152
Anal. Chem. 1984, 5 6 , 152-156
because the purpose of this work was to demonstrate that the control of the electron transfer at the metal solution interface enables the development of a reagentless glucose electrode. Registry No. D-Glucose, 50-99-7; glucose oxidase, 9001-37-0; flavin adenine dinucleotide, 146-14-5;platinum, 7440-06-4. 4:
x
LITERATURE CITED
+
(1) Scheller, F.; Strnad, G.; Neumann, 8.; Kuhn, M.; Ostrowski, W J. Electroanal. Chem. 1979, 6 , 117-122. (2) Scheller, F.; Strnad, G. Adv. Chem. Ser., in press. (3) Bourdillon, C.; Bourgeois, J. P.; Thomas D. J. Am. Chem. SOC.1980, 102, 4231-4235. (4) Ianniello, R. M.; Lindsay, T. J.; Yacynych, A. M. Anal. Chem. 1982, 54, 1098-1101. (5) Durliat, H.; Comtat, M. Anal. Chem. 1982, 5 4 , 856-861. (6) Durliat, H.; Comtat, M. Anal. Chem. 1980, 5 2 , 2109-2112. (7) De Angelis, T. P.; Helneman, W. R. J. Chem. Educ. 1976, 5 3 ,
C
0 L
¶
u
594-597.
(8) Witby, L. G. Blochem. J. 1953, 5 4 , 437-442. 10
5 t i m e , rnin
Figure 7. Typical response curve for an enzymatic glucose amperometric electrode (GO in reaction chamber, 0.1 mM; E = -I-0.45V): (-) glucose 3 mM; (- - - ) residual curve.
the detection level i s about 0.1 mM. Several hundreds of assays have been realized in this concentration range with constant intensities and response time on periods of a week. Many enzymes have been used during 1 month, thus corroborating the low influence of the electric field on the enzyme denaturation. Among these assays a high number has been performed under nitrogen atmosphere, the absence of dissolved oxygen being controlled by means of an electrode in the limit of sensitivity of the commercial oxygen electrode. The results are the same as those performed under air. The difference between glucose concentrations determined on the one hand by the enzyme electrode and on the other hand by an enzymatic method is less than 5 % . The reduction of the depth of the reaction chamber was not used to optimize this sensor,
(9) Stankovitch, M.; Schopler. C.; Massey, V. J. Blol. Chem. 1978, 253, 4971-4979. (10) Dryhurst, G. "Electrochemistry of Biological Molecules"; Academic Press: New York, 1977; Chapter 7. (11) Braun, R. D. J. Electrochem. SOC.1977, 124, 1342-1347. (12) Danckwerts, P. V. "Gas Liquid Reactions"; McGraw-Hill: New York, 1970. (13) Vallot, R.; N'Diaye, A.; Bermont, A.; Jakubowicz, C.; Yu, L. T. Electrochim. Acta 1980, 2 5 , 1501-1512. (14) Plichon, V.; Laviron, E. J. Nectroanal. Chem. 1976, 71, 143-156. (15) Gorton, L.; Johansson, G. J. Electroanal. Chem. 1980, 113, 151-158. (16) Svoboda, B.; Massay, V. J. B i d . Chem. 1966, 241, 3409-3416. (17) Duke, F. R.; Kust, R. N.; King, L. A. J. Nectrochem. SOC.1969, 116, 32-34. (18) Guilbault, G. G. "Handbook of Enzymatic Methods of Analysis"; Marcel Dekker: New York, 1976. (19) Thevenot, D. R.; Sternberg, R.; Coulet, P.; Laurent, J.; Gautheron, D C. Anal. Chem. 1979, 59, 86-100. (20) Kamin, R.; Wilson, G. S. Anal. Chem. 1980, 5 2 , 1198-1205. (21) Ianniello, R. M.; Yacynych, A. M. Anal. Chem. 1981, 53, 2090-2095. (22) Romette, J. L.; Froment, 13.:Thomas, D. Clin. Chim. Acta 1975, 95, 249-253. (23) Nagy, G.; Von Storb, L. H.; Guilbault, G. G. Anal. Chlm. Acta 1973, 6 6 , 443-447. (24) Liu, C. C.; Weaver, J. P.; Chen, A. R. Bloelectrochem. Bioenerq. 1981, 8, 379-386. (25) Marlncic, L.; Soeldner, J. S.; Colton, C. K.; Giner, J.; Morris, C. J. Electrochem. SOC. 1979, 126, 43-49.
RECEIVED for review March 7,1983. Accepted October 3,1983.
Ion-Selective Electrodes for Octyl and Decyl Sulfate Surfactants Gordon C. Kresheck* and Ioannis Constantinidis Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115 Surfactant Ion selectlve electrodes were prepared which gave near-Nernstlan behavior In aqueous solution for sodlum octyl sulfate at 25 OC and sodlum decyl sulfate at 20, 25, 30, and 35 OC. The selectlvlty of both electrodes to other alkyl sulfates was determined. The electrode response varied In golng from water to pH 7.4 In 0.01 M Trls-HCI buffer with and wlthout 0.1 M NaCl or 6 M urea. The blndlng of octyl and decyl sulfate to poly(vlnylpyrro1ldone)(PVP) of dlfferent molecular weights, concentratlon, and temperature was also studied. Flnally, It was shown that slmllar results are glven by using equlllbrium dlalysls or potentiometric titrations with the surfactant electrodes for the blndlng of decyl sulfate to bovine P-lactoglobulin.
Procedures for the preparation of surfactant ion selective electrodes which are simple to use and allow the rapid de-
termination of surfactant ion concentrations for use in physical chemical as well as analytical studies were described by Birch and Clarke (I, 2). This work focused on the use of dodecyl sulfate electrodes. A slightly different electrode system was later developed to study premicellar aggregation and the degree of counterion binding for two cationic surfactants and one anionic surfactant (3). We have confirmed the nearNernstian behavior of the more recent electrodes as well as their potential use for surfactant binding studies with several types of polymers (4,5). However, during the course of these studies occasional unexplained electrode responses were noted, and it was decided to undertake a systematic study of the behavior of octyl and decyl sulfate electrodes under conditions which might be of interest to chemists in view of their potential utility as originally demonstrated by Birch and Clark ( 1 , 2 ) for the dodecyl sulfate electrodes. The results of these studies will be reported.
0003-2700/84/0356-0152$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
153
Table I. Average Values of the Constants from Equation 1 and CMC for Duplicate Titrations of a Decyl Sulfate Electrode with Sodium Decyl Sulfate solvent E " , mV N , mV/(decade M) CMC, M water TrkHC1 Tris.HC1 t NaCl TrisHCl t Urea
-5.76 * -3.71 * 5.92 t 18.90 i
-60.66 -60.60 -58.21 -61.75
3.96 2.28 0.67 2.93
EXPERIMENTAL S E C T I O N The surfactant ion selective electrodes and reference electrodes were constructed as described by Kale et al. (3),and the electrode emfs were measured in conjunction with an Orion digital voltmeter (Model 801 A) equipped with a manual electrode switch (Model 605) to facilitate readings with a counterion selective electrode which was placed into the test solution along with the surfactant selective one. The stirred sample was surrounded by a waterjacketed cell for temperature control. Aliquots of a stock surfactant solution were added to the sample (10 mL or 25 mL) with Eppendorf pipets and stable emf readings were found within 2 to 3 min as reported by Kale et al. (3). The organic solution used for the octyl or decyl ion exchanger was 0.17 M hexachlorobenzene and 0.017 M 4-bromoacetanilide in o-dichlorobenzene (2). The composition of the inner filling solution was similar to the one described by Kale et al. unless stated otherwise, e.g., about two-thirds of the critical micelle concentration (CMC) in 0.01 M NaCl. The same standard Orion silver/silver chaloride reference electrode was filled with 0.5% NH4Cl. The sample of poly(vinylpyrrolidone) (PVP) used for all of our work unless stated otherwise was from Mann/Schwartz and had a reported molecular weight of 40000 g/mol. Another set of samples of PVP was purchased from Sigma Chemical Co. with various molecular weights. Sodium dodecyl sulfate was purchased from BDH and the octyl and decyl sulfates were from Mann/Schwartz or Eastman Kodak Co., and were recrystallized prior to use. Solutions were prepared by diluting the desired weight of material to volume with deionized laboratory distilled water. Other chemicals (reagent grade or better) were used without purification. The electrode's response to the activities of the corresponding ions is related to the observed emf, E , by eq 1, where ai is the E = E" + N log ai (1) activity of the ion in question and E" and N are treated as empirical parameters although they may be identified with the electrode potential when log ai = 0 and a theoretical value of 59.2 mV per decade change of activity for 1:l electrolytes at 25 "C (3, 6). Attempts have not been made to find activity coefficients in the present study since we are only interested in the analytical use of these electrodes below the CMC. Instead, we have substituted the molarity of the neutral surfactant for ai and used this form of eq 1 for presenting our findings. R E S U L T S A N D DISCUSSION The behavior of dodecyl sulfate electrodes was fairly well described by Birch and Clarke ( I ) . Therefore, we focused on the properties of decyl sulfate electrodes, and a limited number of studies with octyl sulfate electrodes. Response Characteristics. The response in water of nearly all of the octyl and decyl sulfate electrodes we prepared was near Nernstian over a 100-fold concentration range up to the CMC (Figure 1). Below 1% of the CMC, curvature developed and the electrodes did not respond below about 2 X 10" M. The useful range of these electrodes thus resembles that of the dodecyl sulfate electrodes previously described (I, 3). The CMC may be identified with the break in the curves which occurs at 0.030 M and 0.115 M for the decyl and octyl sulfate electrodes, respectively. These values compare favorably with previous determinations of the CMC for the two surfactants (7).The slopes for these two titrations were -58.01 and -58.46 mV/decade concentration for the octyl and decyl sulfate electrodes, respectively, indicating near-Nernstian behavior. Results with a counterion electrode (Na+) in the presence of either decyl or octyl electrodes gave results comparable to
* t f
i-
2.25
0.030 t 0.029 t 0.012 * 0.033 t
0.81
0.49 1.26
0.001 0 0.001 0.001
3ooc 250 200 -
> 150-
100 -
50t "5
~
4
3
2
1
0
-log [ D S ]
Figure 1. Plot of the observed potentlal, E , vs. the logarithm of the surfactant anlon concentratlon, [DS-1, for octyl sulfate (filled symbols) and decyl sulfate (open symbols) electrodes in water at 25 "C.
those for sodium dodecyl sulfate studies ( I , 3-5), e.g., a positive slope of near 59.2 mV/(decade concentration) at 25 "C and a break near the CMC. Finally, the concentration of NaDecSOI in the inner filling solution was lowered to and M in an attempt to determine its effect on the extent of the linear region of the electrode response. The concentration change did not alter the dilute solution response of the electrode but produced curvature in the vicinity of the CMC which was the greatest with the more dilute inner filling solution. Therefore, the more concentrated inner filling solutions (about two-thirds of the CMC) were used in all of our studies. Selectivity. The selectivity of the octyl sulfate and decyl sulfate electrodes was determined for comparison with the selectivity of the dodecyl sulfate electrodes reported by Birch and Clarke ( I ) , who used the Nicolsky equation to represent the data obtained from single-component studies as a function of alkyl chain length. The electrode response in the presence of a single surfactant is described by this method as
E = Eo
+ N log ( k l u t z )
(2) where k l is a selectivity constant and k2 provides a measure of nonideality of the surfactant with the liquid ion exchanger. The results obtained for decyl and octyl electrodes are given in Figures 2 and 3, respectively. In every case curvature of the data precluded the use of eq 2 to obtain the constants kl and kz. However, the same trend as observed by Birch and Clarke with dodecyl electrode may be noted. For example, the slopes are steeper in the presence of surfactants with alkyl side chains longer than the test electrode and vice versa, reflecting the alkyl chain length dependence of the nonideality of the surfactants with the liquid ion exchangers. Solvent Effects. The performance of a given decyl sulfate electrode over the course of 40 h was determined in duplicate for four solvents which have been frequently used in biological studies. The average results from these experiments are given in Table I. The solvents chosen were water, pH 7.4 0.01 M Tris-HC1 (TriwHCl), Tris.HC1 + 0.1 N NaCl (Tris.HC1 + NaCl), and Tris.HCl+ 6 N urea (Tris.HCl+ urea). The values of N are all close to the theoretical value of -59.2 mV/(decade M), but a decided solvent effect on E" may be seen. Also, the solvent effects on the CMC are consistent with previous
154
ANALYTICAL CHEMISTRY, VOL.
56,NO. 2, FEBRUARY 1984
Table If. Summary of Data for Nine Consecutive Decyl Sulfate Titrations in the Presence and Absence of PVP (mol wt 40000) in Water at 25 "C runno. 2
3 4 5 6 7 8 9 10
%PVP
E " , mV
N
-15.60 -25.51 -19.42 -25.44 -22.80 -14.30 -20.45 -12.47 -11.57
-56.84 -61.04 -60.15 -59.56 -58.75 -58.46 -61.34 -58.37 -57.10
0
1.2 1.2 0
0.52 0
0.52 0 0.25
EsrnM
mV
9
115 115 118 112 112 121
120 122 119
>
CMCa~p
CMC'
0.029 0.063 0.063 0.028 0.043 0.028 0.040 0.028 0.035
0.029 0.023 0.024 0.028 0.025 0.028 0.024 0.028 0.027
n
0.38 0.37 0.38 0.35 0.36
140 -
E
w
W
120 -
100 L
-log [ DS-]
80 -
Figure 2. Plot of the observed potential, E , vs. the logarithm of the surfactant anion concentration, [DS-1, using a decyl sulfate electrode for aqueous titrations with sodium octyl sulfate (open circles), decyl sulfate (filled circles), and dodecyl sulfate (open squares) at 25 "C.
A0
604
-log[ D S ]
Figure 4. The titration at 25 "C of 1.2% (open circles), 0.52% (open squares) and 0.25% (filled squares) PVP and a reference titration for the dilution of the surfactant into water (filled circles). Each of the upper three curves is displaced 10 mV from the one immediately below it to avoid overlapping of the data.
-log [ DS'
]
Flgure 3. Plot of the observed potential, E ,vs. the logarithm of the surfactant anion concentration, [DS-] , using a octyl sulfate electrpde for aqueous titrations with sodium octyl sulfate (open circles), decyl sulfate (filled circles), and dodecyl sulfate (open squares).
studies, i.e., increased by the addition of urea and lowered by salt (7). These studies demonstrate the sensitivity of E" to solvent changes and indicate the importance of calibrating the electrodes prior to analytical uses for each solvent to be employed. The break in slope at the CMC was sharp with each solvent used. T e m p e r a t u r e . Whereas nearly all of our studies were made at 25 "C, a set of duplicate experiments was performed with a single decyl sulfate electrode over a period of 3 weeks to investigate the effects of temperature on the electrode response in water. The values of N and Eo obtained were -58.65 1.04 and 17.87 f 1.10, -58.75 f 0.18 and -2.56 f 0.06, -59.51 f 0.69 and 13.33 f 0.38, and -60.86 f 0.55 and 11.79 f 1.30 at 20, 25, 30, and 35 "C, respectively. Theoretical slopes at the same temperatures are -58.17, -59.16, -60.15, and -61.14 mV/(decade concentration), respectively. Therefore, the electrode showed the proper response for N with changing temperature, but E" varied considerably. The average value of the CMC was 28.5 f 1.4 mM for the four temperatures in agreement with the literature (7).
*
Poly(vinylpyrro1idone) Binding Studies. The nonionic water-soluble polymer PVP is an important member of the group of water-soluble synthetic polymers of wide industrial and academic interest (8). The interaction of PVP with alkyl sulfates has been studied by several methods, including potentiometry using dodecyl sulfate selective electrodes (2,4, 9). It mimics the binding behavior of surfactants to proteins, and for this reason has been a useful model system (9). We have extended the earlier work with decyl sulfate to include the effects of PVP concentration, molecular weight, and temperature on this interaction. The binding of octyl sulfate at 25 "C was also determined. In order to determine the dependence of surfactant binding on the concentration of PVP, a series of ten titrations with the same decyl sulfate electrode were conducted over the course of 6 days with three different concentrations of PVP (0.25,0.52, and 1.2% in duplicate and four reference trials). Plots of emf vs. -log (DS-) are given for each concentration of PVP used in Figure 4 along with one reference titration in water. Nearly parallel Nernstian lines may be noted at low concentrations of DS- indicating little if any binding up to about 9 X M. The next region in the PVP titrations is also nearly linear, and it becomes less steep with increasing concentration of PVP due to an interaction between the polymer and decyl sulfate. The final break in the curve is similar to the break in the reference titration at the CMC, and is, therefore, ascribed to micelle formation. The surfactant concentration at that point is denoted the CMC,,,, since it is the apparent or stoichiometric surfactant concentration at which micelles begin to form. The CMC', defined as the monomer concentration at which micelles begin to form in the presence of polymer, is obtained by extrapolation of the data below 9.0 X M to the experimental emf at the CMC,,,. It is equal to the CMC in the absence of PVP. A
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
summary of the pertinent data for all ten titrations is given in Table 11. The values of E" and N are defined by eq 1,E s d is the measured emf when the stoichiometric surfactant concentration was 5 mM, and n represents the number of moles of decyl sulfate bound per mole of PVP monomer and was calculated as CMC,,,
m=
- CMC'
[PVPI
(3)
Values of E" may be seen to fluctuate randomly from run to run (also reflected in the values of EbmM). The average values of N for the four conditions are within the standard deviation for all ten titrations (-59.02 f 1.43). However, extrapolation of the dilute solution data to the emf at the CMC,,, to find the CMC' seems more appropriate than using an average value of N , since E" is not constant. This approach assumes no binding below the break point in the titration which occurred a t (9.5 f 10) X M. The increase in the value of CMC,, with increasing PVP concentration reflects greater surfactant binding with more polymer present. However, the binding ratio, n, is constant within experimental error for the concentrations studied. The value of CMC' was less than the CMC which is the reverse of the CMC,,, increase noted in these studies. The poisoning of the sodium electrode by PVP precluded its use in binding studies involving either sodium octyl or decyl sulfate. Thermometric titration studies (9) have shown that the enthalpy of binding alkyl sulfates to PVP is small. Therefore, little effect of temperature change of the binding curves would be expected. To test this view, duplicate samples of aqueous 0.5% PVP solutions were titrated at 20,25,30, and 35 "C with sodium decyl sulfate, and the results at the other three temperatures were similar to those at 25 "C in that the CMC,,, was greater than the CMC and CMC' was less than the CMC. The values of n were 0.29 f 0.01,0.365 f 0.015,0.285 f 0.005, and 0.24 f 0 at 20, 25, 30, and 35 "C, res?ectively. Since previous workers have been interested in the possible effect of polymer size on ligand binding (8), a set of PVP samples with reported molecular weights of 10OOO, 40 000, and 340 000 g/mol were obtained from Sigma Chemical Co., and 0.5% aqueous solutions were titrated with sodium decyl sulfate a t 25 "C. No dependence on molecular weight was observed within the limits of experimental error. The average values of CMC,,,, CMC', and n were 0.037 f 0.002, 0.027 f 0.015, and 0.20 f 0.03 for the three different molecular weight samples, respectively. For some unexplained reason, the amount of surfactant binding to the PVP 40000 g/mol sample from Sigma Chemical Co. is less than that found with the sample from Mann/Schwartz. Finally, in order to determine the effect of the size of the alkyl group on the binding of alkyl sulfates to PVP, duplicate samples of 1% aqueous PVP were titrated with sodium octyl sulfate at 25 "C using an octyl sulfate selective electrode and 0.20 f 0.04 mol of surfactant was bound per mole of PVP monomer. The values for the CMC,,, and CMC' were 0.130 and 0.112 f 0.004, respectively, compared to the CMC of 0.1175 f 0.0025 without PVP. The first break point in the titration was observed a t a concentration of 0.075 M sodium ocyl sulfate. &LactoglobulinBinding Studies. Our previous circular dichroism studies (10) have shown that the addition of both sodium octyl and decyl sulfate abruptly increases the amount of a-helical and @-structureof @-lactoglobulina t the expense of the unordered form in aqueous solution at 22 + 1 OC. Therefore, studies were performed under similar conditions (IO)with aqueous solutions of @-lactoglobulinAB using the electrodes to determine if the binding behavior paralleled the
155
conformational changes. The results obtained resembled the titration curves shown in Figure 4 for the titration of PVP. After a region at low concentrations of surfactant for which a linear but non-Nernstian electrode response was obtained, an abrupt break in the titration curve of 0.5 and 1.0% protein appeared a t 0.0065 f 0.002 M sodium decyl sulfate. This concentration corresponds to the decyl sulfate concentration required to initiate large charges in the circular dichroism spectra described previously. Similar results were obtained for the titration of 0.5% protein with sodium octyl sulfate, with a break point at 0.06 f 0.015 M, which also approximates the surfactant concentration required to initiate large changes in conformation of @-lactoglobulin,Finally, it was noted that Eo in addition to the initial value of N was altered from the reference values for both octyl and decyl sulfate electrodes by the presence of the protein. Therefore, reliable values of CMC' could not be obtained by the extrapolation procedure used for the PVP studies. A comparison of the more traditional method to study ligand binding to proteins, equilibrium dialysis, was made with the titration method using a decyl sulfate electrode. Ten milliliters of a 1% aqueous sample of 0-lactoglobulin adjusted to pH 5.6 was placed inside a dialysis bag with 25 mL of 0.02 M sodium decyl sulfate outside the sack. The concentration of surfactant in the outer solution was monitored regularly and determined to reach a constant value within 2 weeks. The values of n determined after 9, 13, 16, and 18 days were 19, 19,22, and 19, respectively. The value of n from two titrations of 1%solutions of @-lactoglobulin with a total surfactant concentration of 0.014 M (the same as 25 mL of 0.2 M diluted to 35 mL) was 15 f 2. Another dialysis experiment with 10 mL of 1% @-lactoglobulin inside the dialysis sack and 25 mL of an initial concentration of 0.005 M sodium decyl sulfate on the outside gave values of n equal to 7, 4, 7, and 7, after 13, 18, 20, and 22 days. The comparable value of n at 0.036 M sodium decyl sulfate from duplicate electrode titration studies was 6 f 2.
CONCLUSIONS The results of our studies with the octyl sulfate and decyl sulfate electrodes support their cautious use in certain bioanalytical applications. The decyl sulfate electrode functioned properly from 20 to 35 "C in aqueous solutions, and a t 25 "C in various aqueous mixtures. However, calibrations are necessary in each solvent system to be used. The electrode constants E" and N were found to be characteristic of each individual electrode and showed some drift with time. The useful lifetime of an individual electrode cannot be predicted. The emf of our electrodes did not exhibit any reproducible discontinuity in dilute solution as might be expected from other studies of surfactants well below the CMC (11, 12). Our data for the binding of octyl and decyl sulfate to PVP parallel those employing other methods to study binding by Arai et al. (13) in that transition points were found with each surfactant and the ratio of surfactant bound to the weight of PVP present was constant and did not vary much with the surfactant alkyl chain length. Also, the relationship between surfactant alkyl chain length and the logarithim of the surfactant concentration at the first transition point is the same for the octyl and decyl sulfates as for the dodecyl, undecyl, and decyl sulfates reported by Arai et al. Our results with the sample of PVP from Sigma Chemical Co. do not show a great dependence on molecular weight in agreement with the results of Shirahama et al. (14),who also used techniques other than selective electrodes to study the binding process. The method we used to calculate n is a little different than the one used by Arai et al. (13) in that they used the difference between the two break points as a measure of the amount of surfactant bound to the polymer. Our method attempts to
156
Anal. Chem. 1984, 56, 156-159
directly measure the concentration of free surfactant from the emf at the CMC,,,. This also allows us to determine the true CMC at the CMC,, which was found to be less than the CMC without PVP. The studies with @-lactoglobulinillustrate that comparable results are obtained for the binding of decyl sulfate to @lactoglobulin by equilibrium dialysis or using surfactant selective electrodes. Our results show that a reference titration without protein must be used to determine the amount of binding due to the likely binding of surfactant at low surfactant concentrations. This level of binding must be quite small since it does not produce large changes in the conformation of (3-lactoblobulin as detected by circular dichroism studies. The major limitation to the use of ion-selective electrodes in the presence of polymers (including proteins) is their effect on the electrode response per se. This leads to altered values of Eo and N a n d may preclude quantitative use of the electrodes except for determining break points in the titration. The latter may be useful if their significance can be established by other types of measurements. The use of surfactant electrodes to monitor the surfactant concentration in the outer fluid during dialysis experiments appears to be without cornplications.
ACKNOWLEDGMENT We are indebted to Kalidas M. Kale for assistance in
preparing our first surfactant ion selective electrodes. Registry No. Poly(vinylpyrrolidone),9003-39-8; sodium octyl sulfate, 142-31-4; sodium decyl sulfate, 142-87-0.
LITERATURE CITED (1) Blrch, B. J.; Clarke, D. E. Anal. Chim. Acta 1973, 6 7 , 387-393. (2) Birch, 8. J.; Clarke, D. E.; Lee, R. S.; Oakes, J. Anal. Chim. Acta 1974, 7 0 , 417-423. (3) Kale, K. M.; Cussler, E. L.; Evans, D. F. J . Phys. Chem. 1980, 54, 593-598. (4) Kale, K. M.; Kresheck, G. C.; Erman, J. I n "Solution Behavior of Surfactants"; Mittal, K. L., Fendler, E. J., Ed.; Plenum: New York, 1982; Vol. 1, pp 665-676. (5) Kresheck, G. C.; Kale, K.; Erman, J. I n "Solution Behavior of Surfactants"; Mittal, K. L., Fendler, E. J., Eds.; Plenum: New York, 1982; VOi. 1, pp 677-692. (6) Birch, B. J.; Clarke, D. E. Anal. Chim. Acta 1972, 67,159-162. (7) Mukerjee, P.; Mysels, K. J. "Critlcal Micelle Concentrations of Aqueous
Surfactant Systems"; US. Superintendent of Documents: Washington, DC, 1971. (8) Molyneux, P. I n "Water: A Comprehensive Treatise"; Franks, F., Ed.; Plenum: New York, 1975; Vol. 4, pp 569-757. (9) Kresheck, G. C.; Hargraves, W. A. J . Colloidlnterface Sci. 1981, 8 3 ,
1-8. (10) Damon, A. J. H.; Kresheck, G. C. Biopo/ymers 1982, 27, 895-908. (11) DeLlsl, R.; Perron, G.; Paquette, J. Can. J . Chem. 1981, 59, 1865-1871. (12) Nikolov, A,; Martynov, G.; Ekserova, D. J . Collokj Interface Scl. 1981. 87,116-124. (13) Aral, H.; Murata, M.; Shlnoda, K. J . Colloid Interface Sci. 1971, 37, 223-227. (14) Shlrahama, K.; Tsujji, K.; Takagi, T. J . Biochem. 1974, 75,309-319.
RECEIVED for review May 25,1983. Accepted October 24,1983.
Subtractive Anodic Stripping Voltammetry with Flow Injection Analysis Joseph Wang* and Howard D. Dewald Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
A new approach to subtractive anodic strlpplng voltammetry, utilirlng the manifold of a flow Injection system, is descrlbed. For thls purpose the carrier solution stripping voltammogram Is subtracted from the sample one, yleidlng a purely analytlcai response. The effectlve correctlon for both Faradaic and non-Faradaic background current contributions allows a detectlon limit for cadmlum of about 6 X lo-' M (0.14 ng) with l-mln deposlon. Nondeaerated samples can be used due to the effective correctlon of the oxygen reduction current. The system permits simultaneous measurement of several trace metals in the part-per-billion concentration level, using a 2 0 0 - ~ Lsample volume and an injection rate of 24 samples per hour. Good precision and linear cailbration plots are obtained. Appllcabillty to various real samples Is illustrated.
Anodic stripping voltammetry (ASV) is a powerful electroanalytical technique for trace metal measurements (1, 2). Various approaches have been proposed for further improving its sensitivity, aimed mainly a t correcting the non-Faradaic charging background current. These include the application of potential-time wave forms such as differential pulse (3), phase selective ac ( 4 ) ,or staircase (5) to replace the conventional linear scan during the stripping step. Despite the improved sensitivity offered by these potential-time excita-
tions, the remaining Faradaic background current contributions (e.g., oxidation of mercury, reduction of hydrogen ions or oxygen) limit the detectability. By use of a subtractive stripping mode, it is possible to correct for both Faradaic and non-Faradaic background current contributions and to shorten the deposition time. Subtractive ASV is based upon subtracting a background voltammogram from the analytical one. Different approaches have been suggested for generating the background curve. These include the use of twin working electrodes placed in two cells (sample and electrolyte) ( 6 ) , different deposition times or convection rates on twin electrodes immersed in the sample solution ( 7 - I O ) , or different deposition times on a single electrode (11, 12). The present paper describes a new and convenient approach to generate the background curve for subtractive ASV, utilizing small sample volumes and a rapid sampling rate. The method is based upon the incorporation of ASV with the manifold configuration of a flow injection system. In flow injection analysis (FIA) small volumes of sample solution are injected into a carrier stream that transports the sample toward the detector (13,14). Conventional voltammetric and potentiometric stripping analyses have been incorporated recently as sensitive detection modes for FIA (15, 16). The unique nature of the FIA manifold can be exploited toward achieving the desired subtractive ASV response. For this purpose the "analytical" and "background stripping voltam-
0 1984 American Chemical Society 0003-2700/84/0356-0156$01.50/0
