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: Secondary pinprick hyperalgesia was observed as a marked increase in the visual analogue scale ( VAS ) response to von Frey gauges 60 and 100 g ( P < 0 . 001 ) after glutamate injection .
### Output Text:
Secondary pinprick hyperalgesia was observed as a marked increase in the visual analogue scale ( VAS ) response to von Frey gauges 60 and 100 g ( P < 0 . 001 ) after <span class="chemical">glutamate</span> injection . 