  Optical quasar spectra can be used to trace variations of the fine-structure
constant alpha. Controversial results that have been published in last years
suggest that in addition to to wavelength calibration problems systematic
errors might arise because of insufficient spectral resolution. The aim of this
work is to estimate the impact of incorrect line decompositions in fitting
procedures due to asymmetric line profiles. Methods are developed to
distinguish between different sources of line position shifts and thus to
minimize error sources in future work. To simulate asymmetric line profiles,
two different methods were used. At first the profile was created as an
unresolved blend of narrow lines and then, the profile was created using a
macroscopic velocity field of the absorbing medium. The simulated spectra were
analysed with standard methods to search for apparent shifts of line positions
that would mimic a variation of fundamental constants. Differences between
position shifts due to an incorrect line decomposition and a real variation of
constants were probed using methods that have been newly developed or adapted
for this kind of analysis. The results were then applied to real data. Apparent
relative velocity shifts of several hundred meters per second are found in the
analysis of simulated spectra with asymmetric line profiles. It was found that
each system has to be analysed in detail to distinguish between different
sources of line position shifts. A set of 16 FeII systems in seven quasar
spectra was analysed. With the methods developed, the mean alpha variation that
appeared in these systems was reduced from the original
Dalpha/alpha=(2.1+/-2.0)x10^-5 to Dalpha/alpha=(0.1+/-0.8)x10^-5. We thus
conclude that incorrect line decompositions can be partly responsible for the
conflicting results published so far.
