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: DISCUSSION : Quantifying tremor by using an inexpensive laser pointer is , with the exception of children ( < 12 years ) a sensitive and reproducible method .
### Output Text:
DISCUSSION : Quantifying tremor by using an inexpensive laser pointer is , with the exception of children ( < 12 years ) a sensitive and reproducible method . 