The precise chemical analysis (PCA) was developed to study the layers near the semiconductor surface. This method is based on dissolving of different components in the selective etchants. The solution, containing the products of etching, was analysed with photometric technique. In this paper, we present the photometric measurements of "free" (non-interconnected) Si atoms, which remain at the silicon surface after a typical process of semiconductor technology such as rapid thermal diffusion (RTD) of boron into silicon. The advantage of this method is that it is non-destructive for the silicon matrix. The boron content was also investigated in the volume near the surface. The accuracy is evaluated as 5%.