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: Tissue lipid peroxidation was measured in whole homogenates as well as in lipid extracts from homogenates as thiobarbituric acid reactive substances .
### Output Text:
Tissue lipid peroxidation was measured in whole homogenates as well as in lipid extracts from homogenates as <span class="chemical">thiobarbituric acid</span> reactive substances . 