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: METHOD : This was a cross - sectional study of 85 patients from a lithium clinic who received different dose schedules .
### Output Text:
METHOD : This was a cross - sectional study of 85 patients from a <span class="chemical">lithium</span> clinic who received different dose schedules . 