  We present a series of systematic abundance measurements for 25 hot DA white
dwarfs in the temperature range ~20000-110000K, based on far-UV spectroscopy
with STIS/GHRS on HST, IUE and FUSE. Using our latest heavy element blanketed
non-LTE stellar atmosphere calculations we have addressed the heavy element
abundance patterns making completely objective measurements of abundance values
and upper limits using a 2 fitting technique to determine the uncertainties in
the abundance measurements, which can be related to the formal upper limits in
those stars where particular elements are not detected. We find that the
presence or absence of heavy elements in the hot DA white dwarfs largely
reflects what would be expected if radiative levitation is the supporting
mechanism, although the measured abundances do not match the predicted values
very well, as reported by other authors in the past. Almost all stars hotter
than ~50000K contain heavy elements. For most of these the spread in element
abundances is quite narrow and similar to the abundances measured in G191-B2B.
However, there is an unexplained dichotomy at lower temperatures with some
stars having apparently pure H envelopes and others detectable quantities of
heavy elements. The heavy elements present in these cooler stars are often
stratified, lying in the outermost layers of the envelope. A few strong
temperature/evolutionary effects are seen in the abundance measurements. There
is a decreasing Si abundance with temperature, the N abundance pattern splits
into two groups at lower temperature and there is a sharp decline in Fe and Ni
abundance to zero, below ~50000K. When detected, the Fe and Ni abundances
maintain an approximately constant ratio, close to the cosmic value ~20.
