Chromatography Forum

Dears

We operate a total carbon analyzer here in the lab. Calibration is done using a phthalate/carbonate mixture for TC (total carbon) and IC (inorganic carbon). The TOC (total organic carbon) is calculated as TC minus IC.
Now the question: We can calculate an LOQ for TC and IC from the individual calibration curves. But what is our LOQ for TOC?
LOQ for TC and IC is in the range of 3 mg/L and 0.5 mg/L, respectivly. My feeling is that I cannot just subtract LOQ(TC) from LOQ(IC), but I should look at the uncertainty of the calibration curves. Can anyone point me to the right direction please?

Thanks a lot
Jörg

If you have no inorganic carbon present then your TOC will be equal to your TC. So the lowest TOC detection you can have will be equal to your TC detection limit. We normally set the TOC and TC limits to the same value.