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: Peak concentrations were > 6 microg / ml in 100 % of the OD group and in 67 % of the TD group ( P < 0 . 01 ) .
### Output Text:
Peak concentrations were > 6 microg / ml in 100 % of the OD group and in 67 % of the TD group ( P < 0 . 01 ) . 