Anal. Chem. 1990, 62, 909-914 (4) Lam, K. W. L. Ph.D. Thesis, Wayne State University, 1974. (5) Ferrier, D. R.; Schroeder, R. R. J. Electroanal. Chem. Inferfacial Ebctrochem. 1973, 45,343. (6) Ferrier, D. R.; Chidester, D. H.; Schroeder, R. R. J. Electroanal. Chem. Interfacial Electrochem. 1973, 45,361, (7) Stefani, S.; Seeber, R. Anal. Chem. 1082, 5 4 , 2524. (8) Zipper, J. J.; Perone. S.P. Anal. Chem. 1973, 45,452. (9) Schachterle, S. D.; Perone, S. P. Anal. Chem. 1981, 53, 1672. (10) Surprenant, H. L.; Rigway, T. H.; Reilley. C. N. J . Elecfroanal. Chem. Inferfacial Electrochem. 1977. 75, 125. (11) Seralathan, M.; Osteryoung, R. A.; Osteryoung, J. G. J. Nectroanal. Chem. Interfacial Necfrochem. 1987, 222, 69. (12) Bilewicz, R.; Osteryoung, R. A.; Osteryoung, J. Anal. Chem. 1986, 58,2761. (13) Seralathan, M.; Osteryoung, R.; Osteryoung, J. J. Nectroanal. Chem. Inferfacial Electrochem, 1988, 2 74, 14 1. (14) Murphy, M. M.: O'Dea, J. J.; Arn, D.; Osteryoung, J. G. Anal. Chem. 1989, 67, 2249-2254. (15) Nicholson, R. S.; Shain, I.Anal. Chem. 1964, 36,706.
909
(16) Saveant, J. M.; Vianello, E. Electrochim. Acta 1963, 8 , 905. (17) Nadjo, L.; Saveant, J. M. J. Electroanal. Chem. Inferfacial Nectrochem. 1973, 48, 113. (18) O'Dea, J. J.; Osteryoung, J.; Osteryoung, R. A. Anal. Chem. 1981, 53,695. (19) Nicholson, R. S. Anal. Chem. 1986, 38, 1406. (20) Smith, D. E. Anal. Chem. 1963, 35,602. (21) O'Dea, J. J. Ph.D. Dissertation, Colorado State University, Fort Collins, CO, 1979. (22) Nicholson, R . S.;Olmstead, M. L. I n Necfrochemlstfy: Calculations, Simulation and Instrumentation; Mattson, J. S., Mark, H. B., MacDonald, H. C., Eds.; Marcel Dekker: New York, 1972; Vol. 2, Chapter 5. (23) Brumleve, T. R.; Osteryoung, J. J. Phys. Chem. 1982. 86, 1794.
RECEIVED for review September 18,1989. Accepted January 24, 1990. This work was supported in part by the National Science Foundation under Grant no. CHE8521200.
Conductive Polymeric Tetrakis(3-methoxy-4-hydroxyphenyl)porphyrin Film Electrode for Trace Determination of Nickel Tadeusz Malinski,* Aleksander Ciszewski,' and Judith R. Fish Department of Chemistry, Oakland University, Rochester, Michigan 48309-4401 Leszek Czuchajowski* Department of Chemistry, University of Idaho, Moscow, Idaho 83843
Stable polymer film electrodes were formed from tetrakis(3methoxy4hydroxyphenyi)porphyrinwlth NI( I I ) as the central metal. The nickel-porphyrin polymer films efficiently demetaiated In acidic media at pH 1. The resulting demetalated porphyrln polymer electrodes required no preconditioning treatment before exhlbitlng strong affinity for Ni( I I ) . Electrodes were placed in sample soiutlon to preconcentrate NI( I I ) and transferred to a blank electrolysis solution where the signal due to the Ni( I I)/NI( I I I ) oxidation In the film was observed by dlfferential pulse voltammetry. A detection limit of 8 X 10" M was obtained for a 60-s exposure to the sample solution. Accurate quantitatlon of Ni in certified standard reference material NBS 1643 6 (Trace Elements in Water) was achieved wlth the electrode developed. Electrodes wlth polymeric porphyrin, which can be used as amperometrlc sensors, were easlly and controllably formed and stable with both time and use. The demetalated film could be regenerated with exposure to acid and reused for chemical preconcentration. I n interference studies, a 10-fold excess of Co resulted In partial suppression of the Ni( I I ) signal, but no new signals were observed. Similar concentrations of cations of Zn, Cd, Pb, Cu and Fe did not appreciably influence the NI( I I ) response.
INTRODUCTION Application of chemically modified electrodes for trace analysis offers the advantage of selectivity coupled with the *To whom correspondence should be addressed. Permanent address: Department of Analytical Chemistry, Technical University of Poznan, 60-965 Poznan, Poland.
sensitivity enhancement of preconcentration. Electrode surfaces are designed and fabricated to preconcentrate a particular species by reaction and bonding with specific functional groups. The preconcentration, which has a purely chemical rather than electrochemical mechanism, can be effected by ion exchange ( 1 , 2 ) ,covalent linkage (3),or complex formation (4-8) reactions. Therefore, selectivity is determined by the chemical reactivity of the electrode modifying agents rather than the redox potential of the analyte. This allows construction and use of electrodes specifically optimized for an analyte of interest. These electrodes do not depend on electrochemical processes performed in or on the film and, therefore, make possible selective preconcentration of analytes that are either difficult or impossible to reductively deposit onto untreated electrodes. Quantitation of accumulated analyte is effected by the usual voltammetric methods. However, this process is facilitated because interfering species that would be codeposited a t the negative potentials necessary and/or exhibit overlapping stripping potentials are discriminated against during chemical preconcentration. Exchanging the sample solution with a clean medium prior to performing the actual quantitation step may effectively bypass a host of electroactive interfering substances encountered with stripping voltammetry in which electrolytic preconcentration is employed. Ni(I1) is an essential metal that occurs a t trace concentration levels in physiological and environmental systems. However, its quite negative reduction potential, -1.2 V vs SCE (9, l o ) ,and propensity to form intermetallic compounds with other metallic species codeposited result in complex, matrix-dependent stripping patterns not suited for quantitation (10). Therefore, development of electrodes with selective analyte collection properties is relevant for trace level analysis of this metal.
0003-2700/90/0362-0909$02.50/00 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 9, MAY 1, 1990
Previously reported electrode fabrications for analysis of Ni(I1) were carbon paste containing mixtures of dimethylglyoxime (DMG) coated onto graphite (11). These electrodes were highly selective for Ni(I1) but had several disadvantages. A prior conditioning step was required to achieve optimal sensitivity, and decreased sensitivity was observed with extended use whereby fresh surfaces were needed. Optimization of the DMG-graphite ratios was necessary. The pH range for analysis was pH 5-9 with the former requiring immediate measurement upon insertion of the DME and the latter exhibiting overlap of the Ni response with background DMG reduction. Exposure of the DMGgraphite electrodes to solutions containing oxygen during voltage scanning resulted in “fatal performance” (12). The problem of stability was later overcome by using a computerized flow system (12). However, even this modification required addition of a substantial concentration of CaC12to remove O2 and the use of ammonia buffer to maintain the basic optimal pH condition. With both batch and flow methodologies, paste fabrication was an issue as different backgrounds were observed for different pastes. In the present work, a new electrode for preconcentration and quantitation of nickel is described. Tetrakis(3-methoxy-4-hydroxypheny1)nickel porphyrin (TMHPPNi) is used as an agent to modify either glassy carbon or platinum electrodes. This electrode has suitable properties for practical analytical methodology for trace level determination of nickel by voltammetric methods and can be used as an amperometric sensor as well. Initial oxidation of the monomeric, metalated porphyrin units leads to polymerization and formation of highly conductive polymer film on the electrode surface. The polymer undergoes facile demetalation in acid solution leaving an intact, adherent, conductive film on the electrode surface that then selectively chemically incorporates nickel(I1) cations from analyte solutions. The analytical signal is the current from the Ni(II)/Ni(III) oxidation. Because both Ni(I1) and Ni(II1) cations remain in the porphyrin film electrode, stripping is not involved. Voltammetric detection is best effected by an anodic scan. Amperometric detection can be achieved by maintaining the electrode at an appropriate constant potential, and, under these conditions, the electrode can be used as a sensor. These electrodes were easily prepared and behaved as anticipated in that nickel cations, selectively preconcentrated from NBS standard solutions, were reproducibily and quantitatively determined.
EXPERIMENTAL SECTION Reagents and Materials. Tetrakis(3-methoxy-4-hydroxypheny1)nickel porphyrin (TMHPPNi) was synthesized according to a procedure described previously (13, 14). All acids and bases used were Suprapure (Ultrex, J. T. Baker). Standard solutions of cations were prepared from 1000 fig mL-’ ICP reference stock standards (Spex Industries) for Ni(I1) and for the interference studies. NBS 164313 Trace Elements in Water, used as analytical standard, was obtained from the U S . National Bureau of Standards. Supporting electrolyte for electrochemical procedures was 0.1 M NaOH. Aqueous CH,COONa (Aldrich) was used as the medium for chemical preconcentration. Apparatus. Poly-TMHPPNi/TMHPP electrodes were prepared from either glassy carbon or platinum electrodes (Bioanalytical Systems). Prior to use, electrodes were rinsed in methanol and polished on alumina. Any residual particles of alumina were removed by ultrasonic cleaning. A classical,three-electrode system in a quartz cell fitted with a Teflon cap was used for poly-TMHPP electrode fabrication and Ni(I1) determinations. The working electrode was a poly-TMHPP electrode prepared as described below. Platinum mesh (area about 2 cm2)was used as the auxillary electrode and a saturated calomel
electrode (SCE) as the reference electrode. The SCE was separated from the fabrication or analyticalsolution by a bridge, fitted with a Vycor disk (IBM), containing saturated KC1. All potentials are reported vs SCE. Cyclic and differentialpulse voltammetry were performed with an IBM Model EC 225 voltammetric analyzer and the resulting current-voltage curves recorded with a Houston Omnigraphic 2000 X-Y recorder. Poly-TMHPP Electrode Fabrication. Electrode fabrication consists of deposition of conductive, polymeric TMHPPNi film on the electrode surface and verification of film deposition followed by demetalation and verification of demetalation. Polymeric film is deposited from a solution of 0.1 M NaOH containing 5 x lo4 M TMHPPNi by cyclic scanning between 0.0 and 1.0 V with a scan rate 0.1 V s-l. A peak attributable to the Ni(II)/Ni(III)couple appears at E l / 2= 0.50 V. The number of monolayers deposited is dependent upon the initial concentration of TMHPPNi and the number of cyclic voltammetric scans. After film formation, the electrode is removed from the deposition solution,rinsed, and immersed in 0.1 M NaOH. The presence of poly-TMHPPNi film is confirmed by the Ni(II)/Ni(III) couple, which appears at Ell2 = 0.50 V. The poly-TMHPPNi film is then demetalated in a chemical process by placing the electrode in a stirred 0.1 M HCl solution. Under these conditions, a 1-min demetalation time was found to be sufficient for all electrodes studied. The electrode then is removed from the demetalation solution, rinsed, and immersed in 0.1 M NaOH. The absence of current peaks when scanning between 0.0 and 0.65 V confirms the absence of Ni in the polyTMHPP film. Electrodes were stored in 0.1 M NaOH until ready for use. Electrodes have been stored with intermittant use for periods as long as 1 month without deterioration in response. Chemical Preconcentration. Chemical preconcentration of metallic species was performed in 5-mL solutions contained in quartz cells. A poly-TMHPP film electrode was rinsed by immersing in H20 prior to transfer to analyte solutions containing Ni(I1) in sodium acetate and NaOH (pH 8.5-9.0). Preconcentration times studied varied from 1 to 5 min. Solutions were stirred throughout. Determination of Analytical Signal. The poly-TMHPP film electrode with its incorporated Ni was removed from analyte solution, immersed in H20 and then transferred to 0.1 M NaOH for electrochemical determination of the chemically preconcentrated species. While the presence of nickel was indicated in linear sweep voltammetry, the higher current produced by differential pulse voltammetry makes this the methodology of choice for lowest detection limits. Quantitation was effected by the method of standard additions. Data reported are the average of triplicate determinations and the reproducibility studies are based on seven determinations of the same sample solution.
RESULTS AND DISCUSSION Typically, immobilizing a modifier agent at an electrode surface is accomplished by incorporating it into a conductive material. In the proposed procedure, this is effected by polymerization of TMHPPNi after electrochemical oxidation to form a highly conductive film, poly-TMHPPNi, on either glassy carbon or platinum electrodes. Figure 1 shows the growth patterns for TMHPPNi in continuous scan cyclic voltammetry from 0.00 to 1.00 V at a potential scan rate of 0.1 V s-l. Peaks Ia and IIa correspond to the oxidation of the porphyrin ring on GCE and peaks IIIa and IIIc, observed at E l j z = 0.50 V, correspond to the Ni(II)/Ni(III) redox couple. Film growth is accompanied by increasing current for the latter peaks. The oxidation of Ni(I1) to Ni(II1) in monomeric TMHPPNi is not seen in the range of potential available in aqueous solution. However, the hydrophobic condition that exists within the conductive polymeric porphyrin film allows the facile oxidation of Ni(I1) to Ni(II1) to be observed. This oxidation occurs within the film and does not result in demetalation or “stripping” into the electrolyte solution. The formation of poly-TMHPPNi on the electrode surface was confirmed by spectroscopic and spectroelectrochemical
fi!?!il
ANALYTICAL CHEMISTRY. VOL. 62, NO. 9, MAY 1, 1990
911
1.0
-8
3
60
-
0.5
40
mB ;
0
2000
2
4
6
PH
6
10
12
14
(a) pH dependence of demetalation process for polymeric TMHPPNi (film thickness. 20 equivalent monolayers; time, 60 s). (b) pH dependence of metalation of polymeric TMHPP with Ni(I1) related to peak current of Ni(ll)lNi(Ill) obtained after metalation at pH 9.0 (film thckness was 20 equivalent nwndayers: Inm 60 s; [N(Il)] 1 FM). Flgure 2.
I
1
1.0
0.5 ECV) vs.SCE
0.0
Fmre 1. COntinuous scan cyclic voiiammcgram of TMHPPNi on GCE in 0.1 M NaOH (scan rate 100 mV s-').
methods. Time-resolved, thin-layer absorption spectra obtained for the oxidation pmeess of TMHPPNi in 0.1 M NaOH at 1.0 V show a continuous decrease of the Soret band at 441.3 nm and the two bands at 543.0 and 498.1 nm and the absence of any new hands observed during the oxidation process. These observations indicate exhaustion of the porphine species from solution. UV-visible spectra of the film formed on a transparent semiconductor indium oxide electrode show a characteristic Soret hand at 434.0 nm and a low absorption hand at 538.4 nm. A detailed characterization of the film formation from TMHPPNi has been the subject of previous works (13, 14). The rate of polymerization and the conductivity &d morphology of the film formed strongly depends on the inclusion of a central metal within the porphyrin (25). The free base of THMPP polymerizes slowly and only forms a relatively thin and mechanically unstable film that incorporates nickel poorly. The metalated species, TMHPPNi, quickly forms the thick stable filmsdesired. However, this film is saturated with metal and shows no affinity for binding additional nickel. Therefore, demetalation of the electrochemically produced film is necessary. Monomeric porphyrins are thought to incorporate metal and demetalate according to the overall chemical equilibrium
jz+ w-
Hz(P
2H*
(P)z-
-MH
M(P)
where (P)represents porphyrin and M, metal (16). While eq 1does not consider mechanistic details of either process, the pH environment of the electrode surface would he expected to influence the equilibrium. Demetalation or acid-catalyzed solvolysis involves the replacement of a coordinated metal by protons. Metalloporphyrin stability has been defined as stability toward acids or stability in the presence of another metal compound. Nickel(I1) porphyrins tend to he highly stable. A typical stability order for cations is F't(I1) > Pd(I1) > Ni(I1) > Co(I1) > Ag(I1) > Cu(I1) > Fe(I1) > Zn(I1) > Mg(I1) > Cd(I1) > Liz > Naz > Ba(I1) > K2[Ag(I)], (26).This apparent stability might he expected to make the necessary demetalation of the TMHPPNi film difficult. Figure 2 shows results of experiments to determine the ratio of metalated to demetalated species present in the film over the pH range 1-12 for both the demetalation and chemical preconcentration process.
Figure 2a shows the extent of demetalation observed as a function of pH. The percent of demetalation was obtained from the relative height of the Ni(II)/Ni(III) current peak hefore and after immersion in solution with the appropriate pH for 1 min. Based on eq 1, the shift of equilibrium to demetalated species would be predicted to occur at lower pH. T h e results obtained, an increasing current slope for pH 1-6 and a plateau a t higher pH, follow the equilihrium-based predictions. Demetalation data were confirmed by determination of nickel in solution by inductively coupled plasma (ICP) emission spectroscopy according to a method published previously (27). The UV-visible spectrum of the TMHPP film on indium oxide electrode obtained after demetalation shows a 20% increase of the Soret band after loss of electrochemical signal from nickel in the film, an additional confirmation of demetalation (25). The data obtained from each of these experimental approaches indicate that despite the known stability of the monomeric nickel(I1) porphyrin, demetalation can he effected by immersion of the polymeric film electrode in acid a t room temperature. While the degree of demetalation could he increased by longer immersion times at somewhat higher pH, 0.1 M HCI was used as a convenient solution for demetalation in order to keep the analysis time as short &s possible. Once demetalsted, the poly-TMHPP fh can he expected to readily incorporate nickel from solution (eq 1) and be used for chemical preconcentration. This makes TMHPPNi a suitable material for the fabrication of a sensor when the polymerization process is followed by subsequent demetalation. Figure 2h shows the effect of pH on remetalation. The height of the Ni(II)/Ni(III) current peak obtained following immersion in 10+ M Ni(I1) solution was referenced to the height of the highest current peak obtained for remetalation. Current peaks were measured in 0.1 M NaOH a t pH 8.5-9.0. Sodium acetate was used in the Ni(I1) solution to allow the transfer of counterions, e.g. protons from the TMHPP film, so remetalation could occur (eq 1). The Ni(I1) concentration itself would be expected to effect the equilibrium in favor of the metalated species. However, unlike the situation in porphyrin synthesis where a 10-fold excess of cation is employed to ensure metalation, in trace level analysis, the cation concentration is on the order of 10-6M e inadequate driving force for metalation. Acidity or basicity of solution would he a factor amendable to experimental control. On the basis of only eq 1,the remetalation would be predicted to not occur at pH less than 6 and increase at pH levels higher than 8 where the equilibrium would be shifted to the metalated species. However, the plot obtained experimentally is a steep parahola
ANALYTICAL CHEMISTRY, VOL. 62, NO. 9, MAY 1, 1990
912 I
0
0
I
e-----% 30
c-
,.
d
c
-
-”
60 90 120 Preconcentrationtime, seconds
0
c -
9
d ‘
150
0.0
fl 10.6
10+
10’~
Concentration, M
Figure 3. Plot of peak current vs preconcentration time for initial solution concentration of nickel of (a) 10 pM; (b) 5 pM; (c) 1 pM; (d) 0.2 pM (film thickness was 20 equivalent monolayers, pH 9.0).
Figure 4. Number of monolayers of TMHPP occupied with nickel as a function of nickel concentration in solution (film thickness was 20 equivalent monolayers; pH 9.0; preconcentration time, 60 s).
that decreases rapidly at pH values greater than 10. At high pH, the nickel cation can react with OH- to form insoluble Ni(OHI2 (kSq= 1.6 X lo-’*) decreasing the concentration of Ni and inhibiting the metalation process. Therefore, pH 9.0 f 0.2 was chosen as optimal for chemical preconcentration of trace amounts of nickel in poly-TMHPP. The chemical preconcentration process should depend on both preconcentration time and initial concentration of nickel in solution. Therefore, the preconcentration yield was studied for each of these parameters. Investigated were preconcentration times from 15 to 150 s for initial concentrations 0.2, 1, 5, and 10 WMNi(I1). Figure 3 is a plot of peak current vs preconcentration time for each of the concentrations studied. Generally, at comparable times, higher initial concentrations give higher current signals. Initially, current increases with time of exposure to the cation solution. Each of the concentrations investigated exhibits a linear region on a plot of current versus preconcentration time at short preconcentration times. The slope depends on the concentration levels studied and is larger the higher the initial solution concentration. This suggests that the rate of incorporation of nickel into the polymeric film is fast during the initial period of time and that the rate is dependent on the initial concentration of nickel. This can be due to fast incorporation of nickel in the outermost monolayer of the film.Further incorporation will be controlled by propagation (diffusion) into the film that will be controlled by the gradient of Ni(I1) concentration in the film. The film concentration gradient would in turn be expected to be effected by the concentration a t the film-solution interface. With the fully water-soluble porphyrins, the incorporation rate of metals has been found to be first order in free base, metal, and anion (18). However, it has been suggested that during incorporation of metals in poorly soluble porphyrins, which would correspond to the situation in these studies, intermediates (“sitting-atop’’complexes) can be formed. Incorporation kinetics were suggested to be second order in metal and first in porphyrin. Under conditions of low level concentrations, 104-10-‘ M (Figure 3c,d), cation depletion and dissolution result in low slope and current plateaus observed at preconcentration times greater than 120 s. If the amount of cation available in solution is depleted upon saturation or even before saturation of the outermost monolayer, the concentration gradient would favor dissolution from the film. This would result in an equilibrium concentration level in the film that would be manifested in current plateaus (19). Plateaus are not observed at the higher concentration levels studied. Cation depletion does not occur so the concentration gradient continues to favor cation incorporation into the film. It needs to be emphasized
that at the levels of concentration employed in the present discussion, full occupation of the film would not be expected. Other issues to be considered are the number of polyTMHPP monolayers occupied by Ni following reconcentration and the effect of film thickness on the analytical signal obtained. Figure 4 shows the relationship between the initial concentration of Ni(I1) and the number of monolayers occupied for the typical film thickness and preconcentration time used in this work (20 monolayers and 60 s, respectively). Data plotted were calculated based on coulometric data for the Ni(II)/Ni(III) system following preconcentration for 60 s. As expected, the number of monolayers occupied increases with initial concentration. The effect of film thickness on the peak current obtained is an important issue to be considered in order to determine the precision required in the poly-TMHPP electrode fabrication and saturation effects that need to be considered. Data obtained for 5 X lo* M Ni(I1) and a 60-s deposition time showed 0.64 f 0.02 monolayer occupation for 8, 15, 22, and 29 monolayer films. At trace concentration levels, film thickness is not important as only the first few layers are occupied and the analyte signal does not depend on film thickness. These concentration levels and preconcentration times are not sufficient to exceed the film capacity to incorporate Ni(I1). Saturation is evidenced by observation of the same peak heights in cyclic voltammetry as were produced by the freshly prepared film before the demetalation process. A 10 monolayer film became saturated after immersion in M Ni(I1) for 1 h. Under the conditions of the experiment, the total concentration of Ni(I1) in the saturated film was calculated to be 7 x lo-” M. Therefore, trace level concentrations of lo4 and M should provide sufficient film concentrations for the easy observation of analytical signal. While this occupation may not saturate a full monolayer, the concentration of Ni(I1) in the film is sufficient to afford good analytical signals at these concentrations and detection limits of 8 X M. Differential pulse voltammetry of nickel can be performed in either the anodic or cathodic mode, i.e. electrochemical oxidation of Ni(I1) to Ni(II1) or reduction of Ni(II1) produced by constant potential oxidation of Ni(I1). Both require transfer of the electrode from analyte solution to 0.1 M NaOH. Figure 5a shows differential pulse voltammograms obtained for the oxidation of 1 pM Ni(I1) for two preconcentration times. Voltammograms obtained in the anodic scan are not as well-defied as those obtained for the cathodic scan (Figure 5b). The higher background obtained in the anodic scan is due to catalytic oxidation of water, which occurs very easily on the poly-TMHPPNi electrode (14).At nickel concentra-
ANALYTICAL CHEMISTRY, VOL. 62, NO. 9,MAY 1, 1990
B
b
L 0.6
913
,
0.4
1’
I
0.2
E ( V ) vs.SCE
7’
Flgure 5. Differential pulse voltammograms of nickel obtained with polymeric TMHPPNi film in 0.1 M NaOH: (a) anodic and (b) cathodic scan. Nickel was preconcentrated from 1 pM solution at pH 9 for 30 s (1, 1’) and 60 s (2, 2’); film thickness was 20 equivalent monolayers.
Table I. Optimum Conditions for Determination of Ni(I1) electrode fabrication
demetalation
supporting electrolyte [TMHPPNi] voltage range scan rate number of scans electrode material solution stirring
0.1 M HC1 1 min 600-800 rpm
solution pH
8.5-9.0
time
metalation (preconcentration)
time
stirring Ni(I1) determination
0.1 M NaOH (1-5) x 10-4 M 0.00-1.00 v 0.1 v s-1 20-40 GCE or Pt
sodium acetate
60 s 600-800 rpm
supporting electrolyte 0.1 M NaOH
method scan rate voltage range amulitude
DPV 5 mV
s-’
0.20-0.60 V 40 mV
tions higher than lo4 M, there is no significant difference between the signals obtained for the anodic and the cathodic process. Table I summarizes the optimum conditions employed. Interferences. The use of polymeric porphyrin sensors might be hampered by types of interferences not commonly encountered in anodic or cathodic stripping. In particular, metal cations that can form stronger complexes than Ni with porphyrins might block the coordination centers in the polymeric porphyrin film. However, not only the capability of the cation to form a complex with porphyrin has to be considered but also the capability to be incorporated into and/or diffuse through the polymeric film. The later can be hindered by steric effects as well as hydration of the cation. The chemical conditions in effect during the preconcentration step might be optimized to favor the formation of the Ni(1I) complex over others. Metals are normally “inserted” into a preformed porphyrin to yield metalloporphyrins. Because of differences in behavior among various ions, no general method is available for metal insertion. While almost every metaI can be forced to form some kind of complex with porphyrins and elaborate methods for metal insertion have been described, the conditions employed in these studies are not forcing. Synthetic mechanisms for metal incorporation involve ring deformation, effective collisions, initial monoanion formation, and involvement of
Concentration, M Flgure 6. Calibration curve for range of Ni(I1) solution concentrations of to 2 X lo-’ M obtained with polymeric TMHPPNi film in 0.1 M NaOH; nickel was preconcentrated from solution at pH 9 for 60 s; film thickness was 20 equivalent monolayers.
a receptor for the displaced protons. Essential is the notion that M(P) formation is an example of a sterically controlled substitution process. In this case, any needed deformation or steric control is there a priori due to the rigidity of the film. Selectivity for Ni(I1) would be expected because the site was produced by Ni(I1) leaving. Nonetheless, experiments were performed to check for apparent incorporation of other metals. The sensitivity of the polymeric TMHPP electrode toward zinc(II), cadmium(II), lead(II), copper(II), iron(III), and cobalt(I1) was examined under optimum conditions (Table I). DP voltammograms were obtained after a 60-s chemical preconcentration of 1pM solutions of individual cations. None of the other cations was found to undergo a redox reaction in the polymeric TMHPP in the range of potential where the Ni(II)/Ni(III) couple is observed. After similar preconcentrations of 1 pM Ni(I1) in the presence of 10 pM zinc(II), cadmium(II), lead(II), copper(II), and iron(III), a small suppression of the nickel peak was observed. This suppression was about 6% in the presence of zinc and copper and about 5% in the presence of iron, cadmium, and lead. However, the suppression in the presence of cobalt was more significant (about 80%). On the basis of these data, it has been estimated that a 50-fold excess of cobalt(I1) will change the detection limit of nickel(I1) to about lo4 M. However, no voltammetric peak due to the oxidation of Co(1I) to Co(II1) incorporated in the polymeric TMHPP was observed. Interference of other cations was only observed in polymeric NiTMHPP as a decrease in peak current of the Ni(II)/Ni(III) couple. Transmetalation, the replacement of one metal by another, would not be expected to be a problem due to the analytical conditions. Not only are nickel porphyrins known to be stable in the presence of other metals (16),following a short preconcentration time, the sensor is transferred to a fresh solution of NaOH for determination. Therefore, the dual issues of selective incorporation and interferences have been shown not to present a problem in the use of the poly-TMHPP sensor for the selective determination of trace levels of Ni(I1). A linear relationship between current and concentration for several regions of concentration indicates the applicability of this method for the determination of nickel. One linear region from about 2 X lo4 M to the detection limit has a slope of 1.33 FA/pM and a 0.935 correlation coefficient (Figure 6). A second region exists at higher concentrations, up to M, with a slope of 0.84, pA/pM and a 0.965 correlation coefficient. Detection limits of 8 X M have been achieved by using the conditions shown in Table I. Analysis of Ni(I1) in certified standard reference material NBS 1643 B (Trace Elements in Water) was 47 ng of 49 ng
Anal. Chem. 1990, 62, 914-923
914 T a b l e 11. R e p r o d u c i b i l i t y
series
I I1 I11
of t h e M e t h o d (n = 7)
amt added PM
amt found, p M
10.00
9.90 1.00 0.19
1 .oo
0.20
s t d dev, pM
0.29 0.07
0.03
re1 s t d dev, %
2.9 7.4 15.0
reported. The reproducibility of the method was determined for 10.00,1.00, and 0.20 KMNi(I1) using optimum conditions. Data, based on seven determinations, are shown in Table 11. The detection limit achieved with T M H P P electrodes is comparable to that reported for the DMGgraphite electrode and the poly-TMHPP system is more stable with both time of use and number of measurements. Some T M H P P electrodes have been used for more than a hundred preconcentration/demetalation voltammetric experiments with no significant deterioration in performance. High mechanical durability makes this electrode suitable as a detector in a flow system.
ACKNOWLEDGMENT The helpful comments from Professor Petr Zuman are gratefully acknowledged. LITERATURE CITED (1) Cox, J. A,; Kuleska, P. J. Anal. Chlm. Acta 1983, 154, 71-78. (2) Wang, J.; Greene, B.; Morgan, C. Anal. Chim. Acta W84, 156, 15-22.
, ,
(7)
(8) (9) (10) (11) 12)
13)
14)
(15) (16) (17) (18) (19)
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RECEIVED for review November 6, 1989. Accepted January 29, 1990. This work was supported in part by the State of Michigan Research Excellence Fund.
Polar-Liquid, Derivatized Cyclodextrin Stationary Phases for the Capillary Gas Chromatography Separation of Enantiomers Daniel W. Armstrong,* Weiyong Li, and Chau-Dung Chang Department of Chemistry, University of Missouri-Rolla, Rolla, Missouri 65401 Josef Pitha National Institutes of Health, NIAIGRC, Baltimore, Maryland 21224 A new dess of hydroph#ic, relathrely polar Ilquld, cydodextrln (CD) derlvatlves have been used as hlghly selectlve chlral statlonary phases (CSPs) for capillary gas chromatography (GC). Several posslble requirements for IIquMity In CD derivatives are dlscussed. O-( S )-2-Hydroxypropyl derivatives of a-,8-, and y-cyclodextrlns were synthesized, exclusively characterlred, permethylated, and evaluated for enantloselectlvlty. Seventy pairs of enanthers were resolved. They represent a wide varlety of structural types and classes of compounds Including chlral alkyl amlnes, amlno alcohols, epoxldes, pyrans, furans, sugars, dlols, esters, ketones, bicyclic compounds, alcohols, and so on. Many of these compounds were not aromatlc and cannot be resolved on any known liquid chromatographic CSP. Often, these enantlomers had far less functlonallty than required for LC separation. General properties of these CSPs as well as possible Insights Into the separation mechanism are dlscussed.
INTRODUCTION Most of the early work on chiral stationary phases (CSPs) for gas chromatography (GC) used amino acids, peptides, and
various derivatives thereof (1-7). Some efforts were made to use other naturally occurring chiral molecules such as tartaric acid, malic acid, mandelic acid, and chrysanthemic acid (7-9). Despite a large amount of work in this area the only resulting widely available and commercially viable CSP for gas chromatography was Chirasil-Val (IO,11). This CSP consists of a siloxane copolymer to which L-valine-tert-butylamide is coupled. There were a number of limitations to these early GC-CSPs (7, 12). First, they did not seem to be widely applicable. Most of the reported separations were of racemic amino acid derivatives. Just as significant was the fact that the high column temperature needed for GC often resulted in racemization, decomposition, and bleeding of the CSP. Even the moderately successful Chirasil-Val is not recommended to be used at temperatures much above 200 "C. Also, it was apparent that enantioselectivity decreased significantly a t the higher temperatures needed for GC. Early on there were a number of efforts to use a- and /?-cyclodextrin as GC stationary phases (13-1 7). I t was apparent from the early GC and more recent LC work that cyclodextrins had potential as gas chromatographic stationary phases. Unfortunately, the CD-GC stationary phases were not as successful as the liquid chromatography bonded stationary phases (18-20). Although interesting selectivities,
0003-2700/90/0362-0914$02.50/00 1990 American Chemical Society
