Overview of the ¹³CO₂ ice feature toward the IPA sources studied in this work in order of increasing source luminosity. First column: data before subtracting the H₂ and the CO ro-vibrational lines and the continuum. The dashed gray line shows the position of the H₂ 0–0 S(10) emission line. Second column: bands after subtracting CO gas-phase lines. The dotted blue line shows the local continuum that was traced for each band. Third column: bands in optical depth scale after continuum subtraction.