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: Mean peak concentrations were markedly different : 11 . 00 + / - 2 . 89 microg / ml in OD vs . 6 . 53 + / - 1 . 45 microg / ml in TD ( P < 0 . 01 ) .
### Output Text:
Mean peak concentrations were markedly different : 11 . 00 + / - 2 . 89 microg / ml in OD vs . 6 . 53 + / - 1 . 45 microg / ml in TD ( P < 0 . 01 ) . 