523
Anal. Chem. 1992, 64, 523-527
Sensitive Layer for Electrochemical Detection of Hydrogen Cyanide Jan Langmaier' and J i g Janata*
Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84112
with zero or a finite electric field). Ekcb0chrmlCd)y polymrked polvanHlne wlth metal dusters If a gaseous electron donor or acceptor enters the sensitive hwmporated Mothe bulk of the fibn 88wea w a senshg layer layer, it may change ita work function, forcing a new value for gamow HCN in the concontration range 0.13-20 ppm. of the compensating voltage. The assumption which has to The interaction of the gaa wlth HJI layer causes a reversible be made, and which is relatively difficult to verify, is that the and reproduclbk change of its work function whkh Is meawork function of the reference plate remains unaffected by sured wtth a KoMn probe. The retpome ls logarlthmlc, the change of the gaseous environment. One solution to this augpdngabdkmechaniam. ~sbpedtheconceniratkn problem is to prepare a corresponding "reference" layer and depedwm k between 20 and 35 mV/decade of HCN. The measure the difference of the change of work function between maw factor affoctlng the 8ensltlvMy k the total thickness of these two layers. the layer and the thickness of the space charge Inside the The design of the HCN-sensitive layer follows the concept layer. T h . ~ ~ w a s o b t a l n e d h w n t h e f l h r s of introducing specific HCN-binding sites into an electroutllzlng the pdyanlllne/Hg/phoqhate syrtuin. chemically polymerized layer of organic semicondudor chosen
INTRODUCTION The mode of transduction influences the choice of method by which a selective layer is prepared and integrated with the physical part of a chemical sensor. Chemical modulation of the work function (WF) is one of the modes of transduction on which a whole new class of chemical sensors is based. It requires that the chemically sensitive layer be capacitively coupled to the rest of the sensor a t one interface.l This requirement is satisfied in the vibrating capacitor (Kelvin probe) and in all solid-statedevices utilizing mxalled insulated gate on which the selective layer is in direct contact with the gate insulator. The typical examples are so-called chemically sensitive diodes2 and chemically sensitive field-effect transistors. The Kelvin probe is best known for measurement of surface and contact potentials. Explanation of ita function, in the context of chemical wnsing, has been developed recently? The Kelvin probe differs from the insulated gate structures mainly in the method of measurement of the charge separation. In the former, the distance between the two plates is periodically varied, hence the name vibrating capacitor. This produces an alternating current in the external circuit. If a source of electrons (e.g. a battery) is placed in series with the plates, a compensating voltage can be applied which restores the original distribution of electrons in the structure. With a decrease in intensity of the electric field, the amplitude of the ac current also decreases until it reaches zero, indicating that the compensating voltage equals exactly the difference of the work function. A similar situation esista in insulated gate structures: The WF difference between the silicon and the conducting layer adjacent to the insulator resulta in a charge distribution. In chemically sensitive insulating diodes the amount of excess charge at the semiconductor plate is determined from the capacitance-voltage curves. In field-effect transistors the excess charge is related to the drain-to-source current. The important difference is that with these two structures it is possible to perform the measurement of AWF either in a completely compensated or in an uncompensated state (i.e. *Towhom corres ndence should be addressed. +Onleave from J. %yrovsky Instituteof Physical Chemistry and Electrochemistry, Prague, Czechoslovakia.
from the polypyrrole, polythiophene, or polyaniline family. Apart from analytical applications&' these materials also received attention as possible building materials in electronic devices.+l0 Some of these materials have also been investigated as possible sensing layers for organic vapor~.~J'-'~ The binding sites investigated in this work were metal/ metal cation pairs which are known to form strong cyano complexes, such as Cu, Ag, and Hg.16 They have been incorporated into the bulk of the polyaniline film. Various metal/organic semiconductor composites have been investigated before, primarily as catalysta.16-20 The resulta described in this paper were obtained with a Kelvin probe which is a macroscopic form of a work function sensor. It is a convenient instrument for development and optimization of selective layers. The utilization of the optimized layer in a field-effect transistor will be the subject of a forthcoming publication.
EXPERIMENTAL SECTION Equipment. The electrochemical instrumentation consisted of an EG&G PAR potentiostat/galvanostat Model 273 and a Houston Model 2000 x-y recorder. The cell was a three-electrode type with a saturated calomel reference electrode separated from the solution by a Luggin capillary which was placed 1cm below the center of the working plathum disk A p t spiral wound around the Luggin capillary served as a counter electrode. Electrodea used for the Kelvin probe measurements were made from Pt disks (0.3 cm2,2 mm thick). They were tightly pressed into Kel-F (3M Co.)cylinders (1cm o.d., 1cm high) and cemented in by Epon 825 epoxy resin (E. V. Roberta & Associates, Inc.). The working surface was then polished with emergy paper and finally with alumina (particle size -0.3 pm). A soft iron spring and a metal screw formed an electronic contact to the backside of the platinum. Two typesof Kelvin flow-through probes were used for the work function measurements. The first type was homemade?' It was used in measurements where nitrogen served as the carrier gas (0.015 L/min). The second type (Delta-Phi-ElektronikKelvin Probe Type S, Besocke, GmbH, Julich, Germany) was used for testing in air (flow rate 0.026 L/min). The gas cylinder with 50 ppm HCN in nitrogen (ScottSpecialty Gases) was the calibration source for the measuremenb in nitrogen. The gas was further diluted with nitrogen using the Precision Gas htixer (Edicor, Inc.). Calibration in the air was done bv the Dvnacalibrator Model 340 (VICI Metronics) using appiopriatekCN permeation tubes. All measurementa were done at room temperature, and the gas Concentrations throughout this work are expressed as volume/volume ppm.
0003-2700/92/0364-0523$03.00/00 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64. NO. 5. MARCH 1. 1992
A
PA
I. charactsriaic CyctiC vOltamMgram of PANI preparaw. Conditions: ekmtyte. 0.1 M enlllne In 1 M H2S0,; potentla1 range from -0.1 10 +0.75 v w SCE; scan rate. 20 mvh; 1Oa wamng p&cds at bom potential hlts;electropalymerlyltbnstopped at +0.75 V.
Reference plates from both types of the Kelvin probe were covered galvanostatically with polypyrrole films from 0.1 M &butyl tetrailuoroborate and 0.1 M pyrrole in acetanitrile. Thia film was found to be inactive with respect to HCN. Film Preparation. All polyaniline films were prepared by oxidative polymerization from aqueous solution of 0.1 M aniline (Mallinckrodt, Inc.) in 1 M sulfuric acid (Eastman Kodak Co.) by means of potential cycling (20 mV/s) between 4.1 and +0.75 V (va SCE) with a 10-8 waiting period at both potential limits of each scan. The polymerization was done in air atmosphere and was always stopped at +0.75 V after the polymer fhmched the required thickness. A characteristic cyclic voltammogram (Figure 112?was obtained, in which the sharp anodic peak at 0.18 V corresponds to the expulsion of protom from the growing film and the level current between +300 and +500 mV is attributed to the capacitive The film was washed shortly with distilled water. immersed in aqueous mercuric chloride (Aldrich Inc.) for 12 h and exposed to gaseous HCN above solid KCN in a closed container, for 24 h. The film thickness was determined using the profilometer (SloanDektak 11). It was found that the oxidation peak current at 0.18 V (Figure1) was linearly proportional to the f h thickness (A(thiclmess)/A(ent density) = 0.045 cm3/A, with correlation coefficientr = 0.92). The films could be prepared quite repro. ducibly (0.22 Bm thick film with standard deviation of 4.7%). Before each new polyaniline deposition the electrodes were polished and cleaned for 30 min in an ultrasonic bath in 30% hydrogen peroxide and then cycled for about 1h in 1M sulfuric acid between +1.7 and 4 . 3 V at the scan rate 100 mV/s.
RESULTS It is hown that the work function of the electropolymerid film decreases bv several hundred millivolta durinn fmt few hours. This phenomenon has been studied in detail for polythiophene218 and haa been found to OCNT also in pnlypymole and in polyaniIine. During chis period electrons in the matrix become less strongly bound; i.e. the matrix becomes strongly reducing. Consequently, the metal salt which is present in the bulk of the polymer is reduced to metallic clusters which can be observed by optical microscopy (Figure 2). It is suspected that clusters which are much smaller than the op tical resolution of the microscope are formed. The metal dusters together with the exma of the metal salt form a redox couple which electronically communicates with the electronicallyconductingpolymermatrix. Thusasubaequent reaction with the gaseous ligand results in the modulation of electronic levels in the entire phase.J
Photomicrograph of the 0.22 prn thick PAN1 layer before (a) and after (b) HgCI, treatment. The relaxation was done In 0.002 M HgCl,. The grooves seen In the picture were caused by polishing the pt substrate with 0.3-pm grade alumina. Flgurs 2.
After the formation of the metal phaae inside the polyaniline (PANI) matrix the layer is conditioned by exposure to HCN atmosphere. This is done by placing the probe tip above solid KCN in a closed container overnight. The probe tip is then mounted in the Kelvin probe apparatus in the stream of air and allowed to reach a stable initial value of AWFinit. The testing is done either in laboratory air or in nitrogen. Response of the Selective Layer. A typical response of the oxidized PANI'Hg layer to stepchmge of HCN concentration from 0 to 19.3 ppm is shown in'Figure3. The dynamic range presented here was limited by the emission r a t a of the permeation tubes. The response such aa this was obtained for period of 24 days without okrvable deerease of sensitivity ( F m e 4). In between the measurements the probe was kept exposed to normal laboratory air. The typical concentration dependence of the PANI'Hg layer is shown in Figure 5. More than 30 layers have been investigated in this study. The slopes of the concentration d h r a t i o n curvea varied between 20 and 35 mV/decade. The relative standard deviation of the response to 19.3ppm HCN WBS 5.33% for one layer and 5.08% for seven layers of the same nominal thickness. The testing was done for both decreasing and increasing HCN concentration steps. No hys. teresis of the response hae been observed. Both the risingand the falling portions of the WF change have a form AWF = k log ( I I 2), indicating the bulk response. The diffusion coefficient calculated for the known thicknesa of the layer L, according to the relationship L = 2(Dt)' z, is 1.1 X l(rl*cmz/s.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
s
i
B
30 -
n
525
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s. a 20
5: E
Y
a
I
0 2
/
4
5
6
3
PH Flgure 8. Dependence of sensitlvtty on pH. Conditions: concentration
Figure 9. Characteristic response of the PANI'Hg layer to concentration step change of HCN. Conditions: layer thickness, 0.22 pm; fiow rate of ak, 25.8 W m i n ; relaxation in 0.01 M HgCI,; Concentration steps from (a)0 to 1.50, 3.77, 8.36, and 19.3 ppm HCN and (b) 0 to 19.3 ppm HCN.
range from 0 to 19.3 ppm in air; flow rate 25.8 mL/min; thickness of layer 0.3 pm; relaxation done in 0.01 M HgCi, and 0.01 M phosphate buffer in pH range from 2.5 to 5.5.
i
I
no
I?a
f.
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Y
> E
L
10
a
0
B
40
3 a
0.1 20
3 0.2
0.3
0.4
0.5
THICKNESS IN MICRONS Flgure 7. Dependence of sensitivity on layer thickness. Conditions:
Concentration range from 0 to 19.3 ppm in ak; flow rate 25.8 mUmin; layer thickness range from 0.13 to 0.43 pm; relaxation done in 0.01
0 3
4
8
7
M HgCi,.
14 15 16 24
DAYS SINCE FORMATION Flgure 4. Long-term stability of the response of the PANI'Hg layer to a concentration step change from 0 to 19.3 ppm HCN in air. Condltlons: flow rate, 25.8 mL/min; thickness of layer 0.22 pm; relaxation in 0.01 M HgCl,.
> E
Y
- 1
0
1
2
LOG [PPM] Flgwo 5. Concentratlon dependence of WF. Conditions: concentration range from 0.138 to 21.1 ppm HCN in air, flow rate 25.8 Wmh;thidvress of layer 0.3 pm; relaxation in 0.01 M HgCi, and 0.01 M phosphate buffer at pH 5.5. Detection limit (for AWF = 0) is 80
PPb.
In the attempt to optimize the performance we have investigated the effect of the following parameters for the PANI*Hg system: concentration of the HgClz solution,
thickness of the layer, and pH of the solution during conditioning. In all cases the layer was electropolymerized under the standard conditions. In was found that the concentration of HgClzhas a defied but weak effect on the sensitivity. For a 2250 A thick layer the slope decreases from 28 to 23 mV/decade as the HgClz solution concentration changes from 0.5 to 20 mM. There is a much stronger effect of pH on the performance of the layer. A 3000 A thick layer was formed under the standard conditions and then soaked in 0.01 M HgCl, and 0.01 M phosphate buffer solutions between pH 2.5 and 5.5. The slope of the potentiometric open cell potential (i.e. the pH sensitivity) is 15 mV/pH as opposed to 38 mV/pH for a PANI layer without the Hg/HgC12 system. This is a result of the formation of the mixed potential at the layer/electrolyte interface (vide infra). The initial value of the WF changes only by 5 mV/pH being slightly higher at pH 5.5. From the analytical point of view the dependence of the sensitivity on pH (Figure 6) is quite important. The thickness of the layer has the strongest influence on ita performance. The dependence of the sensitivity and of the time response on thickness in the range 0.1-0.43 wm of PANI were investigated. There is a scatter in this data due to the uncertainty in relating the thickness of the deposited film to the deposition current; The growth of the PANI is said to follow a 'supralinear expansion".14 There is also a tendency to form a somewhat thicker layer at the edges of the electrode. Nevertheless, there is a clear trend of the increasing slope of the concentration dependence with increasing thickness of the
526
’
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
PAN1 * Hg
*O V
1
-
’
I
.
,
.
I
.
,
0.2
0.3 0.4 0.5 THICKNESS IN MICRONS F@wa 8. Work functkn response to conceniratb step from 0 to 19.3 ppm HCN w layer thldcness in alr. conditions: Row rate 25.8 ml/mln; layer thickness range from 0.13 to 0.43 pm, relaxation done in 0.01 M HgCI,. 0.1
film (Figure 7). The best fit in this case was obtained using a logarithmic interpolation. Similarly, the sensitivity of the layer to the step change from air to 19.3ppm HCN is higher for thicker films (Figure 8). This feature is important if the layer were to be used in a detector or in an alarm. It was verified that none of the above measurements were affected by the flow rate of the carrier gas in the range 2.9-46.5 mL/min. This leads us to believe that the layer will function equally well in sensing situations where the main mode of transport of the sample to the sample is by diffusion.
DISCUSSION It has .been shown before14 that the ability of the matrix to form a charge-transfer complex with the gas molecule depends on the initial value of the work function of the matrix. We have therefore prepared films in various stages of oxidation. In the first set of experiments the f i b reduced either chemically (by ascorbicacid) or electrochemically. In this state the reactions of the film with HCN can be represented as
-‘t
€8
[PI-PANI-M]
Pt
+
ACN
Slow _ . I C
t
I
[PI-PANI-H’M(HCN),J
L l The HCN molecule has a high affinity for the metal, readily forming the M(CN), complex. This process represents a surface oxidation of mercury. During the interaction of HCN with the layer both electrons and protons are transferred to the PANI backbone. The excess of electrons then flows through Pt to the reference plate. This process is seen as the formal decrease of the work function. The weakly basic secondary amine linkages in the PANI presumably act as proton acceptors. The sign and magnitude of this response (decrease of AWF) are similar to those obtained on pure Ag except that on pure metal the response is irreversible. The second set of investigations was carried out on films containing the oxidized form of the metal or an excess of the M(CN), complex. In that case the reactions with gaseous HCN do not involve change of the oxidation state of the metal and can be schematically shown as [Pt-PANI-M/M(HCN),J
+
ACN
fast
l - 5
[PI-PANI-H+, M(HCN),l]
r
The HCN molecule diseociates, and the proton is either shared with, or completely transferred to the PANI backbone. The electroneutrality of the PANI is maintained by the partial transfer of electrons from the reference plate which is equivalent to the increase of the WF of the sample layer.
1
2
3 x=L
x = o
X
Figure 9. Model of the dependence of the contact potentlal difference on the thickness of the PANI layer.
Thus, the interaction of the layer with HCN can result in a decrease or in an increase of its WF and the corresponding calibration curves can have slopes ranging from negative to p ~ s i t i v e .This ~ behavior was observed both for Ag- and Hgbinding sites. None of the other metals which were examined gave a satisfactory result. Their response was either too small, irreversible, or too slow. The thickness of the PANI*M film affects the sensitivity and the time response in a profound way. Yet, the value of the initial WF is almost unaffected. This seeming contradiction of the theory, proposed for the concentration dependence of the WF,3 can be explained by the existence of the space charge which extends from the PANI/Pt interface to the interior of the layer over the distance of approximately 5000 A. Notice that the thickness dependencies (Figures 7 and 8) tend to level off for thicker f i . The solid line in these figures is a logarithmic interpolation which, for all tested functions,yields the highest value of the correlation coefficient. The thickness of the space charge is logarithmically related to its potential profile 0 (Figure 9). If we ignore the contribution to the compensating voltage from the surface adsorption, the signal from the Kelvin probe yields the value of the contact potential aLfor layer of total thickness L
a0- @ =
=
a,, exp(-kx)
(1)
x is the distance parameter, and k is the constant whose value
depends mainly on the conductivity of the layer. It is, therefore, possible to state that a full response is obtained for the films whose total thickness L is equal to or greater than the thickness of the space charge. The physical reasons supporting this conclusion are the same as those operating on the dependence of the interfacial potential on the thiclrness of hydrated layer of weakly conducting “insulabr~”.~ There could be several reasons for the existence of this space charge, the major one being the low specific conductivity of the PANI*M layer. It has not been determined but it is known that PANI is electroactive below pH 3-4,26727 and at pH = 4 PANI has the highest conductivity when polarized below 0.4-0.5 VaZ8Thus the pH of the conditioning solution which determines the degree of protonation of the polymer also plays an important role. The solutions of 0.01 M mercuric chloride are not buffered, and their pHs are not well defied. Because the open cell potential of polyaniline film immersed in the HgClzsolutions for 12 h is about 420 mV, the polymer electrochemical state is apparently somewhere at the edge of the conductivity and of the electrochemical activity region. It is likely that under these conditions the space charge thickness is at its maximum.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
Another reaaon for the formation of the space charge could be a profile of the morphology of the film formed during the electropolymerizationBa and/or during the relaxation period. In that case the gradient of the WF could be formed which would, in turn, support the potential profile and electric field near one or the other interface of the film. The interpretation of the thickness effect on the time response is not straightforward. Thicker filmsappear to respond faster. In any case the dependence on the squareroot of time strongly suggests that the majority of the signal has ita origin in the bulk of the film. The bulk mechanism is also indicated by the shape of the concentration dependence which is logarithmic over almost three decades. The major difference between behavior of the PANI*M system and polypyrrole’* is the difficulty with adjusting the initial value of the WF electrochemicallyin the former. This is probably due to the fact that the rest potential of the PANI system is dominated by both the prcrton and by the anion exchange (Figure 1). In such case a mixed potential is formed. The value of the WF (or of the Fermi level of the film) is given by the magnitude of the redox term in the equation for the rest potential of these materialsB
Erest= E’ + 5’log (ai + K’aox/ared+ CKiiaj) where S is the slope, Kij is the potentiometric selectivity coefficient, and a, and aiare the activities of the primary and interfering ions, respectively. The contribution from the second and third term in the logarithmic argument leads to the formation of the mixed potential which in the case of PANI is apparently dominated by the contribution from the ion exchange. In some cases the WFa can be adjusted chemically by, e.g., reduction with, e.g., ascorbic acid. Although ascorbic acid does not reduce Hg2’ readily, it nevertheless lowere the WFMtof PANI*Hg, albeit not as much as that of PANI*Ag. Both pH and the Hg/HgCl, ratio show a weak effect on the sensitivity of the f h . It can therefore be concluded that two mechanisms affect the sensitivity of the film: first of all, the formation of the space charge and ita confinement within the thickness of the layer and, secondly, the electron affinity of the matrix. The incorporation of the electrically neutral HgClz in the PANI layer by diffusion is relatively more straightforward than the incorporation of the anionic complexes by ion exchange or by potential cycling. For these reasone, and because of ita better long-term stability, the PANI*Hg system is preferable to other PANI*metal systems for sensing of HCN.
527
ACKNOWLEDGMENT Thie work was supported by Grant No. R-816491-01-0 from the Office of Exploratory Research of the Environmental Protection Agency and by a contract from the BWB, Germany. Registry No. HCN,74-90-8; HgCI2, 7487-94-7; polyaniline (homopolymer),25233-30-1;aniline, 62-53-3. REFERENCES Janata, J. prnclpbs of Chemkal Sensors; Plenum Publlehers: New York. 1989; Chapter 4. Zemel, J. Sens. Actuators 1981, 1, 31-75. Janata, J. Anal. Chem. 1991, 63, 2546-2550. Wang, J.; m n , S. P.; Un, M. S. J . Elecb.oenel. Chem. Interfaclel Ektfochem. 1889, 273,231-242. Boyle, A.; Genies, E. M.; Lapkowskl, M. J . Ektronnal. Chem. Inter&&I Ekfrochem. 1989, 28,C7694774. Ye, J.; Baldwh, R. P. Anel. Chem. 1888, 60, 1979-1982. Shaolin, M.; Hualguo, X.; Bldang, Q. J . Ekctroenal. Chem. Intdaclel Ekbvchem. 1991, 304, 7-16. Chao, S.; Wrlghton, M. S. J . Am. Chem. Soc. 1987, 100, 6627-6631. mer, D.; Crooks, R. M.; Wrlghton, M. S. J . Am. Chem. Soc. 1990, 112,7869-7879. Sawal, T.; Shinohera, H.: Ikarlyarna, Y.; Airewa, M. J . Ehsctru8nal. Chem. Interfaclel Ekbochem. 1990, 283,221-230. JOSOWlCZ, M.; Alate. J. A M . Chem. 1988, 58, 514-517. Joeowlcz, M.; Janata. J.; Ashley, K.; Pons, S. Anel. Chem. 1987, 50, 253-258. Bartlett, P. N.: Sim, K.; Llng-Chung. Sens. Actuators 1889, 20, 287-292. Josowicr, M.: Blackwood, D.. J. phys. Chsm. 1991, 05, 493-502. Ookrb, A. M.; K W , H.; Skopenko,V. V. Chemkby ot&euWmldr#r; Elsevk. Amsterdam, 1986. Esteban, P. 0.; L m , J. M.; LatlIY, C.;Genies, E. J . Appl. E k h m.1988, IO,462-464. TSakOW, V.; Milchev, A. E&ct#dth. Acte 1991, 36, 1151-1155. Gholamien, M.; Contrector, A. Q. J . Ektronnal. Chem. Interfadal Ekbvchem. 1990, 280,69-83. KOSt, K. M.; Bartak, D. E. Anel. Chem. 1988, 60, 2379-2384. Tian, Z.Q.; Lien, Y. Q.; Wang, S. J.; Li. W. H. J . Elecbwnal. Chem. Interfacial Eh9ctrccl?em.1991, 308,357-363. Gratzl, M.; Hsu, Duan-Fu; Riley, A. M.; Janata, J. J . Wys. Chem. 1990. 04, 5973-5981. Genles, E. M.; Penneau, J. F.; Viell, E. J . Ektroenal. Chem. 1980, 283,205-219. Hsu, DuaMu; Gratzl, M.; Rlley, A. M.; Janata. J. J . Wys. Chem. 1990. .- ... 94. .. . 5982-5989. ... - .... ZoM, G.; Cattarin. S.; Comlsso, N. J . Electraenal. them. Interfacial EleC@OChem. 1987, 235, 259-273. SendHW, J. A M I . chem.1988, 60, 1553-1562. oheeke,T.; Ohnuki, Y.; Oyama. N.; KatagH, G.; Kamisako, K. J . E k tmnel. Chem. Interfaclel E-. 1884, 161,399-405. Cushman, R. J.; McManus, P. M.; Yang, S. C. Meknxrol. Chem., Rep# Convnun. 1987, 8,89-75. Orata, D.; Butby, D. A. J . Am. Chem. Soc. 1987, 100, 3574-3581. Carlln, C. M.; Kspley, L. J.; Bard. A. J. J . E m . Soc. 1981, 132,353-359.
RECEIVED for review August 23,1991. Accepted November 20, 1991.
