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 samples with benzoylecgonine greater than 150 ng / mL or an amphetamine greater than 500 ng / mL were defined as positive .
### Output Text:
Plasma samples with <span class="chemical">benzoylecgonine</span> greater than 150 ng / mL or an <span class="chemical">amphetamine</span> greater than 500 ng / mL were defined as positive . 