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: Brain levels of 5 - HT and its metabolite , 5 - hydroxyindoleacetic acid ( 5 - HIAA ) , were determined using HPLC .
### Output Text:
Brain levels of <span class="chemical">5 - HT</span> and its metabolite , <span class="chemical">5 - hydroxyindoleacetic acid</span> ( <span class="chemical">5 - HIAA</span> ) , were determined using HPLC . 