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: Only preservative - free methotrexate in Elliott ' s B Solution at a concentration of not more than 1 mg / ml should be used for intrathecal administration .
### Output Text:
Only preservative - free <span class="chemical">methotrexate</span> in Elliott ' s B Solution at a concentration of not more than 1 mg / ml should be used for intrathecal administration . 