1. Nippon Sanso Corporation, Tsukuba Laboratories, Ohkubo, Tsukuba 300‐33, Japan
2. Department of Electronics, Faculty of Engineering, Tohoku University, Sendai 980, Japan
3. Nippon Sanso Corporation, Fine Gas Laboratory, Yokokurashinden, Oyama 323, Japan
4. Nippon Sanso Corporation, Yamanashi Laboratories, Takane‐cho, Kitatama‐gun 408, Japan
5. Horiba, Limited, Miyanohigashi, Kisshoin, Minami‐ku, Kyoto 601, Japan