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: The eluate also reacted weakly with red blood cells in the absence of cefotetan , suggesting the concomitant formation of warm - reactive autoantibodies .
### Output Text:
The eluate also reacted weakly with red blood cells in the absence of <span class="chemical">cefotetan</span> , suggesting the concomitant formation of warm - reactive autoantibodies . 