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 dose was escalated from 100 to 200 mg / m2 / d until dose - limiting toxicity ( DLT ) was observed .
### Output Text:
The dose was escalated from 100 to 200 mg / m2 / d until dose - limiting toxicity ( DLT ) was observed . 