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: Plasma extracts were analyzed by a highly sensitive high - performance liquid chromatographic method for the direct determination of malondialdehyde , oxypurines , and nucleosides .
### Output Text:
Plasma extracts were analyzed by a highly sensitive high - performance liquid chromatographic method for the direct determination of <span class="chemical">malondialdehyde</span> , <span class="chemical">oxypurines</span> , and <span class="chemical">nucleosides</span> . 