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 purpose of this study was to investigate the influence of calcium channel blockers on bupivacaine - induced acute toxicity .
### Output Text:
The purpose of this study was to investigate the influence of <span class="chemical">calcium</span> channel blockers on <span class="chemical">bupivacaine</span> - induced acute toxicity . 