TOC: what is it, why monitor it... and why is it so bad for your ultrapure water?
25 Jan 2006

Monitoring TOC levels in your laboratory pure water is essential for ensuring accuracy in sensitive applications. By incorporating real-time TOC monitoring, the PURELAB Ultra from ELGA LabWater is a state-of-the-art ultrapure water purification system that provides a solution to a frequently underestimated issue.

What is TOC? Total organic carbon, or TOC, gives a measure of the overall level of organic contaminants in purified water. Typical feedwater will contain a wide range of organics originating from a variety of sources, both natural and man-made. Common culprits include humic acids and tannins (arising from the decomposition of leaves), organisms such as bacteria and their by-products, or other materials such as plasticisers and polymers. Many applications are affected directly by organic contaminants, such as HPLC and GC, while other applications can be affected indirectly, such as IC and tissue culture.

How do you counter TOC? A sophisticated water purification system such as the PURELAB Ultra from ELGA LabWater uses a combination of purification technologies to polish pre-purified water to a resistivity of 18.2M§Ù-cm - suitable for the most critical applications. Dual purification cartridge packs incorporating high-quality organic absorbents and, ion exchange resins and membrane processes ensure complete removal of trace impurities and produce water that meets ASTM, ISO, CLSI and USP standards. The dual- pack design ensures the removal of all trace impurities, while providing greater economy, convenience and security. The removal of the final traces of TOC is ensured by recirculation of the water through a photo-oxidation module using full-spectrum UV light for the oxidation of organic contaminants. A feature of the PURELAB Ultra Analytic and Genetic models, produces water with TOC levels <5 ppb, and allows a specification to be chosen which is tailored to your bioscience or analytical needs. But can you really be sure that water with a theoretically pure resistivity has low TOC levels? The answer is no, a quality water purification system will monitor TOC for absolute confidence in water purity.

Why monitor TOC? Most systems exhaust their ability to remove organics before they exhaust their ability to remove ions. The resulting ¡®organic breakthrough¡¯ can go undetected by your system¡¯s resistivity monitor - such that even at 18.2M§Ù-cm at 25¡ÆC, levels of organic contaminants can be high enough to have adverse effects on results. For example, organics in water used for HPLC can directly affect chromatograms but they can also coat the stationary phase and prevent molecules or ions from accessing exchange sites, thus reducing the peak resolution and column capacity. Organic contaminants also interfere with sensitive biological assays, cell culture and enzyme assays. While measuring TOC provides neither the exact composition nor level of particular impurities, it comes closest to a universal indicator of the presence of organic impurities in the water. It provides confidence that contamination is below a given level.

Real-time TOC monitoring is unique to the PURELAB Ultra, allowing continual monitoring of organic purity and continual confidence in the water dispensed. The TOC monitor in the PURELAB Ultra is inbuilt into the photo- oxidation system to provide continuous sampling and analysis. This eliminates the delay in measurement evident in TOC monitors in most other water purification systems; a cycle of measurement (flushing, oxidation, results display) typically results in a lag of anything between 3 and 9 minutes, by which time the on-set of contamination or the passage of transient impurities may have been missed.

Truly continuous TOC monitoring can guard against contamination that can potentially ruin an analysis or experiment. Combined with additional features, such as multi-stage monitoring, recirculation to maintain a consistent peak quality and automatic pre-rinse on start-up to purge static feedwater, the PURELAB Ultra maximises the long-term efficiency of the ultra-purification process. This leaves the researcher free to focus on the experiment in hand and not on potential problems with water purity.

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