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: Twenty - seven patients were given a 600 - mg / m2 busulfan total dose with continuous i . v . infusion of clonazepam ; none had any neurological symptoms .
### Output Text:
Twenty - seven patients were given a 600 - mg / m2 <span class="chemical">busulfan</span> total dose with continuous i . v . infusion of <span class="chemical">clonazepam</span> ; none had any neurological symptoms . 