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: It is often combined with caffeine in diet preparations and " look - alike " pills .
### Output Text:
It is often combined with <span class="chemical">caffeine</span> in diet preparations and " look - alike " pills . 