How do we know that CO2 measurements from ice cores are representative of CO2 in Earth's past atmosphere?

1. The obvious, nobody have cavity ring down spectrometer iinstrument back then

2. 18th and 19th century "wet extraction" old school chemistry method (similar to litmus test) suggests that CO2 can be up to 500 ppm, in stark disagreement with ice core data.

3. Plant stomata index also disagree with ice core data