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: Superoxide and hydrogen peroxide production in neutrophils stimulated with formyl - methionyl - leucyl - phenylalanine ( fMLP ) from a further 18 ALF patients was unaffected compared with control neutrophils .
### Output Text:
<span class="chemical">Superoxide</span> and <span class="chemical">hydrogen peroxide</span> production in neutrophils stimulated with <span class="chemical">formyl - methionyl - leucyl - phenylalanine</span> ( <span class="chemical">fMLP</span> ) from a further 18 ALF patients was unaffected compared with control neutrophils . 