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: Moreover , both hepatitis B surface antigen and e antigen were observed to have disappeared in this patient .
### Output Text:
Moreover , both <span class="chemical">hepatitis B surface antigen and e antigen</span> were observed to have disappeared in this patient . 