  The relativley large spread in the derived metallicities ([Fe/H]) of M dwarfs
shows that various approaches have not yet converged to consistency. The
presence of strong molecular features, and incomplete line lists for the
corresponding molecules have made metallicity determinations of M dwarfs
difficult. Furthermore, the faint M dwarfs require long exposure times for a
signal-to-noise ratio sufficient for a detailed spectroscopic abundance
analysis. We present a high-resolution (R~50,000) spectroscopic study of a
sample of eight single M dwarfs and three wide-binary systems observed in the
infrared J-band. The absence of large molecular contributions allow for a
precise continuum placement. We derive metallicities based on the best fit
synthetic spectra to the observed spectra. To verify the accuracy of the
applied atmospheric models and test our synthetic spectrum approach, three
binary systems with a K-dwarf primary and an M-dwarf companion were observed
and analysed along with the single M dwarfs. We obtain a good agreement between
the metallicities derived for the primaries and secondaries of our test
binaries and thereby confirm the reliability of our method of analysing M
dwarfs. Our metallicities agree well with certain earlier determinations, and
deviate from others. We conclude that spectroscopic abundance analysis in the J
band is a reliable method for establishing the metallicity scale for M dwarfs.
We recommend its application to a larger sample covering lower as well as
higher metallicities. Further prospects of the method include abundance
determinations for individual elements.
