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: Among the 2000 spots compared for statistical difference , 67 spots were significantly changed in abundance and identified using matrix - assisted laser desorption / ionization time - of - flight MS , atmospheric pressure matrix - assisted laser desorption / ionization and HPLC coupled tandem MS ( LC / MS / MS ) .
### Output Text:
Among the 2000 spots compared for statistical difference , 67 spots were significantly changed in abundance and identified using matrix - assisted laser desorption / ionization time - of - flight MS , atmospheric pressure matrix - assisted laser desorption / ionization and HPLC coupled tandem MS ( LC / MS / MS ) . 