A microsampling technique was used to measure enamel fluoride concentrations before, immediately after, 1 week after, and 3 months after an application of an APF or NH4F solution on the maxillary anterior teeth of 63 children. The fluoride treatments were preceded by a 1-min application of 0.05 m H3PO4. It was found that NH4F produced an initial enamel fluoride level much higher than that produced by APF. One week after treatment, the fluoride concentrations produced by NH4F were still higher than those obtained with APF. Three months after treatment there was no difference between the NH4F and APF groups. Both groups, however, were observed to have lower fluoride concentrations than before treatment.
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Stephen H.Y. Wei (2009) studied this question.