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 50 % effective dose of d - tubocurarine did not correlate with muscle mass or AChR expression .
### Output Text:
The 50 % effective dose of <span class="chemical">d - tubocurarine</span> did not correlate with muscle mass or AChR expression . 