Anal. Chem. 1991, 63, 2951-2955 (10) Trlnder, P. A. Ann. Clln. Blochem. 1071. 37,38-41. (11) Purdle, N.; Swallows, K. A. Anal. Chem. 1080, 67, 77-MA. (12) Burke, W.; Diamondstone, E. I.; Velapoldl, R. A.; Menis, 0. Clln. Chem. 1074, 20, 794-801. (13) Chugaev (Tshugaev), L.; Gastev, A. Chem. Eer. 1010, 4 2 ,
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(16) Abell, L. L.; Levey, B. 6.; Brodie, 8. 6.; Kendall, F. F. J . E b l . Chem. 1852, 795, 357-366. (17) Tletz, N. W., Ed. Fundementals of Clinical Chemisby. 3rd ed.: WB Saunders Co.: Philadelphia, PA, 1987; p 457.
.--. .--..
AWRI-ARRA
(14) Cox, R. H.; Spencer, E. Y. Can. J . Chem. 1051, 29, 217-222. (15) Purdie, N.; Purdie, R. N. Unpublished results, Chemistry Department, OSU, 1989.
RECEIVED
for review June 5, 1991. Accepted September 19,
1991.
Elemental Specificity in Dual-Channel Flame Photometric Detection of Gas Chromatographic Peaks Walter A. Aue,* Brian Millier, a n d Xun-Yun S u n Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4 J 3 Canada
Slgnals from a duakhannel flame photometric detector (FPD) can be dlgitally processed by a conditional-access algorithm (CONDAC) to yiekl chromatograms that are speclwlc (meanlng infinltely selective) for any chosen FPD-active element. Usually, one chromatographic separatlon stored In the computer can thus be examlned successlvely for the presence of several heteroatoms.
INTRODUCTION When it first became commercially available (I),the flame photometric detector (FPD) (2) offered selective detection for only two elements: phosphorus and sulfur. Since then, some 20 more elements have been shown to produce significant FPD response. (For this purpose a t hand, “significant response” is defined as being at least 10 times stronger than expected from the carbon portion of the molecule.) Figure 1 presents an overview of “best” molar detection limits as reported in the literature (B, P, S, Cr,As, I) or taken from the measurements of our own group. Similar to other selective GC detectors, the FPD is often used to scan complex environmental or medical samples that may show several hundred peaks in capillary gas chromatography. Out of an overwhelming number of hydrocarbonaceous matrix components, this type of survey analysis can pinpoint trace amounts of compounds that contain a particular, biologically interesting heteroelement. Missing a compound of that element or mistaking it for that of another thus becomes a real possibility. While the FPD was once restricted to volatile analytes, its recent use with techniques such as supercritical fluid chromatography and capillary HPLC has opened the way for the selective detection of many more compounds, including nonvolatile organometallics of industrial or biochemical importance. Because many more of these compounds can now be separated, the need for new detector modes of improved selectivity and unambiguous elemental recognition has greatly increased. An ideal FPD would respond in a specific manner to any element for which its interference filter had been chosen. (Note that the term “specific” is commonly used to mean “designed for” or “selective for” in the current FPD literature. In this study, “specific” retains its original analytical definition of “infinitely selective”-meaning that no other than the chosen analyte element is seen to respond.) Unfortunately, true specificity is found rarely if at all among the selective GC detectors, and in this regard the flame photometric detector is no exception. The spectra of most FPD-active ele0003-270019110383-295 1$02.5010
ments overlap severely, and there exist several wavelength regions where practically all species radiate to a major or minor degree. In fact, the increase in selectivity gained by using an interference filter is often quite small. Elemental selectivity, in all kinds of analytical systems, can be significantly improved by dual-channel differential operation (e.g. ref 3). In the FPD this approach can be most effectively used to discriminate against a complex hydrocarbon matrix and to confirm the identity of suspected heteroatoms. An example can be found in our recent determination of a manganese antiknock compound in gasoline (4). While this dual-channel subtraction method excels at eliminating response from compounds of one particular element, it proves cumbersome and time-consuming to use with multielement samples (5). In these, the analyst often needs to establish the presence of only one element (or of only one element at a time). This study examines the theory that elemental specificity can be achieved for resolved GC peaks from a dual-channel flame photometric detector. While tested only in the FPD, the described approach should be applicable wherever two synchronous channels of a suitably diverse and time-dependent information content are available from some sensing device. EXPERIMENTAL SECTION A gas chromatograph with a dual-channel flame photometric detector (Shimadzu GC-4BMPF) was used with a packed column (100 X 0.3 cm i.d. glass, 5% OV-101 on Chromosorb W, 100/120 mesh) under a nitrogen flow of 22 mL/min. The detector, with its standard quartz chimney removed, was run with 300 mL/min of hydrogen and 60 mL/min of air plus an additional 18 mL/min of nitrogen, under an efficient exhaust duct. The output from the two Hamamatsu R 268 photomultiplier tubes was amplified by two slightly modified GC electrometers (Shimadzu) and, via their “integrator” ports, passed onto a laboratory-made electrometer/computer interface. After being dampened by a simple RC filter to prevent aliasing noise in the digital data, the signal voltage was converted to pulses and sampled every tenth of a second. A full-scale input produced a digital count of 16K, about equal to a 14-bit analog-to-digital converter. The dual-channel counts were processed by a 12-MHz AT-compatible computer with the help of a 1-Mbyte memory, 40 Mbyte hard disk, 80 287 math coprocessor, VGA display adapter, and Multi-Sync monitor. The two necessary programs, named CHROM and CORR, were written in compiled Quick Basic to generate a reasonably fast code. All data acquisition was performed by assembly language routines using interrupts for timing accuracy. CHROM is an existing, laboratory-developed program for highaccuracy display and manipulation of dual-channel chromatograms. It offers up to 1600 s (26 min 40 sec) of chromatographic 0 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 24, DECEMBER 15, 1991
OPEN
-”.
I,
c
Figure 1. Approximate overview of “best”FPD detection limb, in -log (moles of element per second) at SIN = 2, as taken from the literature and from our own measurements. Halogens: as copper or indium halides. Darkened rectangles: no significant response (own experiments). Note: as these data come from different sources, different detector models, etc., they should be used with due caution. Direct comparisons among certain elements may prove misleading. Data from other than the hydrogen-rich, typical GC-FPD flames have not been included.
run and CRT readout time and provides convenient routines such as digital filtering. Most important for its use in improving selectivity, the CHROM program has differential capabilities; i.e. it permits rough (manual, one-step) and fine (automatic, iterative) scaling and color-coded display of the two channels and their numerical intensity ratio. Superimposed on the two chromatographic traces appears a separately magnifiable trace of their amplitudinal difference (subtraction chromatogram). For each element in the present study, the intensity of one channel was adjusted such that a single peak-that of a standard containing the element of interest-assumed the same size in both chromatograms and that the subtraction mode displayed no (or minimum) evidence of its presence. (Note that this subtraction mode had really been developed for generating differential chromatograms in which matrix components, e.g. hydrocarbons, could be suppressed (5). Being available, it was used here as an easy means of producing intensity-matched channels for the subsequent correlation routine.) CORR, the second program, is an algorithm specially written for this study, which correlates the slopes of data trains from two channels that had been previously adjusted by the CHROM program for equal response of a heteroelement standard (see above). CORR’S main operating principle is “conditional acceptance” or “conditional access” (CONDAC): data are accepted by the algorithm and can access the final readout routine only on the condition that the slopes sa (i.e. P R a / A t ) and S b of the two chromatographic responses R, and Rb do not differ from one another by more than an (analyst-chosen) fraction f of their value-otherwise the data are rejected and a flat baseline is drawn. If accepted-Le. if located within the tolerance band of width 2fSa-each data pair is summed (and the result is halved for display convenience) to produce the final response R. To wit, if
then R = (R, + Rb)/2 if not, then
R=O Data preliminarily accepted in this manner gain access to final readout only after further tests against statistical and chromatographic criteria, of which two more have threshold values set by the analyst. First, a minimum percentage of positively correlated data pairs must be obtained, and second, this extent of positive correlation has to be maintained over a specified time. The particular settings of these thresholds may vary widely depending on the type, operating conditions, and quality of the chromatographic separation, on the kind of sample, on the signal/noise level, and on the objectives of the analyst. For general purposes, most analysts may want to choose the most narrow
Re IRU
Fe
3;
immin
160
131
I/ !
1 ~ ~ m i n
,&t
Figure 2. Comparison of simultaneous chromatographictraces from an open (nondispersive)channel (upper half) and a channel equipped with a 530-nm interference filter. Compounds are as listed In Table
I.
threshold definition that still allows all peaks of a chosen element to be recognized. Approximate usage ranges are maximum slope tolerance f = 0.044.1,minimum number of positively correlated data pairs = 5 W % ,and minimum correlated time = 1-4 standard deviations of the average Gaussian peak. In this study, where injected amounts were kept deliberately low to test the system under high-sensitivityconditions in the presence of clearly visible noise, the most often used set of thresholds was 0.08, 80%, and 8 s, respectively.
RESULTS AND DISCUSSION Most of the luminescence monitored by the FPD consists of continua and broad molecular bands, which cover wide wavelength regions (although resonance lines of transition metals do show up in some cases). Extensive spectral overlap of emissions from different elements-hence inadequate selectivity in single-channel modes-is thus the rule rather than the exception for the FPD. However, the detrimental effect of spectral overlap does not have to preclude specificity in dual-channel modes. The necessary condition for obtaining specific response is only that the two-channel intensity ratio of the one selected radiation (element) is sufficiently different from that of all the others. If the relevant spectra are known, any number of optical combinations can be suggested that will satisfy this requirement. In fact, it is not even necessary to select a separate combination of two wavelengths for each analyte element. Most spectra, though expansive, are sufficiently different from one another. This is illustrated in Figures 2 and 3 with an example taken a t random from a series of similar experiments. The figures show chromatograms, both initial and derivative, that all result from a one-time injection of a mixture of transition-metal compounds. The demonstration mixture also contains a hydrocarbon (hydrocarbons are frequent components of various sample matrices) and a sulfur compound (sulfur is the element most prominently determined by the FPD, besides being a quadratically responding analyte and a common interferent). Table
ANALYTICAL CHEMISTRY, VOL. 63, NO. 24, DECEMBER 15, 1991
Figure 3. Upper half same as in Figure 2. Lower half: six conditionalaccess chromatograms, using ditferent channel-matching ratios on the two inltial chromatograms shown in Figure 2. Wedge marks: see text for explanation.
Table I. Test Mixture“ C, H co Re S os
Mn Mn Fe Cr Ru
1.6 X lo4 g 2.0 x 10-7 2.4 X lov8g 1.7 X 10% g 4.0 X lo4 g 4.0 X lo4 g 3.2 X lo4 g
3.2 x 10-9 g 1.3 X IO4 g 1.6 X
g
“ Conditions:
Shimadzu FPD, open mode, no chimney, H2 300 mL/min, air 60 mL/min, 1 meter 5% OV-101 nlass column. I lists the compounds and amounts injected. The latter were chosen to be on the low side and to produce peaks of comparable (though not equal) height in nondispersive mode. Note that there are 4 orders of magnitude difference between the amounts of the best and the worst responding elements. Figure 2 shows the two initial chromatographic traces of luminescence generated infabove the FPD flame. No interference filter was used for the first channel (“open” mode, upper chromatogram); i.e. the wavelength regime was defined by the photomultiplier’s response profile. Although using a nondispersive channel may seem counterproductive to the avowed intent of raising selectivity to specificity, it does provide a spectrally “unbiased” representation of the various components of the mixture; i.e. it displays the “innate” elemental sensitivities and selectivities in the common 300650-nmrange. The use of an open channel p e s an additional challenge to the present method development, and for practical analysis it assures the consideration of all light-emitting species (elements). The only wavelength discrimination in this experiment was provided by a 530-nm interference filter (Ditric three-cavity, band-pass about 10 nm) located in the second channel (lower Chromatogram). That wavelength corresponds to one of the strong bands from ruthenocene. Note, however, that the 530-nm interference filter, though expressly chosen for ruthenium, allowed all other elements to show up as well-an illustration of the earlier statement that single FPD channels
2953
often suffer from poor interelement selectivity. Though this particular run was meant to establish specificity only for ruthenium, once stored in the computer it was tested for other elements as well. It-like several other separations that were carried out with different optical channels and are not shown here-was spectrally diverse enough to achieve individual specificity for several more metals. This is illustrated by the six (stacked) CONDAC chromatograms, located in the bottom half of Figure 3 below an orienting repeat of the initial open-mode chromatogram. Individual specifity could thus be sequentially achieved for Mn, Co, Cr, Re, and Os despite the fact that 530 nm does not correspond to any particular emission maximum in the respective spectra of these elements. The ruthenium CONDAC track picks up the very front of the solvent peak (cause unknown-but, as the absence of the nonane peak demonstrates, not owing to hydrocarbon response). To produce specificity for the remaining heteroelements S and Fe, a different interference filter can be used (not shown). The lower part of Figure 3 thus demonstrates how easy it is to attain specificity for a larger number of elements, on the basis of only one injection and only two detector channels. The results shown in Figure 3 are quite typical of a wider variety of other multielement separations done in this and other studies. Wedge-shaped marks inserted into Figure 3 point to characteristic artifacts of the technique. In these high-sensitivity runs, the very beginning and end of each peak drowns in the noise level. (Less graphically expressed, the very gentle slope at the start and finish of a Gaussian concentration profiie is altered beyond acceptance thresholds by the random baseline fluctuation.) The algorithm therefore commences late and terminates early the access of a bona fide peak. The resulting vertical ascent/descent connections from/ to the artificial zero baseline show up to a larger or smaller extent in all CONDAC chromatograms. Here, however, they are marked on the cobalt peak only. Another interesting phenomenon, again pointed out by a wedge, can be observed on the chromium peak. As is obvious from the upper chromatogram (open mode), the chromium peak overlaps a slightly later eluting, small peak of unknown origin. A t that point in the lower chromatogram, the computer-controlled trace abruptly breaks off and descends to zero. In other words, the small peak did not contain chromium and was therefore eliminated by the algorithm. This emphasizes the implicit need for (at least partially) resolved peaks-a need that is shared by most though by no means all chromatographic detection techniques. To deal with severely overlapping or axially coincident peaks, spectral deconvolution or subtraction routines would have to precede the CONDAC algorithm. Note that, as expected, both manganese-containing compounds show up on the Mn trace. Other separations of several compounds containing the same element displayed similar behavior, confirming the implicit assumption that all peaks of the same element behave the same. That assumption would run into trouble only if an element produced more than one emitter in the flame and if those emissions had different response kinetics, or if the ligands (substituents, structural features, etc.) influenced the relative intensities of disparate radiative systems. A case in point is carbonfhydrogen: aromatics luminesce stronger than aliphatics at certain (but not all) wavelengths. It is no problem to eliminate carbonbased responses in a conditional-access mode geared to recognize another element. However, if the objective were to accept only carbon-based responses-an unlikely proposition in the FPD context-then the two channel wavelengths would have to be chosen so that only one of the carbon-containing
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 24, DECEMBER 15, 1991
30 100CImin 20b"C Flgure 4. Upper half: Chromatogram of a local super nonleaded gasoline, doped with 0.65 pg of tetraethyllead and 3.4 ng of triethyl phosphate, as seen by a 490-nm interference filter. The second initiil channel, equlpped with a 524-nm widaband interference filter, is not
shown. Lower half: stacked CONDAC chromatograms for lead and phosphorus.
emitters is registered and, consequently, none of the hydrocarbon peaks is lost. Obviously, also, the presence of carbon in compounds that contain other, stronger emitters (as in the organometallics of the test mixture) is masked and cannot be discerned by the CONDAC algorithm. The efficacy of the algorithm in suppressing all but the element of interest is demonstrated in Figure 4, which uses gasoline to represent a complex sample matrix. This particular gasoline contains (in addition to the usual aliphatics, aromatics and oxygenates) compounds of sulfur and manganese; and it was doped for this experiment with compounds of lead and phosphorus. The latter elements, as shown below, can be easily recovered by the algorithm (and so can sulfur and manganese by using different wavelengths). Note that one initial channel-the one shown in the upper part of Figure 4-registers but a very small peak for the phosphorus compound. Yet, this proves good enough for the CONDAC algorithm to retrieve it. (Re peak size: note that the second initial channel, which is not shown here, uses the more effective 524-nm wide-band filter. The phosphorus peak on the CONDAC chromatogram represents the averaged response of two channels.)
ANALYTICAL CRITERIA ( 6 ) The algorithm regulates the flow of chromatographic peaks according to analyst-defined conditions. The computer does not create these peaks: in essence, it only accepts or rejects them. Peaks will therefore retain their shape and signal/noise characteristics as they pass through the CONDAC gate. As defined for this study, the peaks include contributions from both optical channels. In terms of quantitation, this is innocuous. If the injected amount is within linear range, any linear combination of the two channels will also produce a linear response. But it need not always be a combination of channels. A trivial change in the software would allow the analyst to commit to readout-within the acceptance limits imposed by the algorithm-either one channel, or the other channel, or their combination. The latter choice carries a slight theoretical advantage, provided the peaks in the two channels are of similar signal/noise ratio and the nature of noise is non-
coincidental (random within each channel). This is why the channel combination was used for this study. However, an analyst faced with different samples and following different objectives might well be inclined to stick with just "the better one" of the two channels. This one-channel mode implies that the signal-to-noise ratio has not been changed at all by the CONDAC algorithm: the initial noise still encumbers any accepted initial peak. One could visualize this mode as only one channel being recorded, while the other channel simply turned the signal on and off (with the recorder pen trolling along a t zero during off periods). This image also portrays correctly the quantitative accuracy of a CONDAC chromatogram as being simply that of its mother chromatogram (meaning the original chromatogram in the chosen channel, from which the CONDAC chromatogram was algorithmically excised). (In the case of a twochannel combination, error propagation rules apply.) The only possible difference between a peak from the CONDAC chromatogram and the same peak from the mother chromatogram arises from the operational definition of their baseline(s). Despite their visual similarity, the zero line of a CONDAC chromatogram is not a conventional baseline: it does not represent a noisy background but simply indicates that no signal-peak, background, or noise-has gained access. Yet, by subjecting the chosen channel to a conventional baseline correction routine (prior to being treated by the CONDAC algorithm) the CONDAC zero line can be made coincident with the "true" chromatographic baseline. In this case, the quantitative accuracies of the CONDAC chromatogram and its mother chromatogram are the same. Even in the absence of electronic means of integration, the quantitative accuracies of the two chromatograms can still be made identical. Although not mentioned in the preceding text (and certainly not used for the CONDAC chromatograms in Figures 3 and 4, since that would have seemed like cheating) the algorithm also offers the operator the chance to add user-defined sections of the original signal around each peak. This "skirting" routine can effectively restore the initial baseline preceding and following each bona fide peak, thereby allowing any subsequent method of peak integration to apply. While CONDAC chromatograms are thus not likely to differ from conventional chromatograms in terms of signal-to-noise ratio and quantitative accuracy, the same cannot be said of their detection limits. Traversing the detection limit is a continuous process in conventional chromatography, a discontinuous one in CONDAC chromatograms. The minimum detectable amount in a CONDAC chromatogram can be defined as "the smallest amount of analyte accepted by the algorithm". But this amount depends-aside from the usual filter routines, etc.-on the setting of the three correlation thresholds (which, in turn, influence the algorithm's reliability in accepting the desired and rejecting the undesired elements). A thorough experimental study of this theoretically quite complex problem is beyond the realm of this manuscript. However, it appears safe to predict, on grounds of some preliminary evidence, that the minimum detectable amount in a CONDAC chromatogram will correspond to S I N = 2-3 in the conventional chromatogram. In other words, the difference will be negligible for any practical purpose. Some special "problems and limitations" have already been mentioned, but we would like to add an important general one. As in many methods involving dual-channel instruments and/or analytical data reduction and manipulation, the specificity offered by the CONDAC algorithm is one of appearance, not of substance. It is not that the detector is insensitive to other elements, it is simply that it is not allowed to recognize them. While the CONDAC routine can be very helpful indeed
Anal. Chem. 1991, 63, 2955-2960
in saving chromatographic interpretation time, in tracking down compounds of important elements, and in directing the attention of the analyst to interesting substances, it should serve neither as a final arbiter of elemental identity nor as an inscrutable, infallible "black box". Perhaps more than other types of analytical results, CONDAC chromatograms require an analyst fully aware of their conditional nature. In conclusion, the simple conditional-access approach can be employed-with just a few or perhaps even with only one wavelength selection-for obtaining specific chromatograms from resolved peaks of most if not all of the FPD-active elements. This study deliberately used a worst-case scenario in terms of separation efficiency and noise levels. With capillary separations of larger peaks on cleaner baselines, the CONDAC algorithm will work just that much better. Registry No. Cr, 7440-47-3; Co, 7440-48-4; Fe, 7439-89-6; Pb, 7439-92-1;Mn, 7439-96-5;Os, 7440-04-2;Re, 7440-15-5;Ru, 7440-18-8; C, 7440-44-0; HP, 1333-74-0; P, 7723-14-0; S,7704-34-9;
2955
n-C9Hzo,111-84-2; Co(CgHS)2,1277-43-6; Re2(CO)lo,14285-68-8; ( ~ - B U ) ~110-06-5; S ~ , O S ( C ~ H ~1273-81-0; )~, Mn(CH3C5H,)(C0)3, 12108-13-3; Mn(CSHS)(CO)3,12079-65-1; Fe(CSHS)2,102-54-5; Cr(CO)6,13007-92-6; R u ( C ~ H ~1287-13-4. )~,
LITERATURE CITED (1) Body, Sam S.; Chaney, John E. J . Gas Chromfogr. 1966, 4 , 42-48. (2) Dressier, M. Selectke Gas Chromafogmphic Defectors; Journal of Chromatogrephy Library; Elsevier: Amsterdam, 1988; Voi. 36. (3) Joonson, V. A.; Loog, E. P. J . Chromatcgr. 1976. 720, 285-290. (4) Aue, Waiter A.; Milller, Brian; Sun, Xun-Yun. Ana/. Chem. 1990, 62, 2453-2457. (5) Aue, Walter A.; Miliier, Brian; Sun, Xun-Yun. 73rd CIC Conference, Halifax. NS, July 1990. (6) This section was added in response to the reviewers' requests for additional information.
RECEIVED for review June 19,1991,Accepted September 19, 1991. This study was supported by NSERC Operating Grant A-9604, and was presented in part at the 73rd CIC Conference, July 1990.
Interdigitated Array Electrode Diffusion Measurements in Donor/Acceptor Solutions in Polyether Electrolyte Solvents Hiroshi Nishihara,' Frank Dalton? and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill,North Carolina 27599-3290
A steady-state interdigitated array (IDA) electrode method Is introduced for measurement of the dtffusivity of charge in mixed-valent donor/acceptor solutions In polyether solvents. The method lo based on establishment of steady-state c r d conccmtratkm gradients d donor and acceptor solutes In the 15-pm gap between the IDA finger electrodes. Apparent dtffusion coefficients D,,, are determined for the couples [c~,Fe]+'~,[Cp, Fe]'", TMPD'", TCNQo'-, and DDQo'- in poly(ethyiene glycol dimethyl ether) (MW = 400 and 1000) with lithium perchlorate electrolyte. The D,, values are In the range 10-10-104 cm2/s and exhibit strong variations with solute concentration. The changes are lnterpreted in terms of the concentration dependencies of the and of the electron self-exsolute physical coeffkient DPHYS change reactions between donor and acceptor. The IDA 0, measurements can also be employed to track the crystallization kinetics of the polyether solutions.
The dissolution of metal salts by poly(ethy1ene oxide) (PEO) and its polyether analogues to form ionically conductive polymer electrolytes (I) has spawned considerable research aimed at understanding ionic mobilities in these polymeric solids (2, 3). Our interests (4-8) in this family of polymer electrolyte are in their use as solvents in studies of the methodology of solid-state voltammetry and of the chemistry of redox monomer solutes in rigid or near-rigid solvents. Investigations of polymer-phase rates and mechanisms of transport of molecular redox solutes has been an important part of these studies. Molecular diffusion rates in the poly-
'
On leave from Department o f Chemistry, K e i o University, H i yoshi, Yokohama 223, Japan. *Present address: Grove C i t y College, Grove City, PA 16127.
ether phases are typically much slower and more sensitive to the chemical nature of the diffusant than in more familiar monomeric fluid solvents. Methodology we have adapted for diffusive transport measurements in polymer phases includes potential sweep voltammetry (4-6), chronoamperometry (5), and ac voltammetry (7), all at microelectrodes and with consideration given to linear vs radial transport geometry effecta (5,9). Studies of redox molecule transport mechanisms have included the concentration dependencies of physical diffusion (5b,c)of diffusion-plasticization effects (4), and of coupled diffusion-electron self-exchange reactions (5b,c, 6). Except for electron self-exchange processes, such concentration dependencies are also known (5b, 10) from ionic conductivity measurements. This paper presents an investigation of interdigitated array (IDA) electrodes for redox diffusivity measurements in polyether solvents. The method is based on coating the IDA (fingers and gaps) with a 1:l mixed-valent polymer solution of a redox molecule of concentration C = C, = CRED and thickness h- Applying a potential bias between the electrode finger pairs electrolytically establishes steady-state concentration gradients of the oxidized and reduced forms of the redox species in the interfinger gap (d, cm). The limiting current iLIMflowing under this condition is related to the apparent diffusion coefficient DAppof the donor-acceptor couple by DAPP
=
~ L I M ~
nFCLhfi,(N - 1)
where N , L, and h , are the number of fingers, finger length, and height of the IDA (11). We apply the IDA experiment to mixed-valent solutions of the donor-acceptor couples tetracyanoquinodimethane (TCNQOI-),2,3-dichlor&,6-dicyanyano-p-benzoquinone(DDQo/-), tetramethylphenylenediamine (TMPD+Io), ferrocene
0003-2700/91/0383-2955$02.50/00 1991 American Chemical Society
