TOC Analyzer Pitfalls Part 3

In the second part of TOC analyzer pitfalls, the pitfalls relating to carrier gas used in TOC analyzer’s analysis and reactor of High Temperature Combustion and UV/Persulfate analyzers were mentioned and methods to avoid these pitfalls were suggested. Now, let’s see what pitfalls are associated with non-dispersive infrared (NDIR) analyzers and range selection and implementation.

TOC Analyzers that Use NDIR (Non-Dispersive Infrared)

TOC analyzer “drift” is mainly associated with the NDIR which is the most vital component of the analyzer and is, in itself, also an analyzer, and is affected by all related instrumental performance anomalies. The NDIR must function properly at all times, and must inform the user if one or more of its critical components are malfunctioning. Some NDIR critical components include: infrared sources, infrared detectors and choppers, if used.

If the NDIR cannot self- compensate for drift which results from malfunctions, then you will need to perform frequent calibrations on the TOC analyzer.

TOC Analyzer Ranges

In many TOC analyzers’ total organic carbon analysis, it is important to also consider range selection and implementation pitfalls. Usually, the TOC is used within nominal limits while monitoring, until an ‘upset’ in the process occurs. During this time, there may be more than a ten-fold increase in TOC values. It is the desire of the Plant Operator to achieve both maximum precision in the normal operating range and the ability to track TOC during the “upset”, in order to increase control.

The TOC analyzer’s pitfalls associated with range selection and implementation are as follows: If only electronic scaling were done for the two ranges, it would result in inaccuracies at the lower, normal operating range attributed to the “full-scale” higher TOC range. Typically, the TOC analyzer chemical calibration would be “full-scale” at the highest range. If you use a higher range of 0 -10,000 ppm in process upset and normal range of 0-1,000 ppm, the resulting inaccuracy in at the normal operating range would be 1,000 +/-200 ppm, or +/-20 %.

There can be bigger problems. For instance, if you’re using a UV/PERSULFATE analyzer, you may experience the phenomena of “carbon-loading” and competing reactions which can actually cause a “down-turn” of analyzer TOC results, with the TOC measurements being considerably lower than actual TOC data. This pitfall in TOC analyzer’s analysis can be avoided by using two truly distinct ranges, each calibrated “full-scale” with appropriate different chemical calibration solutions.