Groups of three specimens each were etched for @ minutes in @ % citric acid ( pH @ ) , treated in a @:@ slurry of dentifrice -LSB- @,@ ppm F as stannous fluoride ( SnF@ ) , @,@ ppm F as sodium fluoride ( NaF ) , @,@ ppm F as sodium monofluorophosphate ( SMFP ) , or @,@ ppm F as amine fluoride ( AmF ) -RSB- and saliva for @ minutes , exposed to @ % alizarin red-S ( a calcium-selective dye ) and rinsed again .