[javhert]

Hi, I'm asking advice from people which are experienced measureing total organic carbon and, more specifically, non-purgeable organic carbon (NPOC).

I'm having trouble with water containing natural alkalinity (carbonates) when measuring NPOC. Despite the acid addition and the sparging, even TOC blanks containing inorganic carbon give false positive results. A case of 50 mg / L inorganic carbon gave me 45 mg / L as NPOC, which is truly wrong, the waters I'm expecting to work with will have about 200 - 400 mg / L CaCO₃ as total alkalinity, which is about 25-50 mg / L IC.
I’m sparging for three minutes per measurement at a 80 mL / min rate, but I don’t know whether it is too little or enough. I’m doubtful that increasing the sparging time will give me a big change, given how 3 minutes just sparge like 5 % of the IC. Don’t want to jump into conclusions like NPOC being a inherently flawed technique for anything but ultrapure waters, but so far evidence point toward it. Or maybe my TOC equipment manufacturer is really bad, but won’t badmouth it until I have full evidence.

[shchavel]

Hello! We work with NPOC for purified water analysis. I'm not expert, but this method is realy sensible. You sample can take some NPOC from air (CO2) or just from sufractant after flasks washing. Everthing affects!

[javhert]

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).