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 main problem encountered was haematological toxicity , with three subjects experiencing severe neutropenia ( neutrophil count < 0 . 5 x 10 ( 9 ) / l ) .
### Output Text:
The main problem encountered was haematological toxicity , with three subjects experiencing severe neutropenia ( neutrophil count < 0 . 5 x 10 ( 9 ) / l ) . 