Synthetic spectra with varying carbon (left) and hydrogen (right) abundances overplotted on the observed spectrum (grey). Inset panels highlight the diagnostic regions. Left: Carbon abundances ranging from −8.0 to −3.0. The absence of C I lines near 1280 Å indicates X_C < −5.0. Right: Hydrogen abundances from −9.5 to −3.0. Lyα is outshone by geocoronal emission, and no clear hydrogen diagnostic lines are present, leaving the hydrogen abundance unconstrained.