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: DNA analysis revealed low amounts of a 4 . 8 kb mtDNA deletion but with no differences in frequency or quantity in the control and experimental groups .
### Output Text:
DNA analysis revealed low amounts of a 4 . 8 kb mtDNA deletion but with no differences in frequency or quantity in the control and experimental groups . 