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Department of Chemistry,School of Chemical Science, Universityof Georgia, Athens, Georgia ... derived by Prabhu and Anderson for a solid electrode und...
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Anel. Chem. 1991, 63,1651-1658

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Amperometric Responses of Poly(chlorotrifluoroethylene)/Graphite Composite Electrodes with Varying Compositions and Particle Sizes under Flow Injection and Liquid Chromatographic Conditions Tse-Yuan Ou and James L. Anderson* Department of Chemistry, School of Chemical Science, University of Georgia, Athens, Georgia 30602

(BIFD) method to predict the current responses for pseudoThe amperometrk currenl rmpomes of clrcular dkk heterorandom arrays in a flow channel. When edge diffusion was ~ w o ucompodte r Kd-F/grclphne (Kdgraf) eiectmh have neglected, the resulting current responses were somewhat beon rtudkdexp.rlmMtrr#yand mockkdtheoreticdlyumler lower but within 10% of the response of regular arrays, with steady-state and ikpM chmmatographlcfbw condttbns in a the same fractional active area and number of active zones thin-layer flow channel. The relative current efficlency of a per unit length in the direction of flow. Furthermore, exclrcular dh& mkroeMrode array consisting of itnear drips perimental current responses obtained with Kel-F/graphite perpmdkular to flow k equlvaknt to that for a m a r e array (Kelgraf) electrodes, which can be viewed as heterogeneous of the same overall dhnendons, but havlng 78.5% as many composite microelectrode arrays, are also within 10% of mkroekidmblnsequmce. T h e c u r m t r ~ e q u a t b n theoretical predictions for regular array electrodes with the derived by Prabhu and Anderron for a sdkl electrode under same fractional active area and average active site dimensions liquid chromatagraphlc co"was coupled with the relaunder steady-state conditions (7). Consequently, Anderson tive current response for a regular circular array under et al. have verified that the Kelgraf electrode can be apsteady-state conditions. Kelgraf electrodes of two different proximated as a regular microelectrode array (7). Kei-F partlcie dzes were tested In a thln-layer flow channel The Kelgraf electrode has been used as a LC-EC detector underlamlnrrrfbwcondlknr. ~ c w e n t ~ o f t h e s e for a long time and also has shown a better SIN ratio than electrodes were experimentally evaluated by udng steadysolid electrodes and some other composite electrodes (1-3, date flow injection as well as liquid chromatographic meth5-7). However, successful theoretical prediction of the current odology and compared wHh theoretkal predlctlons. The response of a microelectrode array in a thin-layer flow channel theoretlcaily predicted current responses were in good under LC-EC conditions has not been reported. agreement wHh experiment, and the heterogeneous comIn this study, the relative current efficiency will be derived for a circular disk microelectrode array consisting of parallel porlte arrays can bo approxbnated as orderly arrays. Amlinear strip electrodes perpendicular to flow in a thin-layer perometrlc response can bo wed to Wimate the effective channel, based on the model developed by Fosdick and Annumber of microeMrodes and the fractional active area. derson (9)for a regular redangular microelectrode array. The

INTRODUCTION Microelectrode arrays have been widely used as electrochemical sensors, with potential for enhancement of detection limits and/or selectivity as detectors (1-11). Current per unit active area for an array electrode is enhanced relative to a solid electrode with the same overall geometric area as the fractional active area diminishes, resulting from the edge effects at microelectrodes in stationary solutions or from the replenishment of the depleted diffusion layer between active sites in flowing solution (5-9,12-16). In contrast, dominant noise sources, which are proportional to the active area of an electrode (16-18), are diminished for an array relative to a solid electrode. Thus the ratio of analytical signal to noise improves as active area decreases, so that microelectrode arrays have a better signal/noise (SIN)ratio than solid electrodes (1-7,15,16,19)and are particularly useful detedors for flow injection measurements as well as liquid chromatography with electrochemical detection (LC-EC)(1-11). The theoretical current responses of microelectrode arrays in thin-layer flow channels have been investigated by several workers (8,9,12-16,19,20). Most array geometries in those studies consisted of either regular array electrodes (8,9,12-16) or interdigitated array electrodes (19,201. Moldoveanu and Anderson (13)used the backward implicit finite difference

* To whom correspondence should be addressed. 0003-2700/91/0363-1851$02.50/0

amperometric response of a Kelgraf array electrode will be investigated in a flow channel under chromatographic conditions, with the objective of testing whether the fractional active area of a microelectrode array may be determined from chromatographic peak current response and whether random arrays with different KeEF particle sizes may be approximated by regular array models. From the current response equation derived by Prabhu and Anderson for a solid electrode in LC-EC (21) as well as a relative current efficiency equation for predicting the steady-state current response of a circular regular array, the LC-EC current response and hence the fractional active area of a circular Kelgraf electrode can be predicted. Experimental results are in good agreement with theory. Moreover, the advantage of fabricating Kelgraf electrodes with smaller Kel-F particles is demonstrated.

THEORY Steady-State Current at a Circular Microelectrode Array. An expression for current response at a regular rectangular electrode array (i,,,), in a flow stream, neglecting edge effects, was first investigated by Filinovsky (12). Response depends on the number (N) of equally spaced microelectrode strips oriented perpendicular to the direction of flow and the fractional inactive area (8)between strips. Due to the limitations of the harmonic interpolation scheme in Filinovsky's expression for the relative current efficiency, p,(N,8), of a rectangular microelectrode array relative to a rectangular solid electrode for lower values of 8,Fosdick and Anderson (9) developed a modified expression that agreed well 0 I991 American Chemical Society

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ANALYTICAL CHEMISTRY, VCX. 63, NO. 15, AUWST 1, 1991 L

=

It

2

p,(N,0) in eq 3 is the current ratio (efficiency) of a circular

m

array of electrodes relative to a solid circular electrode of the same geometric area and is the corrected result of integration of the product of the local efficiency p(N,By) and local current over all values of y (Figure l),according to the relationship

L - 4

A

Flguo 1. coordkrates and d i " of a circular sor# ektrcde and a circular microelectrode array. Several electrodes of the array are illustrated.

with both experimental results and more rigorous theoretical predictions based on BIFD calculations (9). With identical geometric area in the flow channel for both a rectangular solid electrode and a rectangular microelectrode array, the expression for current at the microelectrode array can be written (9, 12) as

imayJ = i,p,,(N,W

(1)

where i, is the steady-state current for a rectangular solid electrode and p,,(N,8) is given in eq 2, where a and k are empirical coefficients with values of -2.5577 and -0.0303629, respectively, derived from a least-squares fit to BIFD calculations (9).

p,(N,e) = {N/3[(1 - e ) / e ~ ~ / -g iaNk[(i - e)/e]2/3]x [I - ( 0 . 4 e ~ / 3 / N / ~ ) ] l / { n / ~ [-( e)/e]2/3{i i aNk[(i - e)/e]2/3}+ 1 - ( 0 . @ / 3 / N / 3 ) ] (2) Equation 2 is valid for a regular rectangular microelectrode array and agrees with BIFD calculations within ca. 1% over a wide range of conditions (9). However, many practical electrodes, including composites, are most conveniently fabrimtad as disks. Consider a circular disk microelectrode array containing N parallel electrode strips oriented perpendicular to flow in a flow channel, as partially illustrated in Figure 1. The line parallel to flow that bisects the circle will be referred to as the central axis of symmetry of the disk for this discussion. The number of microelectrodes i n t e r a d by a line parallel to flow has a maximum value (N) on the central axis, and decreases with increasing displacementy from the central axis. Accordingly, the relative current efficiency in eq 2 must be corrected for estimation of current at the circular electrode array. The convective diffusion-limited current at a circular electrode array (i,,,,,) can be expressed relative to a solid circular electrode of identical overall dimensions by the relation imay,c

= pcc(N,e)ic

(3)

Here i, is the steady-state current for a circular solid electrode and can be expressed (21) by

i, = 0.8413.3.22/3nFKfj/3

(4) where n is the number of equivalents of electrons per mole transferred in the electrochemical process, F is Faraday's constant, K is a composite term including the dimensions of flow channel and flow rate, etc., and r is the radius of the electrode (see Figure 1). Equation 4 can be rewritten (22) as

i, = 1.234nFC*WLs13(D/b)2f3(U/ W,)1/3

(5)

where C* is the concentration of the electrochemical active material, WL (=2r) is the diameter of the electrode parallel to flow, b is the channel height, U is the volume flow rate, and W,is the channel width perpendicular to flow. The term

(6) where the (l/x)ll3 term reflects the inverse of the local diffusion layer thickness at distance x from the leading edge of the eledrode at displacement y from the central diameter (see Figure 1). The local relative efficiency p,(N,0,y) in eq 6 can be expressed by substituting the local value of N = "(1 - Cy/ r)2)1/2into eq 2, where N- is the number of electrodes intersecting the central axis of the disk. Integrating eq 6 over x , defining a new variable R = y / r for the displacement from the central diameter, and rearranging yield bmay,c

= 3.22/3r%FKJ1

p,(N,B,y)(l - R2)ll3dR

(7)

Assuming the average distance between individual microelectrodes is equal to x,, the maximum number of microelectrodes (N-l on the central axis of a circular electrode is PIX,. At array relative displacement R (0 5 R I1) from the central axis, N = N-0 - R?'12, assuming that N varies continuously with R. This is not strictly true, since N must be an integer, but it is a reasonable approximation. Substitution of this expression for N in eqs 2 and 7 yields 1

hray,c

= 3.22f316/3nFKx( A / B ) dR

(8)

A = N-1/3(l - R2)'/2[(1- e)/eI2/'{l - ~ [ ( -l e)/e]2/3~-k(i - ~ 2 ) " / 2 } - (1 - ~ 2 ) 1 / 3 [ ( 1 e ) / e 1 ~ / ~ ( 0 . 4 e ~-/ ~~ )[( i(-i e)/e]2/3~-k(i - ~ 2 ) k / 2 )

B = N-l/3(i - R2)1/6[(i - e)/e]2/3{i ~ [ ( -l 8)/0]2f3N-k(1 - R2)k/2] + 1 - (0.404/3/ [~,-1/3(1-

~2)1/61)

The integrated relative current efficiency, p,(N,8), of a circular electrode array relative to a circular solid electrode can be obtained from eqs 3 and 4 with eq 8:

This expression can be evaluated numerically to determine the value of the microelectrode array efficiency p,(N,0). An alternative approximate strategy is to replace the number of microelectrodes(N)in eq 2 with the effective number of active strips on the electrode (Nd) for an equivalent rectangular microelectrode array of the same overall dimensions. The derivation is as follows. According to Figure 1, the distance between the leading and trailing edges of the microelectrode array in the direction of flow at displacement y from the central axis is given by L = 2(? - y2)lI2. The average length of a flow line across the circle is given by

e

= 2 r i 1 ( 1 - ~ 2 ) 1 / 2d~ = ar/2 = 1.5708r (10) The effective average number of active strips on the electrode in the direction of flow is then given by Ne, = (€/2r)N,, = 0.7854" (11)

ANALYTICAL CHEMISTRY, VOL. 63,NO. 15, AUGUST 1, 1091 1653 1.0

0.9

0.e 0.7

5

8.5

Q 0.4

0.3 8.2 0. 1 “ I

0

I

0.2

1

0.4

I

0

8.6

1

1

0.8

1.0

Flgurr 2. Comparison of relative current efficiency vs fractional inactive area 8 predicted for a circular microelectrode array from eq 2 (N = N d ; g = 0.7854) (triangles) to the predlctlons of eq 9 (solid line). Maxlmum number of electrodes: (a) N, = 10, (b) N, = 50, (c)N, = 100.

The value of N,, for a circular electrode array is equal to N for a rectangular array electrode of the same total length. Consequently, replacing N in eq 2 with NOR from eq 11provides a very useful equation to estimate the current efficiency, p,(N,8), of a circular microelectrode array relative to a solid electrode, when both the array and the circular solid electrode have identical maximum width and length. The resulting relative current efficiency is expressed as eq 2 except that N is replaced by NefpIn general, NOR may also be represented by N-g, where g is a geometric factor dependent on the shape of the microelectrode array. Values of g are 0.7854 for p,(N,8) and 1for p,(N,8), respectively. The numerically integrated and approximate current efficiencies expressed in eqs 9 and 2 (g = 0.7854) were calculated by a program written in QuickBasic on an IBM PC. The approximate results calculated by eq 2 agreed with the numerically integrated predictions of eq 9 within l % for all values of N and 8 tested. The excellent agreement between relative current efficiencies calculated from the full numerical expression (eq 91,and the approximate expression (eq 2) is shown in Figve 2. This evidence shows that the approximate expression of p,(N,8) in eq 2 with g = 0.7854 can be a simple and useful tool to estimate relative current efficiency for a circular microelectrode array. It should be noted that the ratio of pec/pn decreases slightly (as much as 5%) as N decreases and 8 increases. This behavior arises from the smaller number of active sites and lower enhancement of current density at the lateral edges of a circular array, in comparison with a rectangular array of the same total width and length. LC-EC Studies at a Circular Solid Electrode. Prabhu and Anderson (21) have shown that the peak current i, at a circular solid electrode in a thin-layer flow channel under LC conditions is given by i, = i,V0/[rfprP(2rLH)1/2(1 + k’)] = i&/[W/2(1 + k’)] (12) where Kd/[H1/2(1+ k’)] in eq 12 is the chromatographic peak dilution factor, Vo is the injected analyte volume, CT is the column porosity, r is the inner radius of the column, k’is the capacity factor, L is the length of the column, and H is the height equivalent of a theoretical plate. Values of H = L / N ’ can be calculated from the column length L and the number of theoretical plates N’, calculated from eq 13 derived by Foley and Dorsey (23),where tRis the retention time, Wo,(=A+ B) is the width of the peak at

of peak height, and A and B are the segment lengths at height before and after the peak. Equation 13 is accurate to f1.5% if 1.00 IB / A I2.76 (23). This approach is considerably more convenient than the approach reported by Prabhu and Anderson (21) and does not require determination of van Deemter parameters. EXPERIMENTAL SECTION Chemicals. House-distilled water was deionized by a Barnstead Nauopure water purification system. Methanol (MeOH) was J. T. Baker %esi-Analyzed” grade. The analyte used was 1,l’-bis(hydroxymethy1)ferrocene(BHMF), synthesized locally (2). All other chemicals were reagent grade. Equipment. The flow system consisted of a Varian syringe pump (Model 8500) to minimize the pump noise and a pneumatically actuated Valco injector or a manually actuated Rheodyne Model 7010 injection valve. The flow injection experiments were performed in two ways. Steady-state experiments were performed with a Teflon sample loop volume of 3.5 mL, coupled with Teflon tubing of 18.5-pL dead volume between the injector and the flow cell. This combination of injector and connecting tubing volumes ensured that peak concentrations were undiluted from injected concentrations (24).Hydrodynamic voltammetric flow injection studies under more conventional flow injection conditions were performed with a 1WpL sample loop (Rheodyne) and a Teflon tubing of 18.5& volume connectingbetween injector and flow cell. The LC investigations were carried out with a 50-pL sample loop (Rheodyne)and an Alltech Ultrasil-ODS CB reversed-phase column (4.6i.d. X 25-cm length) packing with 5 pm diameter C18particle size. The connections between pump, injector, and column were made with stainleaseteel tubing, while the connection between the column and the flow cell was made with Teflon tubing. The potentiostat (Bioanalytical Systems Model LC-3) served to control the applied potential and to monitor current response for both flow injection and LC experiments. Electrodes. Kel-F particle powders (3M Corp.) of two different particle size distributions and average diameters (d ) were used to fabricate electrodes. The average Kel-F particle 8.iameter was determined by averaging over a large number of particle diameters measured from electron microscopic (SEM)micrographs. The average particle dmmeters of 103and 48 pm (see Figure 3a,b respectively) observed for the two distributions were referred to as ‘large” and “small”, respectively, for the two particle size distributions used here. The small diameter particles were a gift from Neil Danielson (Miami University, OH). Kel-F particles of an appropriate size were mixed in various proportions by weight with powdered flake graphite of 1 pm or smaller particle size (UCP-1M “F” purity, Ultracarbon Corp.) and heat-compressed (at 260 OC and 2500 lbs) to fabricate the working electrodes. The 5%(S),15%(S),and 25%(S) Kelgrafelectrodeswere made of small diameter Kel-F powder mixed with 5%, 15%, and 25% graphite by weight, respectively. For comparison, a Kelgraf electrode with large Kel-F particle size (15%(L)),was also made of 15% graphite and 85% Kel-F by weight Except for the 15%(S) electrode,which had a diameter of 3.5 mm, all electrodes had identical diameters of 3.4 mm. All Kelgraf electrodes were fabricated with a surrounding insulating Kel-F sheath, as described elsewhere (25). All these electrodes were abraded on 600 grit sandpaper (3M Corp.) first and then polished with a series of polishes with increasinglyfine particle sizes, i.e. 1,0.3, and 0.05 pm, of a-alumina powder (Buehler Micropolish) on a kitten-ear polishing cloth (Buehler) mounted on a lapping wheel. The electrode surfaces were rinsed with a stream of deionized, distilled water after every polishing process. Cell Design. The Kelgraf electrodes were mounted in a thin-layer flow-through cell as detectors for both flow injection and LC studies. The design of this flow cell was similar to previously reported designs (1-3), with a flow channel volume of 3.0 pL (0.45-cm width (W,) and 1.4-cm length) made from a Tefzel 200 spacer of 50.0-pm thickness ( b ) . A Bioanalytical Systems Model TL-SA flow cell, with a flow channel volume of 3.7 pL (50.0-pm thickness (b),0.46-cm width (Wc),and 1.60-cm length), was used for measurement of current response of a glassy-carbon

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ANALYTICAL CHEMISTRY, VOL. 63,NO. 15, AUGUST 1, 1991

a

Figure 4. (a) Hexagonal array electrode model of Gueshi et al. (28). The shaded areas are the active regions. (b) Schematic diagram of the equivalent strip electrode array. The shaded areas are the inactive zones. Figure 3. (a) Scanning electrode micrograph of a 15%(L) Kelgraf electrode fabricated from “large”Kel-F particles. (b) Micrograph of a 15% (S) Kelgraf electrode fabricated from “small”Kel-F particles. The white scale bars at lower left equal 100 pm in both cases.

electrode (2.9-mm diameter). The reference electrode (Ag/ AgC1/4.0 M KCl) and auxiliary electrode (platinum wire) were mounted in a glass chamber positioned in the downstream outlet of the flow cells. Mobile Phase. An aqueous phosphate-acetate buffer of pH 7.0 was prepared by adding 120 mL of 2.0 M sodium acetate solution to 3.0 L of an aqueous solution containing 6.76 X M Na2HP04and 1.06 X 1W2 M KH2P04and adjusted with NaOH solution (50%, w/w). For the flow injection experiments, the mobile phase used was aqueous phosphate-acetate buffer or various phosphate buffer-MeOH solvent mixtures. For the LC studies, a phosphate buffex-MeOH solvent mixture (l:l, v/v) was used. All solutionswere filtered through Nylon 66 filters (Rainin) with 0.22 pm pore size in an all-glass Millipore filtration apparatus before use.

RESULTS AND DISCUSSION Circular Composite Microelectrode Array. The Kelgraf electrode (Figure 3) has been shown previously to behave like a heterogeneous composite microelectrode array consisting of either a collection of microelectrode islands in a sea of insulating Kel-F (3, 5, 6) or an aggregation of interlocking narrow, irregular, active graphite annuli (black zones in Figure 3a,b) surrounding relatively large inactive Kel-F particles (7) (gray or white zones). The brightest zones reflect electron repulsion from nonconductive regions. The similarity of structure for different Kel-F particles sizes is evident. It is also evident that graphite extends in tortuous, irregular but more or less continuous paths across the surface of the composite. The annular structure arises from the much finer graphite particles forming a thin coating around the much larger individual Kel-F particles, which remain largely intact during the compression molding step of fabrication. Grinding and polishing truncates the particles and reveals the annular

structure. Since a random array such as the Kelgraf electrode can be approximated as a regular electrode array (7,26), eq 2 ( N = N,,g; g = 0.7854) and eq 5 can be used to evaluate the steady-state current response for a circular random microelectrode array relative to a solid electrode of the same electrode shape and geometric area. For Kelgraf microelectrode arrays, the fractional active area of each electrode, 1- 8, is equivalent to the volume fraction of graphite and can be experimentally approximated by the weight fraction (3,5-7,27), since the densities of graphite and Kel-F are nearly identical. As an illustration of the equivalence of a microelectrode array of annular particles with a regular array of parallel strips, we revise the model of Gueshi et al. (28)for a hexagonal array of circular insulators separated by annular active regions that completely fill the gaps between insulators (Figure 4a). Examination of Figure 4a reveals that the annular zones form a continuous, zigzag network across the array regardless of orientation, which can be transformed into equivalent linear strips (Figure 4b). Spacing between the strips depends on orientation of the array relative to the direction of flow. If the shortest nearest-neighbor distance between Kel-F particles surrounded by graphite annuli is 2W, the spacing x, between equivalent strips for hexagonal packing is (3)lI2Wwhen rows of particles are perpendicular to flow and 2W when rows are parallel to flow. However, the spacing for square packing is 2(2)1/2Wwhen rows are at 45O ( r / 4 rad) to flow and 2W when rows are parallel to flow. The number of equivalent strips for these geometries will be in the ratio 1.15:1:0.71:1, reflecting a 15% range for hexagonal geometry and a 29% range for square geometry depending on orientation. Since the packing of a typical disordered composite material will contain a mixture of orientations, the range of the number of equivalent strips will probably not be as wide. For 50% mixtures of two orientations in each packing type, the number of strips would deviate by ca. 8% from either extreme for the hexagonal geometry and by ca. 15% for the square geometry. The length of each equivalent rectangular

ANALYTICAL CHEMISTRY, VOL.

active strip ( W Jand gap (WJis given by = (1- B)(W, W,)

+

w,

w, = ex,

(14a)

(14b)

where 1- 8 is taken as the percent graphite by weight. For the hexagonal packing in Figure 4a, 8 equals d/(2(3)'/*W). The maximum feasible fractional inactive area 8 = 0.907 for hexagonal packing is obtained when the radius of insulator r is equal to half-zone width W.The model described in Figure 4b breaks down for 8 1 0.907. With 8 1 0.907, the treatment of the array as continuous parallel strips breaks down, as the conducting zones become isolated to the concave triangular gaps between insulating particles. This observation helps to account for the rapid increase in resistance of Kelgraf electrodes for 8 1 0.95, as a m u l t of a breakdown in the continuity of the graphite annuli surrounding Kel-F particles. Planar geometries with no voids are feasible for 8 1 0.907 only for irregularparticlea or for nonuniform particle size distributions. Therefore, when 8 1 0.907, the shape of microelectrodes is no longer annular and is better approximated as isolated disks. Rather than assuming regular geometry, the average value of W,+ W Ewas estimated by dividing a scanning electron microscopic (SEM) photograph into many small columns, and the average number of graphite zones per column was calculated. Averages were taken for several orientations. Finally, dividing the length of the column by the average number of strips, the average value of W,+ W,was obtained and the average number of active sites (N-) on the central diameter for a circular Kelgraf electrode was calculated by the ratio of WL/(Wl + W,).It should be noted that the number of hexagons or equivalent rectangular microelectrodeson the electrode surface is dependent on both fractional active area and size of insulating particle. The treatment of current response mentioned above neglects edge effects, since the edge effects due to longitudinal and lateral diffusion are only significant for very small electrode dimensions as well as flow rates under convective diffusion conditions for both a single microelectrode (29) and microelectrode arrays (13-15). The accurate treatment by Aoki et al. (29) of longitudinal diffusion effecta for a single microelectrode was used to estimate an upper limit at channel Kelgraf electrodes, according to the expression for a single electrode i(P) = i,(l + 0.5644P8I4 - 0.2457P) (15) where the longitudinal effect parameter P(10.25), given by P = [0.55(b2W$W~/U)1/3/ W,I2 (16) represents the square of the ratio of downstream diffusion layer thickness to electrode length (13). Equation 15predicts a 5% contribution of longitudinal diffusion to total current when P = 0.054 and 10% contribution when P = 0.15, corresponding to ratios of diffusion layer thickness to electrode length of ca. 0.23 and 0.39, respectively. In this study, eq 15 was used to determine the experimental conditions that can minimize the longitudinal diffusion at the electrode. Current enhancement by lateral diffusion in a flow stream has been little studied. Caudill et al. (4) have demonstrated that combined longitudinal and lateral diffusion is dominant at 10 pm diameter isolated disks only when both slow flow rates mL/s) and thick channel heights (11mm) are employed. Notice that the slow flow rates and thick channel heights required for a dominant role of lateral diffusion differ greatly from the experimental conditions normally used analytically and will not be used here. Finally, when both longitudinal and lateral diffusion are insignificant for an electrode, combining eq 2 ( N = N-g; g = 0.7854), with eqs 3 and 12 provides a very useful equation to predict the current response at a circular random mi-

03,NO. 15, AUGUST 1, 1991

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croelectrode array under liquid chromatographic conditions. The result is

i, = p,,(N,B)i&/[H'/2(1

+ k')]

(17)

The Kelgraf electrodes used in this study were made with either of two sizes of Kel-F particles, mixed with different percentages of graphite. Using the average lengths of W,+ W,perpendicular to the direction of flow, measured from SEM photographs (see Figure 3), the average lengths of active graphite sites were calculated by eq 14. The values of length are 8.8 and 16.0 pm for 15% (S)and 25% (S) electrodes, respectively, and 18.8 pm for the 15% (L)electrode, assuming that active sites may be approximated as narrow strips oriented perpendicular to flow. As mentioned, when 8 1 0.907, the shape of microelectrode is expected to be better approximated as small isolated disks. This expectation is borne out by SEM micrographs, which yielded an average length of active sites of 3.2 pm for the 5% (S) electrode. The maximum number N- of equivalent microelectrode strips on the central axis of the Kelgraf electrode surface was estimated by the ratio of WL/( W,+ W,), as mentioned. These numbers are 55,62, and 53 for the 5%(S), 15%(S), and 25%(S) electrodes, respectively, and 28 for the 15%(L) electrode. For these dimensions, the longitudinal diffusion effect predicted by eq 15 was insignificant for all electrodes except for a 5% enhancement in current with the 5%(S) electrode at 0.5 mL/min flow rate under the experimental conditions used here. Additionally, lateral diffusion is significant only at low flow rate and thick channel height (4,131.Thus, edge effects play a minor role in current for the cell geometry and experimental conditions selected in this study. The relationship among current response, number of microelectrodes, Kel-F particle size, and content of graphite on the electrode surface will be discussed in detail under steady-state and liquid chromatographic conditions. Estimation of Fractional Active Area. Theoretical treatmenta of the Kelgraf electrodes (6, 7)and other Kel-F composite electrodes (30) have been reported, even though the random distribution of active sites (e.g. graphite) and Kel-F particles in the electrode makes theoretical treatments difficult. Kel-F electrodes with large particle size have been successfully approximated as regular microelectrode arrays by backward implicit finite difference simulations under steady-state flow conditions (7),and eqs 1and 2 also have been used in regular microelectrode array studies under steady-state conditions (9). In this study, eq 2 ( N = N-g; g = 0.7854), with eqs 3, 12, and 17, was used to investigate the fraction of active area for Kelgraf electrodes with small Kel-F particle size, assuming that small particle size Kelgraf electrodes can also be approximated as regular electrode arrays. BHMF was used as an analyte. The experimental current efficiency, p,(N,8), can be computed for Kelgraf electrodes with small particle size relative to a circular solid electrode by two methods. One method is to normalize experimental steady-state i, for Kelgraf electrodes relative to i, for glassy-carbon electrodes of the same circular shape under the same conditions, correcting for diameter for the glassy-carbon electrode by multiplying the latter current by the 6/3 power of the ratio of Kelgraf to glassy-carbon diameter (see eq 5). Here, i, values for Kelgraf and glassy-carbon electrodes were obtained in steady-state flow injection experiments. A variation on the above method is to calculate the absolute response directly relative to the theoretically predicted response at a circular solid electrode. In an alternative method, i, values for Kelgraf electrodes were predicted by means of eq 4 from chromatographic peak currents and then normalized in the same way. Comparisons between experimentalvalues of p,(N,8) were obtained by ratioing currents at circular Kelgraf composite

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ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991

1.8 8.9 8.B

8.7

-

20.5

Q

0.41 8.3

Figure 5. Experimental response of Kdgraf electrodes normalized to the area-corrected and channetwldth-corrected response of a glassy-carbon electrode (0.29-cm dlameter corrected to 0.34 cm by multiplying current by 1.30 = (0.34/0.29)5'3:channel width 0.46 cm corrected to 0.45 cm by multiplying current by 1.01 = (0.46/0.45)'/9) under Identical condltlons (points)compared to theoretical response (curves) of mlcroelectrode arrays based on eq 2 with g = 0.7854: (squares) normailzed ratio of experimental I, for steady-state flow lnjectkm experiments with BHMF (0.84 pM) for three small particle size electrodes; (trlangles)normalized ratio of i, (I, calculated by eq 12) of BHMF (4.20 pM) derived from chromatographic peak currents for three small particle size electrodes: (diamond) normalized ratio of experimental I, for steady-state flow injection experiments with BHMF (0.84 pM) for 15%(L)electrode; (hourglass)normalized ratio of i, (I, calculated by eq 12)of BHh4F (4.20 pM) derhred from LC peak currents for 15%(L)etectroda The flow rate was 0.5 mL/mh for fbw injectla? experlments and 1.0 mL/mln for LGEC experiments. All points are the average of three determlnationson three freshly poHshed sutfaces.

electrodes relative to a circular glassy-carbon electrode corrected to equal geometric area. Experimental values (points) and theoretical values (lines) of p,,(N,8)obtained from eq 2 with N = 0.7854Nm, are plotted as a function of fractional inactive area in Figure 5. Experimental data extracted from both steady-state flow injection and from LC-EC peak response are in good agreement. Except for the 5%(S)electrode, it is apparent that agreement between experiment and theory is good for N- and 9 values examined here. The deviation for the 5%(S)electrode is consistent with expectation, since such sparse electrodes are not adequately modeled as continuous strip arrays and the response of sparser electrodes is more sensitive to the fractional active area (see ref 7) and packing geometry. In addition, the heterogeneous structure of Kelgraf electrodes limits the surface reproducibility to ca. 10% RSD of fractional active area (6, 7). As noted above, regular hexagonal and rectangular packing geometries have variations as high as 15% and 29% between the number of equivalent strips as the axis of the array is rotated relative to the direction of flow, corresponding to ranges of current response of 5% and 9%, respectively, for sparse arrays. Assuming equal population of the extreme geometries,average response would be expected to deviate 2-3% and 4-5% negative of the maximum value predicted for the two geometries. Moreover, negative deviations as large as 10% have been predicted for random electrode arrays relative to a regular strip array by BIFD simulations (13). The magnitude of this negative deviation due to randomness is large compared to the edge effect enhancement predicted for the sparser electrodes. For Kelgraf electrodes, the electrical conduction mechanism is based on a percolation process in the electrode. A high resistance was measured for the 5%(S)electrode, since the interconnections between active sites on the surface of the

5%(S)electrode are not complete (i.e. the graphite appears patchy in an SEM micrograph), as expected for 8 1 0.907. Thus, a lower value of experimental p,(N,9) may reasonably be expected. When a measure of N is available from micrographs, the percentage of active area can be determined from the current response relative to predictions for a solid circular electrode of the same dimensions as the array, without need for experimental measurement on a solid electrode as a reference. The observed response agrees well with both absolute predictions and experimentallyobserved ratios relative to a solid electrode. Additionally, results in Figure 5 validate eq 17, which allows estimation of p,(N,8) from the ratio of LC-EC peak currents i, for Kelgraf electrodes relative to the values of i, observed for a solid electrode of the same shape and geometric area under the same LC conditions. It should be noted that the validity of eq 17 is based on the assumption that the variation of bulk analyte concentration from entrance to exit of the detector cell at any instant due to dispersion is negligible. This assumption appears to be justified and is supported by our data for both solid electrodes and microelectrode arrays under LC-EC conditions, as well as under steady-state flow injection conditions. Our results are in contrast with the results of Magee and Osteryoung ( I I ) , who found that solid electrodes and microelectrode arrays exhibit different flow rate dependence under both LC and flow injection conditions and attributed the differences to dispersion in the flow stream. Our LC results explicitly deal with the effeds of dispersion and show that the flow dependence of solid electrodes and microelectrode arrays in low-volume thin-layer cells with laminar flow should be comparable under typical LC conditions with constant potential amperometric detection, where the instantaneous bulk concentration gradient across the detector cell should be negligible (21). On the other hand, a difference might be expected between the flow rate dependence of a solid electrode and a microelectrode array under conditions where significant concentration gradients due to dispersion are present across the length of the electrode. While such conditions are not expected for LC experiments or steady-state flow injection experiments, such conditions may apply for the smallest injection volumes in the flow injection experiments described by Magee and Osteryoung (11). Under these circumstances, the different spatial distributions of the diffusion layers at the two types of electrodes could interact differently with the concentration profile due to dispersion, giving rise to a difference in the dependence of response on flow rate. This condition is likely to occur only for high degrees of dispersion in flow injection experiments, e.g. for small injection volumes and large dead volumes between injector and detector (24). Alternatively, differences might be expected if a microelectrode array had a significant degree of surface roughness, which contributed to the development of stagnant flow zones or a disruption of smooth laminarflow. Solid electrodes or microelectrodearrays fabricated from softer materials (e.g. the Kelgraf electrode) might be expected to be more immune to flow profile disruption than microelectrode arrays based on hard materials such as reticulated vitreous carbon embedded in a softer insulating matrix (II), due to more uniform polishing rates of conductor and insulator. An alternative method for estimation of the average fraction of active area on the electrode surface is based on the relationship between background current and active area of the electrode (7).The background current for aqueous solution, mainly due to the kinetically limited process of solvent oxidation, is frequently directly proportional to the active area of the electrode. Repolishing of the electrode surface changes

ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUWST 1, 1991

0 0.2

I

0.9

I

I

I

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0.5

0.6

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0'7

0.8

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Figure 6. Flow Injection hydrodynamic voltammograms for oxklatlon of 0.84 pM BHMF (in phosphate buffer) at Kelgraf electrodes. The Row rate Is 0.5 mL/min.

the fractional active area, due to the random distribution of active sites, and background current varies accordingly (5,7). Using a series of electrodes fabricated with a common Kel-F particle size but varying percent graphite, Anderson et al. (7) normalized background currents by their graphite percentages, and obtained a common plot of normalized background current v8 applied potential (7).With this plot as a reference, the fractional active area of an unknown electrode can be estimated, by ratioing the background current of the unknown composite electrode to the background current of a composite electrode of known fractional active area. Results for the small particle size electrodes in this study showed that background current response is directly proportional to graphite content, as reported by Anderson et al. (7)for large Kel-F particle size electrodes. It should be noted that variations of background current are also sometimes observed for cell disassembly and reassembly and are influenced by roughness of the electrode surface, incomplete sealing between graphite particles, or bad sealing between the composite electrode and the Kel-F sheath, leakage of the thin-layer gasket, etc. Therefore, the normalized background current method for estimating the fractional active area for a composite electrode should be used with caution, and replicate runs should be made to rule out some of the possible interferences listed above. For accuracy and convenience, diffusion-controlled flow experiments (under both steady-state and LC-EC conditions) are most suitable for the estimation of the average fractional active area on composite microelectrode arrays. This results from the weak dependence of the diffusion-controlled current on fractional active area and greater immunity to the confounding factors observed for the kinetically controlled background current. Effect of Kel-F Particle Size on Diffusion-Controlled Current. In Figures 5 and 6, it is obvious that current efficiency of the 15%(S)electrode is significantly larger than that of the 15%(L) electrode, despite the identical graphite content of both electrodes, because of smaller particle size. This observation results from the larger number of smaller microelectrodes per unit length for the smaller particle size, with less diffusion layer depletion at the trailing edge of each site, and more frequent replenishment of the diffusion layer between inactive sites by diffusion from the bulk. Each microelectrode is used more efficiently in the electrodes made with smaller particles, yielding a higher current response. In addition, the low fractional active area electrode shows significant current density enhancement, and the diffusion current is weakly dependent on the fractional active area of the electrode. This behavior is observed because the di-

1657

mensions of active sites decrease with decreasing percent graphite composition for Kelgraf electrodes of constant KeEF particle size and since the diffusion layer depletion also diminishes for a sparser distribution of active sites. Figure 6 shows hydrodynamic voltammograms of BHMF in phosphate buffer for four M e r e n t Kelgraf electrodes at a constant flow rate. The response of the 15%(S)electrode is 30% larger than that of the 15%(L)electrode because of the former's smaller particle size. In addition, although the fractional active area for the 25%(S) electrode is 5 times larger than that of the 5%(S)electrode with identical size Kel-F particles, the limiting diffusion-controlled current response is only 2.6 times larger. These results are qualitatively consistent with recent results from transient studies of Kelgraf electrodes of small particle size prepared by a new grinding technique (31). Effect of Graphite Content on S I N Ratio. For composite microelectrode arrays, the diffusion-controlled current is weakly dependent on fractional active area, but the kinetically limited background current and dominant noise sources are strongly dependent on active area as mentioned. Hence, electrodes with smaller active areas should exhibit better SIN ratios when compared to electrodes with larger active areas but identical geometric areas until a critical minimum active area is achieved. At a given frequency and band-pass for an EC detector, the noise has both area-dependent and area-independent components (16, 17, 19). Therefore, there will be an optimum fractional active area for a composite electrode to reach the optimum SIN ratio. Recently, Weber (16) predicted that the fractional active area for an optimum SIN ratio should have been a.2% for Kelgraf electrodes. Unfortunately, Kelgraf electrodes with graphite content lower than 5% have limited analytical utility, because of the high resistance for such low active area electrodes. In this study, the experiments evaluating the SIN ratio were performed under LC conditions. The resulting values of SIN ratio per unit of concentration for 5%(S),15%(S),and 25%(S) electrodes are 809,628, and 444 (per micromolar concentration of analyte), respectively, for the LC-EC experimental conditions when 4.20 pM BHMF was used. The same trend of SIN ratio with respect to various graphite contents was also found by Tallman and Weisshaar (5) for Kelgraf electrodes with large Kel-F particle size (ca. 150-450 Nm), which were degassed during electrode fabrication. Although the low fractional active area electrode would enhance the signal/noise ratio, electrode fouling was significant for the 5%( S ) Kelgraf electrode. This phenomenon also was found in the study of Tallman and Weisshaar (5) with Kelgraf electrodes of low fractional active area. Thus, 15% graphite content and small Kel-F particle size are a good compromise for electrode fabrication when both signal/noise ratio and longevity of the electrode are considered. In conclusion, the experimental resulta for a series of circular Kelgraf electrodes agree with theoretical calculations, indicating that the randomly structured electrode can be approximated as an orderly electrode array and that the theoretical model for calculating the geometric efficiency is valid. The results indicate that the effects of on-column dispersion in an LC experiment are functionally the same for both solid and array electrodes under typical LC conditions. Useful estimates of the fractional active area of Kelgraf electrodes are obtained experimentally for both diffusion-controlled and kinetically controlled reactions. Good agreement is obtained between fractional active areas determined by the two methods. Current response for a composite electrode can be calculated by ratio to that theoretically predicted or experimentally measured for a solid electrode from either flow injection or LC experimentswith the same solvent composition. Sparse arrays enhance the signalfnoise ratio, and the most

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Anal. Chem. 1991, 63, 1658-1660

significant improvements in detection limita result from use of a sparse array of many small electrodes.

ACKNOWLEDGMENT We are grateful to Neil Danielson of Miami University, Oxford, OH, for a generous gift of sized Kel-F particles. LITERATURE CITED (1) Andenon, J. L.; CheSnOy, D. J. Anal. Chem. 1080, 52, 2158. (2) M y , D. J.: AndUUOn, J. L.; WeQshear, D. E.; Tallman, D. E. Anal. chkn.A& 1081, 124, 321. (3) WeiEshaar, D. E.;Telhnan. D. E.; Anderson, J. L. Anal. Chem. 1081, 53, 1809. (4) Caudll, W. L.; Howell, J. 0.;Wlghtman. R. M. Anal. Chem. 1082. 54, 2532. (5) Tallman, D. E.; Wdsshaar, D. E. J . Llq. CYmnatogr. 1083, 6, 2157. (8) Webrshrrar, D. E.; Talhnan, D. E. Anal. Chem. 1083, 55, 1148. (7) Anderson, J. L.; Whiten, K. K.: Brewster, J. L.; Ou, T. Y.; Nonidez, W. K. AMI. CWm. 1085, 57, 1368. (8) Foedldc, L. E.; Anderson, J. L. Anal. Chem. 1088, 58, 2481. (9) FOSdICk, L. E.; Anderson, J. L.; Baglnskl. T. A,; Jaeger, R. C. Anal. Chem. 1086, 58, 2750. (10) bbl, F.; Anderson, J. L. AM@t 1086, 110. 1493. (11) M aw,L. J., Jr.; Ostscyoung, J. Anal. Chem. 1000. 62, 2625. (12) FWlnovsky, V. Yu. Ebcbodnkn. Acta 1080. 25, 309. (13) Moldoveanu, S.: Anderson, J. L. J . Electroanal. Chem. Intetfacial Electrochem. 1086, 185, 239. (14) COfm,D. K.; Tallman, D. E. J . Electroanal. Chem. InterfacialElecb.0chem. 1086, 188, 21.

(15) Cope, D. K.; Tallman. D. E. J . Ebetrmnal. C h m . IntetfadpIElecb.0&em. 1086, 205, 101. (IS) Weber, S. 0. Anal. Chem. 1080, 61. 205. (17) Lankelma, J.; Poppe,H. J . clwometogr. 1076, 125, 375. (18) Morgan, D. M.; Weber, S. 0. Anal. Chem. 1084, 56, 2580. (19) Anderson, J. L.; OU. T. Y.; Mddoveanu, S. J . EkwIhJanal. Chem. Interfacial Electrochem. 1085, 196, 213. (20) Ou,T. Y.; Moldoveanu, S.; Anderson, J. L. J . Ehtroenal. Chem. Interfadsl E k t r m . 1088, 247. 1. (21) Prabhu, S.; Anderson, J. L. Anal. Chem. 1087. 50, 157. (22) Ou. T. Y.; Anderson, J. L. J . Electroanel. Chem. Interfacial Electrochem. 1091. 302, 1. (23) Foley. J. P.; Dorsey, J. 0. Anal. Chem. 1083, 55, 730. (24) Meschl, P. L.; Johnson, D. C. Anal. Chlm. Acta 1981, 124, 303. (25) Anderson, J. E.; Tallman, D. E.; Chesney, D. J.; Anderson, J. L. Anal. Chem. 1978, 50, 1051. (26) Amatore. C.: Saveant, J. M.; Tessier, D. J . EkwIhJanel. Chem. Interfads1E l e c t ” . 1083, 147, 39. (27) Petersen, S. L.; Welsshaar, D. E.; Tallman, D. E.; Schulze. R. K.; Evans, J. F.; Desjarlais, S. E.; Engstrom, R. C. Anal. Chem. 1988, 60, 2385. (28) m i , T.; Tokuda, K.; Matsuda. H. J . Electroenal. Chem. Interfecial Electrochem. 1078, 80. 247. (29) Aokl, K.; Tokuda, K.; Matsuda, H. J . Electroanel. Chem. Interfacial Elect”. 1087, 217, 33. (30) Petersen, S. L.; Tallman, D. E. Anal. Chem. 1000, 62, 450. (31) Anderson, J. E.; Montpomery. J. B.; Yee, R. Anal. Chem. 1000. 83, 653.

RECEIVED for review February 15,1991. Accepted May 13, 1991.

TECHNICAL NOTES Comblnatlon Eiectrospray-Liquid Secondary Ion Mass Spectrometry Ion Source Damon I. Papac, Kevin L. Schey, and Daniel R. Knapp* Department of Cell and Molecular Pharmacology, Medical University of South Carolina, Charleston, South Carolina 29425

INTRODUCTION Recent developments in electrospray ionization (I,2) have prompted widespread interest among mass spectrometrista studying large molecules. Most of these investigatorsuse, and will continue to use, liquid secondary ion mass spectrometry (SIMS) ionization in their work. In many laboratories the same mass spectrometer will be used for both electrospray and liquid SIMS work, necessitating reconfiguration of the instrument for changing between the two ionization methods. Faced with this prospect, we decided to implement electrospray ionization on our triple-quadrupole instrument (Nermag R30-10)in a manner that would allow easy switching between liquid SIMS and electmpray modes. This note describes the successful implementation of a combination electrosprayliquid SIMS ion source on a quadrupole type mass spectrometer. We had previously modified our instrument to incorporate a cesium ion gun (3)for liquid SIMS analysis of peptides. The cesium ion gun was mounted on the ion source with the cesium ion beam coaxial with the sample probe and perpendicular to the quadrupole axis. To implement electrosprayionization, we chose the recently published design of Chowdhury, Katta, and Chait (4), which employs a heated metal capillary for desolvation. The repeller on the standard Nermag FAB ion source was replaced by a hemispherical electrode attached to the end of the electrospray source. The resulting device gave comparable performance in liquid SIMS mode to the previous configuration and allowed rapid changeover (30min) between liquid SIMS and electrospray modes within the time required 0003-2700/91/0383-1858$02.50/0

to heat and cool the ion source pellet in the cesium ion gun. This combination ion source permits the use of both modes of ionization without the need to reconfigure the instrument. EXPERIMENTAL SECTION A diagram of the ion source is shown in Figure 1. The components are as follows: (A) Hamilton (Reno, NV) 1705RN syringe with Hamilton 80426 needle (25 gauge) driven by a Harvard Apparatus (South Natick, MA) Model 11 syringe pump (the high voltage required to generate the spray is applied to the syringe needle; a Teflon insulating cap is placed over the syringe plunger handle to prevent discharge through the syringe plunger to the pump); (B) stainless steel counterelectrode disk insulated from the stainless steel capillary with Kel-F bushing; (C) Upchurch Scientific (OakHarbor, W A ) P-640Kel-F adapter, 1/4-28thread to 10-32Fingertight chromatography fitting sealed to flange with Teflon O-ring; (D)20 cm X 0.062 in. o.d., 0.020 in. (0.50mm) i.d. 316 stainless steel tubing (Upchurch Scientific U-103); not shown is heater wire (0.020 in. Nichrome) in fiberglass sleeving wrapped on capillary and an iron-constantan thermocouple to monitor capillary temperature; (E) Kel-F bushing to align capillary end with skimmer (the bushing is mounted in a stainless steel plate mounted on two threaded standoffs); (F) skimmer cone with 0.5-mm orifice [Vestec (Houston, TX) VT 1020Al located ca. 3 mm from end of capillary; (G) hemispherical electrode (radius 0.375 in.; center of radius at entrance of f i t ion source lens) which serve8 as the repeller for liquid SIMS operation [the skimmer cone and hemispherical electrode are electrically connected to the ion source repeller voltage supply (0-40 V)];(H) Teflon washer (0.030 in.) which insulates the skimmer mounting plate from the housing (the mounting plate is attached with six 2-56Nylon screws); (I) standard lenses of the Nermag R30-10FAB ion source; (J) first 0 1991 American Chemical Society