Anal. Chem. 1990, 62, 1424-1428
1424
Determination of Trace Amounts of Vanadium in Natural Waters and Coal Fly Ash with 2-(8-Quinolylazo)-5-(dimethylamino)phenol by Reversed-Phase Liquid Chromatography-Spectrophotometry Jun'ichiro Miura Department
of
Applied Chemistry and Biotechnology, Fukui University, Bunkyo-3, Fukui, 910 J a p a n
I n reversed-phase high-performance liquid chromatography (RP-HPLC) for neutral and catlonlc metal chelates wlth azo dyes, tetraalkylammonlum salts (TAASs) are added to an aqueous organic mobile phase. The TAASs are dynamically coated on the reversed statlonary support. As a result of the additlon of TAAS, the retention of the chelates Is remarkably reduced. Tetrabutyiammonlum bromide permlts rapid separation and sensltlve spectrophotometrlc detectlon of the vanadlum(V) chelate with 2 4 Oqulnolylazo)b-(dlmethylamlno)phenol, making It possible to determlne trace vanadlum(V). When a 100-mm3 aqueous sample was injected, sensitivity and precision were as follows: peak height calibration curves of vanadlum(V) were linear up to 800 pg at 0.005 absorbance unlt full scale (AUFS) and up to 160 pg at 0.001 AUFS; the relative standard deviation for 10 determlnatlons at 0.005 AUFS was 2 . 3 % at a level of 320 pg of vanadium(V); the detectlon limit was 2.6 pg at 0.001 AUFS. Many catlons Including Iron( II I ) and alumhum( I 11) do not interfere with the determination. Vanadium in coal fly ash and natural waters can be successfully determined without preseparatlon and preconcentratlon of vanadium.
INTRODUCTION Reversed-phase-high-performance liquid chromatography (RP-HPLC) provides a highly selective and sensitive method for the spectrophotometric determination of metal ions when complexed with various chelating agents. Some azo dyes are accepted to be useful for HPLC because they react nonspecifically with various metal ions to form chelates having high molar absorptivities on the order of lo5 L mol-km-'. Recently, tetraalkylammonium salts (TAASs, R4N+)have been employed in the RP-HPLC system for the separation of charged metal chelates ( I ) . However, the use of TAAS is mostly limited to the separation of negatively charged chelates for the enhancement of their retention on a column of an alkyl-bonded reversed stationary support. This HPLC mode is well-known as the ion-pair RP-HPLC. In this study, TAASs were intentionally added to an aqueous organic mobile phase for controlling polarity of the nonpolar stationary support in the RP-HPLC for metal chelates with azo dyes. In the present system, any ion-paired complex between metal chelates with azo dye and R4N+does not form, because the metal chelates with azo dyes are neutral and cationic (2). The retention of some chelates with azo dyes was reduced by addition of TAAS, and, especially, the retention of the positively charged cobalt(II1) chelate remarkably decreased. On the basis of these findings, I propose a novel method for the highly sensitive and selective determination of trace vanadium(V) with 2-(8-quinolylazo)-5-(dimethylamino)phenol. There seems to be no reports on the use of
TAAS in the practical analysis of metal ions as neutral and cationic chelates with RP-HPLC.
EXPERIMENTAL SECTION Apparatus. The chromatographic system consisted of a Shimadzu Model LC-6A pump, a Rheodyne Model 7125 injector with a 100-mm3sample loop, and a Shimadzu Model SPD-6AV variable-wavelength spectrophotometric detector with a 10-mm flow-through cell (8 mm3). A Licrocart column (4-mm bore X 125-mm length, Merck) was used by packing LiChrosorb RP-18 (ODS type, particle size 7 Fm, Merck). The column was preceded by a guard column (4-mm bore X 10-mm length) packed with LiChrosorb RP-18. Prior to use, the column was allowed to equilibrate at a flow rate of 1.0 cm3m i d under various conditions of the mobile phase used. Spectrophotometric data were obtained by using a Shimadzu Model 260 spectrophotometer. All glassware was kept in nitric acid (1 + 1)for a day and more and then was rinsed with deionized water before use. Reagents and Materials. 2-(8-Quinolylazo)-5-(dimethylamino)phenol (DMQAP) was synthesized according to the method reported by Shibata (3). The compound obtained was characterized by elemental analysis: Cl7HI6N40requires 69.8% C, 5.5% H, 19.2% N, 5.5% 0; found 70.1% C, 5.6% H, 18.7% N, 5.6% 0. 2-(8-Quinolylazo)-5-(diethylamino)phenol(QADAP) was provided by Professor T. Yotauyanagi, Tohoku University ( 4 ) . Other azo dyes, 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol (5-Br-PADAP),1-(2-pyridylazo)-2-naphthol(PAN), 2- (2-thiazo1ylazo)-p-cresol(TAC), and 1-(2-thiazolylazo)-2-naphthol (TAN), were purchased from Dojin Laboratories (Kumamoto, Japan). All azo dye solutions were prepared by dissolving the respective compound into an aqueous solution of poly(oxyethy1ene) 4nonylphenyl ether (Tokyo Chemical Industry, Tokyo, Japan) with 10 (PONPE-10) or 20 oxyethylene units (PONPE-20) as nonionic surfactant: A 0.029-g portion of DMQAP was dissolved into a small amount of 1 M sodium hydroxide (1-2 cm3) in a beaker, and then to this solution, a 10-g portion of PONPE-10 was added. An appropriate amount of water, up to about 150 cm3,was added to the mixture with warming on a water bath and stirring. The entire mixture was the stirred gently until a clear solution was obtained. The solution obtained was neutralized by addition of a desired amount of 1 M hydrochloric acid (1-2 cm3). The resultant solution was transferred into a 250-cm3volumetric flask and then made up to the mark with water; QADAP solution was prepared by the manner as in the case of DMQAP except that PONPE-20 was used in place of PONPE-10; 5-Br-PADAP solution was prepared according to a previous report (5), and PAN, TAN, and TAC solutions were prepared as in the case of 5-Br-PADAP. These surfactants were also employed for solubilization of the chelates with azo dyes in the sample solutions. TAASs used were tetramethylammonium (TMA), tetraethylammonium (TEA), tetrabutylammonium (TBA) and tetraoctylammonium (TOA) bromide (Tokyo Chemical Industry, Tokyo, Japan). TMA, TEA, and TBA were dissolved in water, and TOA was dissolved in acetonitrile. Standard metal ion solutions were 1.00 mg cme3 solutions for atomic absorption spectrophotometry (Kanto Chemical Co., Inc., Tokyo, Japan) and were accurately diluted by keeping 0.1 M acid concentration. The mobile phase was aqueous acetonitrilesolution buffered at pH 7.5 f 0.3 with sodium acetate and ethylenediaminetetraacetic acid (EDTA) and in which,
0003-2700/90/0362-1424$02.50/00 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 14, JULY 15, 1990
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Table I. Chromatographic Separation of Metal Chelates with Various Azo Dyes on a LiChrosorb RP-18 Columno azo dyes and their chelates (capacity factorb) in the presence of TBA/lO-* mol kg-' in the absence of TBA
azo dye QADAP DMQAP 5-Br-PADAP PAN TAC TAN
(3.81, R (17.4) (2.0), R (9.7), Ni (15.2) (0.7),R (13.8), Ni-Fe (19.4-19.9) (0.7), R (8.8), Ni (17.3),Fe (18.8) Fe (4.0), Ni-R (5.4) V (0.8),R (9.9),Fe (26.4), Ni (29.0)
V V V V
V V V V
(3.6), R (14.9), Co (26.7), Fe-Ni (35.8-36.7) (1.7), R (6.7), Co (8.7), Fe-A1 (11.5),Ni (12.5) (0.7), Co (5.5),R (9.7), Ni (15.1),Fe (16.8)
(0.7),R (6.8),Co (10.0),Ni (16.41,Fe (18.1) Fe (3.6),Ni (4.0), R (4.4) V (0.7), R (8.6),Fe (21.2), Ni (23.8)
OEach metal ion, 1000 ng; symbol R, azo dye. bCapacityfactors given in parentheses were by triplicate injections.
QADAP
k-PA DAP
a
PAN
TAC R
TAN e
R I
,
0
I
,
20
,
,
40
,
I,&
20 0
,
I
,
20
, I , & , 20 0
, 20
Retention t ime/min Flgure 1. Chromatographic separation of various azo dye systems on LiChrosorb RP-18 column with acetonitrile-water mobile phase containing TBA: TBA, lo-' mol kg-'; CH,CN, 50% w/w; flow rate, 1.0 cm3 min-'. (a) QADAP, (b) DMQAP, (c) 5-Br-PADAP, (d) PAN, (e) TAC, and (f) TAN. Symbol R in figure, azo dye.
if required, TAAS was added. EDTA acts as a masking agent for the metal contaminants from the HPLC apparatus. The mobile phase was degassed under vacuum prior to use. All the aqueous solutions were prepared with distilled deionized water purified with a Milli-Q system (Millipore Corp., Bedford, MA). Recommended Procedure. A sample solution containing less than 160 ng of vanadium(V) was pipetted into a 50-cm3screwcapped glass botlle. To this solution, 5 cm3of 4 x lo4 M DMQAP solution (in 4% w / v PONPE-10 solution) and an acetate buffer solution (pH 5.3) were successively added. If required, 1 M hydrochloric acid or 1 M sodium hydroxide solution was added to neutralize the sample solution and to keep the pH of the final solution at 5.3. The whole mixture was diluted to about 20 cm3 with water and then heated at 90 "C for 10 min. After cooling, the solution was diluted to 25 cm3with water and then injected with a 100-mm3loop injector onto the column. The mobile phase was 50% w/w acetonitrile-water that contained 7.5 X low3mol kg-' of TBA, 5 x mol kg-' of sodium acetate, and 1 x lo-* mol kg-' of EDTA. The flow rate of the mobile phase was 1.0 cm3m i d . The detector setting at 0.005 absorbance unit full scale (AUFS) at 537 nm was used for recording chromatograms. If a sample solution containing less than 40 ng of vanadium(V) was to be analyzed, the AUFS was 0.001. The capacity factor (k? was calculated from values of retention time (TR) and column dead time (To): k'= (TR- T o ) / T pValues of Towere determined from the first base-line disturbance on injection of each sample. Values of TRwere measured from the distance between the injection point and the peak maxima on the chromatogram. Preparation of Coal Fly Ash Sample Solution. The National Institute of Standards and Technology Standard Reference Material 1633a (NIST SRM 1633a, coal fly ash) was used as a sample for the determination of vanadium. The certified values (in pg g-') in NIST SRM 1633a are as follows: V, 300; Al, 140000; Fe, 94000 f 1000; Ca, 11100 f 100; Ti, 8000; Zn, 220 i 10; Mn, 190; Ni, 127 f 4; Cu, 118 f 3; Co, 46. An aqueous sample was prepared according to the method reported by Sato and Sakata
(6). A 0.1952-g portion of NIST SRM 163'3a was digested at 100 "C for an hour in a Teflon-lined bomb with a mixture of 2 cm3 of hydrofluoric acid and 2 cm3 of nitric acid. After cooling, a 25-cm3aliquot of 4% w/v boric acid was added to the solution in the bomb, and then the whole solution was heated again at 100 "C for an hour. Then, the solution was filtered and diluted to 50 cm3. A 10-cm3portion of this solution was diluted to 250 cm3 with 200 cm3 of 0.1 M hydrochloric acid and an appropriate volume of water. A 2-cm3portion of this solution thus obtained was submitted to the analysis with the recommended procedure.
RESULTS A N D DISCUSSION Chromatographic Separation of Azo Dyes and Their Metal Chelates. The present method is based on the precolumn derivatization of metal ions with azo dyes and the subsequent separation of the metal chelates using a LiChrosorb-RP 18 column and an acetonitrile-water mobile phase that contains TBA and no azo dye. A sample solution containing 1.0 wg each of vanadium(V), iron(III), cobalt(II), copper(II), cadmium(11),nickel(II), zinc(II), manganese(fI), and aluminum(II1) was used. Chromatograms of samples prepared a t pH 7-7.5 were measured a t 550 nm with 50% w/w acetonitrile-water mobile phase in the absence and presence of TBA. Table I shows the values of capacity factor, k', for azo dyes and their metal chelates in the absence and presence of TBA in 50% w/w acetonitrilewater mobile phase. Though most metal ions shown above react with azo dyes to form neutral and positively charged chelates in this p H range (2), some chelates such as the zinc chelate decomposed on the column, and in the absence of TBA, a few chelates were adsorbed onto the column. In the presence of TBA, new peak were observed; all or most the values of k'for the chelates were reduced in all systems, the retention time of positively charged cobalt(II1) chelates remarkably decreased. TBA is useful to
1426
ANALYTICAL CHEMISTRY, VOL. 62, NO. 14, JULY 15, 1990
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Flgure 2. Effects of concentration of acetonitrile and addition of TBA on separation of metal chelates with DMQAP. (a) 50% wlw CH,CN, (b) 65% wlw CH3CN, (c)75% w/w CH,CN, and (d) 50% w/w CH&N containing TBA (lo-* mol kg-'). Flow rate, 1.0 cm3 min-'. establish simultaneous and rapid detection system for metal ions with azo dyes. Figure 1 shows the chromatograms of various azo dye systems ((a) QADAP, (b) DMQAP, (c) 5Br-PADAP, (d) PAN, (e) TAC, and (f) TAN) in the presence of TBA. DMQAP was chosen for the determination of vanadium because of its excellent selectivity and sensitivity. DMQAP is a tridentate ligand, with the structure. N=N 3
Retention t irnelrnin
Figure 3. Effect of tetraalkylammonium salt on retention of metal chelates with DMQAP: tetraalkylammonium salt, lo-* mol kg-'. (a) Without tetraalkylammonium salt, (b)TEA, (c) TBA, and (d) TOA. Flow rate, 1.0 cm3 min-'.
DMQAP
DMQAP reacts with various metal ions to form neutral and positively charged chelates such as [VO,(DMQAP)], [Ni(DMQAP),], and [Co(DMQAP),]+. Figure 2 shows the effect of concentration of acetonitrile on the retention of chelates with DMQAP. The most familiar means for controlling retention time is to vary the concentration of a modifier such as acetonitrile in the mobile phase. Acetonitrile always reduces the retention time, but acetonitrile at concentrations higher than 50% w/w leads to lower resolution as is seen in the chromatograms a-c in Figure 2. The iron(III), aluminum(III), and cobalt(II1) chelates were adsorbed firmly on the top of the column. Even if the concentration of acetonitrile was increased in the mobile phase, these chelates could not be eluted from the column. As a result, the strong adsorption of these chelates prolongs the time for the analysis inclusive of washing the column and curtails the life of the column. On the other hand, when TBA was present in the mobile phase, the accumulation of these chelates on the column was completely prevented (Figure 2, chromatogram d). Addition of TBA is very useful, not only for simultaneous detection of these chelates but also for maintenance of the column. Effect of Kind of TAAS. To find more useful TAASs, various TAASs were examined on their effect on the retention of the metal chelates. Figure 3 shows chromatograms of chelates with DMQAP: chromatogram a shows separation of the chelates with 50% w/w acetonitrile-water in the absence of TAAS; chromatograms b-d are those when TEA, TBA, and TOA were added to 50% w/w acetonitrile-water, respectively. Although not shown in the figure, TMA was ineffective for improving the retention. It is seen that the TAAS having the longer alkyl chain gives shorter retention time of the chelates with DMQAP, and the retention of DMQAP was scarcely affected by the length of the alkyl chain. In all cases using other azo dyes, decrease of the retention of their chelates was observed and, especially, that of positively charged cobalt(II1) chelates was conspicuous. When DMQAP is used, the time
A
0 , 0
.
-
5 .
. .%
z
2.5 5.0 7.5 10.0 Concentration of TBAI 103rnoi kg'
Figure 4. Effect of concentration of TBA on k'of DMQAP and its metal chelates: (1) iron(II1)and aluminum(III),(2) cobalt(III),(3) nickel(II), (4) DMQAP, and (5) vanadium(V). CH,CN, 50% wlw. needed for analysis is limited by the retention time of the iron(III), the aluminum(III), and the cobalt(II1) chelates. TBA is excellent in peak resolution and requires a shorter time for detection. Effect of Concentration of TBA. The retention of DMQAP and its metal chelates was investigated under various conditions of TBA concentration in the mobile phase. As shown in Figure 4, the retention of the iron(III), aluminum(III), and cobalt(II1) chelates decreased with increasing concentration of TBA. Hence, it is possible to control the retention by varying the concentration of TBA. I t has been known that TAAS was dynamically retained onto the chemically bonded reversed stationary phase. Two mechanisms on the retention of TAAS have been proposed (7): the first is the hydrophobic interaction of alkyl chains of TAAS with alkyl chains bonded onto silica; the second is the ion exchange between R4N+ and a proton of unreacted silanol group (8). Bartha and Vigh have already reported that the amount of TBA adsorbed onto the stationary support increases with increasing its concentration (9). When mobile phase was switched to 50% w/w acetonitrile-water mobile phase containing no TBA after equilibration of stationary phase with 50% w/w acetonitrile-water mobile phase containing TBA, the effect of TBA gradually disappeared with a repeat of
ANALYTICAL CHEMISTRY, VOL. 62, NO. 14, JULY 15, 1990
r
7 7 r
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Figure 6. Typical chromatograms of standard and coal fly ash sample solutions: (a) Standard solution of V (80.0 ng of V was added in 25 cm3), and (b) sample solution (NIST SRM 1633a, 92.7 ng of V was found in 25 cm3). TBA, 7.5 X mol kg-'; CH,CN, 50% wlw; flow rate, 1.0 cm3 min-'.
Table 11. Determination of Vanadium in Rainwater, Sea Water, and Coal Fly Ash (NISTSRM 1633a) sample
V,bng added found'
rainwater 2.0 seawater 8.0
NIST SRM 1633a0
1.3 3.2 17.2 25.2 92.7
rsd,d % 5.8 3.9 1.8 1.4 3.5
recovery, % 95
100 99
"NIST RSM 1633a was dissolved in acid by the manner described in the text. *Vanadium was respectively added and found in 25 cm3 solutions prepared by recommended procedure. Average of three determinations. Relative standard deviation.
natural water samples were analyzed a t 0.001 AUFS, both alone and spiked with 2.0 ng of vanadium(V) for rainwater and 8.0 ng of vanadium(V) for sea water. A 2-cm3 portion of the NIST SRM 1633a sample solution was analyzed by the recommended method. Figure 6 shows typical chromatograms obtained a t 0.005 AUFS for the determination of vanadium in a standard solution (80.0 ng in 25 cm3, chromatogram a) and in coal fly ash sample solution (chromatogram b). For the coal fly ash sample solution, the amount of vanadium(V) found in 100 mm3 of sample injected was 371 pg; the rsd for the three determinations was 3.5%; the concentration of vanadium found in NIST SRM 1633a was 297 ppm, which agrees closely with the certified value (300 ppm). Large amounts of aluminum(II1) and iron(II1) do not interfere with the determination of vanadium(V). The results are shown in Table 11. Satisfactory recovery (95-10070) was achieved for the samples analyzed. The use of TBA is very effective for the rapid and sensitive determination of trace vanadium. CONCLUSIONS Recently, vanadium has been noticed as the index element in urban environmental pollution, especially air pollution (IO). However, vanadium concentration is so low in air (ng m-3 level) and natural waters such as sea and river waters (sub parts per billion). In Japan, for instance, the vanadium concentration in air obtained by neutron activation analysis (NAA) is on the order of 6.7-14.0 ng m-3 (average 10.0 ng m-3) in Tokyo and 2.6-6.2 ng m-3 (average 4.1 ng m-3) in Sapporo (reported by National Air Sampling Network, Environment Agency, Japan, 1985); the concentration of vanadium in rainwater is down to a few parts per billion in Sendai (11). Vanadium in the environmental samples has been determined by NAA (121,
Anal. Chem. 1990, 62,1428-1431
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inductively coupled plasma atomic emission spectrometry (13), atomic absorption spectrometry (AAS) (14,15),and catalytic spectrophotometry (16-18). The first two methods are disadvantages in terms of cost and instruments in the routine analysis. AAS is often lacking in sensitivity and affected by a matrix of samples such as salinity. Catalytic methods are highly sensitive but are generally lacking in simplicity. The proposed method using DMQAP not only is one of the most sensitive methods for the determination of vanadium but also is excellent in terms of selectivity and simplicity. Therefore, this method will be successfully applied to the monitoring of vanadium in small amounts of environmental samples, for example, 1 m3 of air and 10 cm3 of natural waters. This technique with TAAS further permits an improvement of selectivity for RP-HPLC separation in addition to the choice of a column used and of an organic modifier and its concentration in a mobile phase.
ACKNOWLEDGMENT I thank Professor Hiroto Watanabe of Hokkaido University for helpful discussions and for criticizing the manuscript. I also thank Professor Hidehiko Kitajima and Toshio Morita of Fukui University for suggestions for the synthesis of DMQAP and its elemental analysis. LITERATURE CITED (1) Yotsuyanagi, T.; Hoshino, H. Bunseki 1983, 566-572, and references therein.
Shibata, S. Chelates in Analyricel Chemistty; Barnard, A. J. Jr.; Flaschka. H. A., Ed.; Marcel Dekker: New York, 1972; Vol. 4. Shibata, S.;Furukawa, M.; Toei, K. Anal. Chim. Acta 1973, 66, 397-409. Hoshino, H.; Yotsuyanagi, T. Bunseki Kagaku 1982, 37,E435-E438. Miura, J. Analyst 1989, 174, 1323-1329. Sato, K.; Sakata, M. Bunseki Kagaku 1985, 34, 271-275. Kiel, J. S.;Morgan, S.L.; Abramson, R . K. J. Chromatogr. 1985, 320, 313-323. Hansen, S.H.; Helboe, P.;Thomsen, M. Trends Anal. Chem. 1985, 4 , 233-237. Eartha, A.; Vigh, G. J. Chromatogr. 1983, 260, 337-345. Division of Chemistry and Chemical Technology, Environmental Studies Board, National Research Council, Medical and Biologic Effects of Environmental Pollutants, Vanadium; National Academy of Sciences: Washington, DC, 1974; Japanese Edition, Tokyo Kagakudojin: Tokyo, 1977. Miura, J.; Hoshino, H.; Yotsuyanagi, T. International Symposium on New Sensors and Methods for Environmental Characterization, Kyoto, Japan, November, 1986; S1-03. Greenberg, R. R.; Kingston, H. M. Anal. Chem. 1983, 55, 1160-1165. Wang, C.-F.; Miau, T. T.; Perng, J. Y.; Yeh, S. J.; Chiang, P. C.; Tsai, H. T.; Yang, M. H. Analyst 1989, 174, 1067-1070. Torninaga, M.; Bansho, K.; Umezaki, Y. Anal. Chim. Acta 1985, 169, 171-177. Yamashige. T.; Yamamoto, M.; Sunahara, H. Analyst 1989, 774, 1071-1077. Hirayama, K.; Unohara, N. Bunseki Kagaku 1980, 29, 733-737. Fukasawa, T.; Kawakubo, S.;Okabe, T.; Mlzuike, A. Bunseki Kagaku 1984, 33, 609-614. Nakano, S.;Yamada. C.; Sakai, M.; Kawashima, T. Anal. Sci. 1988, 2 , 61-65.
RECEIVED for review December 28, 1989. Accepted April 2, 1990. This work was supported by a grant from Saneyoshi Scholarship Foundation.
Ion-Selective Electrodes Using an Ionophore Covalently Attached to Carboxylated Poly(viny1 chloride) Sylvia Daunert and Leonidas G. Bachas* Department of Chemistry and Center of Membrane Sciences, University of Kentucky, Lexington, K e n t u c k y 40506-0055
An Ion-selective electrode was prepared by using a polymer membrane on which an Ionophore had been covalently attached. Specifically, benro-15-crown4 was modified chemically to yield 4'-aminobenzo-l5-crownb, whlch was then attached covalently to carboxylated poly(viny1 chloride). The prepared electrode responded selectlvely to potassium wlth a Nernstian slope. This polymer-bound ionophore had an Increased llfetlme when compared to the one obtalned by using a poly( vinyl chloride)-based matrlx impregnated wlth benzo-15-crown-5.
Ion-selective electrodes (ISEs) based on ionophore-impregnated polymer membranes (typically, plasticized poly(vinyl chloride) (PVC)) are now commonly employed in a variety of analyses (1-4). The operational lifetime of these ISEs is affected by the solubility of the ionophore in the plasticizer and can be limited by the leaching of the ionophore and the plasticizer from the polymer matrix (2,5,6). Usually, this leaching also worsens the detection limits of the electrode and results in gradual deterioration of the response (7). Covalently grafting the ionophore to a polymeric backbone has been suggested as a possible solution to the problem of leaching ( 2 ) . The first report of chemical immobilization of
an ionophore on a polymer matrix involved the use of phosphorylated VAGH to develop an electrode for Caz+ (8, 9); VAGH is a copolymer composed of vinyl chloride, vinyl acetate, and vinyl alcohol. An electrode for calcium was also prepared by cross-linking a styrene-n-butadiene-styrene triblock elastomer (SBS) with triallyl phosphate (10). A graft copolymer of cellulose and poly(acrylonitri1e) containing hydroxamic acid was used by Volovik et al. in a Ni2+-selective electrode ( I I ) . Further, anion-selective electrodes have resulted by covalently attaching quaternary ammonium moieties to polymer matrices such as poly(vinylbenzy1 chloride) (71, SBS (12), chloromethylated cross-linked styrene (13), and sulfonated PVC (14). Finally, polymers with functional groups such as Nafion (a perfluorosulfonate polymer) (5),poly( 1,2diaminobenzene) (15), and polypyrrole (16) have been used without plasticizers in the development of ISEs for tributylammonium ions, protons, and chloride, respectively. Typically, the above electrodes were reported to have extended operation lifetimes when compared to PVC-based ISEs prepared with the same ionophore. Some of the reports also indicated that covalent immobilization of the ionophore may result in electrodes that have better detection limits and a wider pH working range (7, IO). In this study, the ionophore was covalently attached to carboxylated poly(viny1chloride) (PVC-COOH), a polymeric
0003-2700/90/0362-1428$02.50/00 1990 American Chemical Society
