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: Two samples were studied : 12 patients received oxitropium at 600 micrograms ( 6 subjects ) or at 400 micrograms t . i . d .
### Output Text:
Two samples were studied : 12 patients received <span class="chemical">oxitropium</span> at 600 micrograms ( 6 subjects ) or at 400 micrograms t . i . d . 