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: Magnitudes of secondary pinprick hyperalgesia and brush - evoked allodynia were investigated using von Frey filaments ( gauges 10 , 15 , 60 and 100 g ) and brush strokes .
### Output Text:
Magnitudes of secondary pinprick hyperalgesia and brush - evoked allodynia were investigated using von Frey filaments ( gauges 10 , 15 , 60 and 100 g ) and brush strokes . 