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: Serial audiograms should be performed every six months in those without problems and more frequently in young patients with normal serum ferritin values and in those with auditory dysfunction .
### Output Text:
Serial audiograms should be performed every six months in those without problems and more frequently in young patients with normal serum ferritin values and in those with auditory dysfunction . 