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: CONCLUSIONS : During this study period , routine plasma screening for cocaine and amphetamines in adult seizure patients had a low yield .
### Output Text:
CONCLUSIONS : During this study period , routine plasma screening for <span class="chemical">cocaine</span> and <span class="chemical">amphetamines</span> in adult seizure patients had a low yield . 