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: OBJECTIVE : This study was designed to determine whether patients maintained on a regimen of lithium on a once - per - day schedule have lower urine volumes than do patients receiving multiple doses per day .
### Output Text:
OBJECTIVE : This study was designed to determine whether patients maintained on a regimen of <span class="chemical">lithium</span> on a once - per - day schedule have lower urine volumes than do patients receiving multiple doses per day . 