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: Participants were then tested on 2 occasions after 12 - hour abstinence from caffeine in each of 2 protocols ; this required a total of 4 laboratory visits .
### Output Text:
Participants were then tested on 2 occasions after 12 - hour abstinence from <span class="chemical">caffeine</span> in each of 2 protocols ; this required a total of 4 laboratory visits . 