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: This study was undertaken to determine whether oxidants are associated with the hepatic injury and to determine the relative value of several indirect methods of assessing oxidant exposure in vivo .
### Output Text:
This study was undertaken to determine whether oxidants are associated with the hepatic injury and to determine the relative value of several indirect methods of assessing oxidant exposure in vivo . 