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 methotrexate was administered as a loading dose of 6 , 000 mg / m2 for a period of one hour followed by an infusion of 1 , 200 mg / m2 / h for 23 hours .
### Output Text:
The <span class="chemical">methotrexate</span> was administered as a loading dose of 6 , 000 mg / m2 for a period of one hour followed by an infusion of 1 , 200 mg / m2 / h for 23 hours . 