Groups of three specimens each were etched for 10 minutes in 1 % citric acid ( pH 2.3 ) , treated in a 1:3 slurry of dentifrice [ 1,100 ppm F as stannous fluoride ( SnF2 ) , 1,100 ppm F as sodium fluoride ( NaF ) , 1,000 ppm F as sodium monofluorophosphate ( SMFP ) , or 1,400 ppm F as amine fluoride ( AmF ) ] and saliva for 2 minutes , exposed to 2 % alizarin red-S ( a calcium-selective dye ) and rinsed again .