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Coated-Wire Ion-Selective Electrodes for Levobetaxolol Paul J. Missel* Alcon Research Ltd., R2-45, 6201 South Freeway, Fort Worth, Texas 76134 Received June 29, 1999. In Final Form: August 26, 1999 Electrodes were fabricated to be selective for Levobetaxolol. The coated-wire approach of Freiser et al. was used, but the polymeric materials were very similar to those used by the Salford group to construct membrane electrodes, less susceptible to interference, by virtue of covalent attachment of the counterionophore to the PVC membrane. The electrodes obtained combined the advantages of quick response and greater selectivity. A commercially available carboxylated poly(vinyl chloride) polymer behaved nearly identically to the end-sulfated polymer used by the Salford group.
Introduction 4-6
have reported that ion-selective Recent reviews electrodes are being used increasingly for the analysis of pharmaceutical products. Further studies7-26 seem to be increasing at a steady rate all over the world, quite possibly due to the low equipment costs of this simple analytical technique. Most of the electrodes fabricated for analysis of drug compounds are of the liquid membrane PVC type with an ionically active species incorporated in the membrane, quite often codissolved with a plasticizer before the solvent is evaporated. Since many drugs are charged, this active species is quite often a hydrophobic ion of the charge opposite that of the analyte. The role of the plasticizer was thought to render the membrane material in a noncrystalline, liquid-like state that would enable the free mobility of analyte into the membrane so that it can interact with the active species and generate the desired electrode response (though this interpretation has been challenged recently by a view that mainly surface phenomena are involved27). Electrodes typically place the membrane against an inner chamber containing either an electrochemical reference solution or a reference * Fax: (817) 568-6952. Phone: (817) 551-4926. E-mail: paul.
[email protected]. (1) Martin, C. R.; Freiser, H. Anal. Chem. 1980, 52, 562. (2) Freiser, H. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1217. (3) Takisawa, N.; Hall, D. G.; Wyn-Jones, E.; Brown, P. J. Chem. Soc., Faraday Trans. 1 1988, 84, 3059. (4) Shoukry, A. F. Sci. Pap. Univ. Pardubice, Ser. A 1995, 1, 5. (5) Stefan, R.-I.; et al. Crit. Rev. Anal. Chem. 1997, 27, 307. (6) Menon, S. K.; et al. Rev. Anal. Chem. 1997, 16, 333. (7) Watanabe, K.; et al. Anal. Chim. Acta 1995, 316, 371; Bunseki Kagaku 1997, 46, 1019. (8) Amin, A.; Zareh, M. M. Monatsh. Chem. 1996, 127, 1123. (9) Li, C.; et al. Huanjing Kexue 1996, 17, 71. (10) Katsu, T.; Mori, Y. Talanta 1996, 43, 755. (11) Wang, M.; et al. Fenxi Huaxue 1997, 25, 448. (12) Katsu, T.; Mori, Y. Anal. Chim. Acta 1997, 343, 79. (13) Hopkala, H.; et al. Pharmazie 1997, 52, 307. (14) Kataky, R.; et al. Electroanalysis 1997, 9, 1267. (15) Erdem, A.; et al. Electroanalysis 1997, 9, 932. (16) Gerlache, M.; et al. Anal. Chim. Acta 1997, 349, 59. (17) Hur, M.-H.; et al. Arch. Pharmacol. Res. 1997, 20, 68. (18) Magnuszewska, B.; Figaszewski, Z. Chem. Anal. (Warsaw) 1998, 43, 265. (19) Li, G.; et al. Fenxi Shiyanshi 1998, 17, 13. (20) Masadome, T.; et al. J. Flow Injection Anal. 1998, 15, 242. (21) Min, S.; et al. Fenxi Yiqi 1998, 2, 22. (22) Katsu, T.; et al. Pharm. Biomed. Anal. 1999, 19, 585. (23) Huang, C.-L.; et al. Talanta 1996, 43, 2061; Fenxi Huaxue 1996, 24, 1993; Uaoxue Xuebao 1996, 31, 535. (24) Rover, L., Jr.; et al. Anal. Chim. Acta 1998, 366, 103. (25) Fernandes, J. C. B.; et al. J. Braz. Chem. Soc. 1998, 9, 249. (26) Khalil, S. Analyst 1999, 124, 139. (27) Pungor, E. Microchem. J. 1997, 57, 251; Anal. Sci. 1998, 14, 249.
electrode suspended in an organic solution containing a ballast of the ionically active sensing species. Unfortunately, our experience has shown that though these types of electrodes possess the desired sensitivity, difficulties are frequently encountered when they are employed in complex formulations containing solubilizers, preservatives, and other excipients. The key weakness appears to be the fact that the active agent and plasticizer are not perfectly immobilized in the membrane; thus, when exposed to a realistic formulation, the membrane composition could very well change composition with time, by allowing either for extraction of the active species into a solubilizing phase or incorporation of a surfactant preservative into the membrane (which amounts to a type of interference). There are two strategies that can be employed to overcome these weaknesses. The first is to eliminate the inner compartment by casting the polymer membrane film directly onto a conducting substance such as silver,23 graphite,24,25 or copper.26 In this way only one surface is subject to depletion by exposure to aqueous solution. The resulting electrode is simpler to maintain, since no filling solution is required, and if the membrane film is cast thinly enough, such electrodes usually equilibrate much faster with the solution. The second strategy (that seems not to have been employed very widely) is to immobilize the electroactive species by attaching it covalently to the polymeric material, such as employed by the Salford group.3 The unique contribution of the present work is to attempt to use both of these strategies in combination and to provide data evaluating how well the carboxylmodified PVC works using this technique. A previous study28 described the fabrication and use of ion-selective electrodes for measuring the free concentration of the preservative benzalkonium chloride in realistic ophthalmic formulations. The electrodes fabricated for this work were for Levobetaxolol, an important active substance for the treatment of glaucoma. The coated-wire approach of Freiser et al.1,2 was used initially; the electroactive counterspecies to the analyte is the hydrophobic ion dinonylnaphthalenesulfonic acid (DNNS). This proved not to be useful in the current application, because of the interference of certain formulation components, most notably the preservatives. Another difference between the membrane materials for the two types of electrodes is the plasticizer used to impart liquid-like properties to the PVC membrane. Freiser et al.1,2 used dioctylphthalate; the Salford group (28) Missel, P. J. Appl. Spectrosc. 1996, 50, 1203.
10.1021/la990847c CCC: $18.00 © 1999 American Chemical Society Published on Web 09/16/1999
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employed a high-molecular-weight polymeric plasticizer. Hence, both the counterion and the plasticizer are more securely anchored to the electrode membrane in the second type of electrode. Coated-wire electrodes were thus fabricated using the new materials to determine if the interferences could be overcome. Materials High-molecular-weight poly(vinyl chloride) suitable for ionselective electrodes was obtained from Fluka (Ronkonkoma, NY). The plasticizer and electroactive species for the Freiser type electrodes were described in the previous paper.28 Two different types of PVC were used to provide the electroactive species for the new electrodes. The first type was the end-sulfated PVC used in the Salford studies,3 which was kindly supplied by Professor Denver Hall. The second was a lightly carboxylated PVC available from Aldrich (Milwaukee, WI) under catalog number 18,955-3, labeled to have an average molecular weight of 220 000 by GPC and a carboxyl content of 1.8%. Elvaloy 742, the plasticizer (chemical name poly(ethylene/vinyl acetate/carbon monoxide)), was provided as a sample from Dupont (Wilmington, DE). Levobetaxolol ((-)-1-[2-(cyclopropylmethoxy)ethyl]-3-(isopropylamino)-2-propanol) base and hydrochloride were obtained from Laboratoire D’Etudes Et De Recherches Synthelabo, Paris, France. Polyquaternium-1 was obtained from the Alcon Manufacturing supply. Dibasic sodium phosphate and monobasic potassium phosphate salts were obtained from Baker (Phillipsburg, NJ).
Figure 1. EMF response of DNNS-based Levobetaxolol ISE against a calomel reference electrode to varying concentration of Levobetaxolol HCl.
Methods Fabrication Methods. Freiser type electrodes were also fabricated as described previously,28 substituting Levobetaxolol base in place of the benzyldodecinium bromide. The new type of coated-wire electrodes were fabricated as follows. Copper wires were coated with unmodified PVC by repeated dipping in a 7.5% solution of PVC in THF and allowed to dry overnight. The tip of this coating was cut, exposing about 1 mm of bare wire. The wire was repeatedly dipped in a THF solution containing 0.18 g of Elvaloy, 0.12 g of charged PVC, and 0.3 g of Levobetaxolol base. The quantity of THF used was the minimum amount required to dissolve most of the solids upon heating to 55 °C; in the case of the carboxylated PVC, this was about 9 mL, and in the case of the end-sulfated PVC this was about 23 mL. The THF solutions were filtered through 0.1 µm glass acrodisc syringe filters to remove undissolved solids (most likely undisclosed stabilizers and lubricants in the Elvaloy plasticizer) before use in coating wires. Measuring Electrode Response. All solutions contained a very low concentration of phosphate buffer (1.3 mM KH2PO4 and 2.9 mM Na2HPO4) as a supporting electrolyte for EMF measurements and were read against a calomel reference electrode (Radiometer REF401). Calibrating Electrode Response. The electrode response to an increasing concentration of drug was measured using an autotitrator (Radiometer ABU93 Autoburet with SAM90 Sample Station) equipped with two electrode input channels and three separate burets having capacities of 1, 5, and 25 mL of titrant, respectively. Each buret was supplied with a different concentration of drug in buffer as follows: 6 × 10-5 M Levobetaxolol HCl in the 1 mL buret, 1.5 × 10-3 M in the 5 mL buret, and 3 × 10-2 M in the 25 mL buret. The titration started with 25 mL of background buffer in the titration cup. Titrant was added from the smallest buret first; 41.7 µL was required to bring the initial concentration of Levobetaxolol in the titration cup to 10-7 M. Titrant aliquots were increased in volume and/or concentration (by switching to the next buret) as follows: (1) the titrant concentration in the cup was increased uniformly to provide eight measurements per decade increase in titrant concentration; (2) no buret had to be refilled more than once. For example, the final aliquot of 3 × 10-2 M Levobetaxolol added had a volume of 7.44 mL. Two minutes time was allowed for equilibration after the addition of each aliquot before the EMF was read. The autoburet was
Figure 2. EMF response of COOH-PVC and OSO3H-PVC Levobetaxolol ISEs against separate calomel reference electrodes to varying concentration of Levobetaxolol HCL (data obtained from a single titration). controlled with an IBM-type 386 computer using the ABU9X programmer’s pack. Measuring Susceptibility to Interference. The response of the electrode/reference pair was compared for two solutions, each containing (in addition to the electrode background buffer) 1 mM Levobetaxolol HCl with one also containing 0.01% polyquaternium-1.
Results Sensitivity. Sample calibration curves are plotted in the figures for each of the electrode types. The curve for the Freiser type electrode (using DNNS as the electroactive species) appears in Figure 1. The curves for each of the new types of electrodes appear in Figure 2 (obtained in a single titration by applying each electrode to a separate input channel and measuring the EMF against separate reference electrodes placed in the titrant cup simultaneously). The curves shown are fits to a variant of the Nikolsky29 equation (eq 1). Parameter values appear in Table 1. All three electrodes demonstrated an approximately ideal response (having limiting slope S0 close to the Nernstian value of 59.1 mV/decade) and displayed a limit of detection on the order of 10-5 M Levobetaxolol against the background electrode buffer. The newer type of electrodes had slightly lower limiting slopes but were (29) Koryta, J. Ions, Electrodes and Membranes; Wiley: New York, 1982; p 140.
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Table 1. Values of Parameters Deduced from Fits of Electrode Response to Levobetaxolol Concentration Using eq 1
Table 2. Difference in EMF Induced by Addition of 0.01% Polyquaternium-1 to a Solution Containing Electrode Buffer and 1 mM Levobetaxolol HCl
electrode type
E0
S1
log10(A0)
electrode type
difference in EMF (mV)
DNNS-PVC COOH-PVC OSO3H-PVC
108.5 ( 1.1 228.1 ( 1.3 216.5 ( 0.2
59.9 ( 0.4 52.8 ( 0.4 54.0 ( 0.2
-4.74 ( 0.13 -5.25 ( 0.02 -5.10 ( 0.01
DNNS-PVC COOH-PVC OSO3H-PVC
+12.0 +3.8 -0.2
more sensitive (lower A0 values) by up to half a decade as compared to the DNNS electrode.
MV ) E0 + S1 log10(C + A0)
(1)
Selectivity. Table 2 shows the difference in EMF registered by the electrode pair when immersed in a solution of 1 mM Levobetaxolol without versus with 0.01% polyquaternium-1. The new electrode types show little (if any) interference compared to that for the DNNS electrode. Conclusion One might draw the following conclusions from the present study:
(1) All three electrodes display comparable responses (limiting slope) and sensitivity (limits of detection). (2) Greater immobilization of ionic species and/or plasticizer in the membrane reduces interference from polyquaternium-1. (3) Commercially available carboxylated PVC may be a useful substitute for end-sulfated PVC (which unfortunately is no longer available commercially30). Acknowledgment. The author acknowledges kindly input from Professors Denver Hall, Phillip Meares, and Derek Bloor, and the gift of a small sample of end-sulfated PVC provided by Professor Hall. LA990847C (30) Meares, P. Private communication.
