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Anal. Chem. 1990, 62,1637-1640
Determination of Platinum in Blood by Adsorptive Voltammetry Olle Nygren,' Gary T. Vaughan,* T. Mark Florence, Gregory M. P. Morrison,2Ian M. Warner, and Leslie S. Dale Centre f o r Advanced Analytical Chemistry, CSIRO Division of Fuel Technology, Private Mail Bag 7, Menai, NSW 2234, Australia
Thls work descrlbes a sensltlve method for the determlnatlon of platlnum In blood, whlch can be used for determlnlng the natural levels of platlnum In human blood, for monltorlng patlents treated wlth piatlnm cytotoxic drugs, and for monltorlng occupatlonal exposure to these drugs and other platlnum compounds. The method Involves dry ashlng of blood samples In a muffle furnace and determlnatlon of platlnum by adsorptlve voltammetrlc (AV) measurement of the catalytic reductlon of protons by the platlnum-formazone complex. The detection llmlt for a 100-pL sample of blood Is 0.017 pg/L, wlth a recovery of 94% and a relative standard devlatlon of 7 % at a platlnum level of 1 pg/L. By uslng this method, the natural levels of platlnum In human blood were found to be In the range 0.1-2.8 pg/L (medlan = 0.6 pg/L). These results were verified by Inductively coupled plasma mass spectrometry ( ICP-MS)wlth blood prepared by wet ashlng and uslng gold as an Internal standard.
INTRODUCTION The increased medical and industrial use of platinum has led to a growing need for the determination of platinum levels in biological materials. Platinum coordination complexes such as cisplatin [cis-dichlorodiammine platinum(II)] are potent antineoplastic agents and are frequently used in the treatment of a variety of tumors. The administration of platinum drugs is often accompanied by undesirable side effects including nausea ( I ) , hearing loss ( 2 , 3 ) ,and nephrotoxicity ( 4 ) . There is also concern about the mutagenicity and potential carcinogenicity of these drugs especially in relation to occupational exposure (5, 6). Complex salts of platinum, especially chloroplatinates, are potent sensitizing agents. The allergenic effects of these complexes have resulted in high rates of occupational asthma and dermatitis in workers handling them (7). Concern about the release of toxic material from catalytic converters in motor vehicles led to attempts to establish the background levels of platinum in human tissues and body fluids before these devices were widely used. The determination of platinum in body fluids is important for controlling the dose of chemotherapeutic agents received by patients, for pharmacokinetic investigations, and for monitoring occupational exposure to these drugs. Several techniques have been used to determine platinum in body fluids or tissues after administration of platinum-containing drugs, including X-ray fluorescence (8), atomic absorption spectrometry (9-11), differential pulse polarography (11,12), neutron activation analysis (13),and high-performance liquid chromatography with electrochemical detection (14,15),with UV detection (16,17),or interfaced to an inductively coupled
* To whom correspondence may be addressed.
Present address: National Institute of Occupational Health, Chemical Unit, P.O. Box 6104,S-900 06 Umei, Sweden. 2Presentaddress: Department of Sanitary En 'neering, Chalmers University of Technology, S-41296 Goteborg, fweden.
plasma spectrometer (18). Although some of these techniques are selective for the drugs and their metabolites, none of them gives a detection limit low enough for the measurement of natural levels of platinum in body fluids. Johnson and co-workers (19, 20) measured platinum in blood, urine, hair, and feces samples from 283 volunteers. The levels were below the detection limit (31 pg/L) of the atomic absorption spectrometric technique that was used. However, composite samples of 750 mL of blood from populations in Los Angeles and Lancaster, CA, had platinum levels of 0.49 and 1.80 pg/L, respectively. A study of the platinum contents of autopsy tissues from 97 individuals found that almost half had detectable levels (>3 pg/kg) in one or more tissue samples (21).
A sensitive polarographic method for platinum was described by Zhao and Freiser (22)with a detection limit of 20 ng/L. In this method, a formazone complex of platinum is adsorbed on the surface of a dropping mercury electrode and yields a catalytic hydrogen evolution current at around -1.04 V vs Ag/AgCl. By optimizing conditions and using adsorptive preconcentration on a hanging mercury drop electrode (HMDE), van den Berg and Jacinto (23) improved the detection limit to 8 pg/L for the determination of platinum in seawater. However, the utility of this method for analysis of blood samples is limited by its extreme sensitivity to traces of organic matter. The aim of the present work was to develop a method sensitive enough to determine base-line levels of platinum in blood and other body fluids. The method should be suitable for monitoring occupational exposure to platinum compounds and, therefore, should require a minimal sample size. EXPERIMENTAL SECTION Apparatus. Polarographic measurements were made with a 646 VA Processor and 647 VA Stand (Metrohm Ltd., Herisau, Switzerland). The instrument was fitted with a HMDE, a glassy carbon counter electrode, and a Ag/AgCl (3 M KCl) reference electrode. Inductively coupled plasma mass spectrometry (ICPMS) was performed by using a VG Plasmaquad PQ2 instrument (VG Isotopes, Winsford, U.K.). Reagents and Samples. HN03,HCl, H2S04,and HCIOI were of Suprapur (Merck) or Aristar (BDH) grade. Water was purified with a Milli-Q reaeent water svstem (Millbore). Platinum standard solutions were prepared by dilution of 'a platinum atomic absorption standard solution containing 1010 mg of Pt/L in 5% HC1 (Sigma Chemicals). All other chemicals were of Suprapur or analytical grade. Blood samples were collected by venipuncture and stored in 10-mL heparinized polystyrene tubes at 4 O C . Otherwise, blood samples (25-100 pL) were obtained by fingerprick sampling. When required, erythrocytes and plasma were separated by centrifugation (700g, 10 min). For recovery studies and quantification, aliquots of pooled reference blood (collected from two individuals) were spiked with known amounts of inorganic platinum or cisplatin in saline. Gold standard solution (an internal standard for ICP-MS) was prepared by dissolving 24-kt Au in aqua regia and diluting to the required concentration with 5% (w/v) HCl. Adsorptive Voltammetric (AV) Procedure. A sample (100-500 pL) of blood, spiked blood, or a standard in saline was transferred to a silica crucible, 24 mm (i.d.) x 20 mm. Nitric acid
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1,
1990
(300 pL of 15 M) was added, and the crucible was covered with a silica watch glass and slowly evaporated to dryness on a hot plate. The sample was then ashed in a laboratory muffle furnace by using the heating regime described by Denniston et al. (24) except that the final temperature was increased to 800 " C to completely remove material that interfered with AV measurements. The sample was heated at 20 "C/min as outlined in the following scheme: 1 h at 200 "C, 30 min at 250 " C , 1 h at 350 "C, 30 min at 425 "C, and 3 h at 800 "C. When cool, the sample was dissolved in 1.5 mL of aqua regia (1 mL of 15 M HNO, and 0.5 mL of 12 M HCl), left overnight, and then taken just to dryness on a hot plate. The residue was dissolved in 600 pL of 12 M HC1 to give a final acid concentration of 0.7 M in the voltammetric cell. Water (5 mL) was added, the solution was transferred to the cell to which 0.4% (w/v) hydrazine (80 pL) and 3.2% (w/v) formaldehyde (80 pL) were added, and then water was added to give a final volume of 10 mL. The sample was stirred, deaerated for 5 min, and preelectrolyzed at -800 mV vs Ag/AgCl for between 30 and 600 s. The stirring was stopped, and after 10 s of quiescent time, stripping was recorded in a differential pulse mode with the following instrumental settings: modulation amplitude, 25 mV; pulse repetition rate, 200 me; effective scan rate, 20 mV/s. The platinum concentration was determined by using three standard additions to each sample and subtracting a blank taken through the dry ashing procedure. Inductively Coupled Plasma Mass Spectrometry Procedure. Blood was desalted by ultrafiltration to avoid ion suppression in ICP-MS by sodium chloride. Aliquots (5 mL) of blood were centrifuged to separate the plasma from the erythrocytes and buffy coat. After measuring its volume, the plasma was transfered to a Centricon-30 (30000 M , cutoff filter) microconcentration tube (Amicon, Danvers, MA) and centrifuged at 2000g for 30 min. The retentate was washed off the filter with water and made up to the volume of the original plasma. This solution was added back to the erythrocytes t o give "desalted blood". Platinum was determined in the desalted blood (200 pL) and in the ultrafiltrate by AV. Platinum in the desalted blood was also determined by ICP-MS after dry ashing. In a separate experiment, samples were wet ashed in HN03/HCL04before analysis by ICP-MS. A blood sample (1 mL) was digested with 7 M HNO, (8 mL) in a beaker covered with a watch glass on a hot plate and evaporated almost to dryness. After adding 15 M HNO, (5 mL), the sample was left overnight before being dissolved in water (10 mL) and adding 15 M HNO, (5 mL) and 72% HC104(2 mL). The solution was heated to fumes of HC104,and fuming was continued for 3 min. The sample was transferred to a 10-mL volumetric flask, diluted to volume, and analyzed by ICP-MS. A standard curve was prepared with solutions of platinum from 0.01 to 100 pg/L and containing 10 pg/L Au and 200 mg/L Na. Platinum concentrations were calculated after blank subtraction (0.02 A 0.0024 pg/L). Solutions were introduced into the ICP-MS by a concentric glass nebulizer and a peristaltic pump set to a flow rate of 0.7 mL/min. Isotopic analyses were carried out in the peak jumping mode with 15 points for each peak. The dwell time for each point was set at 10 ms with 50 sweeps per run. The instrumental conditions were as follows: rf power, 1400 W; auxiliary gas, 0.7 L/min; cool gas, 15.6 L/min. On each solution, five sequential isotope measurements were made.
RESULTS AND DISCUSSION Adsorptive Voltammetry. Adsorptive voltammetry of platinum following its complexation with formazone is extremely sensitive to sample matrix effects, especially the presence of surface-active agents, and could not be used for the determination of platinum in blood samples unless the organic matrix was destroyed. Attempts to remove matrix effects by wet ashing blood samples in various mixtures of HNO,, HC104, H2S04,HCl, and HzOz in combination with UV irradiation for 12 h were unsuccessful in producing a sample in which base-line levels of platinum could be measured by AV. Combustion of the sample in a muffle furnace a t 800 "C for 3 h was required to completely remove interferences from the residual matrix. After combustion, the platinum was dissolved in aqua regia and the sample taken
I
-0.8 -0.9 -1.0 - 1 . 1
E / V vs. Ag/AgCI Figure 1. Determination by AV of platinum in a 1OO-pL sample of Mood after dry ashing. The scans of the sample solution (curve 1)and three subsequent standard additions of platinum (20pg) were obtained after 120 s of deposition in the presence of 0.026% formaldehyde, 0.0032% hydrazine, and 0.7 M HCi (curve 2: curve 1 + 20 pg of Pt, curve 3: curve 1 + 40 pg of Pt, curve 4: curve 1 + 60 pg of Pt). to dryness on a hot plate. I t was necessary to evaporate the sample to dryness to remove residual nitrate, which interfered with subsequent voltammetric measurements. However, prolonged heating of the sample at this stage led to decreased recoveries of platinum, and samples should be removed from the hot plate as soon as the acid has evaporated. Dilute H 8 0 4 has been recommended as the supporting electrolyte for AV measurements of platinum in the presence of formazone (22, 23) because it gives higher peak currents than HC1. The reason for this difference is not clear, but one suggestion has been the formation of a platinum(I1) complex with chloride (22). This complex formation cannot, however, be the dominating factor, since the determination of platinum in seawater, having a chloride concentration around 0.55 M, showed sensitivity similar to that in HzS04 (23). In our application, although peak currents in HCl were lower than those in H 8 0 4 , better reproducibility was obtained in HC1. A deposition time of 120 s was used for the measurement of background platinum levels in blood. However, when this deposition time was used for levels above 300 bg/L, the evolution of bubbles of Hz interfered with the AV measurements. For the measurement of higher concentrations, the deposition time had to be reduced. Although dry ashing of the sample removed most of the matrix effects, the peak heights of samples were below those obtained for platinum standards. In addition, the peak potential was found to occur between -920 and -980 mV, depending on the degree of residual matrix effects. As the peak potential shifted toward more positive values, peak heights for equivalent platinum concentrations were progressively lowered. Therefore, standard additions of platinum were required for an accurate measurement of the platinum concentrations (a typical voltammogram is shown in Figure 1). The calculation of the platinum content of samples determined by AV (Tables I-V) was carried out after
ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1 , 1990
Table I. Recovery of Platinum Added to Blood before Dry Ashing
added compd Pt concn, rg/L inorganic Pt
cisplatin
0.1 0.5 1.0 2.0 5.0 0.1 0.5 1.0
2.0 5.0
recovery,
Table 111. Determination of Blood Platinum by ICP-MS after Wet Ashing and AV after Dry Ashing
%
RSD, %
n
sample
88 89 94 96 95 a2 83 85 89 92
10 8 7 7 7
6 5 5 4 3 6 6 10 5 4
A B
10 10 8 7
8
Pt concn, pg/L sample
AV
ICP-MS
AV (untreated blood)
A B A + 1 rg/L Pt
2.6 0.7 3.6
2.5 0.8 3.5
2.8 0.8 3.8
" Reconstituted blood samples, without the plasma ultrafiltrate,
were analyzed in parallel by AV and ICP-MS. A sample of untreated blood was also analyzed by AV.
Inductively Coupled Plasma Mass Spectrometry. Procedures based on ICP-MS were developed to verify the measurements of platinum in blood by AV. In order to avoid the ion suppression in ICP-MS by sodium chloride, which is present mainly in the blood plasma, the plasma was separated from the erythrocytes by centrifugation and then desalted by ultrafiltration. The erythrocytes and plasma retentate were combined, ashed in a muffle furnace, and analyzed by ICP-MS and AV. In addition, a sample of blood was analyzed by AV without the ultrafiltration step. The identification of platinum in the samples was obtained by ICP-MS; the relative intensities of the ions at mass-to-charge ratios of 194, 195, and 196 were similar to those of platinum standards. The results showed good agreement between AV and ICP-MS measurements (Table 11). These results verify that the measured peak in AV is highly correlated to the platinum content of the samples, and it can therefore be assumed that the peak observed is due to reduction of protons catalyzed by the platinum-formazone complex. Slightly higher values were obtained for the platinum concentration in the untreated blood samples, probably because a small fraction of the total platinum content in the blood was present in the plasma ultrafiltrate. AV measurement of the saved ultrafiitrates confirmed these findings (mean = 0.08 pg of P t / L , n = 3). To verify the accuracy of the dry ashing procedure, samples in which platinum was measured by AV after dry ashing were compared to samples in which platinum was measured by ICP-MS after wet ashing. The problem of sodium interference
1.71 f 0.04 0.61 f 0.03 0.47 f 0.01 0.85 f 0.02 2.33 f 0.03
D E
1.66 f 0.11 0.58 f 0.03 0.51 f 0.05 0.74 f 0.06 2.24 f 0.16
Table IV. Concentration of Platinum in Blood
Pt concn, rg/L M F M
+F
n
median
mean
SD
range
12
0.64 0.54
0.84 0.51 0.73
0.68 0.29 0.60
0.2-2.8 0.1-0.9 0.1-2.8
6 18
0.59
Table V. Platinum Content of Other Biological Samples
sample
n
median Pt concn, r g J L
range
urine scalp hair
11 8 1 1 1
0.11 3.02" 0.02
0.044.61 0.33-8.82"
sweat saliva fingernails fig
subtraction of a blank, which had gone through the same dry ashing procedure as the samples. The mean platinum concentration of the blank was 0.001 15 pg/L (SD = 0.000058). The recovery of platinum by the procedure described above was determined by spiking blood with different amounts of inorganic platinum or cisplatin (Table I). The recovery of both the inorganic platinum and the cisplatin spikes was greater than 80% and close to 90% at natural levels of inorganic platinum in blood. The relative standard deviation (RSD) of the AV procedure was determined by analyzing 10 aliquots of the same sample and was found to be 7% and 8% at 1 pg/L for inorganic platinum and cisplatin, respectively.
Pt concn, rg/L ICP-MS AV
C
sex
Table 11. Comparison of AV and ICP-MS Determinations of Platinum"
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0.07 19.0"
of Pt/kg of fresh weight.
was overcome by matrix matching the platinum standard solutions to the blood samples by the addition of 200 mg/L sodium chloride. Gold (10 pg/L) was added as an internal standard to the blood before wet ashing, and the platinum content was calculated from the ratio 1g5Pt/197A~. Once again, there was good agreement between the ICP-MS and AV measurements (Table 111), indicating that no significant contamination or losses of platinum in the samples during the dry ashing procedure had occurred. Base-Line Levels of Platinum in Blood. The background concentrations of platinum in the blood of 18 volunteers were measured by AV (Table IV). These subjects, who were residents of Sydney, Australia, were not occupationally exposed to platinum compounds and had not been exposed to platinum drugs. There was no appreciable difference in the platinum content of the blood of these individuals whether it was obtained by fingerprick or venipuncture. Platinum concentrations were in the range 0.1-2.8 pg/L with a median value of 0.6 pg/L. The two values (0.49 and 1.80 pg/L) reported by Johnson et al. (19,20) for composite samples from populations in California lie within this range. The concentrations of platinum in blood are of the same order as elements of much higher crustal abundance, and we are currently investigating the contribution of diet to these levels. Preliminary investigations have demonstrated that the technique can be used for the determination of total platinum of blood plasma, urine, hair, fingernails, saliva, and animal and plant tissue samples (Table V).
CONCLUSIONS The catalytic reduction of protons by the platinumformazone complex provides a sensitive method for the determination of platinum in blood by AV. For biological materials, complete combustion of the organic matrix by ashing samples in a muffle furnace is required before the AV method can be used to measure the total platinum content of these materials. In the present work, two methods are described for the determination of platinum in blood. Both ICP-MS and AV methods can be used to determine platinum at
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background levels in biological materials, but AV has many advantages for the routine analysis of blood samples. The AV method is more sensitive, uses less sample, is less expensive in terms of equipment and reagent costs, and is not affected by the salt content of the sample. After complete combustion of the organic matrix, it can be applied to a wide range of biological samples. Registry No. Pt, 7440-06-4.
LITERATURE CITED (1) See-Lasiey, K.; Ignoffo, R . J. Manual of oncology therapeutics; C. V.
Mosby Co.: London, 1981. (2) Evans, W. E.; Yee, 0. C.; Crom, W. R.; Pratt. C. 8.; Green, A. A. Bug Intell. Clin. pherm. 1982, 76, 448-458. (3) Loehrer, P. J.; Einhorn, L. H. Ann. Intern. Med. 1984, 700, 704-713. (4) Safirstein, R.; Winston, J.; Goldstein, M.; Mod, D.; Dikman. S., Guttenpian, J. Am. J. Kidney 01s. 1986,8 . 356-367. (5) IARC. Some Antineoplastic and Immunosuppressive Agents. IARC Monographs on the Evaluation of fhe Carcinogenic Risk of Chemicals to Humans; IARC: Lyon, France, 1981; Vol. 26. (6) Vaughn, M. C.; Christensen. W. D. Am. Ind. Hyg. Assoc. J . 1985, 4 6 , B8-BI8. (7) Dhara, S. C. Proceedings of the 7th International Precious Metals I n stitute Conference, San Francisco, 1983; Reese, D. G., Ed.; Pergamon: Toronto, 1984; pp 35-55. ( 8 ) Seifert, W. E., Jr.; Stewart, D. J.; Benjamin, R. S.;Caprioli, R. M. Cancer Chemofher. Pharmacol. 1983, 7 7 , 120-123. (9) Vermorken, J. B.; Van der ViJgh, W. J.; Klein, I.; Gall, H. E.; Van Groeningen, C. J.; Hart, G. A. M.; Pinedo, H. M. Clin. Pharmacol. Ther. (Sr. Louis) 1988,3 9 , 134-144. (IO) Preisner, D.; Sternson, L. A.; Repta, A. J. Anal. Len. 1981, 74, 1255- 1268
(11) Shearen, P.; Smyth, M. R. Ana&st(London) 1988, 773, 609-612. (12) Brabec, V.; Vr&na, 0.;Kieinwachter, V. Collect. Czech. Chem. Cornmun. 1983,48 2903-2908. (13) Kucera, J.; Drobnik, J. J. Radhnal. Chem. 1982, 7 5 , 71-80. (14) Kruil, I. S.; Ding, X.-D.; Baverman, S.; Selavka, C.; Hochberg, F.; Sternson, L. A. J. Chromatogr. Sci. 1983,27, 168-173. (15) Parsons, P. J.; Morrison, P. F.; LeRoy, A. F. J. Chromatogr. 1987, 385, 323-335. (16) Drummer, 0.H.; Proudfoot, A.; Howes, L.; Louis, W. J. Clin. Chim. Acta 1984, 136, 65-74. (17) Marsh, K. C.; Sternson, L. A.; Repta, A. J. Anal. Chem. 1984, 5 6 , 491-497. (18) De Waal, W. A. J.; Maessen, F. J. M.; Kraak, J. C. J. Chromatogr. 1987,407, 253-272. (19) Johnson, D. E.; Tillery, J. B.; Prevost, R. J. Environ. Health Perspect. 1975, 72, 27-33. (20) Johnson; D. E.; Prevost, R. J.; Tillery, J. B.; Camann, D. E.; HosenfeM, J. M. EPA Report 60011-761019; EPA: Washington, DC, 1976. (21) Duffieid, F. V. P.; Yoakum, A.; Bumgarner, J.; Moran. J. Environ. Health Perspect. 1976, 75, 131-134. (22) Zhao. 2.; Freiser, H. Anal. Chem. 1986,5 8 , 1498-1501. (23) Van den Berg, C. M. G.; Jacinto, G. S. Anal. Chim. Acta 1988,277, 129- 139. (24) Denniston, M. L.; Sternson, L. A,; Repta, A. J. Anal. Len. 1981, 74, 451-462.
RECEIVED for review December 27,1989. Accepted March 22, 1990. The support (to Olle Nygren) of the National Swedish Institute of Occupational Health and the Swedish Work Health Fund is gratefully acknowledged. Greg Morrison acknowledges the receipt of a fellowship awarded by the Natural Environment Research Council, U.K.
Spectroelectrochemical Characteristics of the Reticulated Vitreous Carbon Electrode Janet Weiss Sorrels and Howard D. Dewald* Department of Chemistry, Ohio University, Athens, Ohio 45701-2979
The spectroelectrochemlcal characteristics of retlcuiated vitreous carbon optlcally transparent electrodes (RVCOTE’s) are described with respect to both thickness and porosity of the RVC. RVC-OlE’s with thicknesses between 0.5 and 3.5 mm and porosities of 100,80,60,45, and 30 pores per llnear Inch are examined. Cyclic voltammetry and chronocoulometry on sdutions of potassium ferrlcyanlde indicate a trend from thin-layer behavlor toward semWnflnHe dmuslon behavior as the sire of the pores increases. Chronoabsorptometry and spectropotentloetatlcexpertments uslng solutions of o-tolidine give comparable results for all thlcknesses and poroslties studied, resulting In values of n = 1.94and E o ’ = +OB39 V vs Ag/AgCi. Therefore, thicker electrodes with larger pore sires can be employed In experlments in which weakly absorbing species are monitored.
INTRODUCTION Reticulated vitreous carbon (RVC), a porous, vitreous carbon foam material, is well-suited for various electrochemical applications (1). It was first utilized in UV-visible transmission thin-layer spectroelectrochemistry by Norvell and Mamantov (2) as an electrode material in the construction
* Author to whom correspondence should be addressed.
of an optically transparent electrode (OTE). RVC is available in several porosities, ranging from 10 to 100 pores per linear inch (ppi), and can be easily sliced into various shapes and thicknesses. Nearly all spectroelectrochemical applications of RVC-OTEs, however, use solely the 100 ppi grade of RVC with 0.5 to 1 mm thicknesses (2-7), even though the optical characteristics and surface-area-to-volume ratio of the electrode can be controlled by appropriate choice of thickness and porosity of the RVC (2). Hobart and co-workers (8)report that the light transmittance of a porous metal foam (PMF) electrode is dependent upon both electrode thickness and pore size. They also indicate that larger pore sizes result in increased electrolysis times for these electrodes, which are similar in structure and optical properties to RVC. In another application the effects of pore size and thickness of 45 and 100 ppi RVC substrates on the distribution of zinc in Zn-Br2 batteries were reported (9, 10). However, the effects of different thicknesses and porosities of RVC on the electrochemical and spectroelectrochemical characteristics of RVC-OTE’s have not been previously reported. This paper evaluates the spectroelectrochemical characteristics of RVC-OTE’s with porosities ranging from 30 to 100 ppi and with thicknesses varying between 0.5 and 3.5 mm, with respect to their cyclic voltammetric, chronocoulometric, spectropotentiostatic, and chronoabsorptometric behavior in
0003-2700/90/0362-1640$02.50/0 0 1990 American Chemical Society
