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: The biochemical assays on the influence of four analgesics on the brain concentration and turnover of noradrenaline ( NA ) were also performed .
### Output Text:
The biochemical assays on the influence of four analgesics on the brain concentration and turnover of <span class="chemical">noradrenaline</span> ( <span class="chemical">NA</span> ) were also performed . 