subsequently undergo radi-cal-radical coupling to form oligomeric and polymeric speciesof low solubility. Direct and indirect oxidation reactions producingsoluble products, such as hydroquinone and catechol, are alsopossible.23,24 The fact that diamond functioned stably in thismeasurement is a major attribute of this electrode for water qualitymonitoring. There was no need for reconditioning or self-cleaningas is the case for electrochemical detection with other electrodematerials.23-25 The low limit of detection, wide linear dynamicrange, excellent response precision, and response stability enabledthe SPE/CE-EC method to be successfully employed for theanalysis of these pollutants in river water.

Figure 3. Electropherogram for an SPE extract containing chlori-nated phenols at the 0.1-0.2 ppm level (0.8 µM each). River watersample volume applied to the SPE cartridge was 50 mL. Theseparation and detection conditions were the same as those listed inFigure 2. Peak designations a-e correspond to the same solutes aslisted in Figure 2. The preconcentration factor was 50:1.

Table 1. Percent Recoveries for the ChlorinatedPhenols Extracted from a 50 mL River Water Sample(pH 2.5) Spiked with 0.1-0.2 ppm of the Pollutants

analyte % recovery % RSD (n ) 6)

2-chlorophenol 97 2.43-chlorophenol 96 3.04-chlorophenol 95 2.12,4-dichlorophenol 98 3.92,4,6-trichlorophenol 97 3.8Pentachlorophenol 100 1.0

Table 2. Separation and Amperometric Detection Figures of Merit for the Chlorinated Phenols Analyzed by Off-LineSPE and CE-ECa

analyteLDR(ppb)

sensitivity(pA/nM)

responseprecision

(RSD) (n ) 9)LOD (ppb)(S/N g 3)

separationefficiency

(plates/m)

2-chlorophenol 0.02-100 3.2 ( 1.4 4.6 0.02 179 000r2 ) 0.9984

3-chlorophenol 0.02-100 5.0 ( 1.2 5.2 0.02 188 000r2 ) 0.9988

4-chlorophenol 0.02-100 5.3 ( 0.8 4.5 0.02 185 000r2 ) 0.9979

2,4-dichlorophenol 0.04-130 1.9 ( 0.5 5.0 0.04 173 000r2 ) 0.9982

2,4,6-trichlorophenol 0.20-150 1.2 ( 0.3 5.4 0.20 292 000r2 ) 0.9998

pentachlorophenol 0.05-200 1.7 ( 0.8 5.2 0.05 172 000r2 ) 0.9991

a CE-EC parameters: Einj ) 10 kV; tinj ) 3 s; capillary dimensions 30 µm i.d. × 76 cm long; Esep ) 20 kV; and Edet ) 1.05 V. LOD, limit ofdetection. The data are based on the response of three diamond microelectrodes. The separation efficiency, N, was determined from the equation,N ) 41.7 (tr/w0.1)1/2/(b/a + 1.25). The figures of merit were determined for extracted samples using a preconcentration factor of 250:1.

Analytical Chemistry, Vol. 77, No. 20, October 15, 2005 6547

Finally, it is important to evaluate how this method comparesagainst others reported on in the literature. In general, it appearsthat no analytical method, without some form of sample enrich-ment, is capable of achieving the low detection limit required forPCP (1 ppb), as mandated by EPA. SPE is now a common sampleenrichment method that can be utilized in combination with anyanalytical method to increase the linear dynamic range and tolower the detection limit for an analyte.43 Chlorinated phenolanalysis has been carried out using SPE and gas chromatography(GC), liquid chromatography (LC) and CE,2,3,46,50-58 with electro-chemical, UV/visible, and mass spectrometric detection. Chlori-nated phenol limits of detection in the mid ppt to low reported onherein ppb range (S/N g 3) are typical for preconcentrationfactors of 250-500, depending on the analyte. For example,Cheung and Wells reported a detection limit for PCP of 0.05 ppbusing GC/MS.50 Glaceran and Jauregui reported a detection limitof 0.04 ppb for PCP using LC-EC.52 Bruijnsvoort et al. reporteda detection limit of 0.2 ppb for PCP using CE-EC (MEKC).58 Thedetection limits reproducibly obtained with the CE-EC methodreported on herein are as good or better than those reported inthe literature. Moreover, the response reproducibility for this CE-EC method is superior to that reported for the other methods.The most noteworthy feature of this method, as compared to theothers using electrochemical detection, is the stable response andthe fact that no reconditioning or self-cleaning is necessary foroptimum electrode performance.

CONCLUSIONSAn analytical method is described for chlorinated phenol

analysis in river water. SPE was employed for sample enrichment,and CE-EC was used for efficient separation and detection ofthe electroactive chlorinated phenols. A key enabling feature ofthis method is the use of a diamond microelectrode for electro-chemical detection. The method provides detection figures ofmerit superior to other methods reported on in the literature andexceeds EPA requirements for drinking water monitoring in termsof the MCL for pentachlorophenol (1 ppb) and the health advisoryaction levels for 2-chlorophenol, 2,4-dichlorophenol, and phenol(40, 20, and 4000 ppb, respectively). Diamond outperforms allother bare metal and carbon electrodes for the amperometricdetection of chlorinated phenols, in large part, because of itsresistance to deactivation and fouling. Rapid deactivation andfouling plague EC detection with other electrode materials andlimit its application for phenol and chlorinated phenol analysis.Diamond exhibited good electroanalytical performance with a lowpeak-to-peak noise (0.5-1 pA), a low and stable backgroundcurrent under the electrophoretic conditions imposed, and asensitive, reproducible, and stable oxidation response for thechlorinated phenols over many days to weeks of use. Reproducibledetection limits for all the chlorinated phenols were in the midppt range with the exception of 2,4,6-trichlorophenol which wasin the high ppt range. The power of CE separations, the simplicityof electrochemical detection, and the properties of this newelectrode material render this method viable for chlorinatedphenol analysis in real water.

ACKNOWLEDGMENTThis work was supported by a grant from the Office of Naval

Research through the Exploratory Unit Water Purification Pro-gram (EUWP) (N000140310995).

Received for review March 21, 2005. Accepted August 8,2005.

AC050473U

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