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 aim of the present study was to conduct the first investigation of facial expression recognition performance in recreational cocaine users .
### Output Text:
The aim of the present study was to conduct the first investigation of facial expression recognition performance in recreational <span class="chemical">cocaine</span> users . 