1615
Anal. Chem. 1987, 59, 1615-1620
Repetitive in Situ Renewal and Activation of Carbon and Platinum Electrodes: Applications to Pulse Voltammetry Melanie Poon and Richard L McCreery* Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210 Glassy carbon and platlnum electrodes were renewed and actlvated in sltu by using a previously reported technlque involving pulsed laser Ilght. The laser pulse dellvered dlrectly In the solutlon of Interest was able to repeatedly remove passivating films or oxide layers and result In an actlve, reproducible solid electrode surface. A differential pulse voltammetry technique was developed In which the electrode Is laser actlvated before each potentlal pulse during a conventional potentlai scan. Thls laser-asslsted voltammetric experiment,provided enhanced resolution and greatly extended electrode life for the voltammetry of several organic specles, lncludhg phenol, dopamlne, ascorbic acid, and hydroquinone. Some physlcal effects of repetltlve laser pulses were observed and local melting and ablatlon were posslble at higher laser powers. Laser activation may be useful for exploiting the wlde potentlal wlndow of sdld electrodes, while hnprovlng thelr reproduclblllty, Ilfetlme, and electron transfer klnetlcs.
The prominence of the dropping mercury electrode (DME) in analytical applications of voltammetric techniques stems largely from its renewable, reproducible surface. In a wide range of applications, the continually exposed fresh mercury surface and the entirely new drop every few seconds are important for maintaining an analytically useful response. In contrast to the DME, solid electrodes are often deactivated by either species in solution or those generated by a redox process. The result is passivation of the surface which can cause highly irreproducible measurements. In extreme cases, the charge transfer process is completely inhibited and the electrode must typically be removed from solution and restored by one of a number of pretreatment methods. Poor stability can interfere with the performance of solid electrodes to the point where their advantages of mechanical rigidity and wide potential window are difficult to realize. Two general approaches have been pursued to activate and stabilize solid electrodes for analytical or electrocatalytic applications. First, the electrode may be protected by a selectively permeable membrane which excludes deactivating materials. The Clark oxygen electrode is a classical example, and electrodes protected by Ndion ( 1 , 2 )or cellulose (3,4)are more recent cases. Second, a carbon or platinum electrode may be reactivated by a variety of procedures, most of which involve removal from the cell. Numerous procedures for cleaning and activating carbon (5-26) or platinum (27-35) electrodes have been presented and reviewed (10) and include polishing (5-7, I O ) , plasma treatment (24),heat treatment (22,23),laser activation (25, 26), flaming (30,351, electrochemical cycling (13, 14, 17-19, 21),and various chemical reactions (20,30). These procedures drastically affect electrode performance, with, for example, rate constants for the Fe(CN)63-/4- electron transfer rate varying by 3 orders of magnitude on carbon (10)and 2 orders of magnitude on platinum (30)as a function of pretreatment procedure. The majority of these procedures are fairly tedious and time-consuming, with most requiring meticulous attention to cleanliness. An exception is carbon paste, where a new surface may be prepared quickly, albeit outside the cell (31). 0003-2700/87/0359-1615$01.50/0
A few of these procedures may be carried out in situ, including laser activation of glassy carbon (GC) (25,26),electrochemical cycling of platinum by pulse (33,34)or scanning waveforms (28,35-37), and mechanical removal of electrode material (32). We have reported a new in situ method for electrode pretreatment that uses a short laser pulse to irradiate the electrode directly in the solution of interest (%), and we discussed a particular application to glassy carbon (26). The objectives of the present work are two, both directed primarily a t analytical voltammetry using solid electrodes. First, laser activation was investigated as a means to repeatedly renew solid electrodes passivated by adsorbed or polymeric materials. Second, we sought to improve analytical voltammetry by enhancing the electron transfer rate using laser activation. EXPERIMENTAL SECTION The electrochemical cell, optical arrangement, and lasers have been described previously (26). For some of the present work, the fundamental output of the Nd:YAG laser (1064 nm) was frequency doubled by using a standard potassium dihydrogen phosphate crystal and separated from the 532-nm light with a dichroic mirror. For both wavelengths, a He-Ne pilot beam was coincident with the Nd:YAG beam to aim the laser onto the electrode. The laser pulse duration was 7-10 ns, with total energy ranging from 5 to 60 mJ, and only the center of the unfocused Nd:YAG beam impinged on the electrode. In addition to the Nd:YAG laser, a small N2laser (Laser Science VSL 337, pulse duration 3-4 ne, peak power 40 kW)was used for one experiment with its beam focused to a 200-pm diameter spot. The electrochemical cell was constructed of Teflon and was able to accommodate electrodes of varying diameter sealed in glass. Platinum electrodes were made by sealing Pt wire (Engelhard or Goodfellow Metals) in type 0120 potash soda lead glass (Corning) followed by annealing. The glassy carbon working electrode design used previously was changed due to the entrapment of gas bubbles in the Teflon washer after a large number of laser pulses. Glassy carbon (GC20-s, Tokai) was cut with a low-speed diamond saw (Buehler) into small rectangular pieces of approximately 1 x 1 X 5 mm and sealed in the same Corning glass with Torr seal (Varian Associates), leaving the 1 X 1 mm surface exposed. Electrical connections to electrodes were made with silver epoxy. All electrochemical experiments using GC were performed with the 1 X 1 mm exposed area, which was fully illuminated by the laser pulse. For scanning Auger and profilometer experiments, a large (ca. 1 X 1 cm) GC square was illuminated on a ca. 1 mm diameter area, to permit comparison between illuminated and dark regions. Cyclic voltammetry was performed with a CV-1B (Bioanalytical Systems) potentiostat with Ag/AgCl (3 M NaCI) reference and a Pt wire auxiliary electrode. The reference electrode was isolated from the cell by a sintered glass frit (Porosity E, Ace Glass). As explained below, laser activation was combined with differential pulse voltammetry (DPV) by preceding the potential pulse with a laser pulse. Due to fundamental aspects of NdYAG laser design, it is not simple to trigger the laser at will; however the Q-switch may be gated by a TTL pulse. With the laser flashlamp running continuously at 10 Hz,the drop knocker pulse from a PAR 174A polarograph was used to trigger a 100-ms monostable vibrator, which gated the Q switch. The result was a laser light pulse arriving at the electrode within 0-100 ms after the drop knocker signal. Since the "drop time" on the polarograph was set to 1 s, a laser pulse preceded each potential pulse by 0.9-1.0 s. For all DPV experiments reported here, the modulation amplitude was 10 mV and the dc scan rate was 2 mV/s. 0 1987 American Chemical Society
1616
ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987
// Flgure 1. Voltammetry of 3 mM ferrocyanide in the presence of 3 mM phenol in 1 M KCI at a 125 pm diameter R electrode. Curve A is prior to and cuve 6 is after phenol film formation by phenol oxidation. Curve
C is after a single 30 MW/cm2 laser pulse (532 nm). The curves labeled D were taken after 40 film formation and removal cycles.
Electrodes were polished conventionally with 600 grit Sic paper followed by 1.0-,0.3-, 0.05-pm deagglomerated alumina (Baikalox) in a slurry with Nanopure water on a Texmet polishing cloth (Buehler). For the experiment demonstrating Pt oxide removal, the electrode was also cleaned in hot concentrated sulfuric acid before potential cycling. In all cases, thorough rinsing and sonication were performed with Nanopure water. Ascorbic acid, phenol, hydroquinone, eugenol, o-chlorophenol, potassium ferrocyanide, sodium dihydrogen phosphate, sodium monohydrogen phosphate, and potassium chloride were reagent grade and used as received. Dopamine hydrochloride and serotonin hydrochloride were obtained from Sigma Chemical Co. All solutions were prepared fresh daily with Nanopure water (Sybron Barnstead, 16 MQ) and degassed with nitrogen. Auger analysis was performed on a Model 595 physical electronic scanning Auger microprobe with an accelerating voltage of 3 kV and a base pressure of 1 X lo4 torr. The sensitivity factors for appropriate peaks are C (270 eV) = 0.2,O (510 eV) = 0.5, and C1 (180 eV) = 1.0. The scanning electron microscope and profilometer have been described previously (26).
RESULTS AND DISCUSSION Three examples of laser renewal of modified electrode surfaces will be presented here: removal of phenoxy films from GC and Pt, removal of the oxide film from GC formed during electrochemical anodization, and removal of an oxide layer from Pt. In all cases, the laser pulse(s) were delivered in situ without cell disassembly or changing solutions. After discussion of these examples, a solid electrode analogue of the DME will be presented, involving laser activation before each potential pulse in a differential pulse voltammetry experiment. Finally, the long-term effects of laser activation on GC and Pt surfaces will be discussed. The oxidation of phenol at a Pt electrode results in a phenoxy radical which couples to form a passivating polymeric film on the electrode. After only one potential scan from -0.1 to +1.2 V in the presence of phenol, the oxidation of solution species is completely inhibited. Previous methods of restoring such a passivated electrode involved treatment with a flame (38), anodic polarization in an acidic solution of ferric chloride (39),or polishing. Figure 1 demonstrates repeated renewal of the polyphenoxy-coated electrode by in situ laser irradiation. A Pt electrode was immersed in a solution containing Fe(CN)fi"-I4-and phenol, and voltammetric scans were conducted
. 300 Klwtk E w y ,
I
I
60
€00
rV
Figure 2. Scanning Auger microprobe survey spectra of nonirradated
(upper)and irradiated (25 MW/cm2)(lower)regions.of GC which had been coated with a film of o-chlorophenol. Irradiated region was about 1 mm2 in area. either in the Fe(CN)2-/4- potential range (-0.1 to +0.5 V vs. Ag/AgCl) or through the phenol oxidation (-0.1 to +1.2 V). The Fe(CN),3-/4- couple thus acted as a marker of electrode activity toward solution components. The first two curves are voltammograms prior to (curve A) and after (curve B) phenol film formation. After curve B, the electrode was subjected to a single 30 MW/cm2 laser pulse in situ,after which the Fe(CN)63-/4-couple reappeared (curve C). The voltammograms in Figure 1D are similar to those in B and C but were obtained after 40 complete cycles of film formation and removal. Each cycle consisted of the following steps: seven voltammetric scans from -0.1 to 1.2 V, to deposit a phenol film and passivate the electrode, a single 30 MW/cm2 laser pulse delivered in situ at 532 nm, a single voltammetric scan from -0.1 to +0.5 V to observe the Fe(CN)63-/4-couple. Film removal was demonstrated for 40 cycles at a Pt electrode and the voltammogram looks essentially the same even after 1000 consecutive laser pulses. Throughout the 40 film formation and removal cycles and subsequent 1000 laser pulses, the Fe(CN),3-/4- peak current had a relative standard deviation of 3%. For a similar reactivation of a GC electrode, a slightly lower power density is required (24 MW/cm2) to remove the phenol film. With cycle repetition on GC, there also appears to be a small steady increase in the background and anodic peak current for ferrocyanide (variation f 5 % for 35 cycles). The elemental composition of a glassy carbon electrode surface was determined by scanning Auger microscopy before and after phenol film removal by laser treatment. An electrode
ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987
+
1.2
8 Potential
I
+
1.0
.5
Potential
0
(v
.5 v8
- 1.0
AIJ/Agoa)
Figure 3. Removal of surface film and redox couple from electrochemically pretreated GC. Voltammogram A is after 5 min at +1.75 V vs. Ag/AgCI in 0.1 M KNO,. Voltammogram 6 is after subsequent treatment with three 25 MW/cm2 laser pulses. Scan rate was 100 mV/s.
was coated with a polymeric film of o-chlorophenol by scanning to +1.2 V, then laser irradiated. The chlorophenol derivative was chosen as a marker because the oxygen signal may result from untreated carbon (13,17,40) and the Auger signal displayed by chlorine is relatively strong. Figure 2 shows Auger survey spectra of the irradiated and nonirradiated areas of a chlorophenol film on GC. Comparison of these spectra shows considerable reduction of oxygen in the laser-treated area as reported earlier (26) and no detectable C1 in the laser-treated reg;on. It cannot be stated from these results that laser irradiation leaves the electrode surface atomically clean, but from the voltammetry and the Auger spectra, we can conclude that the passivating effects of the film have been removed, and the presence of the chlorine marker was reduced to immeasurable levels on the laser-treated surface. Electrochemical oxidation of GC surfaces has been shown to enhance electron transfer rates for a variety of redox couples, and good evidence exists for formation of an oxygen-rich surface film (13, 17, 19, 21). This film results in a higher background current and may be responsible for enhanced charge transfer. Figure 3 demonstrates that laser treatment can remove the effects of electrochemicalpretreatment of GC (5 min at +1.75 V in 0.1 M KNOB)at GC. Laser treatment reduces background current and results in the loss of the surface waves a t ca. +0.3 V usually attributed to quinoid species. The PtOH film formed on Pt during p i t i v e potential scans may also be removed with laser activation. After the film was formed during a positive scan to +1.4 V, the potential was held at +0.60 V, where the PtOH film was not yet reduced. As shown in Figure 4, three 10-ns pulses of 60 m J results in removal of most of the PtOH film as demonstrated in the subsequent potential scan from +0.6 to 0 V. It was necessary to hold the electrode at +0.6 V for approximately 5 s during laser treatment, but a control experiment in the absence of laser treatment showed negligible decrease in PtOH reduction current. These examples demonstrate fast, facile, in situ, repeated renewal of platinum and GC electrode surfaces under a variety of conditions. When combined with pulse voltammetry, laser renewal would provide a DME analogue for solid electrodes with a repetitively renewed electrode surface free of history
1617
0
4
( V vs #ql#qClI
Figure 4. Removal of PtOH layer by laser irradiation. Upper curve Is a typical voltammogram of Pt in 1 M H,SO, at a scan rate of 50 mV/s. Lower curve solld line shows effects of three 45 MW/cm2 laser pulses delivered while the potential was held at +0.6 V after scanning to +1.4 V. Lower curve dashed line is a control experiment where the electrode was held at +0.6 V for 5 s after PtOH formation and then scanned negatively with no laser treatment. V
Figure 5. Differential pulse voltammetry of 1 mM dopamine in 0.1 M phosphate, pH 7.4 on GC. Curve A solid line is first scan with no laser treatment and dotted line is the second scan. Curve 6 is the first scan with a 20 MW/cm2 laser pulse preceding each potential pulse. Curve C is the 24th scan with the laser on. Curve D shows the decay of the DPV response after the laser is turned off.
effects. Figure 5, curve A, shows an ordinary differential pulse voltammogram of dopamine at a GC electrode at pH 7.4. The electrode response decays rapidly with time due to filming of the products of the oxidation reaction (41),and after only one scan, severe passivation has occurred. Scan B and C of the same figure are the results of an experiment designed so that a laser pulse is delivered to the electrode at the time corresponding to the new Hg drop for a DME, such that each potential pulse occurs on a freshly activated surface. As shown
1618
ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987 0
A
'
2
4 %
6
.a
-
+
1.0
-.2
.05 pA
E
1
.1
0
.2
+.3
v
A
:
V Figure 6. Differential pulse voltammetry of 2 mM phenol in 1 M KCI on GC. Curve A is with no laser treatment. Curve B is the first scan with 25 MW/cm2 laser pulses before each potential pulse. Curve C is the 22nd such scan. Curve D is the first scan with the laser turned off. Curve E is the first scan after the laser treatment is resumed. Curves B-E have the same vertical scale.
in Figure 5 no degradation of response was observed on successive scans, with 30 voltammetric scans yielding an average peak response of 0.518 f 0.027 PA. The same behavior was observed for the oxidation of serotonin on GC, with an ordinary electrode lasting for one scan and a laser-renewed surface lasting for at least 28 scans. Once the laser was turned off, the response rapidly degraded. For the phenol oxidation with DPV, the first scan exhibits very poor response, due to electrode passivation during the voltammetric scan. As shown in Figure 6, excellent response is possible on a repetitive basis with laser activation. More dramatic effects of the combination of laser activation with DPV are apparent in the voltammetry of a mixture of dopamine, hydroquinone, and ascorbic acid, shown in Figure 7. On an ordinary polished GC electrode the three species are severely overlapped due to slow heterogeneous electron transfer kinetics. When a laser pulse precedes each potential pulse, the three components are resolved and peak potentials are close to the thermodynamic values. The improvement in resolution provided by activation has been noted previously for both highly polished GC (IO)and laser-activated GC (26) and results from the increase in electron transfer rate from the activation procedure. The additional features apparent in Figure 7 are two: longevity and effects on adsorption. First, the laser effects are persistent, lasting a t least tens of scans and thousands of potential or laser pulses, whereas a polished surface would require disassembly and renewal. Second, the laser reduces adsorption by decreasing the time available for reactants to adsorb. As shown in curve C hydroquinone can exhibit anomalously large peaks due to reactant adsorption after the laser is turned off. In the presence of laser pulses the peak height for hydroquinone is reduced significantly, since any adsorbed material is removed with each laser pulse.
I I
U
Flgure 7. Differential pulse voltammetry of 1 mM ascorbic acid, hydroquinone, and dopamine in 0.1 M phosphate, pH 7.4 on GC. Curve A shows the decaying response for three scans of the mixture with
no laser treatment. Curve B is a voltammogram with 25 MW/cm2 laser pulses preceding each potential pulse. Curves C are the first (solid), second (dashed), and thiid (dotted) vokammograms after the laser was turned off.
The cumulative physical effects of many laser pulses on the electrode surface during the above experiments were examined with a profilometer and a scanning electron microscope. Figure 8 shows a profilometer trace of a 1 X 1 cm GC piece after 3000 laser pulses at several power densities were applied a t a rate of 10 Hz to a 1 mm diameter area of the electrode immersed in water. A well-defined crater with a diameter equal to that of the laser spot was observed, indicating removal of GC from the substrate. The crater depth was power density dependent, but a t powers employed for film removal (ca. 25 MW/cm2) the average ablation rate was about 10 A/laser pulse. Thus 3000 pulses resulted in a crater of 3 pm depth. A profilometer trace on a laser-treated Pt surface did not show net ablation of the surface but rather surface roughening to the extent of a few micrometers (Figure 9). Note that the abscissa in the profilometer traces is severely compressed relative to the ordinate, so the peaks and valleys are much more gradual than they appear in the traces. While ablation of GC or melting of Pt occur at higher powers, they are not necessarily required for activation or film removal. The power density required to remove polyphenoxy films as shown in Figures 1 and 2 (25 MW/cm2) causes no observable change in the Pt surface at the profilometer or SEM level. Most of the beneficial effects of laser pulses on GC were at power densities which produce some ablation, but it is not yet clear that ablation is necessary for activation. We are presently attempting to accurately correlate physical and chemical surface effects of lasers with electrochemical performance. The cost of the Nd:YAG laser used up to this point is high, in the range of $30-35K. The high pulse energy (5-100 mJ)
.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987
+ D
I
0.6
a5
I
irradiated region
=P
Figure 8. Profilometer traces of GC surface treated with 3000 laser pulses in water: trace A, 10 MW/cm2; trace B, 13 MW/cm2; trace C, 22 MWlcm2; trace D, 44 MW/cm2.
f
2000
u irradirtd p i o n
Figure 9. Profilometer traces of a Pt surface treated with 3000 laser pulses in water: curve A, 44 MWlcrn'; curve B, 66 MW/cm2.
used here is sufficient to achieve the ca. 30 MW/cm2 power density over a typical voltammetric electrode with a diameter of a few millimeters. It is useful for the present work because the center 1 mm of the beam could be used to maximize the uniformity of the power density. If a smaller, less even beam is acceptable, much smaller lasers may be used. Figure 10 shows laser activation of a 3 mm diameter GC electrode activated by 150-4 pulses from a small Nz laser (Laser Scientific VSL 337, list price $3K) focused to a spot of 200 pm diameter. To produce the bottom voltammogram, approximately 100 laser pulses were distributed over the electrode surface during
vs &/&CI
0
)
a period of ca. 5 s. The ca. 30-90 MW/cm2 power densities achieved with this small laser were sufficient to activate the oxidation of ascorbic acid where the laser spot impinged on the electrode. If the electrode size were 200 pm, single laser pulses from a small Nz laser should produce effects comparable to those observed with the Nd:YAG system used for most of the work reported here.
CONCLUSION
8
I
(v
ai
0.2
Figure 10. Effect of N, laser irradiation on voltammetry on 1 mM ascorbic acid in 0.1 M H2S04. Curve A is a conventionally polished electrode. Curve B is after irradiation of a small fraction (ca. 5 % ) of electrode area. Curve C is after irradiation of a large fraction of the total electrode area by ca. 100 laser pulses.
Laser pulses from a Nd:YAG laser can repeatedly renew and activate solid electrode surfaces. It is also shown that similar effects can be observed when a low-cost N2 laser is focused down to a small area on the electrode. The approach is fast (10 ns), in situ, and repeatable on a 100-ms time scale, for combined a totalwith of at differential least several pulsethousand polarogaphy, repetitions. several benefits When
3 *
0.3
0.4 Potential
10 pn
1619
result compared to conventional DPV on solid electrodes. First, history effects from potential pulse to potential pulse are removed, permitting voltammetry on severely passivating materials. Second, the laser pulse activates electron transfer permitting more redox systems to exhibit close to their thermodynamic potentials and in many cases improve resolution. Third, electrode stability is greatly improved due to repetitive renewal of the surface. For a 1 mm thick GC electrode and a 25 MW/cm2 power density, the electrode would last about lo6 laser pulses, or 103-104 DPV scans. The results reported here indicate that a variety of films and superficial substrate layers may be removed by in situ laser treatment. The excellent behavior of GC or Pt electrodes after thousands of pulses implies negligible long-term degradation of electrode performance, at least in terms of analytical response and electron transfer rate. In addition, it does not appear likely that laser-desorbed materials are returning to the surface in quantities sufficient to affect electrode performance. The SAM results on chlorophenol films indicate effective removal of the chlorine marker, at least to levels below the SAM sensitivity of about 1 atom % on the surface. Without further spectroscopic data, it is not possible to determine how completely various films or adsorbates are removed, except to say they have no observable effects on the electroanalytical
Anal. Chem. 1987, 59, 1620-1626
1620
response. In the case of extremely strongly adsorbed materials on GC, it should always be possible to increase laser power until substrate ablation removes all surface adsorbates. Any technique involving a research quality Nd:YAG laser will be expensive and would not be considered routine. However, the N2 laser results indicate promise for reducing cost and complexity of a laser activated pulse voltammetry method for small electrodes. Perhaps a more fundamental long-term result of this work will be a better understanding of the relationship between surface properties and electrochemical performance. The effects of laser activation on carbon surface chemistry are presently under investigation.
LITERATURE CITED (1) Gerhardt, G. A.; Oke, A. F.; Nagy. F.; Moghaddam, B.; Adams. R. N. Brain Res. 1984, 290, 390. (2) Nagy, F.; Gerhardt, G. A.; Oke, A. F.; Rice, M. E.; Adams, R. N.; Moore, R . B.; Szentirmay, N. M.; Martin, C. R. J . Electroanal. Chem. 1985, 188. 8 5 . Hutchins-Kumar, L. D. Anal. Chem. 1986, 58, 402. Hutchins-Kumar, L. D. Anal. Chem. 1985, 5 7 , 1536. (5) Rusling. J. F. Anal. Chem. 1984, 56, 575. (6) Kumau. G. N.; Willis, W. S.: Rusiing, J. F. Anal. Chem. 1985, 57,545. (7) Thornton, D. C.; Corby, K. T.; Spendel, V. A.; Jordan, J.; Robbat, A,; Rutstrom, D. J.; Gross, M.; Ritzler, G. Anal. Chem. 1985, 5 7 , 150. (8) Laser, D.; Ariel, M. J . Electroanal. Chem. Interfacial Electrochem. 1974, 52, 291. (9) Gunsingham, J.; Fleet, B. Anawst (London) 1982, 107, 896. (IO) Hu, I.F.; Karweik, D. H.; Kuwana, T. J . Electroanal. Chem. Interfacial Electrochem. 1985, 188, 59. (11) Plock, C. E. J . Nectroanal. Chem. Interfacial Electrochem. 1989, 22, 185.
(12) Taylor, R. J.; Humffray, A. A . J . Electroanal. Chem. InterfacialElectrochem. 1973, 42,347. (13) Engstrom, R. C.: Strasser, V. A. Anal. Chem. 1984, 5 6 , 136. (14) Blaedel, W. J.; Jenkins. R. A. Anal. Chem. 1974, 46, 1952. (15) Moiroux, J.; Elving, P. J. Anal. Chem. 1978, 50, 1056. (16) Wightman, R. M.; Paik, E. C.: Borman. S.;Dayton, M. A. Anal. Chem. 1978, 50, 1410. (17) Cabaniss, G. E.; Diamantis, A. A,; Murphy, W. R., Jr.; Linton, R. W.; Meyer. T. J. J . Am. Chem. Soc. 1985, 107, 1845.
(18) Wang, J.; Hutchins. L. D. Anal. Chim. Acta 1985, 767, 325. (19) Gonon, F. G.; Fombarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1981, 5 3 , 1386. (20) Rice, M. E.; Galus, Z.; Adams, R. N. J . Nectroanal. Chern. 1983, 143, 89. (21) Faiat, L.; Cheng, H. Y. J . Electroanal. Chem. 1983, 157. 393. (22) Stutts, K. J.; Kovach. P. M.; Kuhr, W. G.; Wightman, R . M. Anal. Chem. 1983, 55, 1632. (23) Fagan, D. T.; Hu, I.F.; Kuwana, T. Anal. Chem. 1985, 57, 2759. (24) Evans, J.; Kuwana, T. Anal. Chem. 1979, 51, 358. (25) Hershenhart, E.; McCreery, R. L.; Knight, R. D. Anal. Chem. 1984, 56, 2256. (26) Poon, M.; McCreery, R. L. Anal. Chern. 1988, 58, 2745. (27) Bishop, E.; Hitchcock, P. H. Analyst (London) 1973, 98. 475. (28) Kinoshita, K. I n Modern Aspects of Electrochemistry; Bockris, J. O’M., Conway, 8. E., White, R. E., Eds.; Plenum: New York, 1982: Vol. 14,
p 557, and references therein. (29) Amatore, C.; Saveant, J. M.; Tessier, D. J. J . Hectroanal. Chem. Interfacial Electrochem. 1883, 746, 37. (30) Goldstein, E. L.; Van de Mark, M. R. Electrochim. Acta 1982, 27, 1079. (31) Adams, R. N., Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1969. (32) Petrii, 0. A.; Khomchenko, I. G. J . Electroanal. Chem Interfacial Electrochem. 1980, 706, 277. (33) Austin, D. S.;Polta, J. A,; Polta, T. Z.; Tang, A. P. C.; Cabelka, T. D.; Johnson, D. C. J . Nectroanal. Chem. Interfacial Electrochem. 1984, 168, 227. (34) Neuberger, G. G.; Johnson, D. C. Anal. Chlm. Acta 1986, 779,381. (35) Gilman, S. I n Nectroanalytlcal Chemistry. A Series of Advances; Bard, A. J., Ed.; Marcel Dekker: New York, 1967, Vol. 2, p 111. (36) Conway, B. E.; et al. Anal. Chem. 1973, 45, 1331. (37) Hoare, J. P. Nectrochim. Acta 1982, 27, 1751. (38) Hedenburg, J. F.; Freiser, H. Anal. Chem. 1953, 25, 1355. (39) Koile. R. C.; Johnson, D. C. Anal. Chem. 1979, 5 1 , 741. (40) Van der Linden, W. E.; Dieker, J. W. Anal. Chlm. Acta 1980, 179,1. (41) Lane, R. F.; Hubbard, A. T. Anal. Chem. 1976, 4 8 , 1287.
RECEIVED for review January 2, 1987. Accepted March 23, 1987. This work was supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by the OSU Materials Research Laboratory. Partial support from The Chemical Analysis Division of the National Science Foundation is also acknowledged.
Maximum Likelihood Quantitative Estimates for Peaks: Application to Photoacoustic Spectroscopy P. E. Poston and J. M. Harris* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
The method of maximum Ilkellhood provldes an optlmum technique for combining measurements of dlff erlng uncertalnty. The method is approprlate for the quantitative lnterpretatlon of peaks from analytical measurements where the peak shape Is known or can be measured. Maxlmum Ilkellhood estlmates of sample concentration are derived in this work for several classes of noise havlng both homogeneous and Inhomogeneous variance. The theory Is tested by uslng numerically simulated data and then applled to detection of weak absorbances in ilqulds using pulsed laser-excited photoacoustlc spectroscopy. Maxlmum llkeiihood estimates produce detection limits whlch are a factor 5 lower than a gated measurement at the peak maximum, in agreement wlth theoretical predictions.
A large fraction of the data produced by analytical chemistry methods appears in the form of peaks rising from a base line as a function of time, wavelength, or applied voltage. 0003-2700/87/0359-1620$0 1.50/0
Numerous examples could be listed from spectroscopy, chromatography, voltammetry, and flow injection analysis. To determine the amount of sample or analyte responsible for the observed signal is a standard goal of a quantitative analytical procedure. For data that appears in the form of peaks, a number of strategies may be implemented to obtain an estimate of the sample concentration. The most straightforward approach is a measurement at a single point, usually a t the signal maximum, corresponding to the peak height ( I ) ; this strategy may be adequate when the signalto-noise ratio is large. Recently, the theory and practice of peak integration or peak area measurement were investigated for improving quantitation in chromatographic applications (2-4). Since integration has the property of averaging noise while summing up the available signal, it is a simple but potentially powerful method for reducing detection limits when flicker (l/f)noise is not predominating. The effectiveness of a data processing strategy generally improves as more “a priori” knowledge is utilized in the analysis. The concept has been implemented in quantitation 0 1987 American Chemical Society
