  Current X-ray observatories make it possible to follow the evolution of
transient and variable X-ray binaries across a broad range in luminosity and
source behavior. In such studies, it can be unclear whether evolution in the
low energy portion of the spectrum should be attributed to evolution in the
source, or instead to evolution in neutral photoelectric absorption. Dispersive
spectrometers make it possible to address this problem. We have analyzed a
small but diverse set of X-ray binaries observed with the Chandra High Energy
Transmission Grating Spectrometer across a range in luminosity and different
spectral states. The column density in individual photoelectric absorption
edges remains constant with luminosity, both within and across source spectral
states. This finding suggests that absorption in the interstellar medium
strongly dominates the neutral column density observed in spectra of X-ray
binaries. Consequently, evolution in the low energy spectrum of X-ray binaries
should properly be attributed to evolution in the source spectrum. We discuss
our results in the context of X-ray binary spectroscopy with current and future
X-ray missions.
