A M . Chem. 1986, 58, 2099-2101 Vt 1
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20
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ACKNOWLEDGMENT One of us (C.G.) wishes to thank J. F. Muller for giving free access to the FTMS-2000 installed in his laboratory during the time left free from the normal research activity.
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142 156 252
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15
allowing or not requiring a statistical equilibration of the ions between source and analyzer.
MASS
12)
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2099
LITERATURE CITED
10
c
Figure 4. Plot of the trapping frequencies (kHz) vs. the square root of the ratio between trappins voltage 01) and ion mass (am). The bns observed were generated by &O, Ne,Ar, benzene, methyl lode, ethyl lodMe,and benzo[e]pyrene. The slope of the line is 37.98 amu1’*. V-1’2.kHz, and the correlation coefficient Is 0.998.
2)), the magnetron motion, and the space charge created in the source (9, 10). It is evident that it is possible to change opportunely the opening time of the conductance limit during the spectral acquisition in order to have a complete, high-resolution spectrum in the analyzer side with minimal mass discrimination with respect to the population originally in the source. In summary, a method is presented for the passage in high yield of selected ions between the two cells of a “double-cell FT/ICR”. This method can be applied in all experiments not
(1) . . Settine. R. L.; Kinsinger. - J. A.; Qhaderi, S. Eur. Srnctrosc. News 1985, 58, 16-18. (2) Qhaderi, S.; Littiejohn, D. P. Contribution ROC 12, ASMS Conference; San Dwo, CA, June 1985. (3) Settine, R. L.; Ghaderi, S.; Littiejohn, D. P. 1985 Pittsburgh Conference, New Orleans, LA, Feb 25-March 1, Contribution 1286. (4) Cody, R. 6.. submitted for publicatlon in Anal. Chem. (5) Schwlnberg, P. 6.: Van Dyck, R. S., Jr.; Dehmelt, H. 0. Phys. Rev. Lett. 1981, 47, 1679. (6) Sharp, T. E.; Eyier. J. R.; Li, E. Inf. J . Mass Specfrom. Ion Processes 1972, 9 , 421. (7) Hunter, R. L.; Sherman, M. 0.; McIver, R. T., Jr. Int. J . Mass Spechorn. Ion Processes 1983, 50, 259. (8) Dunbar, R. C.; Chen, J. H.; Hays, J. D. I n f . J . Mass Specfrom. Ion Processes 1984, 57, 39. (9) Jeffries. J. B.; Barlow, S. E.; Dunn, 0 . H. I n f . J . Mass Specfrom. Ion Processes 1983, 5 4 , 169. (IO) Oiancaspro, C.; Verdun, F. R., unpublished results.
’
Present address: Department of Chemistry, The Ohio State University, Columbus, OH 43210.
Carlo Giancaspro* Istituto di Chimica Nucleare del C.N.R. Area della Ricerca di Roma C.P. 10,00016 Monterotondo Stazione, Rome, Italy Francis R. Verdun’ LSMCL-UniversitR de Metz B.P. 794, 57012 Metz Cedex, France RECEIVED for review September 13,1985. Resubmitted January 27, 1986. Accepted March 7, 1986.
AIDS FOR ANALYTICAL CHEMISTS Reverse Pulse Voltammetry at a Solid Electrode Zenon J. Karpinski
Department of Chemistry, University of Warsaw, Pasteura 1 , 02-093 Warsaw, Poland Reverse-pulse polarography (RPP) has been shown recently to be a valuable method of studying complicated electrode processes (1-3) as well as a useful analytical technique (4). Reverse pulse experiments usually have been performed at DME or static mercury drop electrodes (SMDE) (for the latter experiment the term reverse-pulse voltammetry, RPV, is used). In both these cases the RP potential program is successively applied to a new mercury drop, thus assuring constant initial concentration boundary conditions. This requirement of constant initial conditions is difficult to fulfill at solid electrodes, and with RPV at stationary electrodes only qualitative results could be obtained (5). It recently has been found, however, that RPV a t solid electrodes can also give quantitative results, useful in studies of complicated electrode processes (6-8). In the latter experiments, constant initial conditions a t the working electrode surface were always established before an application of the RPV potential program. This was accomplished by rotating the working electrode 0003-2700/66/0358-2099$01.50/0
under computer control for the first 1 s of 10-s delay time, during which the electrode was kept a t a potential chosen so that no Faradaic process occurred. In this work a simple method of using the drop knocker output of a polarograph to control reestablishing of constant initial conditions at a solid electrode surface is described. This method allows the use of any pulse polarograph and a rotating disk electrode in RPV experiments. EXPERIMENTAL SECTION The potential program in a RPV experiment is the usual pulse train of normal pulse polarography (I),thus any pulse polarograph may be used (in this work a Telpod PP-04polarograph was employed). For RPV with a solid electrode, additional delay time ( t d ) needed to be included in the waiting period between pulses (Figure la). The working electrode was disconnected from the potentiostat during the delay time and was rotated over the initial part of this period (tJ. This procedure reestablished the initial concentration boundary conditions (vide infra). It was executed 0 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986 I
I
a
l
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1 I
. .1.
lf
b
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05
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00 IL
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Figure 1. (a)Scheme of the program appUed to the working electrode in RPV experiment: t,, delay time over which the electrode is dis-
connected from the potentiostat; t,, time of the electrode rotation, 7,
period of the potential progam application (equivalent to the drop time in RPP); and t,, the pulse duratbn. (b) Normal (l’, 2’, 3’) and reverse pulse (1, 2,3),and 1”) vokammograms for 1 catechol in acetate buffer pH 6. E , and pulse direction are indicated by arrows: t , = 1, t , 7 5, T = 2 s; t,, (1, l’, 1”) 30,(2, 2‘), 50, (3, 3’) 104 ms; (1”)
statlonary electrode.
by two timing relays controlled by the drop knocker output of the polarograph. The low-voltage signal from the polarograph activated both relays. One of them turned on the working electrode rotator, while the other one disconnected this electrode from the potentiostat. The use of two timing relays allowed an independent variation oft, and td. The potential program was applied to the working electrode for only a part of the “drop time” set at the polarograph, with the latter time being equal ( t d + 7 ) (Figure la). Weighed samples of catechol (Merck) and DOPA ((3,4-dihydroaryphenyl)alfinine, Reanal) were added to deaerated solutions prepared from reagent grade chemicals and triply distilled water. Glassy carbon RDE (Laboratorni Pristroje) served as the working electrode. A saturated calomel electrode (NaCl) was the reference electrode, and all potentials are referred to this electrode. All experiments were performed at 25 O C .
RESULTS AND DISCUSSION Normal and reverse pulse voltammograms recorded for catechol solution a t varied pulse widths are shown in Figure lb. Differences in half-wave potentials between normal and reverse pulse waves indicate quasi-reversible electron transfer, and small maxima evident at 30-ms pulse time may result from weak substrate adsorption (9-1 1). Nevertheless, the limiting currents (iNP, idc and iRp) agree well with the diffusion-controlled current ratios for double-potential-step chronoamperometry (1, 12), thus showing 1:l stoichiometry of the electrode reaction and a stability of the catechol oxidation product on the time scale of the experiment. Most notably, the well-defied diffusion plateau as of the dc portion of the R P waves gives the same it,1/2 product as the NP limiting currents (t, = 7 for idc and t , = t, for iw), thus showing that the simple procedure indeed reestablishes constant initial conditions at the electrode surface and does not cause a convection affecting the diffusion current. This criterion of
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Figwe 2. Normal (4) and reverse pulse ( 1 , 2, 3) voltammograms for 1 mM DOPA in acetate buffer pH 6: t , = 1, t , = 5 s, t , = 50 ms; 7, (1, 3) 1, (2)5 s;(1) stationary electrode.
a constancy of for i N p and idc can be used to optimize t , and td. The former parameter depends on a rotation rate and an inertia of RDE, and for the electrode used, a t 1600 rpm, could not be made shorter than 1 s. Under these conditions, a t t , = 1 s and T = 5 s, deviations of idc from the constant product, and usually also fluctuations of the dc current, were observed for t d shorter than 4 s. The RPV curve recorded at a stationary (not rotated) electrode did not exhibit a plateau of the dc current (Figure Ib, plot 1”). The decreasing anodic current reflecting a gradual depletion of the solution near the electrode surface was observed. Voltammograms shown in Figure 2 illustrate advantages of RPV in studies of electrode processes involving homogenous chemical reactions coupled to electron transfer steps. Two cathodic RP waves indicate the formation of two electroactive products of DOPA oxidation, and the dependence of RP limiting currents on the generation time (7- t,) clearly shows a chemical reaction of the primary product of DOPA oxidation. This reaction could be identified as the ring closure of dopaquinone (13-16).Additionally, and increase of for idc at longer 7 indicates a mechanism involving electron transfer steps following the chemical reaction, thus confiiing the reaction scheme inferred from CV (15) and chronoamperometric (16) results. This mechanism involves, in addition to the EC steps, the homogenous oxidation of leucodopachrome by dopaquinone. Besides such qualitative conclusions from RPV results, rivaling diagnostic capabilities of CV, RPV limiting currents, independent of electron transfer rate (seldom reversible at solid electrodes), can be simply analyzed quantitatively. The half-implicit (Crank-Nicolson method (17)) finite difference simulation of RPV limiting currents for the mechanism with rate determining dopaquinone cyclization and fast redox equilibrium ( K , = yielded for k = 0.25 s-’ the values marked X in Figure 2. The cyclization rate constant, thus obtained, was close to the value inferred from single-step chronoamperometric results (16). On the contrary, the RP voltammogram obtained at stationary GCE (Figure 2, plot I) only qualitatively indicates the chemical transformation of the DOPA oxidation product. Difficulties
Anal. Chem. 1986, 58, 2101-2103
in defining time of the experiment impede quantitative analysis of the results. Previous attempts to emulate the conditions of polarographic techniques in applications of pulse methods a t solid electrodes involved slow continuous rotation of the working electrode (18-20) or stirring of the solution (21). These methods can be used only in forward potential scan experiments, because they cause the convection-disturbing concentration gradients during the relatively long generation step of reverse pulse experiments. On the other hand, the proposed method of restoring the initial conditions a t the working electrode surface between successive current measurements, tested in normal and reverse pulse experiments, can be used also in applications of other pulse techniques at solid electrodes. A current response during short pulses is generally less affected by the convection than a dc current in RPV.
ACKNOWLEDGMENT I acknowledge the helpful comments of Zenon Kublik and the assistance of Janusz J. Borodzinski. Registry No. DOPA, 59-92-7; catechol, 120-80-9; carbon, 7440-44-0; dopaquinone, 25520-73-4. LITERATURE CITED (1) Osteryoung. J.; Kirowa-Eisner, E. Anal. Chem. 1080, 5 2 , 62-66. (2) Osteryoung. J.; Talmor, D.; Hermolin, J.; Klrowa-Eisner, E. J. f h y s . Chem. 1981, 8 5 , 285-289.
2101
KashtCKaplan, S.;Hermolin, J.; Kirowa-Eisner, E. J. Electrochem. SOC. 1981, 128, 802-810. Wojciechowskl, M.; Osteryoung. J. Anal. Chem. 1085, 5 7 , 927-933. Kulkarni, C. L.; Scheer, E. J.; Rustling, J. J. Electroanel. Chem. 1082, 140. 57-74. Karpinski, 2. J.; Osteryoung, R. A. J. flectroanal. Chem. 1984, 164, 28 1-298. Karpinski, 2. J.; Osteryoung, R. A. J. ffectroanal. Chem. 1084, 178, 281-294. Karpinski, 2. J.; Nanjundiah, Ch.; Osteryoung, R. A. Inorg. Chem. 1084, 2 3 , 3358-3364. Barker, G. C.; Bolzan, J. A. 2. Anal. Chem. 1066, 216, 215-238. Lovric, M. J. Electroanal. Chem. 1084, 170, 143-173. Lovric, M. J. flectroanal. Chem. 1084, 187, 35-49. Schwarz, W. M.; Shain, I , J. Phys. Chem. 1985, 6 9 , 30-40. Raper, H. S. Blochem. J. 1927, 2 1 , 89-96. Mason. H. S. J. BIOI. Chem. 1048. 172. 83-99. Brun, A.;Rossett, R. J . Electroanai. Chem. 1974, 49, 287-296. Young, T. E.; Griswold, J. R.; Hulbert, M. H. J. Org. Chem. 1974, 3 9 , 1980- 1982. Britz, D. Digital Simulation In Electrochemistry; Springer-Verlag: Berlin, Heidelberg, New York, 1981; Chapter 5. Myers, D. J.; Osteryoung, . - R. A.; Osteryoung, . - J. Anal. Chem. 1074, 46, 2089-2092. Stojek, 2.; Llnga, H.; Osteryoung, R. A. J. flectroanal. Chem. 1081, 1. 1 .-9 ,. 365-370. - - - - . -. Thornton, D. C.; Corby, K, T.; Spendel, V. A,; Jordan, J.; Robbat, A., Jr.; Rutstom, D. J.; Gross, M.; Ritzel, C. Anal. Chem. 1985, 5 7 , 150-1 55. Brumleve, T. R.; Osteryoung, R. A.; Osteryoung, J. Anal. Chem. 1982, 5 4 . 782-787.
RECEIVED for review December 23,1985. Accepted March 20, 1986. This work was supported by the MR 1-11Research Project.
Carbon Paste Coated Wire Selective Electrode for Nitrate Ion Yong-Keun Lee,* Jung-Tae Park, and Chang-Kue Kim Department of Chemistry, College of Science, Yonsei University, Seoul 120,Korea
Kyu-Ja Whang Department of Manufacturing Pharmacy, College of Pharmacy, Sookmyung Women’s University, Seoul 140,Korea An increase in the use of ion-selective electrodes as an analytical tool has been observed in recent years. The increased interests in ion-selective electrodes has led to the development of new sensor materials that show selectivity for a variety of anions and cations and new methods for the construction of electrodes from these materials. Davies, Moody, and Thomas incorporated a commercially available liquid ion exchanger in a poly(viny1 chloride) (PVC) matrix to prepare a nitrate ion selective electrode (1). Coetzee and Freiser (2) have prepared PVC-CWE (CWE, coated were electrodes) to determine various anions by mixing a liquid anion selective ion exchanger with PVC. Kneebone and Freiser coated a platinum wire with a nitrate ion selective liquid ion exchanger in a poly(methy1 metacrylate) (PMMA) and used the electrode to determine nitrogen oxides in ambient air (3). Suzuki et al. constructed nitrate-CWE utilizing epoxy resin as a supporting material (4).These CWE’s have limited relative response characteristics, selectivity, and pH range. In order to compensate for the above limitations, this study deals with the optimum condition for the construction of improved nitrate-CWE in which plasticizer was added. But the result was far from satisfactory. Ansaldi and Epstein prepared a calcium selective electrode b y coating a graphite rod with a calcium exchanger in PVC (5). Mesaric and Dahmen constructed carbon paste electrodes for halides and silver(1) ions (6). 0003-2700/86/0358-2101$01.50/0
In this paper the behavior of a carbon paste coated wire nitrate ion selective electrode (carbon nitrate-CWE), which was prepared by coating a copper wire with the nitrate ion selective electroactive paste, was described. The electroactive paste was made from graphite powder, epoxy resin, liquid ion exchanger, and plasticizer. The advantages of using the carbon nitrate-CWE instead of the commercial nitrate ion selective electrode were ease of construction, increased portability, sturdiness, and economy.
EXPERIMENTAL SECTION Apparatus. All electrode potentials and pH measurements were made with an Orion microprocessor ionanalyzer (Model901)
using an Orion double junction electrode (Model 90-02) as the reference electrode. Resistance measurements of polymer membrane were measured with a Hewlett-Packard high resistance meter (Model 4392 A). The glass cell thermostat was maintained at 25 f 0.5 OC by means of Haake constant temperature circurator (Model T 31). Materials and Reagents. Aliquat 336s (tricaprylmethylammonium chloride), triethylenetetramine (TETA), tetrahydrofuran (THF), and dioctyl phthalate (DOP) were obtained from Tokyo Kasei Kogyo Co., Ltd. Epoxy resin (Bisphenol A, Shell Co.) and amorphous graphite powder (200 mesh, Pyungtaick Mining) were also used. Unless otherwise noted, all reagents used in this study were of analytical reagent grade. Ion Exchanger. A 0.4-g portion of Aliquat 336s was dissolved in 0.6 g of 1-decanol and was shaken with the same ratio of 1.0 M KNOBto affect the exchange of NO3- for C1- in Aliquat 336s 0 1986 American Chemical Society
