Ratio of the titanium metallicity relative to the overall metallicity (green distribution, cross marker) as well as the hydride (blue distribution, squared marker) and free electron (red distribution, dotted marker) abundance multipliers retrieved from the joint HARPS + NIRPS retrieval. The top panel shows the enrichment relative to solar (red dashed line) in logarithmic scale for titanium. The bottom panel shows the abundance profile for H⁻ (bound-free) and free electrons multipliers as predicted by chemical equilibrium chemistry with FastChem. Here we see that the H⁻ (bound-free) retrieved abundance is about 0.5 dex higher than that of the metals, as predicted by equilibrium chemistry (grey dashed line). This indicates that, while species such as Fe have very strong lines in the optical, others that only have weaker spectral features may be harder to detect with transmission spectroscopy on ultra-hot Jupiters like WASP-189b. The error bars, which are shown as black markers, were calculated as the standard deviation around the median value of each distribution.