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: Timolol ( but not pilocarpine ) was detected in a sample of plasma removed during surgery ; the plasma concentration of timolol ( 2 . 6 ng ml - 1 ) was consistent with partial beta - adrenoceptor blockade .
### Output Text:
<span class="chemical">Timolol</span> ( but not <span class="chemical">pilocarpine</span> ) was detected in a sample of plasma removed during surgery ; the plasma concentration of <span class="chemical">timolol</span> ( 2 . 6 ng ml - 1 ) was consistent with partial beta - adrenoceptor blockade . 