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: In all patients with variant angina , coronary spasm was provoked at a mean dose of 28 + / - 13 micrograms ( mean + / - SD ) .
### Output Text:
In all patients with variant angina , coronary spasm was provoked at a mean dose of 28 + / - 13 micrograms ( mean + / - SD ) . 