We used an air background obtained separately from each channel to observe the water and DMF spectra similar to section S3. This enabled a more direct route to validation of functionality of the device without cross-contamination. Figure shows the individual spectra from each channel. No water bands at 3400 cm -1 are observed in the DMF spectra. Likewise, no DMFspecific bands (e.g. 1100 or 1400 cm -1 ) are observed in the water channels. This proves the measurement independence of each channel and expands the range of compatible solvents with the technique.