diffusion time scales that strongly depend on the chosen layer. [@koester+wilken06-1] and [@koester09-1] defined the Rosseland optical depth $\tau=5$ as the “standard” layer, assuming that no trace of any heavy element below this would be seen in a spectrum.

However, a more consistent way to determine the abundances in the accreted material, which is the quantity ultimately desired, is the assumption of a steady state between accretion and diffusion throughout the whole atmosphere. At Rosseland optical depth $\tau =
2/3$, and typical conditions for the observed ultraviolet spectra, the diffusion times in the four white dwarfs analysed here are $\simeq0.4$ to four days. Assuming that the accretion rate does not vary over such time scales, we can use the condition of constant flow of an element with mass fraction $X(\tau)$ $$\rho X v = \mbox{const}$$ with $\rho$ and $v$ the mass density and the diffusion velocity of this element. $\rho$ and $v$ are known from the atmosphere model and diffusion calculations, and $X(\tau=2/3)$ is derived from the spectral analysis. This determines the diffusion flux at $\tau = 2/3$. In steady state, as it is the case for the DAZ analysed here, the diffusion flux is constant throughout the atmosphere, and 