Hi, thanks for the response.
Which levels of NPOC are you talking about? I'm aware that an ultrapure water analysis (µg / L levels) everything can add carbon to the sample, but I'm focused mostly on groundwater, natural water and treated wastewater, where measurements are done at the mg / L level, and where even a interference of, say, 200 µg / L is not such a big deal.
My lowest calibration curve starts at 1 mg / L and my blank measurements done with ASTM type I water do give me negative results, so I'm sure I'm not picking up contamination from flasks or the system. It's the carbonates-bicarbonates which are interfering despite the purging process.
Technically a groundwater with about 25 mg / L IC and 2 mg / L TOC is still better suited for NPOC measurement than TC - IC measurement, since the IC is still much larger than the TOC, but I want to know the maximum amount of IC that the purging process can realistically remove (I mean, maybe a 20 minute purging time could suffice, but if I'm spending like 20 minutes per sample then the utility of the process is heavily undermined).