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: In order to determine if PPA / caffeine can lead to stroke in normotensive and / or hypertensive rats , we administered the combination in six times the allowed human dose calculated on a per weight basis for the rats two times per day for five days .
### Output Text:
In order to determine if <span class="chemical">PPA</span> / <span class="chemical">caffeine</span> can lead to stroke in normotensive and / or hypertensive rats , we administered the combination in six times the allowed human dose calculated on a per weight basis for the rats two times per day for five days . 