Anal. Chem. 1992, 64, 533-540 (8) Kwak, J.; Bard, A. J. Anel. Chem. 1989, 61, 1221. (9) (a)WMle, H. S.; Leddy. J.; Bard, A. J. J . Am. Chem. Soc. 1982, 104, 4811. (b) Anson. F. C.; Tsou, Y.-M.; SavCnt, J.4. J . Electroanel. Chem. 1984. 178. 118. (c) He, P.: Chen. X. J . Electroanel. Chem. 1888, 2 5 6 , 353. (d) Sharp,'M.; Lindhoim, 6.;Llnd, W. L. J . E k t r o a ne/. Chem. 1989, 2 7 4 , 35. (IO) (a) Naegeii, R.; Redepenning,J.; Anson, F. C. J . Phys. Chem. 1986,
533
9 0 , 6277. (b) Redepenning, J.; Anson, F. C. J . Fhys. Chem. 1987, 91, 4549. (11) Tsou, Y.-M.; Anson, F. C. J . Ektrochem. Soc.1984, 131, 595.
RECEIVED for review September 19,1991. Accepted November 26, 1991.
Selective Ionophore-Based Optical Sensors for Ammonia Measurement in Air Steven J. West,?Satoshi Ozawa) Kurt Seiler, Susie S. S. Tan, and Wilhelm Simon* Department of Organic Chemistry, Swiss Federal Institute of Technology (ETH), Uniuersitdtstrasse 16, CH-8092 Zurich, Switzerland
Optical sewn (optodes) based on the incorporation of a m monlum Ionselective ionophores and hydrogen ion-selective chromoionophores in plasticized poiy(vinyi chloride) (PVC) membranes are applied to the measurement of ammonla In alr. Dynamic response characteristics and selectivities for ammonla with respect to other normally occurring gases under varying relative humldity are studied for several membrane formulations. No significant interference occurs from relevant ievds of SO2, NO2, or C02, but a trade-off between selectivity over other amines versus insensitivity to changes in rdatlve humidity is found. An optode formulated with the ionophore valinomycin, which forms a comparatively strong complex with ammonium ion, prefers ammonia over the aikylamines tested but ls affected significantly by humidity changes. An optode based on the bnophore EM 157, which fomrcl a weaker ammOnklm complex shows no hunldlty etlect but responds approxlmateiy equally to low levels of ethylamine, methylamine, and ammonia. I n the experimental configvatkn dascdbd, tlw latter optode has a range of 0.002 to 100 ppm, and f,, response times varying from 230 s at 0.05 ppm, to 15 s at 100 ppm,. A proposed optimization of the optical geometry promises to yield sub-ppb, detection limits and faster response times in future studies. There Is no deterioration In response after 4 months in laboratory air.
INTRODUCTION The measurement of ammonia in air is important over a wide range of concentrations. Ammonia in the atmosphere arises chiefly from natural s0urces.l Its concentration at ground level averages 0.002-0.010 ppm? and decreases monotonically with altitude? It is the only significant alkaline gas in the atmosphere, where its average residence time is only 7-14 days due to aerosol formation and neutralization by the more abundant acidic species.'l2 These reactions make the measurement of ammonia important in studies of smog and acid rain formation. Higher ammonia levels are of analytical interest in indoor environments where industrial operations such as refrigeration or fertilizer manufacture are carried out. The short-term exposure limit (STEL) for occupational exposure to ammonia is a time-weighted average (TWA) of 35 On leave from Orion Research Incor orated, Boston, MA 02129.
* On leave from Central Research Lagoratmy, Hitachi Ltd., KO-
kubunji, Tokyo 185, Japan.
ppm, over a 15-min period: and yet the lower l i i i t of human perception is 53 ~ p m , . ~ Numerous chemical sensors for the detection of ammonia in the gas phase or dissolved in aqueous solution have been described in recent literature. Primary sensing elements include solid-state,polymeric, and aqueous compositions. They operate by either surface or bulk-phase recognition processes and utilize optical or electrical signal transduction. A general problem with these sensors is that they are insufficiently selective for many applications; they respond to changes in relative humidity or to other relevant gases. Solid-state ammonia sensors based on changes in electrical properties of both metalized and nonmetalized metal oxidesemiconductor devices have been de~cribed."'~ They must operate at temperatures well above ambient (140-500 "C)or else suffer long recovery times, interference from changes in humidity, and poor limits of detection. Some of these sensors have been shown to respond to ammonia at ambient temperatures, but limits of detection are high (10 p ~ % ) ,and the effect of relative humidity is substantial.'*Js Sensors based on the effect of ammonia on the properties of organic thin films have been studied. The photoconductivities of various metal-modified phthalocyanines change in the presence of ammonia, but only at high levels in air (lo00 ppmv),16and there is a strong dependence on relative humidity.17 An investigation of thin films of N-docosyl pyridinium, TCNQ,18has shown that changes in its electrical and optical properties as a function of ammonia concentration are not selective or reversible. Polypyrrole, a conductive polymer, shows a decrease in conductivity when exposed to ammonia, but this effect is observable only at 100 ppm, and above, and response to nitrogen dioxide also occurs.1g Many sensors for ammonia are based on the reversible equilibration of ammonia with a thin film of aqueous solution, usually separated from the sample medium by a gas-permeable membrane. The change in an electrochemical or optical property of the solution is measured and related to the ammonia concentration. Potentiometric detection can be used to measure a pH change20or to directly measure the ammonium ionz1as in the well-known ammonia gas-sensing electrodes. Various amperometric detection systems have also been employed.22-24Incorporation of a pH-indicating chromoionophore or fluoroionophore into the aqueous film allows optical detection (optodes or optrodes), and signal transduction is usually accomplished with fiber optics.25 All of the aqueous schemes, however, can be applied only to the measurement of ammonia in equilibrium with an aqueous sample
0003-2700/92/0364-0533$03.00/00 1992 American Chemical Society
554
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
proportions per 0.75 mL of T H F 40 mg of PVC, 80 mg of plasticizer, and 3 mg of chromoionophore;ionophore and anionic sites, where used, were added to give approximately a 10% molar excess over the chromoionophore unless otherwise noted. Sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]bwas used as a source of anionic sites. Membranes were cast by spinning0.2 mL of membrane cocktail onto 35-mm diameter quartz plates in a THF-saturated atmosphere using an apparatus described previ~usly.~~ Glass plates were found to be unsatisfactory for low-level measurements in some instances due to gradual deprotonation of the chromoionophores by ion exchange of sodium from the glass. The resulting membranes were estimated to be 1-2-pm thick.% Optode membranes formulated with salts of the active componentswere placed in 0.01 M HC1 for 15 min to remove the inorganic counterions and to ensure complete protonation of the chromoionophorebefore gas-phase experiments. At ammonia levels below 0.5 ppm,, the optodes can take up a significant quantity of ammonia from the air phase. For this reason and also to eliminate edge effects, the excess membrane material, that which did not intercept the incident light beam at the center of the quartz optode plate, was removed by trimming with a straight razor, leaving a 12-mm square of membrane in the center. Apparatus. A Perkin-Elmer Lambda I1 UV/vis spectrophotometer (Kiisnacht, Switzerland)was used for all absorbance measurements. The optode (quartz plate plus membrane) and a blank glass plate were mounted as windows in a flow-through optical cell, the design of which has been described previously,36 although for this work it was fabricated from stainless steel instead of plastic and with poly(tetrafluoroethy1ene) gaskets instead of elastomeric O-rings in order to minimize absorption of ammonia and other gases. This cell was placed in the sample light beam, and a second identical cell containing two blank glass plates was placed in the reference beam. Gas regulating and metering components were purchased from the following suppliers: pressure regulators,Model SP-1,Carbagas (Riimlang, Switzerland); Whitey valves and Swagelok fittings, Arbor AG (Niederohrdorf,Switzerland);flow meters, series 1100 and 1300, Wisag AG (Ziirich, Switzerland). Relative humidity was monitored with a Model SA-100 hygrometer from Rotronic AG (Bassersdorf,Switzerland). Titrations of ethylamine samples were performed on a Model 960 Autochemistry System from Orion Research AG (Uetikon am See, Switzerland). Experimental Procedures. All gas-phaseoptode experiments were carried out by metering certified mixtures of the minor components (ammonia and potential interfering species) into a flowing airstream as described below, except for the ethylamine test atmosphere which is described separately. Humidity was controlled and continuously monitored. All experiments were carried out at 22 f 1 “C. Compressed air was passed through two 250-mL gas-washing flasks containing silica gel and then split into two streams. One EXPERIMENTAL SECTION was husnidifkd by pasage through four 250-mL gas-washing flasks containing distilled water, followed by two empty flasks and one Reagents. Aqueous solutions were prepared with doubly with glass wool to ensure complete removal of suspended water quartz-distilled water and salts or solutions of the highest anadroplets. The streams passed through separate flow meters, were lytical grade available. Valinomycin, nonactin, ETH 157 (N~’-dibenzyl-N~-diphenyl-l,2-(phenylenedioxy)diacetamide), recombined, and passed through a homemade, machined nylon ETH 149 (N,N’-diheptyl-N,N’,5,5-tetramethyl-3,7-dioxanon- flow-cell block containing the relative humidity sensor and another flow meter to measure the fiial combined flow. Up to this point, anediamide), 3’,3’’,5’,5’’-tetrabromophenolphthalein ethyl ester ordinary laboratory glassware, rubber and plastic tubing, con(TBPE), bis(2-ethylhexy1)sebacate (DOS), o-nitrophenyl octyl ether (0-NPOE), sodium tetrakis[3,5-bis(trifluoromethyl)nectors, and stopcocks were used, since they would be exposed only to air. After this point, the airstream flowed to the specphenyl]borate, poly(viny1 chloride) (PVC), 2’,4’,5’,7’-tetratrophotometer in 5-mm i.d. stainless steel tubing and was split bromofluorescein ethyl ester sodium salt (ethyl eosin), and tetrahydrofuran (THF) were purchased from Fluka AG (Buchs, to flow through the sample and reference cells. The minor comSwitzerland). THF was redistilled before use. Syntheses of the ponent gases were introduced with ordinary stainless steel Tfollowing compounds are described in the respective citations: fittings into the background airstream. No difference was found ETH 7075 (4’,5’-dibromofluoresceinoctadecyl ester) and ETH between using a 3- or 1-m length of tubing to allow mixing between 7058 (2’,4’,5’,7’-tetraiodofluorescein octadecyl ester;” ETH 5350 the T-fittings and the cells, so the 1-m tube was used. Humidity (1,2-benzo-7-(diethylamino)-3-( 2-octyldecylimino)pheno~azine).~ was controlled with individual metering valves for the two airThe following certified, analyzed gas mixtures were obtained from streams, and gas concentrationswere calculated based on the f d Carbagas (R~imlang,Switzerland): 1O00,50,and 5 ppm, ammonia combined airstream flowmeter reading and those for the minor in nitrogen; 10 ppm, SO2in nitrogen; 10 ppm, NOz in synthetic component gases as a simple volumetric dilution. air; 100 ppm, methylamine in nitrogen; and pure COz. It was assumed that all gases behaved as ideal gases and that no gas-phase reactions occurred. Total air flow rates of 1-10 Optode Membrane Preparation. Membrane cocktails were made by dissolving the components in THF in the following L/min were used, and the minor components were introduced
solution which is approximately iso-osmolal with the sensing solution. They cannot simply be placed in an air sample because water transport across the semipermeable membrane changes the composition of the internal electrolyte. Ammonia optodes in which colorimetric or fluorometric indicators are incorporated onto or into a polymer matrix such as silicone polymethylmethacrylate,%or polyvinyl or onto g l a s ~ ~have l - ~ been ~ described. In these systems, ammonia deprotonates the indicator to produce a change in absorbance or emission that is monitored using an optical fiber. In cases where the gas-phase response behavior of the sensor including the effect of humidity has been de~ c r i b e d , ~ ~a O significant -~ humidity effect has been observed except in one instance,28where the effect of humidity was reported only for the base-line response in a background of nitrogen with no ammonia present. In the other cases it is proposed that water vapor plays a role in the deprotonation reaction, and a humidity effect is observed in the presence of ammonia. Where investigated, hydrogen chloride response has also been reported.BJO Ammonia detection limits ranging from 60 to 0.7 p p m y 3 l ~in~ ~nitrogen have been estimated. In this report, the application of ionophore-mediated, polymer-based optodes to the measurement of ammonia in air is described. These sensors consist of an optical indicator and an ionophore contained in a thin plasticized PVC membrane which is cast on a glass or quartz plate so it can easily be mounted in a spectrophotometer for absorbance measurements. The ionophore is a neutral carrier of the type commonly used in ion-selective electrodes and the indicator is a H+-selective chromoionophore which changes its absorption spectrum markedly upon protonation. When expceed to the air sample, ammonia diffuses into the membrane. A reaction occurs in which the indicator is deprotonated and ammonia is protonated. The degree of deprotonation of the indicator a t equilibrium is mediated by the complexation of the resulting ammonium ion by the ionophore. This complexation induces selective response to gases which can be protonated to form a cation. The ratio of the protonated to deprotonated form of the indicator is a function of the ammonia concentration which can be deduced by measuring absorbance at an appropriate wavelength for either form. An optimized optode formulation is described which has a useful range of measurement from 0.002 to 100 ppm, and is unaffected by changes in relative humidity. Modifications to the optical system are discussed which promise to yield sub-ppb, detection limits. A detailed description follows under Results and Discussion.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
at flow rata from 2 mL/min to 1L/min. Based on the flow meter supplier's specifications,the estimated accuracy of the gas concentration values obtained at the inlet to the optical measuring cells varied from i5.5 to f11.370, depending upon the flow rates. No corrections were made for back-pressure at the point of mixing, since the p m u r e exceeded ambient by leas than 1% at the highest flow rate. However, efforts were made to run comparative experiments under the same flow conditions. No phenomena were observed which suggested that these assumptions were not valid. The humidity sensor was not recalibrated during this study although readings of 2% below zero and 2% above 100 were sometimes observed in atmospheres believed to be very close to zero and 100%, respectively. Errors of this magnitude were not considered important to this work, and humidity values reported are therefore not corrected. Slow adsorption and desorption of ammonia at the walls of the tubing and components were detected at ammonia levels below 0.5 ppm,. In particular, it was found that when recovering to background air with no added ammonia after having run ammonia mixtures in dry air, a band of ammonia would pass through the system if the humidity were then increased. This suggested that water vapor aded as an eluent for adsorbed ammonia. Thereafter, the adsorption state of the system was taken into consideration during experiments, and, when neceasary, a purge with humidified air was performed. No further problems were then encountered. Ethylamine is a liquid at ambient temperatures, so its test atmosphere was generated from aqueous solution. Humidified air at 5 L/min was diverted from the apparatus described above and passed through two gas-washingflasks which each contained 100 mL of 0.14 M ethylamine hydrochloride in pH 7.45 phosphate buffer, then through an empty gas-washing flask with glass wool to remove liquid droplets, and finally through the optode cells. The concentration of ethylamine in the test atmosphere was determined by diverting the flow for a timed period through two 100-mL volumes of 0.001 M HC1 in gas-washing flasks. The quantity of ethylamine collected was determined by titrating the excess HCl with NaOH, and the ethylamine vapor concentration was then calculated. A result of 0.5 ppm, with an estimated accuracy of f10% was obtained.
535
as a counterion. Therefore a lipophilic anion, R-,is added and plays an indirect role in the response by fixing the total cation concentration in the electrically neutral membrane. In this case, the response is described by the following equation:
+ + IndH+ + R- e
NH4+aq L
LNH4+
+ Ind + R- + H+,,
(2)
where R-is written on both sides to emphasize the difference between the two systems. Each case can be described as an ion-exchange process: exchange of ammonium and hydrogen ions between the membrane and aqueous phases. The degree of deprotonation of the indicator at equilibrium is determined by the ammonium to hydrogen ion activity ratio in the aqueous phase, by the stability of the complex LNH4+,and by the acidity of the species IndH+ or IndH in the membrane phase. Since the ammonium to hydrogen ion activity ratio is formally equivalent to ammonia, eqs 1and 2 can be rewritten in terms of ammonia to yield expressions operative for gas-phase response.
+ L + IndH e LNH4+ + Ind+ L + IndH+ + R- ~1LNH4+ + Ind + RNH3,
NH3,,
(3) (4)
Here, ion exchange across the membrane interface is replaced by diffusion of the neutral ammonia molecule, which then competes in the membrane with the chromoionophore for a proton. Taking only the case of the acidic chromoionophore from eq 3 as an example, an equilibrium expression can be written:
where K is the formal, overall equilibrium constant; brackets represent concentration in ppm, or molarity in the gas or membrane phases respectively, both of which are considered RESULTS AND DISCUSSION ideal; k N H 8 is a formal Henry's law constant for dissolution General. The development of a novel class of ion-selective of ammonia in the membrane phase expressed in these units; optode has recently been reported by thislaboratory (reviews, ?! , is the formation constant for the ammonium-ionophore refs 37-40). Utilizing plasticized PVC membranes containing complex; and KaNq' and KahdHare the acid dissociation neutral-carrier-type ligands as ion-selective ionophores, these constants in the membrane for NH4+and IndH, respectively. optodes draw upon the technology of ion-selective electrodes The absorbance of the system at a particular wavelength can and extend the substrate recognition process from potentiobe expressed in terms of a,defined here as the fraction of the metric to optical signal transduction. This is accomplished indicator in the deprotonated form: by incorporating H+-selective chromoionophores or fluoA = A0 a(A1- A,) (6) roionophores into the same membrane as the ionophore. The where A,, and Al are the limiting absorbance values for a = selectivity of an ion-selective electrode, based on a particular 0 and a = 1,respectively. If L and IndH are initially equimolar ionophore, is found to be duplicated in the corresponding ion-selective optode. Optodes for amm~nium,~ pota~aium,4~*~* in the membrane, then their concentrations will be proportional to (1- a ) ,and the concentrations of LNH4+and Indsodium,4s c a l ~ i u m ,nitrate,48*47 ~ * ~ ~ chloride," and carbonate will be proportional to a itself. Therefore, eq 5 can be solved ions37s48have been described, and these investigations have for a in terms of K and the ammonia concentration in the gas led to the development of sensors for nonionic species such phase and then combined with eq 6 to yield the following as ethanol,@water vapor,5oand ammonia.51 The ammonia expression for absorbance: gas-phase optode described here is derived from the ammonium ion-selective optode developed by Seiler et al.= and from ita subsequent modification for the measurement of ammonia in aqueous solution as reported by Ozawa et al.5l The responses of the ammonium and other cation-selective optodes are based on either of two similar mechanisms, deThe response to gaseous ammonia was first confirmed and pending upon whether an acidic or basic chromoionophore exploited by covering the PVC membrane with a microporous poly(tatrduomethy1ene) membrane and sensing the ammonia is used as indicator. With an acidic indicator, IndH, the response is described by the following equation: partial pressure of aqueous solution^.^^ In the present report, the behavior of uncovered membranes in a gaseous sample NH4+aq L IndH e LNH4+ Ind- H+aq (1) medium is described for the first time. Figure 1, parts a and where L is the ionophore, subscript uaq" denotes aqueous, and b show spectra obtained for optodes using valinomycin as other species are in the membrane phase. In the case of an ionophore with (a) acidic chromoionophore tetrabromoelectrically neutral, basic indicator, Ind, it is the conjugate phenolphthalein ethyl ester (TBPE) and (b) basic chroacid, IndH+,which becomes deprotonated during the reaction. moionophore ETH 5350,a Nile blue derivative, a t different This neceasitates the presence of a third membrane component ammonia concentrations. In the case of TBPE in Figure la,
+
+ +
+
+
536
ANALYTICAL CHEMISTRY, VOL. 64,NO. 5, MARCH 1, 1992
ABSORBANCE MEMBRANE : VALINOMYCIN
20
PPmv"3
0.6 0.4-
(a)
:I
NoNA
0.4
0.2-
0.2
0-
0
400
500
600
700
A [nm]
I
I
-2
-1
I
I
I
0
1
2
I
I
3 log
CNH~
CONCENTRATION[ppmV]
OCI
:"j
ABSORBANCE
ETH
0.4-
1
0.6
5350
(b) ETHYL EOSIN
PPm" "3
0.3-
PPmv "3 PPm" "3
0.2-
PPmv "3
0.1-
0.4 ETH 7075
0.2 I
0-
-2 400
500
600
700
A [nm]
-1
0
1
log
CNHj
CONCENTRATION [ppm,]
Flgure 1. Absorption spectra at dlfferent ammonia concentrations for two optode membranes incorporating valinomycin as ionophore and different chromoionophores: (a) acldic chromoionophore TBPE, and (b) basic chromoionophore ETH 5350 with tetrakis[3,5-bls(trifluoromethyl)phenyl]borate anionic sites.
Flgure 2. Response curves obtained with (a)chromoionophore TBPE without ionophore as well as with four ionophores with different ammonium Ion binding strengths; from weakest to strongest: ETH 149, ETH 157,valinomycin,and nonadn, and (b) Ionophore valinomycln wlth three chromoionophores wlth different acidities; from weakest to strongest: ETH 7075,TBPE, and ethyl eosin.
greatest sensitivity is obtained a t 616 nm, the absorbance maximum (Am=) of deprotonated TBPE. In this band, the absorbance varies directly with ammonia concentration. In the case of ETH 5350 in Figure lb, the greatest sensitivity is obtained as an inverse relationship between absorbance and concentration at 648 nm, the A,, for protonated ETH 5350. In parts a and b of Figure 2, the behavior of optodes with different ionophores and chromoionophoresis illustrated. In this figure, the log of the ammonia concentration is plotted against a. Absorbance at the A- for the deprotonated forms of the respective chromoionophores was measured as a function of ammonia concentration. The curves were generated by fitting the data sets to eq 7, allowing Ao, AI, and K to be refmed by the least-squares method. Using the values of A. and AI thus obtained, a was calculated for both the raw and calculated absorbance values. The five curves in Figure 2a were obtained with acidic chromoionophore TBPE both in the absence of ionophore and with four ionophores having different ammonium complexing strengths. The sigmoid curves are positioned along the concentration axis in an order which corresponds to the relative ammonium binding strengths of the ionophores used. The curve at the far right on the concentration axis, obtained for the membrane without ionophore, fits the data poorly, as may be expected since eq 7 was derived for ionophore-mediated response. The second curve from the right, obtained with the ionophore ETH 149 which binds ammonium relatively weakly, fits the data better but shows deviation which may indicate the presence of some uncomplexed ammonium in the membrane. Proceeding to the left, to lower concentration values, the three remaining curves were obtained with ionophores which form successively stronger complexes with ammonium.
Good agreement with eq 7 is obtained in these cases. These curves demonstrate how the ionophore controls the range of ammonia concentrations that can be measured with a particular chromoionophore. In the absence of ionophore, complete deprotonation did not occur a t the highest ammonia concentration. With nonactin and valinomycin, the indicator was substantially deprotonated even at the lowest concentration. In Figure 2b, results with three different acidic chromoionophores and valinomycin as ionophore are shown. These curves are shifted, depending upon the acidity of the chromoionophores. A lower ammonia measurement range is obtained at greater chromoionophore acidity. Again, the range of ammonia concentrations generated was not sufficient to completely define the curves. Note that the most acidic chromoionophore,ethyl eosin, was almost completely deprotonated a t the lowest ammonia concentration, whereas the least acidic, ETH 7075, was not completely deprotonated at the highest. Membrane Optimization. Of the optode formulations tested in this study, one based on TBPE as indicator and ETH 157 as ionophore showed desirable overall performance characteristics. An important advantage was that it showed no effect from changes in relative humidity and no deterioration in response function after 4 months storage in laboratory air. It was capable of detecting ammonia at the lowest levels that could be reliably produced in the test apparatus and had a fast, repeatable response. Therefore, in studies of selectivity and dynamic response behavior, emphasis was given to fully characterizing this optode system. Interferences. The measurement of ammonia in air must be carried out in the presence of other reactive gases, some
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992 ABSORBANCE (616 nrn)
Table I. Gas-Phase Optical Selectivity Coefficients for Ethylaminenand Methylaminebwith Respect to Ammoniac
optode components
100 pprn, NH3 0.1- &- c - *
1
537
10 PPmv "3
BACKGROUND
,/
AIR
O I
I
I
I
I
I
0
20
40
60
80
100
R H [%]
concn 0.~1 concn 1.; ppm, ppm,
concn 61 concn 65 ppm,
ppm,
-0.6 -1.4 -1.4 -1.4 nonactin-TBPE -0.3 -1.4 -1.4 valinomycin-TBPE -1.0 ETH 157-TBPE 0 >2 0 >2 An ethylamine (EtNH2)atmosphere of 0.5 ppm, was produced from aqueous solution. It was diluted with humidified air to produce the 0.1 ppm, atmosphere. All tests were therefore carried out at approximately 100% relative humidity. Selectivity was determined by first establishing a response curve with ammonia and then measuring absorbance in the ethylamine atmospheres. The ratios of ammonia to ethylamine concentrations which gave the same absorbance values were taken as the gas-phase optical selectivity coefficients, KJ&EmH2, for that amine concentration. bMethylamine (MeNHJ selectivity tests were carried out in dry air with gases metered from premixed cylinders and the selectivity coefficients were then determined by the same method as for EtNH2,explained above in footnote a. See discussion in text under Other Amines.
ABSORBANCE (613 nm)
0.6
1
1
0'5 0.4
100 pprn, NH,
J \20 ppm,
"3
5 ppm,
"3
1
TNIpOADFOPHORE - TBPE
- ETH 157r BACKGROUND
0.1
AIR I
0
20
40
60
,
80
I
100
RH [OL]
Flgure 3. Effect of relative humidity (RH) on the response of three optode systems with ionophore-chromoionophore combinations (a) valinomycin-TBPE, (b) valinomycin-ETH 5350 (and tetrakis[3,5-bis(trifluoromethyi)phenyl] borate anionic sites), and (c) ETH 157-TBPE.
of which interfere with the operation of other types of sensors. In this study, a range of possible interfering species was selected for testing. The selection encompassed gases which are reactive as acids, bases, oxidants, or reductants, or which are expected to be present in ambient or workplace air samples. Relative humidity, sulfur dioxide, nitrogen dioxide, carbon dioxide, ethylamine, and methylamine were tested. Humidity. Parts a and b of Figure 3 show the effect of humidity on the response of optodes with valinomycin as ionophore and TBPE and ETH 5350 as acidic and basic indicators, respectively. An inverse relationship between humidity and apparent ammonia concentration is evident at all concentrations. With both optodes, an increase in humidity of 50% at an ammonia concentration of 1 ppm, results in a reduction in the apparent ammonia concentration by nearly half. An effect of similar magnitude but in the opposite direction was observed with an optode containing no ionophore, using the strongly acidic chromoionophoreETH 7058 (a slightly stronger acid than ethyl eosin, compare Figure 2b). The best results with respect to humidity behavior were obtained with chromoionophore TBPE and ionophores which form weaker ammonium complexes than nonactin or valinomycin. Optodes with TBPE and ETH 149 as ionophore showed only a slight negative interference from humidity. With ETH 157 as ionophore, no humidity effect was detectable over the entire range of humidities and ammonia concentrations (Figure 3c). Sulfur Dioxide. Sulfur dioxide behaves as an acid and reducing agent and is a common air pollutant. It is a toxic gas with a threshold limit value (TLV) of 2 ppm, in air.
Optodes based on TBPE as chromoionophore and ETH 157 or valinomycin as ionophore were exposed to levels of SO2as high as 5 ppm,, both in the presence and absence of ammonia and in both dry and moist air. No effect was observed. Nitrogen Dioxide. Nitrogen dioxide is also a common pollutant and behaves both as an acid and a strong oxidant (approximatelyas strong as chlorine). It has a TLV of 5 ppm, Although no direct interference by NO2 on TBPE optode systems was found, a gradual diminution of signal was observed upon prolonged exposure. It is likely that this is due to oxidation of the TBPE. For the ETH 157-TBPEoptode system, the rate of this process is such that exposure of the membrane to an atmosphere of 1 ppm, NOz results in a halving of the sensitivity after approximately 16 h in dry air or 5 h at 98% RH. At 0.1 ppm, NOz, this effect was not observable (the annual mean US. Federal Air Quality Standard for NOz is 0.0487 ppm,'). In cases where NOz is present at ppm, levels, a shortened lifetime would be expected and more frequent calibration would be required to correct for the decreasing chromoionophore concentration. It might be expected that other strong oxidants such as ozone would have a similar effect. Carbon Dioxide. TBPE optode systems were unaffected by COz in the range of zero to lo00 ppm,, whether ammonia was present or absent, in both dry and moist air. Other Amines. TBPE-based optodes using nonactin, valinomycin, or ETH 157 as ionophore were tested for selectivity with respect to methylamine and ethylamine. The results are summarized in Table I. In all cases, the response to the alkylamines was erratic in comparison with the response to ammonia, and therefore the selectivity coefficients are approximate. It can be stated, however, that the ionophores nonactin and valinomycin induce a preference for ammonia such that optodes based on these ionophores could be used to measure ammonia in the presence of comparable or slightly higher concentrationsof these amines. Optodes based on ETH 157, on the other hand, show approximately equal response to ammonia and methyl- or ethylamine when the amine concentrations are 0.1 p p q , but a preference for the amines is exhibited when their concentrations are higher. The use of optodes based on ETH 157 would therefore be restricted to applications where these amines are absent or below 0.1 ppm, and below the ammonia concentration. Dynamic Response Characteristics. It was not always possible to determine dynamic response characteristics of the
60 -
CHROMOIONOPHORE
:
_-_.. LIMITING
TBPE
-
600
40 -
IONOPHORE
0
VALUES NONACTIN
0
VALINOMYCIN
:
400 -
VALINOMYCIN
200 -
20 -
0-
0-2 I
-2
-1
I
I
0
1
log
CNH~
TIME
0
-1
log
1
CNH)
CONCENTRATION [ppm"]
[s]
CONCENTRATION [ppm"]
Figure 4. Volume of sample air requlred to load the membrane with ammonia, when the concentration is Increased 10-fold from a lower concentration to that shown on the abscissa, for three optode membranes containlng chromoionophore TBPE and Ionophores having different ammonium binding strengths: strongest to weakest: nonactin, valinomycin, and ETH 157.
optode membranes independently of properties of the experimental apparatus. At a low ammonia concentration, the quantity of ammonia present in the membrane at equilibrium is equivalent to that contained in a large volume of sample air. At 10 L/min air flow rate (5 L/min through each cell), periods of time well in excess of the actual optode response times are required to supply enough sample to load or unload the membrane when the concentration changes. This difficulty cannot be overcome simply by working a t higher flow rates with an improved flow-cell geometry because of backpressure and the enormous consumption of compressed gases that would result. Comparison of apparent response times determined in this system is nonetheless instructive. An important compromise in the membrane formulation becomes apparent: in seeking to achieve lower limits of detection by choosing ionophores which form strong ammonium complexes, or chromoionophores with high acidities, larger sample volumes are required, because of the higher value of a at a given concentration. In Figure 4,the minimum volume required to effect the response to a 10-fold step-change from lower to higher concentration is shown for TBPE optodes with nonactin, valinomycin,and ETH 157, using membranes with the thickness and surface area employed in this study. As demonstrated by the results described below, when considered within a specific set of experimental constrainb, the need for higher sample volumes and concomitantly longer measuring times can overwhelm the inherently lower detection limits of optodes with lower measuring ranges by leading to poorer measurement precision. A different experimental system could be devised to take advantage of the optodes with lower measwing ranges. Optics with a very narrow light beam would permit the use of membranes with orders of magnitude smaller surface areas, and therefore orders of magnitude less ammonia loading. A flow-cell which permitted faster mass transfer by accommodating multiple membranes of reduced thickness would further improve dynamic performance without reducing sensitivity. Response Time. In F i v e 5a, tS response times determined for TBPE optodes with nonactin, valinomycin, and ETH 157 are plotted against log of concentration. These data were obtained in steps going from a lower concentration up to that shown on the abscissa. Also depicted in Figure 5a are curves showing the calculated limiting tg5values which could be obtained, in consideration of the time required to transport
0
[
600
0
0.2
0.4
0
NONACTIN VALINOMYCIN ETH 157
0.6
0.8
10
d
Apparent t,, response times for concentration step changes from lower to higher concentratlon for three optodes with TBPE as chromoionophore and three different ionophores, plotted against (a) kg of hi@mconcentratkn showing calculated limiting values as broken lines and against (b) a value of higher concentration. Figure 5.
Table 11. Apparento t g 5Response Times for Concentration Step Changes initial NH3 optode components concn [ppm,] nonactin-TBPE
0.020 0.10 0.50
valinomycin-TBPE
0.020
ETH 157-TBPE
2.0 0.20 0.050 2.5 50 0.50 0.050
final NH3 concn [ppm,] 0.10 0.50
background airb 0.20
tg5[SI
460 120 m
390 150
10 0.020 1100 0.50 100 5.0 36 100 15 0.050 230 background airb 2000
a See Dynamic Response Characteristics in text. bAirstream with no added ammonia.
the necessary amount of gas to load the membranes, if it is assumed that no gas-phase concentrationgradients are present and that reaction time in the membrane is limited only by diffusion in the membrane. In Figure 5b, the same data are plotted against a. In both cases, the ETH 157 optode shows lower tg5values. When comparing a t equal concentrations, one must consider that ETH 157 has a lower a at a given concentration, and therefore a smaller quantity of ammonia in the membrane. When comparing at equal values of a, one must consider that the gas-phase ammonia concentration is higher with ETH 157, and therefore less depletion of the airstream is occurring. At high concentrations, the response times of the ETH 157 optode approach the values expected when diffusion inside the membrane is limiting.35
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992 ABSORBANCE (613 nm)
Table IV. Drift"
0.50ppm, NH3 A
optode components
0.3024 i 0,0019
valinomycin-TBPE ETH 157-TBPE
01275
01276
01271
0050 pprn, NH3 A = 0 1272 2 00005
01264
BACKGROUND
TBPE.
Table 111. Repeatability"
valinomycin-TBPE ETH 157-TBPE
NH3 concn [pprn,]
drift [abs/hl
[ % concn/h]*
drift
0.05 1.0 0.1 2.0
-0.0057' -0.0017' -0.0058d -0.0076d
-7.9 -1.9 -11.1 -6.6
" In each case, the gas stream with the stated concentration was allowed to flow at 10 L/min (5 L/min through each cell) for 6 h and the drift was calculated from the absorbance values at the three and 6-h marks. bd See notes b-d, Table 111.
0 2000 4000 6000 TIME[s] Fbun 6. Response of ETH 157-TBPE optode to concentration step of the deprotonated form of changes monitored at 613 nm, the ,A,
optode components
539
NH3 concn [pprn,]
std dev [abs]
[ % concn]*
re1 std dev
0.02 0.2 0.05 0.5
0.0025' 0.0083' 0.00054d 0.0019d
2.5 9.4 1.2 3.7
" The standard deviation (n = 4) in absorbance units was calculated from data generated by switching back and forth between the two concentrations shown in the table for each optode (ETH 157TBPE data are ala0 depicted in Figure 6). bThe absorbance data were converted to concentration units by interpolation from a calibration curve obtained by fitting absorbance versus concentration data to eq 7 (as in Figure 2, parts a and b). Measured at 616 nm. Measured at 613 nm. Since the loading and unloading of the membrane ammonia content influences the apparent response time, the magnitude and direction of a step change will also play a role. Table I1 contains some additional response time data showing the starting and ending concentrations used. Note that in the worst case situation, recovering to background air without added ammonia with a nonactin optode, the response is so slow as to be practically irreversible. Repeatability and Drift. Figure 6 illustrates some of the response dynamics of the ETH 157-TBPE optode, when subjected to repeated step changes between 0.05 and 0.5 ppm, ammonia. The valinomycin-TBPE optode was subjected to a similar test using concentrations of 0.02 and 0.2 ppm,. Repeatability data from both tests are summarized in Table 111. The poorer precision shown for the valinomycin-TBPE optode may be a function of the longer time needed to perform the repeatability experiments. In performing concentration steps, a period of time equal to at least three times the tgsvalue was allowed for stabilization after each step. This required approximately 1h for each step down from 0.2 to 0.02 ppm, for the valinomycin-TBPE optode. The steps from 0.5 to 0.05 ppm, for the ETH 157-TBPE optode required less than 12 min. In all cases where drift was studied, a negative drift was found. The data are tabulated in Table IV. Since significant d r i i is not observed when the optodes are stored in quiescent air in the dark, this drift could be related to the volume of air to which a membrane is exposed. Evaporative or oxidative processes could deplete one or more of the membrane components. Also, the effect of the incident lightbeam on TBPE stability has not been studied.
In practice, drift of this magnitude would necessitate fairly frequent recalibration or other means of drift compensation. If, as the data suggest, the drift rate is a function of the sample air-flow rate, then recalibration frequency would be dictated by the desired measurement range. Lower concentrations would require greater volumes of air to load and unload the membrane. Low-level continuousmeasurements are the worst case. Under the conditions shown in Table IV for the ETH 157-TBPEoptode at 0.1 ppm, recalibration would be required a t intervals of slightly less than 1 h or after exposure to approximately 300 L of air to maintain an accuracy of better than 10%. With the valinomycin-TBPE optode at 1.0 ppm,, recalibration a t 5-h intervals or after exposure to 1500 L of air would suffice. Less frequent calibration would be required when measurements were periodic rather than continuous or when ammonia concentrations were higher, since lower flow rates could be used. If the cause of drift were depletion of the chromoionophore, a dual-wavelength technique could be employed. The change in absorbance a t the isosbestic point could be measured to correct for drift. Limit of Detection. Practical detection limits, operative within the limitations of the experimental apparatus, were estimated for the ETH 157-TBPE and valinomycin-TBPE optodes, based on the repeatability data. The relative standard deviation in percent of concentration, at the lowest concentration where repetitive tests were carried out, was taken as a quantitative expression of noise, and the limit of detection was calculated as three times that value. Using data at 0.05 ppm, for the ETH 157-TBPE optode, and at 0.02 ppm, for valinomycin-TBPE optode, limits of detection of 0.0018 and 0.0015 ppm, were obtained, respectively. The fact that the valinomycin optode is the more sensitive at low levels (Figure 2a), and yet did not yield a significantly lower limit of detection under these experimental conditions, underscores the need to reduce membrane thickness and surface area in order to take advantage of systems which have inherently greater sensitivity at low levels. Optode Lifetime. The ETH 157-TBPE optode undergoes no response deterioration in 4 months of storage in the dark, in laboratoryair. The ultimate lifetime of the ionophore-based optodes in air can be expected to depend upon the stability of the membrane components. The PVC, plasticizer, and ionophores used in this work have been shown to have sufficient stability in ion-selective electrodes to give lifetimes of more than 1 year. The stability of chromoionophores is probably the limitingfactor, since gradual degradationthrough oxidation might be expected. The fact that significant drift is observed during long-term continuous measurements in flowing air suggests that membrane lifetimes may be determined by the volume of air to which the membrane is exposed. As discussed under Drift, low-level continuous-flow measurements present the most severe conditions. Extrapolation of the results obtained for the ETH 157-TBPE optode at 0.1 ppm,, as shown in Table IV, suggests that a 50% reduction in sensitivity might be expected after approximately 88 h or 26 400 L of exposure. For the valinomycin-TBPE membrane
540
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
at 1.0 ppm,, a similar extrapolation leads to an estimate of 260 h or 78000-L exposure. CONCLUSION Ionophore-mediatedoptodes can detect ammonia preferentially in the presence of other normally occurring gases. In cases where other amines are not present at levels comparable to that of ammonia, the optode based on chromoionophore TBPE and ionophore ETH 157 is capable of measuring from 0.002 to over 100 ppm, ammonia without interference from relevant levels of SO2, NO2, or C02, and without the requirement for humidity compensation. In the presence of comparable levels of ethylamine or methylamine, the use of valinomycin instead of ETH 157 allows ammonia to be measured if humidity is taken into account. Given the wide selection of ionophores and chromoionophores available, further improvements can be expected. In further work, investigation into the cause of the negative drift is required. Elucidation of the mechanism of drift could lead to improved lifetime and less frequent recalibration. Although the optical system used in this study was not optimized for the exploration of the ultimate limits of detection for these optodes, results were obtained which suggest that detection of sub-ppb, concentrations of ammonia is possible. Wet-chemical methods requiring collection of large volumes of air over long periods of time, or spectroscopic methods requiring optical path lengths in the kilometer range, are usually used for measurementsat such low concentrations. Membrane-based optodes could provide an inexpensive and simple alternative. ACKNOWLEDGMENT
S.W.is grateful to Orion Research Incorporated and S.O. to Hitachi Ltd. for supporting them at ETH. Registry No. TBPE, 1176-74-5;DOS, 122-62-3; ETH 5350, 132234-44-7;ETH 157,61595-77-5;ETH 149,58821-96-8;ETH 7075,138337-12-9;NH,, 7664-41-7;valinomycin, 2001-95-8;nonactin, 6833-84-7. REFERENCES Llppmann, M.; Schlesinger, R. B. Chemlcal Contemlnation in the Human Environment; Oxford Unlversky Press: New York, 1979. Cadle. S. H.; Countess, R. J.; Kelly, N. A. Atmos. Environ. 1082, 76
(lo),2501-2506.
Harward, C. N.; McClenny, W. A,; Hoell, J. M.; Williams, J. A,; Wllliams, B. S. At-. Mvkon. 1082, 18 (lo),2497-2500. Air Contamlnants - Permissable Exposure Llmits (Title 2 9 of Federal ReouIatbns Part 1910.70001.US. Dent. of Labor. OccuDatlonal Safety and Health Adrnlnlstration; 1080. & H A 3172. The Merck Index, 9th ed.; Windholz, M., Ed.; Merck 8 Co., Inc.: Rahway. NJ, 1976, p 513. Peschke, M.; Lorenz, H.; Riess, H.; Eisele. I . Sens. Actuators, 1000, B7, 21-24. Arya, S. P. S.; D'Amico, A.; Verona, E. Thin SolM Films 1088, 157,
169-1 74. Ross, J. F.; Robins, I.; Webb, B. C. Sens. Actuators 1087, 7 7 ,
73-90. Spetz, A.; Armgarth, M.; Lundstrom, I . Sens. Actuators, 1087, 7 7 ,
349-365.
Abdulleyev, A. 0.; Evdoklmov, A. V.; Murshudly, M. N.; Scheglov, M. I . Sens. Actuators 1087, 1 7 , 339-347. Wlnqulst, F.; Spetz, A.; LundstrBm, I. Anal. Chim. Acta 1084, 764,
Mlura, N.; Worrell, W. L. Chem. Lett. 1087, 319-322. Brina, R.; Collins, G. E.; Lee, P. A.; Armstrong, N. R. Anal. Chem. 1000, 6 2 , 2357-2365. Belghachi, A.; Collins, R. A. J. Phys. D : Appi. Phys. 1000, 2 3 ,
223-227.
Henrlon, L.; Derost, G.; Ruauedel-Telxier, A.; Barraud, A. Sens. Actuators 1088, 14, 251-257. Maisik, J. J.; Hooper, A.; Moseley, P. T.; Tofield, B. C. Conducting Polymers 1087, 189-198. Ross, J. W.; Riseman, J. H.; Krueger, J. A. Pure Appl. Chem. 1073, 3 6 , 473. Meyerhoff, M. E. Anal. Chem. 1080, 52, 1532-1534. Pfennlng, D. M.; Deprez, J.; Kitzeimann, D. Ber. Bunsen-Ges. W y s . Chem. 1000, 94 (9),988-991. Yamamoto, Y.; Nuno, T.; Osakai, T.; Senda, M. Bunseki Kagaku 1980,
38 (ll),589-595. Osakai, T.; Kakutani, T.; Senda, M. Anal. Scl. 1087, 3 (6),521-526. Arnold, M. A.; Ostler, J. 0. Anal. Chem. 1086, 58, 1137-1140. Relchert, J.; Sellien, W.; Ache, H. J. Fresenius J. Anal. Chem. 1000, 337, 74. Blyler, L. L., Jr.; Lieberman, R. A.; Cohen, L. G.; Ferrara, J. A,; Macchesney, J. B. Polym. Eng. Sci. 1080, 29(17), 1215-1216. Zhou, Q.;Kritz, D.; Bonnell, L.; Slgel, G. H., Jr. Appl. Opt. 1080, 28
(1I), 2022-2025. Sawada, H.; Tanaka, A.; Wakatsukl, N. Fujitsu Sci. Tech. J . 1080, 25
(2),163-169. Muto, S.; Ando, A.; Ochiai, T.; Ito, H.; Sawada, H.; Tanaka, A. Jpn. J . Appl. Phys. 1080, 28(1),125-127. Shahriari, M. R.; Zhou, Q.; Sigel, G. H., Jr. Opt. Left. 1088. 73 (5),
407-409.
Glullanl, J. F.; WohRjen, H.; Jarvis, N. L. Opt. Lett. 1083, 6 (l),54-56. Shahrlari, M. R.; Zhou, Q.; Sigel, G. H., Jr. SPIE Vol 838 Fiber Optic and Laser Sensors V 1087, 346-352. Tan, S. S. S.; Hauser, P. C.; Wang, K.; Fluri, K.; Seller, K.; Rusterholz, 8.; Suter, G.; Kriinli, M.; Spichiger, U. E.; Simon, W. Anal. Chlm. Acta 1001, 255, 35-44. Seller, K. Diss. No. 9221,ETH Zurich, 1990. See also: Seller, K. Ionenseteh?ive Optodenmembranen; Fluka Chemle A G Buchs, S w b erland, 1991. Seiier, K.; Morf, W. E.; Rusterholz, B.; Simon, W. Anal. Sci. 1080, 5 .
557-561. Morf, W. E.; Seiler, K.; Lehmann, 6.; Behrlnger, C.; Hartrnan, K.; Simon, W. Pure Appl. Chem. 1080, 67 (9),1613-1618. Morf, W. E.; Seller, K.; Lehmann, 8.; Gehrlg, P.; Roullly, M.; Suter, 0.; Rusterholz, 6.; Splchlger-Keller, U.; Pretsch, E.; Simon, W. Proc, 2nd Int. Wkshp. Biosens.. Braunschweig-Stiickheim, FRG, Biosens Med. Env. R o c . Cont.; Schmld, R. D., Scheller, F., Eds.; 1080, 73, .(
321-329.
Simon, W.; Morf, W. E.; Seiler, K.; Spichiger-Keller, U. E.; Haug, J.-P.; Buhlmann, P. Proc. 3rd Int. Mtg. Chem. Sens.; Cleveland, OH, Sept. 24-26, 1990,pp 9-12. Simon, W.; Morf, W. E.; Seiler, K.; Spichlger-Keller, U. E. Fresenius J . Anal. Chem. 1900, 337, 26-27. Wang, K.; Seiler, K.; Morf, W. E.; Spichiger-Keller, U. E.; Simon, W.; Lindner, E.; Pungor, E. Anal. Sci. 1000, 6 , 715-720. Pklsset, P. M. J.; Hauser, P. C.; Tan, S. S. S.; Seller, K.; Morf, W. E.; Simon, W. Chimia 1080, 43(1-2), 10-11. Seller, K.; Wang, K.; Bakker, E.; Mod, W. E.; Rusterholz, 8.; Spichiger, U. E.; Simon, W. Clin. Chem. 1001, 3 7 , 1350-1355. Mod, W. E.: Seller, K.; Rusterholz, 8.: Simon, W. Anal. Chem. 1000, 6 2 , 738-742. Seller, K.; Eugster, E.; Morf, W. E.; Wang, K.; Czosz, M.; Rusterholz, 6.; Simon, W.; Spichiger-Keller, U. E. Fresenius J. Anal. Chem. 1000, 3 3 7 , 109. Tan, S . S. S.; Hauser, P. C.; Chaniotakis, N. A.; Suter, G.; Simon, W. Chimia 1080, 43 (9).257-261. Hauser, P. C.; Pgrisset, P. M. J.; Tan, S. S. S.; Simon, W. Anal. Chem. 1000, 6 2 , 1919-1923. Behringer, C.; Lehmann, B.; Simon, W. Chimia 1087. 47 (11).
397-398. Seiler, K.; Wang, K.; Kuratll, M.; Simon, W. Anal. Chim. Acta 1001, 2 4 4 , 151-160. Wang, K.; Seller, K.; Haug. J.-P.; Lehmann, 6.; West, S.; Hartmann, K.; Simon, W. Anal. Chem. 1001, 6 3 , 970-974. Ozawa, S.; Hauser, P.; Seller, K.; Tan, S. S. S.; Morf, W. E.; Simon, W. Anal. Chem. 1001, 6 3 , 640-644.
127-138.
Moseley, P. T.; Wllliarns, D. E. Sens. Actuators 1000, B7, 113-115. Moseley, P. T.; Wllllarns, D. E. PolyMron 1080, 8 (13),1615-1618. Mlura, N.; Worreii, W. L. SoM State Ionics 1088, 2 7 , 175-179.
RECEIVED for review August 1,1991. Accepted November 22, 1991.
