The present invention provides a process for isolating kukoamine by solid phase extraction using a polyamide or a derivative thereof as an adsorbent. The invention also aims at products of metal coated with the iron based material mentioned above. The hydroxyapatite fibers can be formed by a sol-gel process, followed by spinning of the gel to form the hydroxyapatite fibers. Setting information of the marking position is stored in a structure of management information, and is stored in a storage section together with the recording data. The optical band gap of the first gate insulating film has a value in the range of 3. A differential gain amplifier may be included to provide an output signal from the gain signal.