Context : PG 1159 - 035 is the prototype of the PG1159 spectral class which consists of extremely hot hydrogen-deficient ( pre- ) white dwarfs .
It is also the prototype of the GW Vir variables , which are non-radial g-mode pulsators .
The study of PG1159 stars reveals insight into stellar evolution and nucleosynthesis during AGB and post-AGB phases .

Aims : We perform a quantitative spectral analysis of PG 1159 - 035 focusing on the abundance determination of trace elements .

Methods : We have taken high-resolution ultraviolet spectra of PG 1159 - 035 with the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer .
They are analysed with non-LTE line blanketed model atmospheres .

Results : We confirm the high effective temperature with high precision ( T \mathrm { \hskip { -1.72 pt } { } _ { eff } } =140 000 \pm 5000 K ) and the surface gravity of \log g =7 .
For the first time we assess the abundances of silicon , phosphorus , sulfur , and iron .
Silicon is about solar .
For phosphorus we find an upper limit of solar abundance .
A surprisingly strong depletion of sulfur ( 2 % solar ) is discovered .
Iron is not detected , suggesting an upper limit of 30 % solar .
This coincides with the Fe deficiency found in other PG1159 stars .
We redetermine the nitrogen abundance and find it to be lower by one dex compared to previous analyses .

Conclusions : The sulfur depletion is in contradiction with current models of AGB star intershell nucleosynthesis .
The iron deficiency confirms similar results for other PG1159 stars and is explained by the conversion of iron into heavier elements by n-capture in the s-processing environment of the precursor AGB star .
However , the extent of the iron depletion is stronger than predicted by evolutionary models .
The relatively low nitrogen abundance compared to other pulsating PG1159 stars weakens the role of nitrogen as a distinctive feature of pulsators and non-pulsators in the GW Vir instability strip .