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: STUDY DESIGN : First a randomised trial , later periconception care including in total 12225 females .
### Output Text:
STUDY DESIGN : First a randomised trial , later periconception care including in total 12225 females . 