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: OBJECTIVE : The objective of this study was to determine the prevalence of positive plasma drug screening for cocaine or amphetamine in adult emergency department seizure patients .
### Output Text:
OBJECTIVE : The objective of this study was to determine the prevalence of positive plasma drug screening for <span class="chemical">cocaine</span> or <span class="chemical">amphetamine</span> in adult emergency department seizure patients . 