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: The patient ' s observations were within normal limits , he was administered oxygen via a face mask and glyceryl trinitrate ( GTN ) .
### Output Text:
The patient ' s observations were within normal limits , he was administered <span class="chemical">oxygen</span> via a face mask and <span class="chemical">glyceryl trinitrate</span> ( <span class="chemical">GTN</span> ) . 