Whitepaper: USP 643: the wrong end of the stick
To ensure that USP/EP standards are met, TOC of highly purified water can be determined by internal purification-process monitoring.
Online TOC analysers used for measuring the water quality of purified water production systems generally take and analyse discrete water samples and are designed to test accurately at levels around and exceeding 500ppbC. Each individual test requires several minutes to complete and values displayed on the screen are, at best, from water passing the analyser several minutes beforehand.
In contrast, measurements given by process monitoring are instant, continuously updated and the software is designed to give the most accurate TOC values at levels typically found within an ultra-pure water system (≤20ppbC).