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 average recovery time of BAEP waveforms to pre - papaverine baseline values was 39 min .
### Output Text:
The average recovery time of BAEP waveforms to pre - <span class="chemical">papaverine</span> baseline values was 39 min . 