In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: During an 18 - month period of study 41 hemodialyzed patients receiving desferrioxamine ( 10 - 40 mg / kg BW / 3 times weekly ) for the first time were monitored for detection of audiovisual toxicity .
### Output Text:
During an 18 - month period of study 41 hemodialyzed patients receiving <span class="chemical">desferrioxamine</span> ( 10 - 40 mg / kg BW / 3 times weekly ) for the first time were monitored for detection of audiovisual toxicity . 