  This paper is the first one in a series investigating the properties of the S
stars belonging to the Henize sample (205 S stars with delta<-25 deg. and
R<10.5) in order to derive the respective properties (like galactic
distribution and relative frequencies) of intrinsic (i.e. genuine asymptotic
giant branch) S stars and extrinsic (i.e. post mass-transfer binary) S stars.
High-resolution (R=30 000 to 60 000) spectra covering the range 4230-4270AA
have been obtained for 76 S stars, 8 M stars and 2 symbiotic stars. The 4262AA
and 4238AA blends involving a Tc I line were analysed separately and yield
consistent conclusions regarding the presence or absence of technetium. Only
one `transition' case (Hen 140 = HD 120179, a star where only weak lines of
technetium are detectable) is found in our sample. A resolution greater than R
=30 000 is clearly required in order to derive unambiguous conclusions
concerning the presence or absence of technetium. The Tc/no Tc dichotomy will
be correlated with radial velocity and photometric data in a forthcoming paper.
