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: Finally , we quantified both nuclear and mitochondrial DNA ( mtDNA ) as well as superoxide production and the 4834 base pair ' common ' mtDNA deletion .
### Output Text:
Finally , we quantified both nuclear and mitochondrial DNA ( mtDNA ) as well as <span class="chemical">superoxide</span> production and the 4834 base pair ' common ' mtDNA deletion . 