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: INTERVENTION AND OUTCOME : An 18 - gauge Touhy needle was inserted until loss of resistance occurred at the L4 - 5 level .
### Output Text:
INTERVENTION AND OUTCOME : An 18 - gauge Touhy needle was inserted until loss of resistance occurred at the L4 - 5 level . 