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: The residual serum concentrations of oxazepam , temazepam and zopiclone were analysed .
### Output Text:
The residual serum concentrations of <span class="chemical">oxazepam</span> , <span class="chemical">temazepam</span> and <span class="chemical">zopiclone</span> were analysed . 