Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 593-598 Wynblatt, P.; Ku, R. C. In “Interfaclal Segregation”; Ed. Johnson, W. C.; Blakely, J. M. Ed.; American Society for Metals: Metals Park, Ohio, 1979; D 115. Yao, H. C.; Shelef, M. “Proceedings of the Seventh International Congress on Catalysis”; Tokyo, 1980, Preprints, Paper A. 21. Yermakov, Y. I. “Proceedings of the Seventh International Congress on
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Catalysis”; Tokyo, 1980, Preprints, Paper P5. Yu, K. Y.; Helms, C. R.; Splcer, W. E. SoM State Commun. 1976, 78, 1365.
Received for review April 16, 1981 Accepted June 30, 1981
Electrochemical Detectors in Liquid Chromatography. A Short Review of Detector Design Stephen G. Weber’ DePaftf?Wnt of Chemistry, University of Pittsburgh, Pittsburgh, Pennsyivania 75260
William C. Purdy Department of Chemistry, McGill UniversW, Montr6a1, QuBbec, Canada H3A 2K6
This review covers design criteria for the construction of electrochemical (amperometric and coulometric) detectors which operate in flowing streams. Equations are given for the sensitivity of each electrode design as a function of the geometrical parameters (cell dimensions, electrode area) and physical parameters (liquid flow rate, diffusion coefficient, kinematic viscosity). It is shown that the sensitivities of common detectors are similar; on the order of 0.5 pA/pM/l .O cm2electrode area for a one-electron oxidation or reduction. The use of multiple electrodes is discussed. Selectivity can be gained by using two electrodes operating at different potentials. Selectivity and increased precision result from using one electrode to oxidize or reduce the constituents of the flow stream while a second electrode downstream detects the products of the first electrode reaction. Practical points on the operation of the detector, especially noise reduction, are given.
Introduction In fewer than eight years detection of solutes in a liquid chromatographic eluent by electrochemical methods has become one of the most sensitive, specific, and reliable procedures in the analytical laboratory. The work done to date has covered some of the theoretical aspects of how they work, and many applications to compounds which are easily oxidized or reduced. The last three or four years has seen an increasing inquisitiveness concerning how to build a better detector and how to use the available detectors to give more accurate or precise results. It is the intent of the authors that this review should allow a scientist to understand enough about the principles of the operation of such detectors so that he or she could design and build a detedor to obtain information of a desired sort. For good reviews of the history and applications of electrochemical detectors and good arguments of advantages and disadvantages, see Rucki (1980), Heineman and Kissinger (1980), and Kissinger (1977). Two companies, Bioanalytical Systems and Brinkman Instruments, publish information on application of these detectors. This review will begin with a simplified discussion of the basic operating parameters. A guide to deciding whether electrochemical detection is suited to one’s purpose is next, followed by a discussion of various designs and their uses. 0196-4321/81/1220-0593$01.25/0
Finally, a section on practical aspects covers the use of the detectors.
Operating Parameters All the detectors which will be discussed operate by generating a current% the presence of a solute. Thus the detector is a transducer which converts concentration information into electrical current information. The detectors consist of at least a pair of electrodes separated by a solution. This solution is flowing, usually but certainly not necessarily, out of a liquid chromatographic column. The solution is part of an electrical circuit through which current flows. There is, then, a requirement for charge carriers in the solution. This is why most applications are found in chromatography with polar eluents which may be made conductive by adding salts, acids, or bases. Less polar eluents may be used if they can be rendered conductive (Lemar and Porthault, 1977). The quantity of current generated in an electrochemical detector depends on the concentration of solute. This relationship is often linear over 4-5 orders of magnitude of concentration. The ratio of current generated to concentration present is called the sensitivity. The sensitivity depends on two sorts of processes. Recall that the solution is a part of an electrical circuit; electrons flow in the external circuitry while ions carry the charge in solution. The conversion of electron current to ion current occurs at the interface between an electrode (actually part of the external circuitry) and the solution. In order for this to occur there must be a molecule in solution at the interface capable of donating or receiving one or more electrons to or from the circuit. In general, one is only interested in one of the pair of electrodes in solution; this is the one at which the solute of interest engages in electron transfer, called the working electrode. The two sorts of processes on which sensitivity depends are (1)mass transfer; how easily can a solute get to the interface in order to undergo electron transfer? and (2) charge transfer or electron transfer; once at the surface of the working electrode, how likely is electron transfer? 1. Mass Transfer. The transport of matter from the bulk of solution to the electrode surface arises due to bulk 0 1981 American Chemical Society
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motion of the solution, and due to the diffusion of solute to the surface. (Migration, the motion of charged solutes in the electric field, is a small effect when the ionic strength of the solution has been made large by the addition of salts.) In general the molar flux of solute to the electrode surface may be given by J(mol/cm2s) = h(cm/s)[C” - C(0)](mol/cm3) The measured current is proportional to the flux
i ( C / s ) = J(mol/cm2 s)n(equiv/mol)F(C/equiv)A(cm2) The term h is the mass transfer coefficient, C” is the concentration of solute in the bulk of solution, C(0) is the concentration of solute at the electrode surface, A is the electrode area, n is the number of electrons transferred per molecule of solute, and F is the Faraday (about 96500 C/equiv). The term h depends on detector geometry and size, solution flow rate viscosity, and temperature. A quantitative discussion of electrode response using this term borrowed from treatments of heat transfer has been given (Blaedel and Engstrom, 1978). 2. Electron Transfer. It is difficult to convey all the necessary information in one paragraph; this is one entire branch of the science of electrochemistry. A selected list of references covering these topics (in the order theoretical to applied) is Bockris and Reddy (1970), Bard and Faulkner (1980), Adams (1969), and Sawyer and Roberts (1974). The factors which are important for determining whether or not electron transfer will occur once a molecule is at the surface of the working electrode are the same as one would consider for any other reaction: thermodynamics and kinetics. A table of reduction potentials displays the thermodynamic driving force for oxidation or reduction. By convention, an electrode which has a potential more positive than that of a particular half-cell reaction will have a thermodynamic tendency to oxidize (remove electrons from) the molecule. Whether or not the oxidation actually occurs at an analytically useful rate depends upon the kinetics of that particular electrode reaction. This is a complex issue, and no generalizations can be made. It not only depends upon the molecule being oxidized but also upon the solvent and electrode material. The analytically important experimental observable is that for an electrode reaction which is kinetically slow, the application of a greater potential difference (positive working vs. reference for an oxidation and negative working vs. reference for a reduction) will increase the reaction rate, thereby increasing the current. For a reaction which occurs in one direction only J = k(E)C(O) where k(E)is the potential dependent heterogeneous electron transfer rate constant for the oxidation of a species, the concentration of which is C(0) at the electrode surface. k ( E ) has units of centimeter per second. If the concentration of electroactive species at the electrode surface is the same as the bulk concentration, then the flux due to mass transfer is zero and the current is controlled by kinetics. If the reaction rate is rapid, then the concentration of electroactive species near the electrode surface is depleted. If that concentration approaches zero, then the current is entirely mass transfer controlled. In the former case the major experimental control over sensitivity is the working electrode-reference electrode potential difference. In the latter case the major experimental controls over sensitivity are electrode and cell design and solution flow rate or velocity. Figure 1 shows the effects of electron transfer rate and mass transfer rate
Figure 1. Detector sensitivity as a function of mass transfer coefficient, h, and heterogeneous charge transfer rate k. The sensitivity is in arbitrary units.
on the sensitivity of a typical detector. It is in the region of mass transfer control that most electrochemical detection occurs; thus it is h which affects the sensitivity of a particular electrochemical detector and the design of these detectors often centers on h. Will Electrochemical Detection Solve Your Problems? The first question to answer is whether or not the compound or compounds of interest are or can be made electrochemically active. Consulting the literature (vide supra) for analogues to one’s compound is a logical first step. In the absence of information from this source, the series by Bard and Lund entitled “Encyclopedia of Electrochemistry of the Elements” published by Marcel Dekker is a valuable source of oxidation-reduction reaction data. Also one may wish to consult any of several texts on organic electrochemistry, a lit of which is provided at the end of the first chapter of Bard and Faulkner (1980). If your compound is not electroactive, can it be made so? Several indirect reactions may be useful, e.g., hexacyanoferrate(1II) oxidation of sugars, displacement of Hg2+ from HgEDTA by metal ions, reaction of amino acids with copper(I1) (Takato and Muto, 1973), and reaction of Br2 with unsaturated compounds (King and Kissinger, 1980). A discussion of other precolumn and post column reagents has been published (Kissinger et al., 1979). For an electroactive compound the lowest quantity which can be measured with about 10% relative standard deviation depends on the noise level in the measurement. For something which is easily oxidizable (potential difference between the working electrode and Ag/AgCl reference electrode less than +0.7 V) around 0.1-1 pmol are detectable at this level of precision. At higher potentials the precision becomes worse. For reductions one can obtain the same precision at around the 10 pmol level using a gold amalgamated mercury electrode (MacCrehan and Durst, 1978; Funk et al., 1980). The above estimates are intended only as a guide and are dependent on many factors other than electrode design and operating potential. For a nonelectroactive compound in which case one must use a chemical reaction system, detection limits are poorer. The worst case is that in which one detects the disappearance of an electrochemically active species, e.g., Brz (King and Kissinger, 1980) because of the necessarily large background current. In this case a 10% relative standard deviation is obtained at the 50 pmol level. The upper limit is governed by considerations of linearity of the concentration vs. current curve and by the chemistry of the electroreacted solute. For systems not
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b
a
rectangular duct, one wall of which contains the working electrode (Figure 2A). There are many applications of this basic design. A list of references in which this cell (among others) is used is available from Bioanalytical Systems, Inc., W.Lafayette, IN 47907. The mass transfer coefficient for the working electrode of a channel cell is h = 1.467(D/b)2/3(0/A)’/3
Figure 2. Amperometric detector schematics: A, channel; B, tubular; C, “wall-jet”; a, entrance; b, exit; c, working electrode; d, spacer.
I
C
b B I
C
Figure 3. Coulometricdetector schematics: A, open; B, reticulated; a, entrance; b, exit; c, working electrode.
requiring a chemical reaction the linear range is often 4-5 orders of magnitude. The linear range may be somewhat lower for chemical reaction systems. To summarize, for compounds of low molecular weight (
