658
Anal. Chem. 1986. 58. 658-661
solution caused a dramatic, progressive reduction in the aluminum signal from a new graphite furnace. The effect of the sulfuric acid on the aluminum signal is in contrast to the observation by Kaiser e t al. ( 4 ) that aluminum in a sulfate matrix (500 pg/mL) gave essentially the same working curve as aluminum in a 0.16 M nitric acid matrix. Nitrate has been reported to depress significantly the signal from aluminum in GFAAS (6). Kaiser et al. state that nitrate was the dominant anion in their work even in the presence of the sulfate matrix. Possibly the aluminum response in a 0.16 M "OB solution was depressed so much due to the nitrate that the difference between sulfate and nitrate was not observed. In summary, these results indicate that the addition of micromolar concentrations of phosphoric acid improves significantly the GFAAS determination of aluminum a t trace level concentrations in water. Registry No. Al, 7429-90-5;H20,7732-18-5;H3P04,7664-38-2.
LITERATURE CITED (1) Rahn, K. A.; Lowenthal, D. H. Science 1984, 223, 132-139. (2) Harte, J. Atmos. Environ. 1983, 17, 403-408. (3) Slavin, W.; Carnrick, G. R.; Manning, D. C. Anal. Chem. 1982, 5 4 , 621-624.
(4) Kaiser, M. L.; Koirtyohann, S. R.; Hinderberger, E. J.; Taylor, H. E. SDeCtfOChim. Acta, Part 8 1981. 368. 773-783. Siavin, W. "Analytical Techniques in Environmental Chemistry 2"; Pergammon Press: New York, 1962; pp 397-405. Persson, J. A.; Frech W.; Cedergren, A. Anal. Chim. Acta 1977, 9 2 , 95-104. Slavin, W.; Carnrick, G. R.: Manning, D. C. Anal. Chim. Acta 1982, 738, 103-1 10. Sperling, K. Fresenius' 2.Anal. Chem. 1982, 311, 656-664. Czobik, E. J.; Matousek, J. P. Anal. Chem. 1978, 5 0 , 2-10. Lawson, S. R.; Woodriff, R. Spectrochim. Acta, Part B 1980, 3 5 8 , 753-763. May, T. W.; Brumbaugh, W. G. Anal. Chem. 1982, 5 4 , 1032-1037. Eaton, D. K.; Holcombe, J. A. Anal. Chem. 1983, 55, 946-950. Manning, D. C.; Slavin, W.; Carnrick, G. R. Spectrochim, Acta, Part B 1982, 378,331-341. Craney, C. L.;Glennon, A. E.; Zuhoski, S. P., manuscriut submitted to En viron Sci . Techno/. Severin, G.; Schumacher, E.; Umland, F. Fresenius' 2.Anal. Chem. 1982, 371, 205-208. Slavin, W.; Carnrick, G. R.; Manning, D. C. Anal. Chem. 1984, 56, 163- 168. Slavin, W.; Carnrick, G. R. Spectrochim. Acta, Part B 1984, 3 9 8 , 271-282. Garmestani, K.; Blotcky, A. J.; Rack, E. P. Anal. Chem. 1978, 5 0 , 144-147.
.
RECEIVED for review March 14,1985. Resubmitted October 1, 1985. Accepted October 21, 1985.
Propylation Technique for the Simultaneous Determination of Tetraalkyllead and Ionic Alkyllead Species by Gas Chromatography/Atomic Absorption Spectrometry M. Radojevib, A. Allen, S. Rapsomanikis, and Roy M. Harrison* Department of Chemistry, University of Essex, Colchester C04 3SQ, United Kingdom Gas chromatography/atomic absorption spectrometry (GC/AAS) is a sensitive and species-specific method applied in the determination of organometallic compounds in environmental samples ( I , 2). I t has been used extensively for the detection and measurement of organolead compounds because of the concern for the fate of tetraalkyllead (R4Pb) compounds released into the environment as a result of their use as gasoline additives. In the environment R4Pb compounds degrade to inorganic lead according to the following simplified scheme:
R4Pb
- R3Pb+
-
R2Pb2+ Pb2+
R = CH3, CzHs, singly or in combination Only R4Pb species can be determined directly by GC/AAS, and to determine ionic alkyllead species, these must first be derivatized to volatile R4Pb compounds using alkylating reagents. The only technique capable of simultaneous determination of R4Pb and ionic alkyllead compounds involves extraction into an organic solvent in the presence of sodium diethyldithiocarbamate (NaDDTC) and derivatizations of ionic species to R4Pb forms using a butylating Grignard reagent (n-butylmagnesium chloride) followed by GC/AAS analysis ( 3 , 4 ) . This technique has been applied to many environmental samples with some degree of success (4-8), although for many less polluted samples even greater sensitivity is required. In this communication we present an improved and novel derivatization technique for the simultaneous determination of R4Pb and ionic alkyllead species involving extraction into n-hexane and propylation to the tetraalkyllead form using propylmagnesium chloride with GC/electrothermal AAS 0003-2700/86/0358-0658$01 S O / O
analysis. The technique is compared to the previously reported butylation method, and procedures of extract concentration are investigated.
EXPERIMENTAL SECTION Tetramethyllead, tetraethyllead, and their mixed derivatives in diisopropyl ether, pure tetramethyllead and tetraethyllead, trimethyllead chloride, triethyllead chloride, dimethyllead dichloride, and diethyllead dichloride were obtained from the Associated Octel Co., Ltd., S.Wirral, England, and stored in the dark at 4 "C. Stock solutions of the individual ionic alkyllead compounds were prepared by dissolving the salts in water to give 100 fig mL-l Pb. In the case of the dialkyllead salts, it was found necessary to dissolve them in some methanol first. These solutions were stored in the dark at 4 "C and found to be stable for up to 1 month without noticeable deterioration. It was, however, observed that the alkyllead salts decomposed with time and it was therefore necessary to standardize them by selective extraction of alkyllead species into iodine monochloride and analysis by graphite furnace AAS (9). Calibration standards of R4Pb species were prepared by diluting the mixed R4Pb compounds with nhexane t o give -1 ng fiL-l Pb of each species. The Grignard reagents, propylmagnesium chloride in diethyl ether (2 M) and n-butylmagnesium chloride in tetrahydrofuran (2.18 M) were obtained from Aldrich and Alfa Products, respectively. High-purity Milli-& water was used throughout the study. Sodium diethyldithiocarbamate(NaDDTC) and the other reagents employed in the study were obtained from BDH Chemicals. Instrumentation. A Perkin-Elmer F17 gas chromatograph was equipped with a silanized 1 m long glass column (6 mm 0.d. X 2 mm i.d.) packed with 10% OV-101 on Chromosorb W (8CrlaO mesh) with a carrier gas flow rate of 120 mL min-'. The injector port was at 150 "C and the oven was programmed from 80 "C
-
0 1966 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
659
Table I. Summary of Investigation into the Extraction, Alkylation, and Analysis of Organolead Compounds by GC/AAS
compound
retention time, min
detection limit" g of Pb x 10-9
(CH314Pb (CH3)dCzHdPb (CH3)z(CzHM'b (CH3)(C$M" (CZH5)dPb
0.5 0.8 1.2 1.7 2.3
0.01 0.01 0.02 0.03 0.03
f f f f
1.1 2.1 2.9 3.4 4.8
0.02 0.03 0.05 0.08 0.27
104 f 95 f 100 98 f 6
1.5 3.4 3.8 4.9 7.1
0.03 0.04 0.05 0.11 -e
extraction/ alkylation efficiency,b%
f
concentration efficiency,' % KudernaDanish purging with Nz evaporation 50 f 3 73 f 5 84 f 4 92 f 5 94 f 5
aqueous detection limit,dg L-' Pb X lo-'
24 f 3 30 f 3 36 f 6 41 f 13 42 f 14
0.2 0.1 0.2 0.3 0.3
5 10 8 12
73 f 4 80 f 8 81 f 14 81 f I 84 f 1 2
0.3 0.4 0.6 1.0 54
92 f 1 2
96 f 12 91 83 f 7 104 -
0.3 1.4 0.6 6 -
*
32 f 3 95 f 9 18 f 4 -
Detection limit = 3 X standard deviation of noise/sensitivity. bMean and standard deviation of five determinations, except butylated dialkyllead (three determinations) and propylated Pb2+(one determination). Mean and standard deviation of three determinations except butylated dialkyllead (one determination). dBased on 1 L sample, 5 mL of hexane, and concentration by purging with N2 to 0.5 mL; 50 WL injection. e Not determined. f Extraction efficiency 100%. cm
16-
shaken mechanically for 30 min. The organic and aqueous phases , were separated and the n-hexane extracts transferred to small
.--irrmmmmrrmmmTn
Y..
I
I I
+
L l l l omium wire 'Nichrome' lllL1)sL
Ut !t from
Figure
1.
GC
H2 80 m l rnin-'
Electrothermal detector cell.
to 180 OC a t 20 "C min-'. The atomic absorption spectrometer was a Perkin-Elmer 305 equipped with a hollow cathode lamp operated a t 8 mA, a wavelength of 283.3 nm, and a slit width of 3 mm. A Perkin-Elmer deuterium arc background corrector was used in some runs. The GC was interfaced to the AAS by means of a heated stainless steel transfer line ('12 m X 0.6 mm i.d.) heated to 120 OC and the eluate was introduced into an electrothermally heated quartz atomization cell, which lay along the light beam. Hydrogen was also introduced into the cell where it burns as a small diffusion flame. No difference in performance of the system was noted when either helium or nitrogen was employed as the GC carrier gas. Nitrogen was used in the work reported here. Peaks were recorded on a Rikadenki Mitsui chart recorder operated a t 10 mV and a chart speed of 2 cm mi&. The GC/AAS system was optimized using mixed TAL and butylated TriAL standards in the same way as reported for the GC/flame AAS system ( 4 , IO). Slight day to day variations in performance were noted, and the system was calibrated on a daily basis using standards. In the present study we employed an electrothermal atomization cell shown in Figure 1. The quartz tube was wrapped with 22 gauge nichrome wire and insulated with a porcelain sleeve and glass fiber. The electrothermal cell was controlled by a Variac transformer a t 40 V and 300 W and a temperature of 950 "C was measured inside the tube. The GC was interfaced to the cell via a T-piece with side arm, shown in the figure. The effect of a 2-m GC column was also examined. Procedure. One hundred milliliter solutions containing trialkyl- and dialkyllead compounds individually and in combination were prepared to give concentrations of 20-4Opg L-l P b of the individual species. To some of these solutions mixed R4Pb in diisopropyl ether or in methanol was added to give similar concentrations. Five milliliters of NaDDTC (0.5 M), 5 g of NaCl, and 5 mL of n-hexane were added to the solutions, and these were
glass vials. One-half milliliter of the alkylating Grignard reagent (either propyl- or n-butylmagnesium chloride) was added, and the tubes were shaken gently for 8 min. The extract was washed with 5 mL of HzS04 (0.5 M) to destroy any excess Grignard reagent. The phases were separated, and the organic phase was dried with anhydrous Na2S04. Microliter aliquots (0.5-5 pL) of extracts were injected into the GC/AAS system for analysis.
RESULTS AND DISCUSSION GC/AAS Analysis. A typical GC/AAS trace of a propylated standard containing R,Pb, R3Pb+,and R2Pb2+species as well as propylated Pb2+ is shown in Figure 2, , illustrating that the technique is suitable for the simultaneous determination of these species and can be employed for the total speciation of organic lead in aqueous solution. The first peak is due to the n-hexane solvent and as much as 50 pL of hexane could be tolerated without interference with the (CH3)4Pb peak in the present system. For the five R4Pb compounds (R = CH,, C2H5,or their combination), we found that equal quantities gave equal peak areas (f4%) and peak areas may therefore be employed as an absolute measure of organic Pb. T h e precisions of determination of R4Pb compounds were found t o be f7 t o f12% (relative standard deviation) based on peak height determinations from seven experiments using the five R4Pb compounds a t 1 ng (Pb) each. Retention times and detection limits for the various alkyllead compounds employed in our study are given in Table I. Because of peak broadening with increasing retention time, the detection limits were based on peak heights and were estimated as (3 x standard deviation of the base line noise)/sensitivity. Both peak areas and peak heights could be used for the quantification of environmental samples, although we preferred the use of peak areas. Propylation of ionic lead species results in compounds more volatile than the corresponding butylated products, and it is therefore the preferred derivatization technique for GC/AAS analysis. With our technique, peaks due to propylated (CH3)zPb2fand (C2H6),Pb+are much better resolved than those of the corresponding butylated species, and propylation may be used to validate results obtained with the butylation method. The shorter retention times for the propylated species imply a faster analysis, and the elution of alkylated Pb2+is achieved considerably more rapidly due t o
660
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
the greater volatility of (C3H,),Pb than of (C4Hg),Pb. In many environmental samples we found it necessary to hold the final temperature for long periods in order to remove (C4Hg),Pb from the column. The use of a 2-m column containing the same packing and a t the same conditions improved the separation of the peaks, particularly those due to the butylated species; however, the increase in retention time results in longer analysis and a much slower "cleanup" of the column. Other organometallic compounds may also be extracted from environmental samples but their interference is circumvented by the use of the element-specific detector. Quantification of environmental samples may be achieved by comparison with external R4Pb standards, internal standardization, and standard additions. The effect of background correction was investigated and this was found to eliminate the solvent peak but to result in worse detection limits due to an increase in background noise. No effect on sensitivity was noted. Extraction and Derivatization. In the butylation method first proposed by Chau et al. (3) R4Pb and ionic alkyllead species are extracted into benzene in the presence of NaDDTC and NaC1. In view of the toxicity of benzene and its restricted use in many laboratories, we investigated the use of several other solvents. n-Hexane gave the best recoveries and the peak due to its nonspecific absorption did not interfere with the R4Pb peaks. Recoveries of ionic alkyllead species from water into n-hexane were determined for both the propylation and butylation procedures and these are given in Table I. They represent the extraction/alkylation efficiencies of the procedures. R4Pb compounds (R = CH,, C2H5,or their combinations) were recovered quantitatively and were unaffected by the derivatization procedures. With the propylation technique we found quantitative recoveries of all the ionic alkyllead species but a very low extraction/propylation efficiency was observed for inorganic Pb2+. Inorganic lead may be conveniently determined by graphite furnace AAS. The recoveries determined with the butylation procedure are also shown in Table I, and these agree well with those previously established using a different GC/AAS system ( 4 ) . Poor recoveries of dialkyllead species were found using extraction into n-hexane and butylation. Recoveries of trialkyllead and dialkyllead species from 1 L of Milli-Q and filtered and unfiltered rainwater (100 mL
and 1 L) were similar to those quoted in Table I. Varying the NaDDTC concentration 10-fold from 50 mL L-l to 5 mL L-l of solution did not affect the extraction of ionic alkyllead species. A NaCl concentration of 50 g L-' was maintained in all the experiments. For (CHJ4Pb, (C2H5),Pb,and the mixed R4Pb compounds we found quantitative recoveries from 100 mL and 1 L of laboratory grade waters, and filtered environmental aqueous samples when extracted into 5 mL of n-hexane. From unfiltered rainwater and other aqueous samples these species were recovered with an efficiency of 60%. Because of problems of wall adsorption of R4Pb compounds, it was necessary to carry out the extraction in the collection bottles themselves when analyzing for these species ( 4 ) . NO such problems were encountered with ionic alkyllead species, which were found to be in solution and not adsorbed on the walls of sampling vessels. Concentration Procedures. Solvent extraction of large aqueous samples achieves considerable concentration; however, for many samples collected a t sites far removed from sources this is not sufficient and it is necessary to concentrate the extracts further in order to detect the organolead compounds that may be present a t very low levels. We therefore undertook to investigate several procedures for concentrating sample extracts. Chakraborti et al. (11) proposed a method involving extraction into pentane, rotary evaporation of the extract to dryness, butylation, and extraction into 250 pL of nonane for GC/AAS analysis. We found this method to be satisfactory for the ionic alkyllead species, but the R4Pb species were lost completely during the rotary evaporation. This method is therefore not suitable for the simultaneous determination of R4Pb and ionic alkyllead species in one analysis and we therefore looked a t two other techniques for the concentration of alkylated sample extracts. The first method involved evaporation of the n-hexane in a micro Kuderna-Danish concentrator a t 80 "C. The second method consisted simply of purging the n-hexane extract with N2 to concentrate it. These procedures were examined by concentrating extracts containing known quantities of R4Pb and alkylated ionic species from 5 mL to 0.5 mL. Results of these experiments are summarized in Table I, which shows that bubbling with N2 gave much better recoveries of organolead species than evaporation in the Kuderna-Danish apparatus. For both methods recoveries decreased with increasing volatility of species and evaporation to dryness resulted in complete loss of all organolead species.
CONCLUSION Extraction of R4Pb and ionic alkyllead species into n-hexane in the presence of NaDDTC and NaCl followed by propylation of the ionic species to the volatile tetraalkyl form and analysis by GC/electrothermal AAS was found to be a sensitive and specific technique for the simultaneous determination of these species in aqueous solution. This technique offered several advantages over the previously reported butylation method including better recoveries of dialkyllead species and shorter analysis times as well as better resolutions of some of the peaks than with the butylation procedure in our hands. Propylation may be employed in parallel with the butylation so as to increase the reliability of results of environmental analysis. Sample extracts can be efficiently concentrated by purging with nitrogen. The techniques described here have been applied successfullyto a variety of environmental samples and in laboratory studies of the environmental chemistry of alkyllead compounds and the results of these studies are being reported elsewhere (6-8). Registry No. Pb, 7439-92-1; (CHJ,Pb, 75-74-1; (CHJ,(CZHS)Pb, 1762-26-1;(CH3)2(CzH,)2Pb,1762-27-2; (CH,)(CzHJSPb, 1762-28-3; (CZHb)dPb, 78-00-2; (CH3)3(C3HV)Pb,92820-31-0; (CH3)2(C3H7)2Pb,99619-89-3; (CZH,),(C3H7)Pb, 3440-78-6; (C2-
Anal. Chem. 1988, 58,661-662
H&(C,H,)2Pb, 3440-77-5;(CSH,),Pb, 3440-75-3; (CH,),(C,Hg)Pb, 54964-75-9; (CH3)2(CdHs)zPb,65151-01-1; (C2Hb)S(C4H9)Pb, 64346-32-3; (CzH&(C4Hg)2Pb,65121-94-0; (C,H9)4Pb, 1920-90-7; (CH3),Pb+, 14570-16-2; (C2H&Pb+, 14570-15-1; (CH3)zPb2+, 21774-13-0;(C2H,)2Pb2+,24952-65-6;propylmagnesium chloride, 2234-82-4; water, 7732-18-5.
LITERATURE CITED (1) de Mora, S. J.; Hewitt, C. N.; Harrison, R. M. Anal. froc. 1984, 27, 415-41 8. (2) Harrison, R. M.; Hewitt, C. N.; de Mora, S. J. Trends Anal. Chem. 1065. 4 . 8-11. ...., (3) Chau, Y.'K.; Wong, P. T. S.; Kramar, 0. Anal. Chim. Acta 1983, 146, 21 1-217. (4) Harrison, R. M.; RadojeviE, M. Environ. Techno/. Lett. 1985, 6 , 129-1 36.
881
(5) Chau, Y. K.; Wong, P. T. S.; Bengert, G. A.; Dunn, J. L. Anal. Chem. 1984, 5 6 , 271-274. (6) Harrison, R. M.; RadojeviC, M.; Hewitt. C. N., Papers presented at the International Conference on Heavy Metals in the Environment, C.E.P. Consultants, Athens, 1985. (7) Harrison, R. M.; Hewitt, C. N.; RadojeviE, M. Sci. Total Environ. 1985, 4 4 , 235-244. (8) Harrison, R. M.; RadojeviE, M.; Wilson, S. J. Sci. Total Environ., in press. (9) . , Birch, J.: Harrison, R. M.: Laxen. D. Sci. Total Environ. 1980, 14, 31-42. (10) Hewitt, C. N.; Harrison, R. M. Anal. Chim. Acta 1985, 167, 277-287. (11) Chakraborti, D.; de Jonghe, W. R. A.; Van Mol, W. E.; Van Cleuvenbergen, R. J. A.; Adams, F. C. Anal. Chem. 1984, 56, 2692-2697.
for review August 5, 1985* Accepted October 8, 1985.
Silane Coupling Agents for Attaching Nafion to Glass and Silica Marilyn N. Szentirmay, Leigh F. Campbell, and Charles R. Martin*
Department of Chemistry, Texas A&M University, College Station, Texas 77843 Glass and other silacious materials are frequently used as substrates for polymer coatings (1-5). For example polymer-coated silacious particles have been used as stationary phases in both gas ( 4 ) and liquid ( 5 ) chromatography. In addition, polymer films coated on glass or quartz slides have been used to investigate the spectroscopic properties of polymers or incorporated guest molecules (6). Because the adhesion of organic polymers to glass and silica is often poor, silane reagents are often used to attach polymers to these surfaces (1-3,5). For example, we have recently shown that films of a perfluorosulfonate ionomer (Nafion (7)) would not adhere to silica but would adhere to an alkylsilane-derivatized silica ( 5 ) . Because of our interest in further studying both the ion exchange ( 5 ) and spectroscopic (8) properties of these and other ionomers (9),we have attempted to identify silane derivatives that will serve as coupling agents for producing adherent Nafion films on glass and silica surfaces. Since our fundamental investigations of ionomers nearly always involve exposure of the ionomer film-coated surface to solvent ( 5 , 8 , 9), the effect of solvent on the stability of the Nafion-surface interaction is of particular interest. Because of the widespread interest in Nafion and related polymers we report the results of these studies here.
EXPERIMENTAL SECTION Materials. Nafion 117 (1100 equivalent weight, proton form) was obtained from Du Pont and was dissolved in 5050 ethanol-water (IO). N-(Trimethoxysilylpropy1)-N,N,N-trimethylammonium chloride (TTACl) and octadecyltrichlorosilane (ODS) were obtained from Petrarch Systems, Inc. Five percent (v/v) solutions of these silane coupling agents were prepared in either methanol (TTAC1) or toluene (ODs). The methanol and toluene were dried over molecular sieves prior to use. Glass slides were obtained from Becton, Dickinson and Co. Silica slides were obtained from Esco Products. Ru(bpy),Cl, (bpy = 2,2'-bipyridine) was obtained from G. F. Smith; 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (PS3-)and methylviologenchloride were obtained from Aldrich. All solvents were of reagent grade or better. Distilled water was circulated through a Milli-Q water purification system (Millipore Corp.). Silanization Procedures. Glass and silica slides were washed with Alconox and water and rinsed thoroughly with water before use. Prior to reaction with ODs, all glassware was dried in an oven at 160 O C for 4 h. All steps of the ODS silanization procedure
were carried out in an Nz atmosphere glovebag. Silanizationwith TTA+ was carried out in air, although the silane reagent bottle was opened, sealed, and stored under Nz.Slides were immersed for 1 min in the 5% solution of the desired silane coupling agent, removed from the solution, and then allowed to sit undisturbed for 10-30 min. The ODs-treated slides were then rinsed with dry toluene and dried in the N2 atmosphere. TTA+-treatedslides were rinsed with water and dried in air. Nafion Film Coating. One milliliter of a 1.14% (w/v) Nafion solution was added to a cup formed by clamping a plastic ring onto a glass or silica slide (Figure 1). The solvent was evaporated in air at 40-50 "C. The plastic ring was then removed leaving a circular Nafion film on the glass or silica slide. To test whether the resultant films were strongly attached to the surfaces of the slides, the films were rinsed with water or other solvents; photographs were taken both before and after rinsing. To aid in visualization, the films were stained with a dilute solution of R~(~PY),~+.
RESULTS AND DISCUSSION Silane-Treated Glass. The glass surfaces were characterized by using the anionic fluorescent dye, PS3-. When an as-received (Le., unsilanized) glass slide is exposed to a PS3solution, rinsed with water, and then exposed to UV light, the characteristic green fluorescence of PS3-is not observed; this is expected since there are no anion exchange sites on the surface and PS3-is very water soluble. When TTA+-derivatized slides were treated with PS3-,rinsed, and exposed to UV light, PS3-fluorescence is clearly seen, indicating, as expected, that derivatization with this cationic coupling agent yields a positively charged surface. This is reinforced by the fact that derivatized slides that are treated in an analogous fashion with Ru(bpy),'+ do not show R ~ ( b p y ) , ~fluorescence. + Finally, slides derivatized with ODS are rendered hydrophobic as evidenced by poor wetting of the surface by water. Nafion Films. The Nafion films obtained upon evaporation of the solvent were clear and coherent, but often showed cracks near the film edges (Figure 2). Prior to reexposure to solvent, the films adhered strongly to both the derivatized and underivatized slides; for example, tape could be applied to the film surface and then peeled away, yet the Nafion film remained attached to the slide. When rinsed with water, films cast onto underivatized slides were quickly and quantitatively removed from the surface (Figure 2A, right). In contrast, films coated on silane-treated
0003-2700/86/0358-0661$01.50/00 1986 Arnerlcan Chemical Society
