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: Intra - individual variability was generally lower for the VAS response to von Frey and brush compared with areas of secondary hyperalgesia and allodynia .
### Output Text:
Intra - individual variability was generally lower for the VAS response to von Frey and brush compared with areas of secondary hyperalgesia and allodynia . 