Anal. Chem. 1981, 53, 1809-1813
1809
Kel-F-Graphite Composite Electrode as an Electrochemical Detector for Liquid Chromatography and Application to Phenolic Compounds Duane E. Weisshaar and Dennis E. Taliman' Department of Chemistry, North Dakota State University, Fargo, North Dakota 58705
James L. Anderson Department of Chemistry, University of Georgia, Athens, Georgia 30602
The optimum composltlon of a compresslon-molded Kel-Fgraphlte composite electrode Is investlgated for the electrochemical (EC) detection of effluents, including phenolic corn pounds, separated by reversed-phase high-pedormance liquid chromatography (HPLC). Opthnm compositions of 10-15% graphite by weight can be ratlonaked on the basis of the partlcuiate structure of the graphlte actlve sites. The electrode Is compatible wlth a wide range of organlc Solvents and Its linear response to the target compounds extends Over 5 orders of magnltude In concentration. The detector has been applled for the determlnatlon of a range of phenolic species In natural water and a coal gaslfler waste water sample. Detectlon limlts for a varlety of phenols range from 3 to 15 pg. Repetitive lnjectlons yield peak helghts with RSDs typically less than 1 %. The long-term stablilty of the electrode Is also demonstrated.
A number of electrode materials have been used for electrochemical (EC) detectors in liquid chromatography (LC), including many forms of carbon (1-7).The Kel-F-graphite (Kelgraf) electrode developed in our laboratories (5,6,8, 9) has been demonstrated to be effective for trace detection in both LC (5,6) and flow injection analysis (9). We report here investigations on the optimization of electrode composition for improved signal-to-noise ratio. The suitability of the optimized electrode is demonstrated for detecting low levels of selected compounds in complex matrices, including phenolic species in natural waters and waste waters from a coal gasifier. Phenolic compounds were chosen as test compounds because of their increasing importance as byproducts of energy production technologies based on coal. The potential contamination of natural waters by phenolic wastes from processing of coal and other fossil fuels is likely to become more significant as utilization of these alternate fuel sources increases in the future. Phenols are primary contaminants in coal gasification and liquifaction waste waters (IO). Furthermore, the strip mining of coal as practiced in western states exposes the coal to air and water which may result in the release of phenols into surface and ground water systems. Since phenol itself can be detected by odor and taste a t the 10 ppb level and chlorinated phenols (which can be produced when chlorine is added to drinking water) a t the 1 ppb level (111,it is necessary to be able to monitor such compounds at trace levels. For easily oxidized or reduced compounds, electrochemical detectors for LC are among the most sensitive. Takata and Muto have shown that phenols can be detected electrochemically (7).Several electrode nlaterials have been used for the determination of phenols a t trace levels. Kissinger and coworkers have obtained picogram sensitivity for catecholamines 0003-2700/81/0353-1809$01.25/0
on carbon paste (I). Bollet et al. found a detection limit in the picogram range for phenols on glassy carbon (2). Armentrout et al. have developed a carbon-polyethylene tubular electrode which has detection limits for phenols also in the picogram range (3). Strohl and Curran have used a reticulated vitreous carbon (RVC) electrode for flow injection analysis of some phenols and found detection limits on the order of a few tenths of a nanogram ( 4 ) . If liquid chromatography with electrochemical detection (LCEC) is to be used on a routine basis, the detector electrode material must be compatible with a variety of solvents, useful over a large potential range, and easy to fabricate and machine. Carbon is attractive because it has a good positive potential range. Carbon paste has the drawback that the binder normally used in the fabrication of the electrode is soluble in organic solvents. Glassy carbon, while impervious to organic solvents, is expensive and rather difficult to machine or fabricate. RVC electrodes, due to their relatively high porosity, limit minimizatiop of detector volume unless crushed and packed into a tube. Additionally, the nature of RVC restricts detector design to the flow-through electrode type. The carbon-polyethylene electrode is attractive because it is easy to fabricate and machine and it is inexpensive enough to be discarded when it becomes fouled; however, absolute detection limits for phenolic compounds (3)were somewhat poorer than those obtained in this work with the Kelgraf electrode. The Kelgraf electrode is compatible with a wide range of organic solvents and is easily fabricated and machined (6,8, 9). When optimized with respect to composition, the Kelgraf electrode permits detection of phenols in LC effluents at low picogram levels. EXPERIMENTAL SECTION Equipment. The liquid chromatographconsisted of a Milton Roy Model 396 pump, a Rheodyne Model 70-10 injector with a 20-pL sample loop, a stainless steel Bourdon tube pulse dampener, a Rheodyne Model 7037 pressure release valve, a Rheodyne 7302 inline filter,and a 0-5000 psi pressure gauge. Two columns were used in the course of this study, a Bioanalytical Systems 15 cm X 4.6 mm i.d. fi Bondapak CI8column with 5-pm particles and an Altex 15 cm x 4.6 mm i.d. Ultraaphere-octylcolumn with 5-pm particles. The column, injector, potentiostat, and EC detector were housed in a steel Faraday cage to minimize environmental electrical interferences. The cell and the reference and counterelectrode holder were constructed in-house and have been described elsewhere ( 6 , 9 ) . The potentiostat, also constructed in-house, was a standard difference amplifier controller design. The fabrication and polishing of the Kelgraf electrode have been described previously (8). Chromatograms were recorded on either a Heath Model EU-205-11strip chart recorder with a Health Model EU-200-81 Servoamp Patch Module or an Omniscribe 5000 dual pen strip chart recorder. All potentials are reported vs. Ag/AgC1/3.5 M KCl. 0 1981 American Chemical Society
1810
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 198
Table I. Background, Sensitivity, and Noise as a Function of Electrode Composition“ est diampeak-tobacketer of ground sensipeak active % current, tivit y d noise, sites: graptite nA nA/pM PA Pm 35-50 20 150 24.3 110 35-50 18 100 20.0 100 14.3 35-50 17 30 85 30 35-50 10.2 71 16 10 30 35-50 51 5.4 p-Bondapak column, E, = 1.15 V, 30% acetonitrile. All electrodes had an approximate geometrical area of 20 mma. Diameter of 20.0% electrode particles estimated from experiment; others calculated by normalizing to 20.0% result, assuming that average site area and thus square of site diameter is proportional to percent graphite. Determined by using p-methoxyphenol. a
Reagents. Water was purified by passing house-distilled water through a Milli-Q deionizer system (four cartridge system from Millipore Corp., Beford, M A a major particle filter, activated charcoal, and two mixed bed ion exchange cartridges, in that order) and then distilling first from alkaline permanganate and finally from glass. Acetonitrile was “Distilled in Glass’’LC grade (Budick and Jackson Laboratories, Inc., Muskegon, MI). Eluents contained 0.05 M NaH2P04,pH 4.2, as supporting electrolyte and varying percentages of acetonitrile as organic modifier. The various phenols were reagent grade obtained from several commercial suppliers. Phenols that exhibited discoloration due to oxidation were purified by sublimation. Whatman GF/F glass fiber filters (0.7 pm) were found most useful for filtration of eluents and samples.
RESULTS AND DISCUSSION Optimization of Detector Electrode Potential. To minimize background interference and maximize analyte response, the applied potential should be at the minimum value a t which current reaches the limiting-current plateau of the analyte. The analytical potential can be determined directly from hydrodynamic voltammetry (HDV) or estimated indirectly from cyclic voltammetry (CV) (5). On the basis of CV and HDV measurements for a variety of phenols (5),an applied potential of 1.27 V was selected for analysis of samples containing a mixture of phenols. Use of less positive applied potential results in loss of sensitivity toward certain of the phenols, whereas more positive applied potential leads to detector drift and increased background and noise. The optimum applied potential is dependent upon pH of the eluent. The half-wave potentials of phenols shift less positive and the limiting current decreases as the pH is increased due to a change in mechanism of oxidation (12). The best overall signal-to-noise ratio for the phenols investigated was observed at pH 4.2. Further details pertaining to the selection of optimum electrode potential for detection of phenols may be found elsewhere (5). Background and Noise. Electrode composition was systematically varied to optimize analytical sensitivity, background noise, and steady-state background current, as summarized in Table I. Analytical sensitivity diminishes with decreasing graphite content, as expected for a decrease in active electrode surface area, but significantly less rapidly than expected, assuming that active area is directly proportional to graphite content. A rather dramatic decrease in noise (measured with a 2-s time constant) is observed as the graphite content of the electrode is reduced. The noise reported in Table I appears to be chemical in origin and varies from random low-frequency base line fluctuations at 25% graphite electrodes to predominantly nonrandom flow noise, synchronized with the pump stroke, at 10% graphite electrodes.
As percent graphite is reduced, random contributions to the overall noise become less important and the noise reaches a flow noise limit (30 PA). Background current appears to be relatively independent of electrode composition over the range of composition investigated. The simultaneous gradual decrease in analytical sensitivity and rapid decrease in noise with decreasing graphite content can be rationalized from knowledge of the particulate structure of the Kelgraf electrode and from comparison of the dimensions of graphite active sites and surrounding Kel-F gaps to the thickness of the electrochemical diffusion layer in the flow channel adjacent to the electrode. Electron and visible light microscopy suggest that the Kelgraf electrode consists of islands of graphite particles in a sea of Kel-F (13). Approximately 100 pm average diameter of aggregated graphite active sites, separated by Kel-F’gaps of comparable dimension, and ca. 50% active area are indicated for a 20% graphite Kelgraf electrode, by comparison of experimental current-time response in a potentid step experiment (13)with predictions of a theoretical model due to Gueshi et al. (14). The apparent area of the composite electrode in a thin-layer flow cell depends on the ratio between the electrochemical diffusion layer thickness (xd, governed by flow rate, cell gap thickness, and analyte residence time adjacent to the electrode) q d the diameters of the electrode active sites (d,) and the intervening insulating gaps between active sites (g,). For rapid flow (short residence times and thin diffusion layers), xd
