  We have studied the performance of global chi^2 fitting of low-resolution
X-ray spectra in retrieving intrinsic source parameters, with emphasis on the
coronal metallicity. The study has been conducted by fitting large numbers of
simulated spectra with known characteristics, and studying the distribution of
best-fit parameters. We have studied the behavior of the LECS detector on board
the SAX satellite and the SIS detector on board the ASCA satellite. The fitted
source spectra have either two discrete temperature components or a power-law
temperature distribution, with metallicity variations modeled by a single
global abundance parameter. The model used for the fitting has always been a
two-temperature one, with global varying abundance, to explore the influence of
the a priori ignorance of the actual temperature stratification in the source
being observed. The simulations performed explore the influence of varying
statistics in the observed spectrum (spanning a realistic range of values) as
well as the effect of varying the intrinsic source metallicity, with values in
the range 0.15--1.0 times the solar value. We find that the source metallicity
can be retrieved within few tens of percent from ASCA/SIS spectra of typical
signal to noise ratio, and within few percent from SAX/LECS spectra at the same
signal to noise ratio. However relatively small uncertainties in the detector
calibrations and in the plasma emission codes are likely to potentially cause
large systematic off-sets in the value
