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: Thus , use of FS containing tAMCA for surgery within or close to the CNS may pose a substantial risk to the patient .
### Output Text:
Thus , use of FS containing <span class="chemical">tAMCA</span> for surgery within or close to the CNS may pose a substantial risk to the patient . 