Continuous in Situ Cyanide Monitoring Using a Highly Sensitive and

May 9, 2000 - A flow injection analysis (FIA) system incorporating a gas-diffusion membrane was fabricated for the detection of cyanide anion in aqueo...
0 downloads 0 Views 162KB Size
Download PDF
Environ. Sci. Technol. 2000, 34, 2618-2622

Continuous in Situ Cyanide Monitoring Using a Highly Sensitive and Selective FIA System YOKO NOMURA,† KAORI NAGAKUBO,† HONG-SEOK JI,† ATSUSHI WATANABE,† TAKUO AKIMOTO,† SCOTT MCNIVEN,† KENJI HAYASHI,† YOSHIKO ARIKAWA,‡ AND I S A O K A R U B E * ,† Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904 Japan, and Japan Women’s University, 2-8-1 Mejirodai, Bunkyo-ku, Tokyo 112- 0015 Japan

A flow injection analysis (FIA) system incorporating a gas-diffusion membrane was fabricated for the detection of cyanide anion in aqueous samples. The principle of measurement is based on the reaction of o-phthalaldehyde (OPA) and cyanide in the presence of glycine to produce a fluorescent isoindole derivative. The cyanide concentration of the samples is thus proportional to the observed fluorescence intensity. Although extremely low levels of cyanide could be determined using this system (lower detection limit 0.4 ng mL-1 of CN-), measurements were affected by the presence of sulfite ion and thiols. Therefore, a gas-diffusion membrane was incorporated into the system to separate gaseous hydrogen cyanide from interferents in the sample. Consequently, this system displayed high selectivity for cyanide. The presence of sulfite ion (1 µg mL-1), 2-mercaptoethanol (0.1 µg mL-1), or 2-mercaptopropionic acid (10 µg mL-1) did not significantly increase the fluorescence intensity of cyanide solutions (16 ng mL-1). The sensor was then used for the rapid (150 s per measurement) detection of cyanide in samples of river water. A linear response (Kawashima bridge, y ) 0.174x - 2.54E-2, r 2 ) 0.999, n ) 3; Yaba bridge, y ) 0.170x + 2.75E-2, r 2 ) 0.997, n ) 3) was observed between the concentration of cyanide (0.4-40 ng) added to the samples and the response of the sensor. Furthermore, a device based on this FIA system was constructed and used for the continuous, in situ monitoring of cyanide concentrations in river water for 5 months, taking reading every 5 min, convincingly demonstrating the utility of our sensor. Although the sensitivity (mV/[CN-] of the sensor system tended to decrease over time, replacement of the gas-diffusion membrane restored the sensitivity to its initial level. In all cases, calibration curves yielded correlation coefficients ranging from r 2 ) 0.991-1.00.

1. Introduction Free cyanide anion is extremely and acutely toxic to all aquatic life as well as to humans because it obstructs oxygen supply * Corresponding author phone: +81-3-5452-5220; fax: +81-3-54525227; e-mail: [email protected]. † University of Tokyo. ‡Japan Women’s University. 2618

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 12, 2000

to cells. Cyanide is a byproduct of industrial processes such as electroplating. Waste containing cyanide may accidentally contaminate rivers and reach groundwater reserves, endangering water supplies and poisoning aquatic organisms. A rapid and convenient method of detection is therefore required. However, conventional techniques such as the use of cyanide-selective electrodes (1, 2) and spectrophotometric methods (3-5) are not sufficiently sensitive. Ion chromatography has also been used but is relatively expensive (6). Recently, a convenient method based on flow injection analysis (FIA) and a biosensor was developed for the detection of cyanide in aqueous samples (7). A capillary flow-injection analysis using amperometric detection has also been developed (8). Although these methods were both convenient and sensitive, they lacked selectivity. Gas-diffusion membranes may be used to achieve high selectivity in the detection of cyanide or carbon dioxide (9, 10). Lee and Karube used such a membrane in a microbial cyanide sensor (11), and Sebroski and others have used them in amperometric cyanide detection systems (12-14). The kinetics of the passage of hydrogen cyanide through a gas-diffusion membrane have also been investigated in detail (14). We have developed an FIA system incorporating a gasdiffusion membrane which is capable of detecting ppb levels of cyanide. Cyanide reacts with o-phthalaldehyde (OPA) in the presence of glycine to produce a fluorescent isoindole derivative. The cyanide concentration of the sample is thus proportional to the intensity of fluorescence of the isoindole derivative. Although the cyanide concentration of standard solutions could be measured with great sensitivity using this method, the assays were affected by sulfite ion and thiols. Sumiyoshi et al. (15) applied this reaction to cyanide detection and their method achieved high selectivity and sensitivity, principally because the cyanide was separated from any interferents using ion-exchange chromatography, but it took 20 min to perform one measurement. Narinesingh et al. (16) also fabricated a sensitive flow injection system using the OPA reaction (detection limit 2.3 × 10-7 M CN-), but this system required two columns. In our system, cyanide anion is protonated using a pH 7 buffer, and the resulting hydrogen cyanide gas passes through a gas-diffusion membrane and is thus separated from interfering substances in the sample solution. Consequently, this system is highly selective for cyanide, and an automatic device was constructed and applied to the continuous, in situ monitoring of river water.

2. Experimental Section 2.1. Reagents. o-Phthalaldehyde (OPA) (Aldrich), glycine, KCN (Shuzui, Japan), and all other reagents (Wako Chemical Co., Japan) were used as received. OPA (0.268 g) was first dissolved in 40 mL of ethanol and subsequently diluted with deionized (Milli-Q) water. 2.2. FIA System. Figure 1 shows the FIA system used for the laboratory analyses. Reagents were pumped through the system at 0.5 mL min-1 using a 4-channel peristaltic pump (Gilson minipuls 3) equipped with 1.15 mm i.d. PharMed tubes (Norton). Peek tubing (0.5 mm i.d.) was used throughout the rest of the system. Reagent R1 was 0.2 mM glycine solution, and R2 was sodium borate buffer (0.1 M, pH 9.5) containing 0.2 mM OPA. Samples (100 µL sample loop, Rheodyne) were injected (injector, Rheodyne 7125) into the phosphate buffer (0.1 M, pH 7) solution R3 and pumped to the gas-diffusion unit (Chemiford EX TYPE V Gas-diffusion membrane 500-2875, Tector Co., Sweden). Hydrogen cyanide gas 10.1021/es991039a CCC: $19.00

 2000 American Chemical Society Published on Web 05/09/2000

FIGURE 1. Flow injection system for the determination of cyanide. Reagent R1 was 0.2 mM glycine solution, R2 was sodium borate buffer (0.1 M, pH 9.5) containing 0.2 mM OPA, and R3 was phosphate buffer (0.1 M, pH 7).

SCHEME 1. Cyanide Reacts with o-Phthalaldehyde (OPA) in the Presence of Primary Amines or Amino Acids To Yield Fluorescent Isoindole Derivatives

passing through the membrane was absorbed by receptor solution R2. OPA and cyanide anions in R2 were introduced into a 5.0 m stainless steel coil where they reacted at 50 °C with glycine (R1) to produce an isoindole derivative (Scheme 1). Fluorescence intensity was measured using a JASCO 821FP spectrofluorimeter (λex 328 nm, λem 370 nm), and peak areas were calculated using a Hitachi D-2500 integrator. 2.3. Effect of Buffer Solution pH. To optimize the separation of cyanide from sulfite and thiols, which are known to interfere with the reaction of OPA and cyanide, the effect of the pH of R3 buffer solution was examined using glycine buffer (0.1 M, pH 3.0), citrate buffer (0.1 M, pH 5.0), and phosphate buffer (0.1 M, pH 7.0). The interferents, sodium sulfite (0.02-50 µg mL-1), and the thiols 2-mercaptoethanol (0.02-10 µg mL-1) and 2-mercaptopropionic acid (0.02-10 µg mL-1) were each added to standard solutions containing 16 ng mL-1 of CN1-. 2.4. Calibration Curves. KCN was used for all calibration solutions. Carefully weighed KCN (50 mg) was dissolved in 0.1 M NaOH solution (5 mL) and diluted to 0.4 µg mL-1 CNand finally to 0.4-40 ng mL -1 as CN- using Milli Q water. Cyanide samples (0.4-40 ng mL-1) were examined using phosphate buffer (0.1 M, pH 7.0) as R3. 2.5. Measurement of River Water Samples. River water contains a wide variety of dissolved substances so Cl-, Mg2+, and Zn2+ were investigated as potential interferents in the measurement of real samples. NaCl (2.5-12.5 g L-1 Cl-), MnCl2‚4H2O (0.02-0.1 mg L-1 Mn2+), and ZnCl2 (0.02-0.1 mg L-1 Zn2+) were dissolved in standard solutions of 16 ng mL-1 CN-, and the solutions were assayed. These concentrations were used because they were the levels quoted in an annual report on Japanese river water monitoring (17). River water samples were provided by the Japanese Ministry of Construction. These samples were collected from the Tone river at Yaba and Kawashima bridges in the Kantoh area of eastern Japan in January 1996. The samples did not contain any detectable cyanide so CN- (4-40 ng mL-1) was added. 2.6. In Situ River Water Monitoring. The in situ monitoring of the cyanide concentration of river water was carried out from September 1998 to February 1999 using a device based on the OPA reaction. The device (Figure 2) was con-

FIGURE 2. Device used for continuous in situ monitoring of river water. The same FIA system as in Figure 1 was housed in the thermostatic chamber. Reagent R1 was 0.2 mM glycine solution and R2 was sodium borate buffer (0.1 M, pH 9.5) containing 0.2 mM OPA. R3 solution was phosphate buffer (0.1 M, pH 7). structed by Anatec Yanaco Co. (Kyoto, Japan), in cooperation with the Japanese Ministry of Construction. The precipitation tank was obtained from DKK Co. (Japan). River water was also monitored using an Automatic river water monitoring system (Kantoh Area Technical Office, Japan) incorporating a cyanide-selective electrode during the monitoring period. Continuous monitoring was carried out near the Kogasaki sewage plant, situated at the junction of the Saka and Edo rivers in Chiba prefecture, east of Tokyo (18). After allowing the precipitation of any solids, the samples were pumped to the device. The device measured the cyanide concentration in the river every 5 min during the monitoring period of 5 months. The FIA system used was the same as that shown as in Figure 1. The photodetector and peristaltic pumps were the same type, and the reaction coil (5.0 m) was housed in a thermostatic chamber at 50 °C. To separate solid substances from the river water, a precipitation tank was installed between the river water pumping system and the FIA device. River water was pumped continuously from the Saka river (a branch of the Tone river) to the precipitation tank (15 L). The supernatant river water in the precipitation tank was then filtered and injected (100 µL) into the FIA system every 5 min. Automatic calibration of the sensor was carried out daily using 0.1 M NaOH Milli Q water solutions containing 0 and 40 ng L-1 CN- prior to measurement of the cyanide VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

2619

FIGURE 3. Correlation between Na2SO3 concentration and fluorescence intensity. No gas-diffusion membrane was used in this system.

FIGURE 4. Effect of interfering substances on fluorescence intensity using an FIA system equipped with a gas-diffusion membrane.

concentration of the river water. Periodically, standard solutions of 2, 4, 20, and 40 ng L-1 CN- were also used to generate calibration curves. Several CN- solutions were also measured using the conventional pyridine-pyrazolone method (3). The conventional measurements were carried out in the laboratory after in situ measurement using the automated FIA system.

3. Results and Discussion 3.1. Interferences with the OPA Reaction. 2-Mercaptoethanol and OPA have been shown to react with amino acids to produce fluorescent derivatives, and this has been used as a method for their detection and quantitation (19, 20). Cyanide anion can be used in lieu of 2-mercaptoethanol and leads to the formation of isoindole derivatives (Scheme 1) (15). Although the FIA system lacking the gas-diffusion membranes is able to detect ppb (ng mL-1) levels of cyanide (data not shown), OPA, in the presence of primary amines, also reacts with sulfite ion (15), which interferes with cyanide detection. Sulfite ion is found in river water and is used an index of pollution (3) because of its toxicity in aquatic ecosystems. Thus the FIA system without the gas-diffusion membrane could also detect Na2SO3. Figure 3 shows the linear relationship between the fluorescence intensity and the concentration of Na2SO3 (y ) 2.29E-2x + 9.24E-2, r 2 ) 0.999, n ) 3). Statistical errors (maximum value - minimum value/2) were between 4.21 × 10-2 at Na2SO3 80 ng mL-1 and 1.7 × 10-3 at 20 ng mL-1. Other species such as thiols also interfere with cyanide determinations so separation of cyanide from these compounds was necessary to selectively determine cyanide concentrations. Therefore, a gas-diffusion membrane was incorporated to achieve separation of cyanide in the FIA system. 3.2. Effect of Buffer Solution pH. Cyanide separation using the gas-diffusion membrane depends on the pH of the buffer solution R3. Since the pKa of cyanide anion is 9.22 (25 °C) (21), it is largely converted to gaseous HCN at lower pH values, while, conversely, cyanide ion predominates in solutions of higher pH. As expected, using buffer solutions of lower pH increased the fluorescence intensity, due to the greater production of HCN. However, neither 0.1 M glycine buffer (pH 3) nor 0.1 M citrate buffer (pH 5) prevented the interfering effect of sulfite ion. Using 0.1 M citrate buffer (pH 5) gave interferences at concentrations greater than 0.1 µg mL-1 sulfite ion, while using 0.1 M glycine buffer (pH 3) showed interferences at 0.02 µg mL-1 SO32-. As can be seen in Figure 4, in 0.1 M phosphate buffer at pH 7 the presence of sulfite ion (1 µg ml-1), 2-mercaptoethanol (0.1 µg mL-1), or 2-mercaptopropionic acid (10 µg mL-1) did not measurably increase the fluorescence intensity of 2620

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 12, 2000

FIGURE 5. Correlation between cyanide concentration and fluorescence intensity. The FIA system incorporated a gas-diffusion membrane and the buffer solution was 0.1 M phosphate buffer, pH 7. cyanide solutions (16 ng mL-1). The more powerful interfering effect of 2-mercaptoethanol is most probably due to the fact that it is more volatile than 2-mercaptopropionic acid and therefore more able to traverse the gas-diffusion membrane. Under our test conditions (pH 7 buffer) only free cyanide anion or readily dissociable cyanide species can be detected. For example, the much less toxic potassium ferricyanide would not likely be detected under the conditions employed. Thus our method is a primary method for the detection of free cyanide anion. Once the selectivity of this FIA system for cyanide was confirmed, a calibration curve was constructed using phosphate buffer (0.1 M, pH 7) as the carrier solution R3. 3.3. Calibration of the FIA System. Figure 5 shows a calibration curve for cyanide determination. The reaction takes place in the mixing coil, which was heated using the thermostat. The fluorescence intensity at 50 °C was about 3 times higher than at 30 °C. Determinations were rapid (150 s per measurement) and showed good reproducibility. A linear response (y ) 0.161x + 0.763, r 2 ) 0.999, n ) 3) was obtained for cyanide concentrations from 0.4 to 40 ng mL-1, and the reproducibility was excellent (CN- 4 ng L-1, n ) 8, RSD 1.1%). Statistical errors (maximum value - minimum value/2) were between 0.193 at 32 ng mL-1 and 3.57 × 10-3 at 0.4 ng mL-1 of CN-. OPA was allowed to react with cyanide in the reaction coil (5.0 m) for 1 min, and the fluorescence intensity, using 40 ng mL-1 CN- solution, was 60% that observed in batch-type reactions. The batch measurements, however, required 5 min to reach a constant fluorescence intensity, while using this sensor one measurement cycle took approximately 150 s, approximately 1/8 the time of a similar HPLC analysis using OPA (16). Although the use of

TABLE 1. Data Were Obtained between September, 1998 and February, 1999a date

equation

r2

Sept 17 Oct 8 Oct 29 Nov 5 Nov 24 Nov 26 Dec 1

y ) 11.3x - 15.9 y ) 11.0x - 19.9 y ) 8.68x - 14.1 y ) 8.19x - 11.1 y ) 10.2x - 21.8 y ) 9.85x - 21.2 y ) 9.27x - 21.1

1.000 0.999 0.999 0.997 0.992 0.992 0.991

a x represents the concentration of cyanide, and y represents the sensor output (mV). Calibration curves were generated using standard solutions of 2, 4, 20, and 40 ng mL-1 of CN- in river water. The gasdiffusion membrane was replaced on Oct 8 and Nov 24.

FIGURE 6. Bactchwise measurement of river water samples from the Tone river. No cyanide could be detected so KCN was added to the two river water samples, which were adjusted to pH 14 using solid NaOH. The standard solutions were 0.1 M NaOH in Milli Q water containing KCN. a longer reaction coil might increase the fluorescence intensity, the present measurement time is appropriate for an FIA system. In addition, the detection limit of this FIA system (0.4 ng mL-1) is adequate for the determination of cyanide in aquatic environments. This detection limit is extremely low compared with conventional methods such as the standard batch-type system (10 ng mL-1) (3) and an FIA system (9) based on the pyridine-pyrazolone method (200 ng mL-1). 3.4. Interferents in River Water. Although the well-known interferents of the OPA-cyanide reaction (sulfite and thiols) did not appreciably affect the response of this system to cyanide, other substances found in river water, such as chloride ion and heavy metals, might affect the response, and, therefore, the effects of the most prevalent of these substances were investigated (data not shown). Chloride ion up to a concentration of 17.5 g L -1 (0.5 M), which is near the maximum concentration found in Tone river (18.7 g L-1) (17), did not affect the response of the sensor to 16 ng mL-1 of CN-. Manganese and zinc are the main heavy metals present in river water (the maximum concentrations of Zn2+ and Mn2+ found in river water were 0.44 and 0.20 mg L-1 (17)), so the effects of these metals were examined. Neither Zn2+ nor Mn2+ ions affected the response of the sensor to 16 ng L-1 of CN- at concentrations of 0.50 mg L-1 and 0.10 mg L-1 respectively (results not shown). Although the maximum Mn2+ concentration, according to the annual report (17),was higher than the investigated value, it is not usually found in river water. From these results, it is evident that cyanide in river water could be determined with excellent sensitivity and selectivity using this system. 3.5. Measurement of River Water Samples. Water samples taken from the Tone river (Yaba and Kawashima bridges) were spiked with various amounts of cyanide and analyzed using our system. Figure 6 shows the correlation between the added cyanide concentration and the fluorescence intensity of the river water samples. A linear response was observed from 4.0 to 40 ng mL-1 CN-, and the fitted equations are y ) 0.174x - 2.54E-2 (Kawashima bridge), y ) 0.170x + 2.75E-2 (Yaba bridge), and y ) 0.163x + 2.74E-2 (standard solution). The correlation coefficients of Kawashima bridge, Yaba bridge, and standard solutions were r 2 ) 0.999, 0.997, and 0.999 (n ) 3) respectively, with errors (maximum value - minimum value/2) of 6.92E-2, 7.32E-3, and 7.72E-2 ng mL-1 CN1-.

FIGURE 7. Daily responses of the FIA system to 40 ng mL-1 of CNin river water October, 1998. The gas-diffusion membrane was installed on September 20 and replaced on October 8. The BOD5 value (an index of the level of water pollution by organic substances) of the Yaba bridge sample was 1.04 mg O L-1 (pH 7.9) and that of the Kawashima bridge sample was 2.83 mg O L-1 (pH 7.5). Thus neither the amount of organic material in the river water samples nor the differences in pH significantly affected the ability of this system to determine the cyanide concentrations of the samples. 3.6. Continuous in Situ River Water Monitoring. Calibration curves were generated seven times during the monitoring period from September 17 to December 26 in 1998, and the fitted equations are listed in Table 1. The correlation coefficients of the equations are excellent, being between 0.991 and 1.00. However, as the gas-diffusion membrane became soiled, the value of the slope of the equation (the sensitivity) decreased. After 2 weeks, the response to 40 ng L-1 of CN- was approximately 85% that of the first day, but replacement of the membrane restored the sensitivity to its original level. The responses to 40 ng mL-1 of CN1- are shown in Figure 7. The maximum response was 398.0 mV, the minimum was 329.0 mV, and the average was 358.1 mV, yielding an RSD of 5.3%. These results confirmed that this automated FIA system is eminently suitable used for continuous in situ monitoring. According to both our FIA system and Automation river water monitoring system, no cyanide was released during the 5 month monitoring period. Although cyanide anion was not detected during the monitoring period, the maximum CN- concentration in the Tone river was 0.17 ppm (170 ng mL-1) in 1990 (17), which could easily be detected using this system (lower detection limit 0.4 ng mL-1 of CN-). 3.7. Confirmation using the Conventional Method. Several cyanide solutions were manually injected into the FIA system used on site. Figure 8 shows the correlation VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

2621

Literature Cited

FIGURE 8. Correlation between values measured by the conventional method and the FIA method. Samples were measured using the FIA system in situ, and their concentrations were determined. Afterward, the same samples were measured using the pyridine-pyrazolone method in the laboratory between values from the pyridine-pyrazolone method and our in situ monitoring system. The correlation was excellent (r 2 ) 0.997), especially at higher than 10 ng mL-1 CN(r 2 ) 0.999) because the detection limit of the conventional method is approximately 10 ng mL-1. Measurements using the conventional method were carried out after transporting the samples to the laboratory after in situ measurement were finished, and this may be one reason the values from the FIA system were higher than the those of conventional method.

Acknowledgments We would like to thank the Water Research Division of the Kantoh Area Technical Office, under the authority of the Japanese Ministry of Construction, for their assistance and support. We would also like to thank Hisakatsu Yamazaki of Electrical Engineering Association (Tokyo) for his analytical support using conventional methods.

2622

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 12, 2000

(1) Frant, T. S.; Ross, J. W.; Riseman, J. H. Anal. Chem. 1972, 44, 2227. (2) Fogg, A. G.; Alonso, R. M. Analyst 1987, 112, 1071. (3) Japanese Industrial Standard K-0102, Japanese Industrial Committee (in Japanese) 1993. (4) Ganjeloo, A.; Isom, G. E.; Morgan, R. L.; Way, I. L. Toxicol. Appl. Phamacol. 1980, 55, 103. (5) Takanashi, S.; Tamura, Z. Chem. Pharm. Bull. 1970, 18, 1633. (6) Inoue, Y.; Suzuki, Y.; Ando, M. Bunsekikagaku (in Japanese) 1993, 43, 655. (7) Ikebukuro, K.; Miyata, A.; Cho, S. J.; Nomura Y.; Chang, S. M.; Yamauchi, Y.; Hasebe Y., Uchiyama, S.; Karube, I. J. Biotech. 1996, 48, 73. (8) Backofen, U.; Matysik, F. M.; Werner, G. Fresenius J. Anal. Chem. 1996, 356, 271. (9) Huichang, M.; Jingfu, L.; Jianzhang, F.; Yue, G. J. Flow Injection Anal. 1994, 11, 58. (10) Lingren, E.; Karberg, B. J. Automatic Chem. 1995, 17, 105. (11) Lee, J. I.; Karube, I. Biosensors Bioelectronics 1996, 11, 1147. (12) Sebroski, J. R.; Ode, R. H. Environ. Sci. Technol. 1997, 31, 52. (13) Milosavljevic, E. B.; Solujic, L.; Hendrix, J. L. Environ. Sci. Technol. 1995, 29, 426. (14) Sweileh, J. A. Anal. Chim. Acta 1996, 336, 131. (15) Sumiyoshi, K.; Yagi, T.; Nakamura, H. J. Chromatogr. A 1995, 690, 77. (16) Narinesingh, D.; Saroop, S.; Ngo, T. T. Anal. Chim. Acta 1997, 354, 189. (17) River Bureau, Japanese Ministry of Construction, 1990 Annual Observation Record of River Water Quality. (18) Nomura, Y.; Ikebukuro, K.; Yokoyama K.; Takeuchi, T.; Arikawa, Y.; Ohono, S.; Karube, I. Biosensors Bioelectronics 1998, 13, 1047. (19) Roth, M. Anal. Chem. 1971, 277, 222. (20) Benson, J. R.; Hare, P. E. Proc. Natl. Acad. Sci. 1975, 72, 619. (21) Chemical Report (Kagaku Binran), 3rd ed. (in Japanese); Japanese Chemical Society, Maruzen Press: Tokyo, 1984.

Received for review September 8, 1999. Revised manuscript received February 24, 2000. Accepted March 3, 2000. ES991039A