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: He had been taking VPA for treatment of idiopathic generalized tonic - clonic convulsions since he was 56 years old .
### Output Text:
He had been taking <span class="chemical">VPA</span> for treatment of idiopathic generalized tonic - clonic convulsions since he was 56 years old . 