Dear Flavio Salomao-Miranda thanks for the interesting question. Please see the following useful articles which are both freely available as public full texts on RG:
1. Salivary fluoride concentration after the use of a fluoridated dentifrice, preceded
Article Salivary fluoride concentration after the use of a fluoridat...
2. Measuring fluoride in human saliva and water
Article Measuring fluoride in human saliva and water