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: A total of sixty patients were trated with bromperidol first in open conditions ( 20 patients ) , then on a double blind basis ( 40 patients ) with haloperidol as the reference substance .
### Output Text:
A total of sixty patients were trated with <span class="chemical">bromperidol</span> first in open conditions ( 20 patients ) , then on a double blind basis ( 40 patients ) with <span class="chemical">haloperidol</span> as the reference substance . 