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: This low percentage ( 5 % ) of TDFS must be evaluated with respect to the efficacy of ifosfamide in the treatment of mesenchymal tumors in children .
### Output Text:
This low percentage ( 5 % ) of TDFS must be evaluated with respect to the efficacy of <span class="chemical">ifosfamide</span> in the treatment of mesenchymal tumors in children . 