Anal. Chem. IQQI, 63, 2724-2727
2724
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Hlreta, Y.; Inomata, K.; Fujll, T. M C 8 CC, J . H @ IResolut . Chroma top. Chromatop. Cmmun. 1989, 72, 63-64. Hlrata, Y.; Okamoto, K. J. Microcolumn S e p . 1988, 1 , 46-50. Unger, K. K.; Roumellotls. P. J. Chrometogr. 1885, 282, 519-526. Nalr, J. B.;Huber, J. W. LC-GC 1988, 6 , 1071-1073. Campbell, R. M.; Meunler. D. M.; Cortes, H. J. J. Microcolumn Sep. 1989, 1 , 302-308. Cortes. H. J.: Pfetffer. C. D.; Richter, 8. E.; Jensen, D. J. Chromatogr. 1985, 349, 55-61. Duquet, D.; Dewaele, C.; Verzele, M. HRC 8 CC, J . High Resolut. Chromatogr. Chromatogr. Commun. 1988, 1 1 , 252-256. Cortes, H. J.; Olberding, E. L.; Wetters, J. Anal. Chim. Acta 1990, 236. 173-182. Coies. H. Jlbfeiffer, C. D. Chromatogr. Forum 1986, 4 , 29-33. Cortes, H. J.; Richter, B. E.; Pfeiffer, C. D. HRC d CC, J . H/gh Reso/ut. Chromatogr. Chromatogr. Commun. 1885, 8 , 469-474. Duquet. D.; Dewaele. C.; Verzele, M.; McKinley, S. M C d CC, J . High Resolut. Chromatogr. Chromatogr. Commun. 1988, 1 1 , 824-829. Cortes. H. J.; Pfeiffer, C. D.; Jewett, G. L.; Richter, B. E. J. Microcolumn Sep. 1989, 1 , 28-34.
(27) Cortes, H. J.; Jewett, 0. L.; Pfeiffer, C. D.; Martln, S. M.; Smhh, C. Anal. Chem. 1980, 61, 961-965. (28)Cortes, H. J.; Bell, 8. M.; Pfelffer, C. D.; Olaham. J. D. J. Micfocol. sep. 1989, 1 , 278-288. (29) Ghijs, M.; Van Dljck, J.; Dewaele, C.; Verrele. M.; Sandra, P. On-Line Micro SEC-COC; loth Internatlone1Symposium on Capillary Chromatography, Riva del Garda, Italy, 1989; Huethig Verlag: Heidelberg, Germany, 1989. (30) Cortes, H. J.; Pfeiffer, C. D.; Richter, 8. E.; Stevens, T. S. HRCB CC, J . High Resolut. Chromatogr. Chromatog. Commun. 1987, 10, 446-448.
(31)Bally, R. W.; Cramers, C.A. HRC B CC, J . H/gh Resolut. Chromatogr. Chromatog. Commun. 1888, 9 , 626-632. (32) Formulations and Environmental Chemistry Laboratory. Method ACR 73.5 S1;DowElanco: Midland, MI, 1973.
RECEIVEDfor review March 26, 1991. Accepted September 13, 1991.
Conductivity Response of Porous Electrodes Supported on Perfluorosulfonic Acid Membranes to Acidic Gas Mixtures Armand Bettelheim,* Rachel Harth, Dan Ozer, Uri Mor, and Benjamin Segal Nuclear Research Centre, P.O. Box 9001, Beer-Sheva 84190, Israel
Gold or platlnum film8 depodted on a Naflon membrane were used to measure surface conductance of the membrane. Addc gases such as HCI ar SO,, Introduced as mMures wlth an Inert gas, were found to affect the conductance of the membrane surface faclng thk mlxture whlle the other side of the membrane was constantly supplied wlth a mdst Inert gas. The relatlve conductance (G/G,) Is mOa affected wlthln the 0-0.5 and 3-4.5 vol % ranges both for HCI and SO2. The effect Is higher for the Pt-deposlted than lt Is for the golddeposlted membrane: for an HCI concentration of 1.5 vol %, G / G , Is 5 for Pt deposlted on Naflon while A Is only 1.3 for gold deposlted on Naflon. This Is attributed to dlfferent geometrles and porosity of the Pt and Au electrodes. No conductance response was observed for CO, whlch yields wlth water a much weaker acld than those formed by HCI and
so,. INTRODUCTION The recent emergence of concern over pollution and concerns over safety in industrial activities involving poisonous gases has stimulated substantial research and development in the field of gas sensors. Semiconductor materials whose conductance is modulated directly by the interaction with an active gas have been studied since it was discovered that the reversible chemisorption of reactive gases a t the surfaces of certain metals, oxides, and chalcogenides could be accompanied by reversible changes in conductance (1-3). The solid polymer electrolyte (SPE) techniques involve the use of an ion-exchange membrane with porous contacting electrodes for the construction of electrochemical devices in which ionic migration to maintain charge neutrality occurs within the membrane. SPE technology was applied to fuel cells (41, water electrolyzers (51, electrochemical oxygen separators (6) and to electrosyntheses in low-conductivity solvents (7-9).The membrane usually used in conjunction with SPE
* To whom correspondence should be addressed. 0003-2700/91/0363-2724$02.50/0
is Nafion, a perfluorosulfonic acid membrane composed of polytetrduoroethylene (PTFE) backbone and pendant side chains terminating with -S03-M+ where M+ is a proton or one of the alkali cations. The capability of SPE electrodes to make oxidation and reduction processes possible in the absence of supporting electrolytes (10,lI) has suggested their uses as sensors for analytes present in the vapor phase (12, 13). However, while previous studies have employed the SPE technique for amperometric and voltammetric measurements of electroactive gases (12,13),we show in the present study that acidic gases (such as HC1 and SOz) affect the SPE conductivity.
EXPERIMENTAL SECTION The ion-exchange material used as the SPE was Nafion 117 protonated membrane (0.175 mm thick, Aldrich). Water content in the as-received membrane was determined to be 5.5% by measuring the weight difference before and after drying at 60 "C in vacuo for several hours. Preliminary experiments were also conducted with an anion-exchange membrane (0.5 mm thick, quaternary amine instead of sulfonate groups and a water content of -5%).
The membrane was cut into disks of 30-mm diameter and one face was covered with a porous film of conductive material (Au or Pt). Platinum films were prepared by a chemical plating method by reducing a solution of chloroplatinic acid contacting one face of the membrane with an alkaline solution of sodium borohydride diffusing from the other face (14,15). The membrane was removed when the platinized side had a metallic appearance (a few hours). Gold/Ndion composite electrodes were prepared by dc sputtering deposition (Polaron E5100, Cheshire, England) to product 0.1-0.3 pm thick gold deposits at a deposition rate of 0.05 pm/min. Although the thickness of the platinum films was not measured, these seemed to be much thicker and more porous than the gold ones. Masking with an acetone-soluble paint (Lacomite, Canning) allowed Au or Pt deposition on desired sections of the membrane. The Pt or Au electrode patterns, as illustrated in Figure 1, were designed so as to obtain high surface conductance. The gold patterns were 1 mm wide with gaps 3 mm wide. The Pt was deposited on the whole surface except a 2-mm gap in the middle of the membrane. Also schematically shown in Figure 1is the two-component cell used in all experiments. Between the two half-cells were clamped 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 23, DECEMBER 1, 1991
2725
SIDE VIEW
I
I
Au/SPE
Pt/SPE
epu t b r e d AU
1.01 0
2
FRONT VIEW
E
IO
8
HCI c o n c e n t r a t i o n / v o l %
(focing comportment R)
Figure 3. Dependence of the Au/SPE conductance on HCI concentration in the gas mixture stream (pure Ar HCI, total flow rate of 1
+
Flgure 1. Schematic diagram of the Au/SPE and RISE cells. n
4
L/min).
mr
o
(a) HCI
( b ) SO2
A
"
h
0
so,
4-
0.35 I
0
1
2
3
4
5
E
7
8
e
J IO
t / min Flgure 2. Conductance response of the Au/SPE to 0.4 vol % HCI (a) or 0.4 voi % SO2 (b) in a gas mixture consisting of pure argon and test gas (total flow rate of 1 L/mln). Conductance is expressed in siemens (S =
the Au/SPE or Pt/SPE, two Au wires (0.5mm thick) serving as electrical contacts, and perforated Teflon gaskets. Argon passing through a water bubbler was constantly introduced (1 L/min) to one of the compartments in order to achieve a fixed water content in the membrane. The gold or platinized side of the membrane faced the compartment into which mixtues of argon and the gas to be tested were introduced. These mixtures were prepared by controlling the flows of the two gases (total flow rate of 1 L/min). An ac bridge (Wayne-Kerr, B642) operating at a frequency of 1591.5Hz and connected to an X-t (HP, 7100BM)recorder was used to measure surface conductance, expressed in siemens (S = f2-l). IR spectra were taken with a FTIR (Nicolet, MX-S) spectrophotometer connected to an X-Y (PAR, RE0074) recorder.
RESULTS AND DISCUSSION Figure 2 shows the conductance response of a gold/SPE to the introduction of 0.4vol % HCl in an argon stream (curve a). It can be seen that the conductance increases within 6 min from an initial value of 0.38 pS (0.38 X 10" f2-l) to a steady-state value of 0.46 $3. About half this time is required to flush the reaction tube with a different gas composition. For practical application the response time can, therefore, be reduced by minimizing the volume of the test gas inside the probe. The relative conductance is defined as GIG, where G is the conductance at the instant of observation and Gois the initial conductance prior to the test gas introduction. The depen-
'0
I
2 SO,
3
4
5
E
7
concentration / v o l X
Flgure 4. Dependence of the Au/SPE conductance on SO2 concenSO2, total flow rate of tration in the gas mixture stream (pure Ar 1 L/min).
+
dence of the relative conductance upon the HC1 concentration in the gas stream is presented in Figure 3. Two relatively steep effects are observed in the 0-0.5 vol % and 3-4.5 vol '70HC1 ranges. No significant changes in conductance occur when the HC1 concentration exceeds -5.5 vol %. When HC1 is replaced by SOz, the response of the Au/SPE is larger, as illustrated in Figure 2: a higher steady-state conductance value is obtained with the introduction of 0.4 vol % SOzthan with 0.4vol '70HCl (curves b and a, respectively). However, steady state is achieved faster with HC1 than with SOz (6 and 10 min, respectively). The effect of SO2 concentration in the gas steam on conductance is shown in Figure 4. Similar to the behavior with HC1, the larger effects are in the 0 4 . 5 and 3-4.5 vol % SO2ranges. However, the relative conductance a t high SOz concentrations (>5 vol % ) is approximately twice as large with SOz than with HC1 (approximate values of 4.5 and 2.1, respectively). No conductance response was observed for C 0 2 concentrations up to 10 vol '70. The Pt/SPE was exposed to the same initial conditions as the Au/SPE, i.e. wet argon flowing a t a rate of 1 L/min in one half-cell and dry argon flushing a t the same rate in the other half-cell of the assembly shown in Figure 1. The initial conductance obtained for the Pt/SPE was 0.015 NS, compared to about 0.38 pS for Au/SPE (Figure 2). The different initial conductance can be related to the different structures of the
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ANALYTICAL CHEMISTRY, VOL.
63,NO. 23, DECEMBER 1, 1991
Table I. Conductance Ftesponse of the Pt/SPE to Various HCI Concentrations vol
70
0 1.5 2.0 5.0 8.0 10.0
G, rS,"
GIGO
0.105 0.075 0.100 0.110 0.170 0.200
1.0 5.0 6.7 7.3 11.3 13.3
o r c s = 10" SI-*.
( b ) NH3 In NRFION
b
a
4600
A -
2750 1600 WAVENUMBER / cm''
A, 860
Figure 5. IR spectrum of an as-recelved Nafbn 117 membrane before (a) and after (b) being exposed to NH, vapor for 20 min.
electrode assemblies (Figure 1). However, the conductance response of the Pt/SPE to HCl introduction is much higher for the Pt/SPE. As it can be seen from Table I, the relative conductance at 1.5 and 10 vol % HCl is 5 and 13.3, respectively, for the Pt/SPE while it is only 1.3 and 2.1, respectively, for the Au/SPE. Different geometries as well as a larger HC1 uptake by the thicker and more porous Pt coating could be the reasons for the higher sensitivity of the Pt/SPE. This can also be deduced from the different conditions which had to be applied to regenerate the two assembles: pure argon (1L/min) was flushed for about 40 min after introduction of 10 vol YO HC1 to restore initial conductance (Go)of the Pt/ SPE assembly, and this is about twice the time required for the Au/SPE exposed to a similar HCl concentration. It is expected that gases such as HC1, SOz, COz, and NH3 coexist in equilibrium with the species obtained after dissolution in water, and IR spectroscopy was used to detect the presence of the various species in Ndion. Ndion membranes were exposed to various gases, and IR spectra were recorded in the 4600-860-cm-' wavenumber region. Although HCl and SOz permeate into the membrane, as concluded by the Au/ SPE conductance response to these gases (Figures 3 and 4), no IR absorbance bands characteristic of molecular HC1 and SOz were observed for membranes exposed to these gases as well to COz. However, while no conductance changes were observed for Au/SPE (as well as for Pt/SPE) exposed to ammonia, changes in the IR spectra were recorded before and after exposing a Nafion membrane to gaseous ammonia (Figure 5). The spectrum obtained after ammonia exposure is according to that reported in the literature (16) for NH3 in the presence of water vapor (absorption at wavenumbers 4230,3900,2550,2140,2050,1865,1770,1570, and 920 cm-9, thus suggesting permeation of NH3 into the membrane as well as interaction with water molecules (and to some extent with protons) in the membrane to yield NH4+ ions. The conductivity of highly conductive membranes has been shown to depend on the water content and the concentration of the charge carriers (14). The bulk conductivity of proton-form Nafion membranes immersed in aqueous solutions, as determined by various investigators, is on the order of
1.25-1.4 S m-l (17, 18), and this conductivity has been attributed to protons serving as charge carriers. This suggests that acidic gases (such as HC1 and SO2)which are capable of dissociating in the water present in Nafion increase the charge carrier (protons) concentration in the membrane and therefore increase its conductivity. Lack of conductance response of the Au/SPE toward COz is believed to be due to the lower acidic strength of H2C03compared to that of HC1 and H 8 0 3 When the Ndion membrane in assemblies similar to those shown in Figure 1 was replaced by an anion-exchange membrane (quaternary amine instead of sulfonate groups) with a similar water content (-5%), the initial conductance as well as the conductance response to HCl additions decreased by more than 1order of magnitude. These results indicate that the conductivity of these assemblies is affected both from the parameters related to the membrane conductivity (such as water content) and structure of electrodes (such as geometry, thickness and porosity). Moreover, the nature of the charge carriers in the membrane has an important role: proton conduction seems to be essential in order to detect significant effects of acidic gases on conductivity. Since the IR spectra clearly indicate that ammonia permeates into Ndion, the lack of conductance response suggests an insignificant effect on proton concentration. Lower diffusion coefficients have been reported for alkali cations (such as Na+) compared to those for protons for self-standing Nafion membranes (17). Therefore, the lower concentration and diffusity of NH4+ ions formed in the Nafion membranes compared to those of protons seem to explain the conductivity results. HC1 and SOz are known to interact with water to yield a single and double protic acid, respectively. However, since the two gases show similar conductance-gas concentration relationships, it is improbable that the two regions observed for HCl as well as for SO2 in Figures 3 and 4 result from two dissociation steps but rather more likely from an effect related to the microstructure of the membrane. The microstructure of Nafion has been examined by several authors by means of various methods such as low-angle X-ray scattering, masstransfer experiments, and infrared spectra (19-23). A structure model has suggested that polymeric ions (SO,-)and water contained in Nafion are clustered and separated from the surrounding polytetrafluoroethylene (PFTE)fluorocarbon matrix (19-21). From results obtained for the transport of bisulfate ions in Nafion membranes, Verbrugge and Hill claim that a cluster network morphology does not form in this system (24-26). Instead, they propose a structure in which water is present either in pores with a diameter of about 60 8, or in the form of free water. The presence of two forms of water in Nafion has also been confirmed by differential thermal analysis (27). Evidence for nongaseous species existing at different environmental sites in Ndion according to different charge (%) and hygroscopic/hydrophyllic interactions between the diffusing species and Nafion (29) have been discussed in the literature. Similarly, it is suggested that the different effect of low and high HC1 or SOz concentration on conductance (Figures 3 and 4) reflects the different behavior of gas molecules which have been dissolved in different forms of water in the membrane. Since water molecules in Nafion mostly populate the polysulfonate region, it seems that HCl or SO2 molecules first dissolve in this environment (concentrations lower than 3 vol YO, Figures 3 and 4) and only after saturation of this region does dissolution occur in the hydrophobic fluorocarbon matrix.
CONCLUSIONS Measuring the surface conductivity of a solid polymer electrolyte (SPE) allows the application of relatively simple
Anal. Chem. 1991. 63.2727-2734
devices to the determination of a single acidic component in a gas mixture. Proton-conducting membranes, such as Ndion, seem to be more suitable for this purpose than anion-exchange membranes. Long-term stability of the gold- or platinumcovered Nafion electrodes is satisfactory, as no appreciable change of their conductance is observed after several weeks of continuous use. This stability is attributed to the solid-state structure of the probe. This prevents slow detachment of particles of conductive material from the electrode surface as observed from SPE systems used in liquid media (12). Moreover, stability toward corrosive gases such as HC1 and SOz can be related to chemical inertness of the gas-sensing element (consisting of membrane and electrodes). Although no selectivity has been sought in the present study, lack of conductance response of the metal/SPE toward gases such as Nz, 02, and COz enables possible use of these devices for quantitative measurements of ambient and industrial SO2 pollutant. This method seems to be preferable to one employing liquid melts (30) or high-temperature solid-state devices (31). Future work will be focused upon investigation of this use as well as on geometry for greater sensitivity and more rapid response time. Registry No. Au, 7440-57-5; Pt, 7440-06-4; SOz, 7446-09-5; HC1, 7647-01-0 Nafion 117, 66796-30-3.
LITERATURE CITED N M , M.; Kanefusa. S.; Haradome, M. J . Elecfrochem. SOC. 1978, 125, 1676-1679. Wimdischmann, H.; Mark, P. J . Electrochem. Soc. 1979, 726, 627-633. Duh, J. 0.;Jou, J. W. J . Ekhochem. Soc. 1989, 136, 2740-2747. Appbby, A. J.; Yeager, E. 6. €nergy(Oxford) 1986, 11. 137-152 and references cited therein. Lu, P. W. T.; Srinivasan, S. J . Appl. Elecfrochem. 1979, 9 , 269-263. Fujlta, Y.; Nakamura, H.; Muto, T. J . Appl. Electrochem. 1986, 76, 935-940.
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Enea, 0. J . Ektroanal. Chem. InterfaclelEkfrochem. 1987, 235, 393-401. Ogumi, 2.; Inaba, M.; Ohashi, S.; Uchida. M.; Takehara, Z. f h t r o chim. Acta 1988, 33, 365-370. Kita, H.; Nakajima, H. Electrochim. Acta 1986, 31, 193-200. De Wulf, D. W.; Bard, A. J. J . Electrochem. SOC. 1988. 735. 1977-1965. Kaaret, T. W.; Evans, D. H. Anal. Chem. 1988, 60, 657-662. Schavon. 0.;Zoni. G.; Bontempelli, 0. Anal. Chlm. Acfa 1989, 227, 27-4 1. Schiavon, 0.;Zoni, G.; Bontempelii, 0.;Farnia, G.; Sandona, G. Anal. Chem. 1990. 62, 293-298. Katayama-Aramata, A.; Nakalima, ti.; Fujikawa. K.; Kita. H. Elechochlm. Acta 1983, 28, 777-780. Katayama-Aramata,A.; Ohnishi, R. J . Am. Chem. Soc. 1983, 705, 658-659. Tables of Wavenumbers for the Calibration of Infra-Red Specfromefers; IUPAC; Butterworth: London, 1961; pp 654-657. Cai. 2.; Liu. C.; Martin, C. R. J . Elecfrochem. SOC. 1989. 736, 3356-3361. &ot, W. G. F.; Munn, G. E.; Walmsley, P. N. J . Electrochem. Soc. 1972. 179, 106C-111'2. Yeo. S. C.; Eisenberg, A. J . Appl. folym. Sci. 1977, 27, 675-890. Yeo. R. S. J . Electrochem. Soc. 1983, 730, 533-536. Hsu, W. Y.; Gierke, T. D. J . Membr. Sci. 1983, 13, 307-326. Yeager, H. L.; Kipiing, B. J . phvs. Chem. 1979. 83. 1836-1839. Sakai, T.; Takenaka, H.; Wakabayashi. N.; Kawami, Y.; Torikai, E. J . Electrochem. Soc.1985. 732, 1328-1332. Verbrugge. M. W.; Hill, R. F. J . Electrochem. SOC. 1990, 737, 886-893. Verbrugge, M. W.; Hill, R. F. J . Electrochem. Soc. 1990, 737, 893-699. Verbrugge, M. W.; Hill, R. F. J . Electrochem. Soc. 1990, 737, 113 1-1 138. Randin, J. P. J. Elecfrochem. SOC.1982, 729, 1215-1220. Harth. R.; Mor, U.; Ozer, D.; Belteiheim. A. J . Elecfrochem. Soc. 1989, 736, 3863-3667. Martin. C. R.; Doilard, A. J. Electroanal. Chem. Interfacial E k f r o chem. 1983, 759, 127-135. Saizano, J.; Newman, L. J . Electrochem. SOC. 1972, 779, 1273-1278. Jacob, K. T.; Bhogeswara, R. J . Elecfrochem. Soc. 1979. 126, 1842-1847.
RECEIVED for review December 18, 1990. Accepted August 23, 1991.
Characterization of Sample Heterogeneity in Secondary Ion Mass Spectrometry by the Use of a Sampling Constant Model Frank P. L. Michiels and Freddy C. V. Adams* Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Wilrijk, Belgium David S . Bright and David S. Simons Center for Analytical Chemistry, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
An hdepth study was undertaken to evaluate the appllcablllty of the sampling constant concept for the characterlzatlon of hetemgemhy of ekfnentai distributions In ion mkroscopy. A new model, whlch take8 Into account the Intensity spread of inclusions, Is described. The model is tested on low alloy steel and brass standard reference samples, relylng on Image registratbn wlth a reslstive anode encoder and Image analysls. The agreement between the model and the experhnentai results Is satlsfactory.
INTRODUCTION Despite its marked microanalytical potential, secondary ion mass spectrometry (SIMS) is very often used for the bulk 0003-2700/91/0363-2727$02.50/0
analysis of solids. Commonly, standard samples that have been certified for their bulk composition are used as Calibration samples in quantitative analysis. Theoretical and semitheoretical calibration approaches are mostly evaluated using bulk materials. In both these instances, it is often implicitly assumed that the sample is homogeneous, i.e., that there are no spatial differences in the concentration of the constituents. However, the composition of the sampled subvolume is not necessarily representative for the overall composition of the sample. Due to the very small sampled volume in SIMS, large errors often arise. This problem has been recognized before and some efforts to overcome it have been undertaken (1-6). Since heterogeneity effects are often very important in the overall quantification error, some understanding of the sampling problem is essential to any com0 1991 American Chemical Society
