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: Diagnosis of these patients can be achieved only if specific AX - related reagents are employed .
### Output Text:
Diagnosis of these patients can be achieved only if specific <span class="chemical">AX</span> - related reagents are employed . 