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: Plasma oxycodone concentration peaked between 60 and 90 minutes postdose ( P < . 01 ) and pupil size , an indication of physiological effects of the medication , peaked at approximately 90 to 120 minutes postdose ( P < . 01 ) .
### Output Text:
Plasma <span class="chemical">oxycodone</span> concentration peaked between 60 and 90 minutes postdose ( P < . 01 ) and pupil size , an indication of physiological effects of the medication , peaked at approximately 90 to 120 minutes postdose ( P < . 01 ) . 