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: Oxidative stress was assessed by determining plasma and liver levels of 15 - F ( 2t ) - IsoP , lipid hydroperoxides ( LPO ) , and thiobarbituric acid reactive substances ( TBARs ) .
### Output Text:
Oxidative stress was assessed by determining plasma and liver levels of <span class="chemical">15 - F ( 2t ) - IsoP</span> , <span class="chemical">lipid hydroperoxides</span> ( <span class="chemical">LPO</span> ) , and <span class="chemical">thiobarbituric acid reactive substances</span> ( <span class="chemical">TBARs</span> ) . 