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: Absence of effect of sertraline on time - based sensitization of cognitive impairment with haloperidol .
### Output Text:
Absence of effect of <span class="chemical">sertraline</span> on time - based sensitization of cognitive impairment with <span class="chemical">haloperidol</span> . 