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: Low clearance values found in several patients could be accounted for by their age and their pre - lithium values .
### Output Text:
Low clearance values found in several patients could be accounted for by their age and their pre - <span class="chemical">lithium</span> values . 