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Anal. Chem. 1985, 57, 1768-1770
Third, the [(BP+)zAl&17-]+ion may be formed, but may readily fragment by loss of AlC13,to form the ion at m/z 439. Several attempts were made to plot the data obtained in order to determine if the results obtained by positive FAB-MS correlate to the potentiometric data of Gale and Osteryoung. While the general trends for the intensities of the 307 and 439 clusters as a function of mole percent A1C13could be rationalized, too many variables were present to permit a quantitative comparison between the two sets of data. For instance, assuming that the ions indicative of melt composition result from ion pairing to BPt, one would need accurate data for the extent of ion pairing of each of the three anions to BPt; these data are not presently known. Perhaps the most intangible variable is the relative desorption efficiency for each of the ions produced. For the sake of simplicity one would have to assume they are equal, as a first approximation. Finally, it is difficult to estimate the relative degree of oxidation occurring in situ for each sample. Oxidation is an obvious source of imprecision and does lead to changes in the mass spectra. Nevertheless, the utility of FAB as a probe of molten salt composition has been demonstrated. Since molten salts are nonvolatile ionic liquids of appreciable viscosity, FAB offers a convenient means of sampling “preformed ions” present without requiring a matrix. In addition, the intense, long-lived signals observed by this method make it possible to perform exact mass measurements. Also, it is possible to examine parent/daughter ion relationships in molten salts by using FAB in connection with mass spectrometry/mass spectrometry (MS/MS). A unique property of molten salts is their ability to act as solvents for nonpolar molecules in spite of their polar nature. Thus, molten salts exhibit potential as matrices for FAB-MS analysis of solutes which are insoluble in glycerol. Recently, Todd et al. demonstrated this possibility by successfully using SbC13as a matrix for the SIMS analysis of pyrene (18). The fact that AlCl@PCl mixtures are miscible with benzene suggests the use of this system as a matrix for similar analyses. Although characteristic changes in the positive ion FAB mass spectra reflected changes in melt composition for the A1Cl3/BPC1 system, a quantitative assessment of the anions present was not possible from our data. A more viable approach would be to observe the negative ions produced, since their formation may not be linked to ion pairing; i.e., one may be able to directly detect Cl-, AlC4-, and A12C1-;. Furthermore, since evidence exists suggesting that FAB may be used to examine equilibria occurring in the solution undergoing bombardment, negative ion data may yield quantitative re-
sults. We are currently investigating the instrumental modifications necessary in our laboratory to make this work possible. ACKNOWLEDGMENT The authors wish to acknowledge R. R. Rhinebarger and J. W. Rovang for their contribution through reagent purification and sample preparation. We also give thanks to A. I. Popov for his helpful discussions. Registry No. A1Cl3, 7446-70-0; BPCl, 1124-64-7. LITERATURE CITED (1) Gale, R. J.; Osteryoung, R. A. Inorg. Chem. 1979, 18, 1603. (2) Wilkes, J. S.; Levisky, J. A.; Pflug, J. L.; Hussey, C. L.; Scheffler, T. B. Anal. Chem. 1882, 5 4 , 2378. (3) Taulelle, F.; Popov, A. I. Po/yhedron 1983, 2 , 889. (4) Gale, R. J.; Gilbert, B.; Osteryoung, R. A. Inorg. Chem. 1978, 17, 2728. (5) Gesenhues, V.; Reuhl, K.; Wendt, H. Int. J . Mass Spectrom. Ion Phys. 1983, 4 7 , 251. (6) Caprioli, R. M. Anal. Chem. 1983, 5 5 , 2387. [7) Johnstone, R. A. W.; Rose, M. E. J . Chem. Soc., Chem. Commun. 1983, 1268. (8) Barber, M.; Bordoli, R. S.;Elllott, G. J.; Sedgwlck, R. D.; Tyler, A. N. Anal. Chem. 1982, 5 4 , 645A. (9) Aberth. W.: Straub, K. M.: Burllnaame, A. A. Anal. Chem. 1982, 5 4 , 2029. Rinehart, K. L., Jr. Science 1982. 218, 254. Busch, K. L.; Unger, S. E.; Vincze, A,; Cooks, R. G.; Keough, T. J . Am. Chem. SOC. 1882, 104, 1507. Ackermann, B. L.; Watson, J. T.; Newton, J. F., Jr.; Hook, J. B.; Braselton, W. E., Jr. Biomed. Mass Spectrom. 1984, 1 1 , 502. Karpinskl, Z. J.; Osteryoung, R. A. Inorg. Chem. 1984, 2 3 , 1491. Schueler, B.; Krueger, F. R. Drg. Mass Spectrom. 1979, 14, 439. Lee, T. D.; Anderson, W. R., Jr.; Daws, G. D., Jr. Anal. Chem. 1981, 5 3 , 304. (16) Weith, M. J. Mass Spectrom. Rev. 1983, 2 , 419. (17) Roblnson, J.; Bugle, R. C.; Chum, H. L.; Koran, D.; Osteryoung, R. A. J . Am. Chem. SOC. 1979, 101, 3776. (18) Groenewold, G. S.; Todd, P. J.; Buchanan, M. V. Anal. Chem. 1984, 56, 2253.
Bradley L. Ackermann Anthony Tsarbopoulos John Allison* Department of Chemistry Michigan State University East Lansing, Michigan 48824 RECEIVED for review February 7, 1985. Accepted March 18, 1985. This work was made possible through the financial support of the National Institutes of Health (NTH Grant No. RR00480-16) and the National Science Foundation (NSF Grant No. CHE-8023704). Also, B. L. Ackermann received support from the Dow Chemical Co. in the form of a research fellowship.
Preparation and Investigation of Polymer-Modified Electrodes by Square Wave Voltammetry Sir: In this paper we present the first use of square wave voltammetry (SWV) for the preparation and study of polymer-modified electrodes. Chemically modified electrodes have received a great deal of attention due to their potential use in a wide variety of applications. Modified electrodes have been characterized by many spectral and electrochemical techniques. Of the electrochemical techniques, cyclic voltammetry, chronoamperometry, and chronocoulometry have been used most widely (I). Pulse techniques have seen less 0003-2700/85/0357-1768$01.50/0
use. Differential pulse voltammetry at modified electrodes (2,3)was found to be well-suited for detecting small coverages and for precise measurements of peak potentials (4-7). Square wave voltammetry, introduced by Barker (8),further developed by Ramaley and Krause (9, IO), and brought to its modern form in 1977 (11,12),employs a potential-time wave form consisting of a square wave superimposed on a staircase. The step height of the staircase is Us, its period is 7,and the square wave is symmetrical with amplitude (one-half peak0 1985 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 57,NO. 8, JULY 1985
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5t
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(r
poly-[Ru(bpy)~~py),]~+ film = 4.9 X mol/cm2) on a platlnum disk electrode in 0.1 M Bu,NBF,/acetonitrile VS. Ag/Ag+: A€, = 5 mV; f = 10 Hz; (a) CV, (b-d) SWV; E,, = 25 mV; I (b), 1, (c), I , ( 4 . Flgure 1. Vottammetry of
to-peak) E , and period 7. The forward pulse (Le., in the scan direction) of the square wave is applied simultaneously with the staircase step, and the forward current (if) is the average value obtained by integration over the last one-third of the forward square wave half-cycle. The reverse current (i,) is obtained similarly on the reverse half-cycle, and the net current, i, is the difference, if - i,, for each staircase step. Moderate frequencies (f = 1 / ~and ) step heights permit rapid experiments: for f = 100 Hz and hE, = 10 mV, a range of 500 mV is covered in 0.5 s. Furthermore, the net current voltammogram provides excellent sensitivity and rejection of background currents. In the following we compare SWV and cyclic staircase voltammetry as techniques for studying polymer-modified electrodes. (Here cyclic staircase voltammetry is a surrogate for linear scan voltammetry and will be referred to as CV.) Films of p~ly-[Ru(bpy)~(vpy)~]~+ (bpy, 2,2'-bipyridine; vpy, 4-vinylpyridine) (13)were prepared by reductive electropolymerization of the monomer on platinum electrodes by CV or SWV, and their electrochemical behavior was studied by the same two techniques. The Ru(II)/Ru(III) oxidation occurs at 0.92 V vs. Ag/Ag+ in acetonitrile. The coverages of the modified electrodes ranged from 4.9 X 10-lo to 1.1 X lo-* mol/cm2. There were no observable differences between the electrochemical behavior of the films prepared by SWV and by CV. All films gave linear plots of peak current vs. scan rate (AE,/7) as measured by CV. The most notable characteristic of SWV is the increase in sensitivity over CV. A comparison of CV and SWV for the same values of AE,and 7 of a film of 4.9 x mol/cm2 coverage is shown in Figure 1. The anodic peak current for CV is 0.69 MAwhile the net peak current for SWV is 11.1MA; a 16-fold increase in sensitivity is obtained with SWV. Clearly with electrodes having small coverages, SWV may be used to detect electroactivity rapidly and to obtain precise values of peak potentials. To gain further insight into the use of SWV for the investigation of modified electrodes, the parameters AE,,E,,, and f were varied. At constant amplitude and frequency, the step height was varied from 2 to 20 mV. The forward current increased (became more negative) and the reverse current decreased (became less positive) by a similar amount. The net current is substantially independent of step height. This agrees with the theory of SWV developed for solution species, for which net current depends only weakly on step height (11). The SW amplitude was varied from 10 to 60 mV at constant step height and frequency. Films of fairly low coverage (4.9 X to 6 x lo4 mol/cm2)showed linear increase of the peak
-2
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I
I
I
I
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Esw, mV Peak current vs. square wave amplitude for poly-[RuA€* = 5 mV; f = 10 Hz; mol/cm2 (A), 6 X lo-' mol/cm2 (B). Flgure 2.
(bpy)2(bpy),]2+ film in Bu,NBF,/acetonitrile: (0)net current, (X) i f , (A)I,; I? = 1.3 X
currents with increasing amplitude. The forward and reverse currents each become larger, and the net current is thus markedly enhanced (Figure 2A). Films with coverages above 6x mol/cm2 showed decidedly different behavior with changes in amplitude. A plot of the forward and reverse currents vs. amplitude yields two lines which intersect at about 35 mV (Figure 2B). Below an amplitude of 35 mV the net current is smaller than the forward current. Calculations for solution species predict this intersection at 15/11 mV (11).The value is clearly larger for polymer-modified electrodes. The dependence of peak width at half height on amplitude was determined also. The ratio of peak current to peak width exhibits a maximum at nEsw = 50 mV. This agrees with predictions for reversible electron transfer for diffusing species (14).
Square wave frequencies from 1to 100 Hz (periods 14.01 s, pulse widths 0.5-0.005 s) were investigated. The films of
coverages below 5 X mol/cm2 showed similar behavior. The forward current increased linearly with 7-ll2 while the reverse current became constant at about ?*I2 = 0.1 s - ~ / ~As . a consequence, the net current leveled off at a constant value above the forward current. With thicker films the reverse current was independent of 7-1/2 while the forward current increased linearly with r-lI2. With thick films changes in the peak shape occur at high frequencies. The reverse current peak shifts in the negative direction or splits into two peaks. This causes the net current peak to move to more positive values. This effect may be due to a slow step in the overall electron transfer process due to polymer or electrolyte movement. In the absence of appropriate theory more detailed interpretation is not possible. However, these qualitative as well as quantitative changes suggest the potential of SWV for studies of mechanism.
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Anal. Chem. 1905, 57, 1770-1771
ACKNOWLEDGMENT The authors thank R. A. Osteryoung for helpful discussion. Registry No. [R~(bpy)~(vpy)~1~+, 75687-40-0; Bu4NBF4, 429-42-5; Pt, 7440-06-4. LITERATURE CITED Murray, R. W. I n "Electroanalytical Chemistry"; Bard, A. J., Ed.; Marcel Dekker: New York, 1984;Vol. 13,Chapter 3. Brown, A. P.; Anson, F. C. Anal. Chem. 1977, 49, 1589. Brown, A. P.; Koval, C.; Anson, F. C. J. Electroanal. Chem. 1976, 7 2 , 379. Lennox, J. C.; Murray, R. W. J. Am. Chem. SOC. 1978, 100, 3710. Ianniello, R. M.; Lindsay, T. L.; Yacynych, A. M. Anal. Chem., In press. Jester, C. P.; Rocklin, R. D.; Murray, R. W. J. Electrochem. SOC. 1980, 127, 1979. Lane, R. F.; Hubbard, A. T. Anal. Chem. 1076, 48, 1287. Barker, 0. C. Congress on Analytical Chemistry In Industry, St. Andrews, June 1957.
(9) Ramaley, L.; Krause, M. S., Jr. Anal. Chem. 1989, 41, 1362. (IO) Krause, M. S.,Jr.; Ramaley, L. Anal. Chem. 1960,41, 1365. (11) Christie, J. H.; Turner, J. A.; Osteryoung, R. A. Anal. Chem. 1977, 4 9 , 1899. (12) Osteryoung, Janet; Osteryoung, R. A. Anal. Chem. 1985, 57, 101A. (13)Calvert, J. M.; Schmehl, R. H.: Sullivan, B. P.: Facci. J. S.: Mever. T. J.; Murray, R. W. Inorg. Chem. 1083, 2 2 , 2151. (14) O'Dea, J. J.; Osteryoung, J.; Osteryoung, R. A. Anal. Chem. 1081, 53, 695.
Esther Sans Takeuchi Janet Osteryoung* Department of Chemistry State University of New York at Buffalo Buffalo, New York 14214
RECEIVED for review February 12,1985. Accepted March 1, 1985. This work was supported by the National Science Foundation under Grant No. CHE 8305748.
Frequency of a Quartz Microbalance in Contact with Liquid Sir: Several pioneering articles have recently appeared (1-3) which indicate that the high mass sensitivity of the oscillating quartz microbalance, which is routinely used under vacuum, is also available in a liquid environment. Already several potential applications of quartz microbalances as chemical detectors have appeared (1, 2). That stable oscillation can be obtained in contact with a liquid is particularly noteworthy in view of the general impression in the vacuum community (4-6)of the deleterious effects of liquid films. It was thought that the viscous damping would not only cause large frequency shifts but also large losses in the quality factor Q leading to instability and even cessation of oscillation. Quantitative knowledge of the resonator behavior is requisite for a proper interpretation of experimental results. We present here an outline and the results of an analysis which yields a quantitative description of the influence of the liquid properties on the oscillation frequency. The experimental observable in these measurements is the frequency (or period) of oscillation. Under vacuum, the rigid attachment of a film of mass Am to the crystal surface causes a decrease Af in the resonant frequency. That the relationship between Af and Am is linear in the limit of small Am was first derived by Sauerbrey (7)and has been verified experimentally. This relationship Af = - -Am (1) fo
m
where fo is the resonant frequency and m the mass of the unloaded resonator is routinely used today. When the overlayer is thick, the relationship is no longer h e a r and corrections for this case have been developed (8, 9). The coupling of the crystal surface to a liquid also drastically changes the resonant frequency. The observed changes have been ascribed to the liquid density ( 2 ) ,to a combination of density and conductivity ( I ) , and to the mass of solid deposits (3). Recent papers ( 5 , 6 , 9 )have also treated the response of a quartz microbalance to liquidlike deposits. These analyses are appropriate for thin films and partially elastic films but not for the case at hand, viz., the total immersion of one surface of the crystal in a viscous liquid. We have determined the behavior of the crystal/fluid system by examining the coupling of the elastic shear waves in the crystal to the viscous shear waves in the liquid. In this manner, the resonance condition derives directly from the matching of appropriate 0003-2700/65/0357-1770$01.50/0
-I
-2.00.2 0.4 0.6
0.0
0.8
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Distancelrnicrons Flgure 1. The z profile for the transverse fluid velocity for water at three different times: (solid line) time of peak velocity at the surface; (dotted line) time of zero velocity at the surface; (dashed line) intermediate time.
500 N
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Weight Percent Sucrose Flgure 2. Frequency shift (sucrose in water) vs. sucrose concentration
for a 9-MHz crystal. Circles are Nomura's data. Solid curve is prediction of model.
boundary conditions on the shear waves. Though not as elegant as the previous treatments using the Rayleigh perturbation theory, transmission line analogs, or energy transfer models, this straightforward approach does provide a quantitative and easily grasped physical picture of the way the liquid affects the resonance condition. Details will be presented in a separate publication (10). Here, an outline of the treatment and major results are given. Q 1985 Amerlcan Chemical Society
