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: One patient , aged 23 , had been followed for 2 years before carcinoma was diagnosed ; the second patient , aged 22 , had been seen on a regular basis for 5 years , 8 months .
### Output Text:
One patient , aged 23 , had been followed for 2 years before carcinoma was diagnosed ; the second patient , aged 22 , had been seen on a regular basis for 5 years , 8 months . 