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: This approach allowed investigating the efficacy of alpha - lipoic acid in preventing axonal damage and apoptosis and the function and ultrastructural morphology of mitochondria after exposure to toxic agents and alpha - lipoic acid .
### Output Text:
This approach allowed investigating the efficacy of <span class="chemical">alpha - lipoic acid</span> in preventing axonal damage and apoptosis and the function and ultrastructural morphology of mitochondria after exposure to toxic agents and <span class="chemical">alpha - lipoic acid</span> . 