  In this paper, we consider molecular communications in one-dimensional
flow-induced diffusion channels with a perfectly absorbing receiver. In such
channels, the random propagation delay until the molecules are absorbed follows
an inverse Gaussian (IG) distribution and is referred to as first hitting time.
Knowing the distribution for the difference of the first hitting times of two
molecules is very important if the information is encoded by a limited set of
molecules and the receiver exploits their arrival time and/or order. Hence, we
propose a moment matching approximation by a normal inverse Gaussian (NIG)
distribution and we derive an expression for the asymptotic tail probability.
Numerical evaluations showed that the NIG approximation matches very well with
the exact solution obtained by numerical convolution of the IG density
functions. Moreover, the asymptotic tail probability outperforms
state-of-the-art tail approximations.
