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: By measuring VPA - induced changes of blood NH3 content , it may be possible to identify patients at higher risk of obtundation when VPA is given chronically .
### Output Text:
By measuring <span class="chemical">VPA</span> - induced changes of blood <span class="chemical">NH3</span> content , it may be possible to identify patients at higher risk of obtundation when <span class="chemical">VPA</span> is given chronically . 