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: Prolonged apnea in our case ensued because the information about suicidal attempt by OP compound was concealed from the treating team .
### Output Text:
Prolonged apnea in our case ensued because the information about suicidal attempt by <span class="chemical">OP compound</span> was concealed from the treating team . 