three PCEBs observed within this snapshot survey, that will be used as “abundances standards”: GD448 (HRCam, @maxtedetal98-1), GD245 (MSPeg, @schmidtetal95-3), and PG2257+162 (KUV22573+1613, @wachteretal03-1), with exposure times of 900s, 600s, and 1070s, respectively.

SDSSJ122859.93+104032.9 (henceforth SDSS1228+1040) was observed in Cycle17 as part of a regular Guest Observer programme. We obtained two sets of spectroscopy with the G130M grating with central wavelengths of 1291Å and 1327Å, and both observations were again split among two FP-POS positions (1&4). In addition, we obtained G160M spectroscopy with central wavelengths of 1577Å and 1623Å. The total exposure time of the G130M and G160M observations were 2821s and 4899s, respectively, seamlessly covering the wavelength range $1130-1795$Å.

The data retrieved from the *HST* archive were processed and calibrated with CALCOS 2.15.6. The COS spectra of the four white dwarfs shown in Fig.\[f-cos\] reveal the broad  profile typical of DA white dwarfs, plus a multitude of narrow absorption lines from a range of metals. The peak signal-to-noise ratio (S/N) of the COS spectra is reached in a line-free region near $1320$Å, and ranges from $\simeq25$ for PG0843+516 and PG1015+161 to $\simeq40$ for SDSS1228+1040 and GALEX1931+0117. However, these values only include photon count statistics, and do not account for 