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 mean steady - state plasma and CSF methotrexate concentrations achieved were 1 . 1 X 10 ( - 3 ) mol / L and 3 . 6 X 10 ( - 5 ) mol / L , respectively .
### Output Text:
The mean steady - state plasma and CSF <span class="chemical">methotrexate</span> concentrations achieved were 1 . 1 X 10 ( - 3 ) mol / L and 3 . 6 X 10 ( - 5 ) mol / L , respectively . 