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PET probe discovery using in situ click chemistry uses ( 19 ) F - bearing fragments as ( 18 ) F surrogates .
 Chemical: ( 19 ) F; Chemical: ( 18 ) F
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: ( 19 ) F; Chemical: ( 18 ) F