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: 93 . 4 % pregnant women had the chance to be examined for HBsAg , and the positive rate was 1 . 4 to 1 . 5 % .
### Output Text:
93 . 4 % pregnant women had the chance to be examined for <span class="chemical">HBsAg</span> , and the positive rate was 1 . 4 to 1 . 5 % . 