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: Aortic superoxide production was lower in the Dex + Ato group compared with the group treated with Dex alone ( P < 0 . 0001 ) .
### Output Text:
Aortic <span class="chemical">superoxide</span> production was lower in the <span class="chemical">Dex</span> + <span class="chemical">Ato</span> group compared with the group treated with <span class="chemical">Dex</span> alone ( P < 0 . 0001 ) . 