147
Anal. Chem. 1992, 6 4 , 147-151
(7) Katsu. T.; Kanamitsu, M.; Hirota, T.; Tasaka, K.; Fujita. Y. Chem. Pharm. Bull. 1988, 3 4 , 3968-3970. (8) Yu, B. Blosensors 1989, 4 , 373-380. (9) Walters, R. R.; Johnson, P. A.; Buck, R. P. Anal. Chem. 1980, 52. 1684- 1690. (10) Kovach, P. M.; Meyerhoff, M. E. Anal. Chem. 1982, 5 4 , 217-220. (11) Tatsuma, T.; Watanabe, T. Anal. Chem. 1991, 6 3 , 1580-1585. (12) Smit, M. H.; Cass, A. E. G. Anal. Chem. 1990, 62, 2429-2436. (13) Tatsuma, T.; Watanabe, T. Unpublished results, Tokyo, 1990. 114) . . Ader. F. I n The PorDhvrins: DolDhin, D.. Ed.: Academic Press: New York, 1978; Vol. 3. p p 167-209: (15) Lee, W. A.; Bruice, T. C. J. Am. Chem. Soc. 1985, 107, 513-514. (16) Rougee, M.; Brault. D. Blochemlstry 1975, 14, 4100-4106. (17) Whlte, D. K.; Cannon, J. B.; Taylor, T. G. J. Am, C M m . sot, 1979, 101, 2443-2454.
(18) Ellbeck, W. J.; West, M. S. J. Chem. Soc., DaRon Trans. 1978. 274-278. (19) Dlctbnary of Organic Compounds, 5th ed.; Chapman and Hall: New York, 1982; Vol. 3. (20) Handbook of Bioche”try, 2nd ed.; Sober, H. A., Ed.; The Chemical Rubber Publishing Co.: Cleveland, OH, 1970. (21) Hofmelster, F. Arch. Exp. Pattwl. Pharmakol. 1888, 2 4 , 247-260. (22) SchuWss, P.; Ammann, D.; Krautler, 6.; Caderas, C.; Stepanek, R.; Simon, W. Anal. Chem. 1985, 57, 1397-1401.
RECEIVEDfor review June 10, 1991. Accepted October 24, 1991. This work Was supported in part by the Chemical Materials Research and Development Foundation.
Determination of Ascorbic Acid Using an Organic Conducting Salt Electrode Ulrich Korell and R. Bruce Lennox*
Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, PQ H3A 2K6 Canada
An organic conducting salt electrode (based on tetrathiafuivaiene-p -tetracyanoqulnodimethane) was used for the amperometric determination of ascorbate levels in aqueous samples. The current/concentration relationship was linear up to approximately 50 mM ascorbate at -100 mV (AgIAgCi). At more oxidizing potentials (-75 and -50 mV), the signals were larger, but the linear dependence evolved into a square root dependence at approximately 2.5 mM ascorbate. Steady-state signals were obtained within 10 8 . Optimum results were obtained between pH 7.5 and 9.0 and at high buffer concentrations. The lTF-TCNO electrode was used to determine the ascorbate content of apple Juice. The data were in reasonable agreement with results from a spectrophotometric-based commercial test kit. Neither adsorption nor background reductant interferences were observed uslng a membrane electrode. Sulfite and dopamine are not eiectrooxidized by the electrode at -75 mV and therefore are not interferents.
INTRODUCTION L-Ascorbic acid (vitamin C) occurs naturally in many foodstuffs (fruits, vegetables, dairy products, meat, etc.) and is frequently added to processed foodstuffs as an antioxidant. Determination of ascorbate levels in these matrices is relevant since they are an indicator of freshness ( I ) . Measurement of ascorbate levels in clinical samples (urine, blood, etc.) is also of interest (2,3)as ascorbate concentration is a useful indicator of several pathological states (4). An analytical problem is that ascorbate constitutes a strong “background” when levels of other reductants of interest (sulfite in foodstuffs, (5, 6), catecholamines in brain tissue ( 2 , 3 ) ,oxalate in urine (7-9)) are measured. There is therefore a need to quantify ascorbate levels selectively in the presence of other species so that subtraction from the total signal may be affected. Ascorbate is a moderately strong reducing agent in aqueous media ( E l i z= -0.06 V vs Ag/AgCl at pH 5 ( I O ) ) , and most *To whom correspondence may be addressed. 0003-2700/92/0364-0147$03.00/0
analytical methologies exploit this property. For example, spectrophotometric or potentiometric determination of a dye (2,6-dichlorophenol-indophenol( 1I, 12)),which is converted to the reduced state by ascorbate, is commonly used but requires correction factors for cooxidizable species present in the matrix. The analytical selectivity is greatly improved by using the oxidoreductase ascorbate oxidase (EC 1.10.3.3) as a selective oxidative catalyst for ascorbate. This enzyme catalyzes the oxidation of ascorbate according to the equation L-ascorbate
-
+ Y2O2
L-dehydroascorbic acid
+ H20 (1)
Background levels of reductants are readily ascertained by determination of both total reductant levels (using dye methologies for example) and ascorbate levels alone. This approach is used in packed-bed enzyme reactor flow injection systems with sample-splittingcapabilities (13). An alternative approach to utilizing the intrinsic selectivity of ascorbate oxidase is to couple the enzyme to the 02-sensingcapability of a Pt electrode, i.e. to a Clark oxygen electrode (14). From eq 1, it is evident that the loss of aqueous Oz concentration is proportional to the amount of ascorbate consumed enzymatically. This method is relatively simple, as it employs one electrode only, but difficulties arise for two reasons. Firstly, the strongly reducing potential of the electrode ( ~ - 0 . 9V vs Ag/AgCl, (15))may lead to coreduction of background oxidants. Secondly, nonenzymatically-controlled variations in O2 levels can lead to variations in signal as per eq 1. In this paper we describe the use of an organic metal electrode, TTF-TCNQ (tetrathiafulvalene-tetracyano-pquinodimethane) to determine the concentration of ascorbate. The ?TF-TCNQ electrode offers advantages over other direct electrochemical techniques (3) in that the low operating potentials required for oxidation of ascorbate may in some cases obviate interference problems from other reductants present in the analysis matrix. We observe good analytical figures of merit at low potentials (-100 mV vs Ag/AlCl) in addition to low background currents, rapid response times, and freedom from dopamine and sulfite interferences. Oxidation of ascorbate at electrodes made from TTFTCNQ has been reported recently (I6-I8), but mechanistic 0 1992 American Chemical Society
148
ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992
aspects were emphasized rather than analytical applications. Furthermore those electrodes were poised at +270 mV (vs Ag/AgCl) in 0.5 M phosphate buffer prior to use. Under these conditions, TTF+ is removed from the electrode surface, as TTF-phosphate readily dissolves in water (18, 19). Hence pretreated electrodes probably do not contain TTF and TCNQ in a 1:l ratio at the surface.
+10 pA
1
Figure l a ) Platinum
J
dopamine ascorbate
EXPERIMENTAL SECTION TTF-TCNQ. TTF-TCNQ was prepared according to the procedure described by Ferraris et al. (20). TTF and TCNQ were obtained from Aldrich (Milwaukee, WI) and were used as received. Elemental analysis of the material used in these studies confirmed the 1:l stoichiometry of the salt. The TTF-TCNQ was thoroughly mixed with silicone oil (Aldrich high-temperature silicone oil, d = 1.050g/cm3) in a 1.01.6 ratio (by weight). This organic conducting salt paste electrode has been found to be an excellent, reproducible material for electrode fabrication by us (21) and others (22). The paste was stored at 4 "C and protected from light and air when not in use. Other Chemicals. L-Ascorbic acid, Anal& (BDH, Montreal, Canada), sodium ascorbate (99+ %, Aldrich), dopamine hydrochloride (Regis, Morton Grove, IL), sodium sulfite anhydrate (98%, Anachemia, Montreal, Canada), Tris-HC1 (reagent grade, Sigma, St. Louis, MO), n i s (basic form, ultrapure grade, Aldrich), sodium pyrophosphate decahydrate (ACS reagent, Aldrich), sodium hydroxide solution (1.000 N, Fisher Scientific, Nepean, Canada), hydrochloric acid (36.5-3870, reagent grade, Baker, Phillipsburg, NJ), potassium chloride, Anal& (BDH),potassium hydroxide (ACS reagent grade, ACP, Montreal, Canada) were all used as received. Apple juice (labeled "ascorbic acid added") was obtained from a local grocery store and used without any pretreatment. All solutions were prepared with deionized (18-MQresistivity) water (MilliQ water system equipped with an organic removal column). Tris buffer solutions (up to 100 mM concentration) were prepared from Tris-HC1 and adjusted to pH 8.3 using a standard sodium hydroxide solution. Tris buffer solutions (500 and 1100 mM) were prepared from Tris and Tris-HC1 in a 1:l ratio, respectively. Pyrophosphate buffer solutions were adjusted to pH 8.6 using hydrochloric acid. All buffer solutions were stored at 4 "C for up to 4 weeks. Ascorbic acid/Tris solutions were prepared in N,-saturated Tris buffer (pH 8.3). If the final pH was much lower than 8.3, it was adjusted with NaOH solution or solid KOH. Ascorbic acid/Tris solutions were kept under water-saturated nitrogen for up to 6 h. Solutions of sodium ascorbate, sodium sulfite, and dopamine hydrochloride in pyrophosphate buffer were used without pH adjustment. These solutions were kept under water-saturated nitrogen for up to 2 h. Electrode System/Cell. The electrode system consisted of a coplanar arrangement of a working electrode, a Ag/AgCl reference electrode (BAS, West Lafayette, IN), and a platinum-wire coil counter electrode. The working electrode was based on a cylindrical platinum electrode (BAS, 1.9 mm diameter), pressfitted into a Kel-F holder. A Teflon cap, having a cavity of 2.2 mm diameter, was placed on top of the platinum electrode. The cavity was filled with the TTF-TCNQ/silicone oil paste. The thickness of the paste layer was approximately 0.5 mm. N o further pretreatment (Le. electrooxidation (16-18)) was performed on the TTF-TCNQ electrodes. In membrane electrode experiments, dialysis membranes (Spectrapor type 2, MW cutoff 12-14K, Spectrum, Los Angeles, CA) were soaked in pyrophosphate buffer (0.150 M, pH 8.6) for 24 h prior to use. Instrumentation/Procedure. Cyclic Voltammetry. The electrode system was controlled by a portable potentiostat with sweep generator capabilities, Model OE PP2 (Oxford Electrodes, Hoo, England). All potentials applied were f0.2 mV. Cyclic voltammograms (CVs) were recorded on a chart recorder, Model SE 120 (BBC/Goertz Metrawatt, Austria). Throughout all measurements, the electrode cell was kept under a N2blanket, and the temperature was maintained at ambient (21 f 2 "C). Steady-State Experiments. The solution was contained in a 30-mL Nalgene beaker. The solution was continuously stirred by a magnetic spinbar flea at medium rotation rate. The solution
sulfite, buffer
mV (AgiAg CI)
Figure l b ) TTF-TCNQ
+*O dopamine
ascorbate
sulfite
buffer
mV(AgIAgCI)
Flgure 1. Cyclic voltammograms, sweep rate = 20 mV/s, at (a) a platinum electrode (geometric area = 0.028 cm2)and (b) a lTF-TCNQ electrode (geometric area = 0.038 cm'). Scans begin at -100 mV. Solutions are 1.0 mM in pyrophosphate buffer (0.100 M KCd). At the platinum electrode, blank and sulfite CV's are superimposable.
composition was changed by adding increments of concentrated stock solutions. Unless otherwise stated, the additions were performed by a programmable titrator, Model DL 21 (Mettler, Greifensee, Switzerland). The titrator's electrode signal input was connected to the current output of the potentiostat. The current was converted to a potential using the internal 103Q ( i l % ) resistor. The baseline was considered stable when the drift was 5 nA/min or less (bare electrode) or 0.05 nA/min or less (in membrane electrode experiments). The slopes were estimated from the chart recorder trace. Unless otherwise stated, the electrode cell was kept under water-saturated nitrogen. Spectrophotometric Assay. As a standard comparison method, the ascorbate content of the samples tested was determined using a spectrophotometric determination of a dye [3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide], after reduction by ascorbate (23),using a commercially available test kit (Boehringer Mannheim) and a Varian DMS 300 UV-vis spectrophotometer.
RESULTS AND DISCUSSION Figure 1 shows the CV's of a TTF-TCNQ electrode in pure pyrophosphate buffer (0.100 M, p H 8.6) and in buffered sodium ascorbate solution (1.0 mM in pyrophosphate buffer, 0.100 M, pH 8.6), respectively. For comparison, additional CV's were recorded a t a bare platinum electrode under the same conditions. Measurable oxidation of ascorbate is observed at -100 mV only with the TTF-TCNQ electrode. Steady-state experiments confirm that the TTF-TCNQ electrode yields measurable signals at -100 mV. The time to achieve a steady-state signal is about 10 s with rapid mixing. As expected from the instability of ascorbate in basic solutions (rapid oxidation by air above pH 6 (24)),the electrode signal is strongly dependent on the sample pH. Figure 2 shows the oxidative current as a function of pH. In blank experiments it was found that the electrode at -100 mV is stable between
ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992 200
300
180
r
140
I
: *
160
I
' .
.
140 120
Q"5
. '.
100
E
, -
80 60
0
40
25
50
75
100
125
ascorbate conc. / pM
20 450 i
0 400 350
-20
6
5
7
9
8
300 250
PH
120
I
100
..
00
I
E
......
A
.&*
...*.*e.-
A A
i
.* . * *
...
i
0
I
I
i
1 0 1 1 1 2
Flgure 2. Anodic current as a function of pH. The scan begins at pH 5 (15.00 mL of 100 mM Tris-HCI, 10 mM ascorbic acid; pH adjusted with NaOH). Equal increments of NaOH (1.000 M) were added (one increment of 0.020 mL/30 s). For each potential, a fresh electrode -75 mV; (A)-50 mV. was prepared. Key: (W) -100 mV; (0)
f
150
100
200
300
I
400
500
ascorbate conc. I mM Figure 4. (a, Top) electrode response to low analyte concentrations at -100 mV. The initial solution was 10.00 mL of 0.100 M Tris (pH 8.3). A total of 200 Increments (0.0200 mL each) of ascorbate stock solution (0.500 mM ascorbic acid, 0.100 M Tris) were added in 15-9 intervals. (b, Bottom) electrode response to high analyte concentrations at -100 mV. The Initial solution was 9.00 mL of 0.100 M Trls (pH 8.3). A total of 60 increments (0.0500mL each)of ascorbate stock solution (2.000 M ascorbic acid, 0.100 M Tris, pH 8.3) were added in 20-9
*I
intervals.
2ol'0
-........I.........
0
100
200
300
400
500
'
600
TRlS conc. / mM
m -100 mV; (0) -75 mV; (A)-50 mV. The scan begins with 10.0 mM Trls (10.0 mM Tris-HCI, 90 mM KCI, 10.0 mM ascorbic acid, pH 8.3, volume = 8.00 mL). A total of 9.00 mL of 1.100 M Tris (containing 10.0 mM ascorbic acid, 100 mM KCI, pH 8.3) was added (45 equal increments, one increment per 90 s). Hence, pH and ascorbate concentration were kept constant. The three curves were recorded using the same electrode in the sequence -100, -75, and -50 mV. Figure 3. Ek&odesignal as a function of Trls concentration:
pH 5 and 12 but the current increases (baseline current -300 nA at pH 5 and 0 nA at pH 12, in pyrophosphate buffer). The instability of ascorbate solutions at high pH results however in irreproducible, scattered signals. Hence the optimum window for ascorbate determination using the electrode is between pH 7.5 and 9.0. The form of the current-pH profile at pH < 9 originates from a change in the electrooxidation mechanism of ascorbate by TTF-TCNQ (25). The response
at pH > 9 most likely is also influenced by the second pK, (11.57) of ascorbate. The dependence of the signal on the buffer concentration is illustrated in Figure 3. At all potentials tested, the signal increases upon increasing buffer concentration at low buffer concentrations. However, at high buffer concentrations the sensitivity toward slight buffer dilution is negligible. The role of buffer concentration in modulating the magnitude of the signal is linked to the solubility of the components of the organic salt; detailed mechanistic studies concerning this and related aspects of the mechanism of action of these salts will be reported shortly (25).
Using the experimental conditions as described above, the current was determined as a function of analyte concentration. At -100 mV, the response is linear over the concentration range 0-10 mM. At -75 and -50 mV however, the linear region evolves into a square root dependence at approximately 2.5 mM ascorbate. To evaluate both the detection limit and the response at very high analyte concentrations, further experiments were performed a t -100 mV. At this potential, the slope is 2.0 nA/pM up to ca. 50 mM ascorbate. Hence, measurements in the micromolar range can readily be performed with a standard potentiostat. Figure 4b shows that the current vs concentration relationship is linear up to an ascorbate con-
150
ANALYTICAL CHEMISTEY, VOL. 64, NO. 2, JANUARY 15, 1992
I
7.801
16C
1
1.40
1
1.2c
.>
s t a r d a r d conc / rrcroM ascorbate
~
I .
P
1
0.80
.-
0.60
1
1~ ~
'
0.40
1::
U
-50
1
"
0
-02c 0
1
2
3
4
5
6
7
0
9 '0
sample conc. in buffer / vel%
Figure 5. Electrode response to 20 increments (0.0500 mL each, one per 30 s) of apple juice. The initial solution was 10.00 mL of Tris buffer (0.500 M, 0.10 M KCI, pH 8.3).
'
"
5
"
"
"
'
"
10
"
15
'
20
""'
25
sample conc. in buffer i vol%
Figure 6. Determination of the ascorbate content of apple juice using a membranecovered lTF-TCNQ electrode, operated at -75 mV. The initiil solution was 15.00 mL of pyrophosphate buffer (0.150 M, pH 8.6). Apple juice increments were added after the signal stabilized. The upper traces show the electrode response to 2.00 mM ascorbate solution (0)and apple juice (A)after background correction. The lower traces show the background current of 2.00 mM ascorbate solution (0)and apple juice (A)after removal of ascorbate using 20 units of ascorbate oxidase.
centration of -50 mM at -100 mV. The electrode can, however, be used at higher concentrations (at least up to 500 mMj, as shown by the well-defined calibration curve which fits an [ascorbate]'I2 relationship reasonably well. To evaluate the effect of air oxidation on the sample solution after dilution with Tris buffer (pH 8.3), calibration curves were recorded at extreme conditions (bubbling with air and bubbling with pure nitrogen, respectively). A small signal decrease observed in the presence of oxygen was caused by ascorbate loss due to air oxidation. The influence of air on the TTF-TCNQ electrode response is evidently negligible.
correction, not shown). Furthermore, both background plots are superimposable, indicating that the membrane electrode detects only ascorbate and not other oxidizible species in the apple juice sample. From the graphs, the ascorbate concentration in the sample can be calculated as
APPLICATIONS Fruit J u i c e . The TTF-TCNQ electrode was used to determine the ascorbate concentration in apple juice using the electrode configuration (held at -75 mV) described above without any sample pretreatment. The calibration curve for apple juice (20 increments of sample, 0.0500 mL each, added to 10.00 mL of 0.500 M Tris buffer, pH 8.3) is plotted in Figure 5 . The slope of the current vs concentration graph slightly decreased with time during use with real samples. Rinsing the electrode with pure buffer restored the TTF-TCNQ electrode to its initial slope value. This time-dependent effect is possibly due to adsorption of species on the electrode surface which would result in a lowering of the effective electrode area and hence signal. In an attempt to circumvent the problems caused by adsorption and nondialyzable coreductants possibly present in the matrix, the electrode was covered with a dialysis membrane (molecular weight cutoff 12-14 K). Six appropriate volume increments of juice were added to 15.00 mL of pyrophosphate buffer (0.150 M, pH 8.6). The existence of background reductants in the sample was probed by recording a second calibration curve (using another 15.00 mL of buffer) after adding ascorbate oxidase solution (20 units) under aerobic conditions. The same experiments were performed with 2.00 mM sodium ascorbate solution in water (Figure 6). The use of a membrane slowed the electrode response (50% of the total current within 150-170 s), and the currents were lower. However, the calibration graphs are linear for both the ascorbate standard solution (R = 0.9994, background-corrected; R = 0.9994 without correction, not shown) and for the apple juice ( R = 0.9997, background-corrected; R = 0.9995 without
From the slopes of the background-corrected calibration curves, the ascorbate concentration in apple juice was found to be 1.41 mM (f0.03 standard deviation, n = 7). Using the spectrophotometric assay, the concentration was 1.66 mM (f0.18 standard deviation, n = 3). These observations suggest that the TTF-TCNQ/silicone oil paste electrode may be optimally employed in a flow injection configuration, using an upstream, switchable ascorbate oxidase packed-bed column to provide background measurements (26). This differential method would provide a simple means to measure signals produced by oxidizable species other than ascorbate. The short sample-electrode contact time and the repetitive flow of pure buffer solution past the electrode would also avoid the effects of nonspecific adsorption and resulting signal drift. However, a membrane-covered TTF-TCNQ electrode, held at -75 mV, is useful even without correction for coreductants in a typical fruit juice. Routine measurements may be performed with the simple electrode once this is established for the analytical sample in question. If coreductants are membrane-permeable and constitute an interference, then the ascorbate oxidase methodology described above can be implemented to quantitate the levels of these interferents. P o t e n t i a l I n t e r f e r e n t s - S u l f i t e and D o p a m i n e . The reductant sulfite is a common food additive and thus a possible interferent with ascorbate assays. The monitoring of dopamine levels in living brain tissue requires simultaneous measurement of the ascorbate concentration since ascorbate severely interferes with standard electrochemical dopamine assays and brain ascorbate levels change as rapidly as dopamine levels (27). Brajter-Toth and co-workers (16) have shown that some discrimination between ascorbate and dop-
[ascorbate] = ((slope of t h e sample line)/(slope of t h e standard line)) X 2.00 m M
Anal. Chem. 1992, 6 4 , 151-155
amine can be achieved at an electrochemically pretreated TTF-TCNQ electrode. At -75 mV, neither ascorbate, sulfite, nor dopamine are oxidized a t a bare platinum electrode (Figure la). At the TTF-TCNQ electrode however, ascorbate is oxidized at -75 mV whereas neither sulfite nor dopamine are oxidized (Figure lb). This is clearly observed under potentiostated conditions, where ascorbate produces a large signal whereas no anodic current is elicited by dopamine or sulfite (not shown). Using a bare TTF-TCNQ electrode, routine analysis of ascorbatecontaining samples can be performed within a few seconds without sample pretreatment. The electrode yields a welldefined response over a wide range of ascorbate concentrations, matrix compositions, pH, buffer concentrations, potentials, and sample oxygen concentrations. Therefore, the operation conditions can be optimized for specific applications. This flexibility, together with the option of easy automation, makes the "F-TCNQ electrode a useful tool for the analysis of ascorbate-containing samplea and complements the recently published application of mediator-modified graphite electrodes for the determination of ascorbate (28). ACKNOWLEDGMENT We thank NSERC (Canada) and the McGill Graduate Faculty for support of this research. Registry No. TTF,31366-253; TCNQ, 1518-16-7;L-ascorbic acid, 50-81-7; L-ascorbate, 299-36-5. REFERENCES OIHver, M. I n The Vh%mh"/2nd ed.;Sebrell, W. H.. Harris,R. S., Eds.; Academic Press: New York. 1967; Vol. 1, pp 359-367. Nagy, G.; Rice, M. E.; Adams, R. N. Life Sci. 1082. 37,2611-2616. Schenk, J. 0.; Miller, E.; Adams. R. N. Anal. Chem. 1082, 54, 1452-1454.
151
(4) &in, M. I n Ascorbic acM: Chemistry, metabolism, and uses; Seib, P. A., Toibert, B. M., Eds.; American Chemical Society Advances in Chemistry Series No. 200; American Chemical Soclety: Washington, DC, 1982; pp 369-379. (5) Leinweber, F. J.; Monty, K. J. Methods Enzymol. 1087. 743, 15-17. (6) Smith, V. J. Anal. Chem. 1987,5 9 , 2256-2259. (7) Laker, M. F.; Hofmann. A. F.; Meeuse, J. D. Clln. Chem. 1080,2 6 , 627-630. (6) Obzansky, D. M.; Richardson, K. E. Clin. Chem. 1083, 2 9 , 1815-1819. (9) Crawford, 0. A.; Mahony, J. F.; Gyoery, A. 2. Clin. Chim. Acta 1085, 747, 51-57. (IO) Brdlcka, R.; Zuman, P. Collect. Czech. Chem. Commun. 1050, 75, 766-779. (11) Roe. J. H. I n The Vitamins, 2nd ed.; Gyorgy. P., Pearson, W. N., Eds.; Academic Press: New York, 1967; Vol. 7, pp 27-51. (12) Pongracz, G. 2.Anal. Chem. 1071,253, 271-274. (13) Matsumoto, K.; Kamikado, H.; Matsubara, H.; Osajlma, Y. Anal. Chem. l08& 6 0 , 147-151. (14) Matsumoto, K.; Yamada, K.; Osajima, J. Anal. Chem. 1081, 53. 1974-1979. (15) Hkchman. M. L. I n Chemical Analysis; Eking, P. J., Wlnefordner, J. D., Eds.; J. Wlley & Sons: New York 1976; Vol. 49. (16) McKenna, K.; Boyette, S. E.; Brajter-Toth, A. Anal. Chim. Acta 1088, 206, 75-84. (17) Freund. M. S.; Brajter-Toth, A. Anal. Chem. 1080, 6 7 , 1048-1052. (18) . . Freund. M. S.; Braiter-Toth, A.; Ward, M. D. J . Nectroanal. Chem. 1000. 289, 127-141. (19) Hill, B. S.; Scolari, C. A.; Wilson, G. S. Phil. Trans. R . Soc. London A 1BBO. 333. .- - - - , 63-89. - ... (20) Ferrarls, J.; Cowan, D. 0.; Walatka, V.; Perlstein, J. H. J. Am. Chem. Soc. 1073. 95. 948-949. (21) Zhao, S.;Lennox, R. B. Anal. Chem. 1991,6 3 , 1174-1176. (22) Gunasingham. H.; Tan, C.-H. Anal. Chim. Acta 1000, 229, 83-91. (23) Hennlger, G. Allmenta 1081, 2 0 , 12-14. (24) . . The Merck Index, 10th ed.; Windholz, M., Ed.; Merck & Co. Inc.: Rahway. NJ. 1983. (25) Zhao, S.; Korell, U.; Cuccla, L.; Lennox, R. 8. Submltted for publication In J. Phys . Chem . (26) Matsumoto, K.; Hamada, 0.; Ukeda, H.; Osajima, Y. Anel. Chem. 1088,58. 2732-2734. (27) . . Wbhtman. R. M.: May, L. J.; Michael, A. C. Anal. Chem. 1088,6 0 , 769A-779A. (28) Kulys, J.; Drungiline, A. Nectroanalysis 1001, 3, 209-214.
-.
RECEIVED for review June 19,1991. Accepted October 8,1991.
Mercury-Coated Carbon-Foam Composite Electrodes for Stripping Analysis of Trace Metals Joseph Wang,* Albert Brennsteiner, and Lucio Angnes Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003 Alan Sylwester a n d Robert R. LaGasse Sandia National Laboratories, Albuquerque, New Mexico 87185-5800 Nils Bitsch Radiometer Analytical AIS, DK-2880 Bagsvaerd, Denmark The advantages and characterlstlcs of mercury-coated carbon-foam composne electrodes for stripplng analysis of trace metals are described. The enhanced perimeter-to-area ratlos characterizing these composite surfaces offer hlgh preconcentratlon efflciencles from quiescent solutlons. Additional advantages accrue from the lower oxygen-reductlon and mercuryoxldatlon background-current components. Scannlngtunnellng and scannlng electron mlcroscoples offer valuable lnslghts into the unlque microstructure of the mercury flhn and substrate. Exploratory experknents have shown the dependence of the stripplng response upon numerous experlmental variables. Convenlent quantitatlon of lead In drlnklng water Is accomplished with qulescent solutlon and short depodtlon period. Slnce neither stlrrlng nor deoxygenation is required, composltsbased stripplng electrodes should be valuable for field and remote operations. 0003-2700/92/0364-0151$03.00/0
INTRODUCTION Because of its inherent sensitivity, stripping analysis has been widely used for measuring trace metals in numerous matrices ( 1 ) . A proper choice of the working electrode is crucial for the success of the stripping operation. Mercury-film electrodes, particularly those based on a rotating glassy-carbon disk, have been traditionally used for achieving high sensitivity and reproducibility (2). The investigation of new electrode materials, as substrates for the mercury film, has continued to receive a great deal of attention. In particular, the introduction of ultramicroelectrodes exhibits great potential for stripping analysis. Several studies indicate that the stripping response of microelectrodes compares favorably with that observed at conventional electrodes (3-9). In particular, such electrodes offer several attractive features for stripping analysis, including enhanced diffusional flux (leading to higher 0 1992 American Chemical Society
