Anal. Chem. 1992, 64, 1534-1540
1534
Lead-Selective Bulk Optodes Based on Neutral Ionophores with Subnanomolar Detection Limits Markus Lerchi, Eric Bakker, Bruno Rusterholz, and Wilhelm Simon’ Swiss Federal Institute of Technology (ETH), Department of Organic Chemistry, Universitatstrasse 16, CH-8092Ziirich, Switzerland
An optlcal sensor (optode) based on a plastlclred poly(vlnyl chloride) (PVC) bulk membrane lncorporatlng a metal lonselective Ionophore, a proton-selectlve chromolonophore,and lipophilic anknlc sttee for the measurementof lead Ispresented. W e r e n t Ionophores and chromolonophores were used to dedgn a new optode for envlronmental senslng propertles. The present optode system shows an especially excellent selectMty over akaH and akallne-earth metals. The measulng range for lead was found to be 5.0 X I O 4 to 5.0 X I O 4 M at pH 5.68 wlth a correspondlng detectlon llmlt of 3.2 X M. ThIs reversibly worklng optode shows different dynamlc ranges dependlng on the basiclty of the chromolonophoreand on the pH of the sample solution. Therefore, these sensors can be tailored to a speclflc appllcatlon by varylng the above mentloned parameters. Dynamlc range, detectlon llmlt, response behavior, short-tlme repeatablllty, stablllty, and selectlvky of the optode systems are dlscussed. Comparlson wlth polarography shows good agreement.
INTRODUCT10N The need for the determination of lead and other heavy metals increased during the last few years because of growing environmental problems. The World Health Organization (WHO)and the U.S.Environmental Protection Agency (EPA) tolerate a maximum amount of 0.05 mg of lead/L of drinking For waste water, the maximum tolerable lead concentrations are of about the same order. The Swiss law admits 0.5 mg/L (2.4 X 10-6M) if the waste water is introduced into sewerage and 0.05 mg/L (2.4 X M) if it is introduced directly into surface waters (rivers, lake^).^ Common analytical methods for lead determination are furnace AAS and polarography. But for monitoring purposes no chemical sensor is available. Besides the necessity of very low required detection limits, the other main problem is to reach good selectivity properties. Especially good selectivity over alkali and alkaline-earth metals has to be obtained, as for example the natural concentration of calcium in drinking water is about 10-3 ~ . 5 Some lead ion-selective electrodes (ISEs) based on liquid or solid-state membranes have been published! but all of them with insufficient detection limit and selectivity prop(1)World Health Organization. Guidelines for Drinking- Water Quality, Vol. I: Recommendations; W H O Geneva, 1984. (2)U.S. Environmental Protection Agency, Office of Water and HazardousMateriala. Quality Criteria for Water;U.S.G P O Washington, DC, 1976. (3)U S . Environmental Protection Agency, Criteria and Standards Division, Office of Water Planning. Ambient Water Quality Criteria for U S . G P O Washington, DC, 1980. Lead. EPA 440/5-80-057; (4)Swiss Federal Council. Verordnung aber Abwassereinleitungen. No. 814.225.21;Berne, December 8,1975 (revised April 1, 1989). (5)Zobrist, Jiug. Gas, Wasser, Abwasser 1983,63, 123. (6)Umezawa, Yoshio. Handbook of Ion-Selective Electrodes: Selectivity Coefficients;CRC Press: Boca Raton, Ann Arbor, Boston, 1990.
erties for environmental sensing. Also electrodes based on chalcogenide glasses7and results of recent investigations on lead-selectiveionophores in liquid membrane ISEsgl1 do not fulfil the demanded requirements. Recently, we reported on bulk optode sensors for different cations,12-16a n i o n ~ , ~ and ~ 3neutral ~~ species,1+-22 all of them based on conventional, electrically neutral carriers. Here we report on the first lead-selective optode. This cation-exchangesystem23based on neutral ionophoresconsists of a plasticized solvent polymeric membrane incorporating lipophilic anionic sites and two cation-selective carriers, one of which is selectively interacting with lead (ionophore), the other one being a lipophilizedhydrogen ion-selectiveindicator (chromoionophore) which drastically changes its optical properties upon protonation. Like all earlier described ionexchange system optodes,12-16this sensor responds to the ratio of the two involved ions. To allow the measurement of lead, the pH of the sample solution has to be kept constant by buffering or has to be measured simultaneously. Earlier developed ionophores for lead%and uranyl cations26 (which were used in solvent polymeric membrane electrodes) and new ionophores are investigated for their use in lead optodes. (7)Owen, A. E. J. Non-Cryst. Solids 1980,35 & 36,999. (8)Novikov, E. A.; Shpigun, L. K.; Zolotov, Yu. A. Zh.Anal. Khim. 1987, 42,885. (9)Jaber, A. M.Y.; Moody, G. J.; Thomas, J. D. R. Analyst 1988,113, 1409. (10)Malinowska, Elzbieta. Analyst 1990,115,1085. (11)Kamata, Satsuo;Onoyama,Kazuhiro. Anal. Chem. 1991,63,1295. (12)Morf, WemerE.; Seiler, K u r t ; R u s t e r h o l z , B r ; Simon, Wilhelm. Anal. Chem. 1990,62,738. (13)Wang, Kemin; Seiler, Kurt; Morf, Werner E.; Spichiger, Ursula E.; Simon, Wilhelm; Lindner, ErnB; Pungor, EmB. Anal. Sci. 1990,6, 715. (14)Seiler, Kurt; Morf, Wemer E.; Rusterholz, Bruno; Simon, Wilhelm. Anal. Sci. 1989,5, 557. (15)Seiler, Kurt; Wang, Kemin, Bakker, Eric; Morf, Wemer E.; Rusterholz, Bruno; Spichiger, . - Ursula E.; Simon, Wilhelm. Clin. Chem. 1991, 37,1350. (16)Wang, Kemin; Seiler, Kurt; Rusterholz, Bruno; Simon, Wilhelm. Analyst 1992,117,57. (17)Tan, Susie S. S.; Hauser, Peter C.; Chaniotakis, Nicolas A.; Suter, Gabriela; Simon, Wilhelm. Chimia 1989,43,257. (18)Tan, Susie S. S.; Hauser, Peter C.; Wang, Kemin; Fluri, Karl; Seiler, Kurt; Rusterholz, Bruno; Suter, Gabriela; Kriittli, Martin, Spichiger, Ursula E.; Simon, Wilhelm. Anal. Chim.Acta 1991,255,35. (19)Ozawa, Satoshi; Hauser, Peter C.; Seiler, Kurt; Tan, Susie S. S.; Morf, Werner E.; Simon, Wilhelm. Anal. Chem. 1991,63,640. (20) Wang, Kemin; Seiler, Kurt; Haug, Jean-Pierre; Lehmann, Beatrice; West, Steven; Hartmann, Karel; Simon, Wilhelm. Anal. Chem. 1991,63, 970. (21)Seiler, Kurt; Wang, Kemin; Kuratli, Matthias; Simon, Wilhelm. Anal. Chim. Acta 1991,244,151. (22) West, Steven J.; Ozawa, Satoahi; Seiler, Kurt; Tan, Susie 5. S.; Simon, Wilhelm. Anal. Chem. 1992,64,533. (23)Morf, Werner E.; Seiler, Kurt; Smenaen, Paul R.; Simon, Wilhelm. Inlon-selective electrodes;Pungor, EmB, Ed.; Akademiai Kiado: Budapest, 1989;Vol. 5, pp 141-162. (24)Lindner, Ern& T6th, K k a ; Pungor, Em& Behm, Felix; Oggenfuss, Peter; Welti, Dieter H.; Ammann, Daniel; Morf, Wemer E.; Pretech Ern& Simon, Wilhelm. Anal. Chem. 1984,56,1127. (25)b n k y r , Jarmlav; Ammann, Daniel; Meier, Peter C.; Morf, Wemer E.; Pretsch, Emd; Simon, Wilhelm. Anal. Chem. 1979,51,786. 0 1992 Amerlcan Chemical Society
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