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When do you require Ultra-Low TOC Type I Ultra-pure water?

Published in Water Filtration

Creation Date Wednesday, 20 April 2011.

Pure water is a commodity in many industries in particular in analytical and biologic laboratories. Laboratory grade water is defined by its resistivity, which is determined by the amount of ionic contaminations and by its Total Organic Carbon content (TOC). Resistivity of the water determines the water quality based on American Society for Testing and Material (ASTM) definitions. Many routine laboratory applications use ASTM Type II water that has a resistivity of >1MΩ/cm2 which corresponds to less than 500ppb total ionic contamination. More specialized applications in analytical chemistry and molecular and cell biology require water of ASTM Type I with a resistivity of >18 MΩ/cm2 which corresponds to about 1ppb total ionic contaminations in the water.

ASTM water quality definitions do not give any information about the amount of organic compounds in the water. Organic compounds, however, can cause serious problems for certain analytical chemistry and biological applications. Liquid phases and buffers prepared for HPLC need to have ultralow TOC and ultralow ionic contamination, as these contaminants can show up as separate peaks in the chromatogram and also increase background and decrease sensitivity. These same is true for gas chromatography (GC), Atomic Absorption Spectroscopy (AAS), Ion Chromatography (IC), Electromagnetic Spectroscopy, and mass spectrometry (MS). Several molecular biology applications require water that is not only ASTM Type I, but also nuclease- and pyrogen-free, which is most reliably achieved with ultra-pure water with ultra-low TOC.

Electrophoresis, polymerase chain reaction, DNA and RNA extraction and other methods that use nucleic acids need water that is nuclease-free to avoid degradation of the DNA. Due to the stability and abundance of RNAses as contaminants, a source of water that is nuclease-free is even more important for applications that use RNA. Enzyme Linked Immuno-Sorbent –Assays (ELISAs) can detect minute amounts of specific contaminants that can increase background and decrease sensitivity of the assay. Many analytical assays that use fluorescence for detection also need low TOC as many organic compound fluoresce to some extent in UV light.

Cell biology is another area where low TOC and purity of water is critical. Water used for the preparation of buffers and media for tissue culture does not only need to be sterile (free of microbes like bacteria and viruses), but also free of organic contaminants like endotoxin. Endotoxin contaminations in tissue culture can interfere with cell growth, cytokine responses and significantly alter the results of experiments. Drug preparations intended for the use in animals or humans also need to be made with high-quality ultra-pure, ultralow TOC, and endotoxin-free water, as endotoxin cannot only alter results of in vivo experiments dramatically, but it can even cause toxicity, though endotoxin contaminations at a level that can cause toxicity are rare. FDA requirements for human and veterinary medications restrict the use of water for production of medications to the highest purities.

To purify tap water to ASTM type I and ultra-low TOC standards, purification systems are used that employ a series of different purification methods, each of which has different maintenance requirements. A vital part of any water purification system that is supposed to achieve ASTM Type I grade purity is a deionizing system.

A deionizer removes cationic contaminants in exchange for H+ ions and anionic contaminants in exchange for OH- ions. The purity of the water depends on the quality of the ion exchange resin used and the quality of the maintenance of the system. Ion exchange resins have a limited capacity of ions they can exchange and need to be replaced when they have reached their limit. The resin can be regenerated, but the regeneration process can cause damage to the resin which can lead to organic contaminations. Ion exchangers do not reduce microbial contaminations, and bacteria can accumulate within the resin, leading to a build up of endotoxin within the resin. Virgin ion exchange cartridges, i.e. cartridges with new rather than regenerated ion exchange resin are usually used for the highest purity water. UV-sterilized feed water prevents the buildup of bacteria

Most water purification systems that produce ASTM Type I water use feed water that is already lower in contaminants than tap water. Pretreatment using activated charcoal can remove chlorine and, to a certain extend, organic compounds from tap water and reverse osmosis is a very economical way to reduce about 95% of all contaminants, which makes it a very useful pretreatment for feed water for ASTM Type I purifiers. Reverse osmosis membranes can be damaged by CaCO3 deposits and deposits of organic compounds and colloids.

Water softeners and pretreatment of the water with activated charcoal can significantly increase the live of the reverse osmosis unit. Since ion exchangers cannot remove non-charged organic compounds, these need to be removed in a separate step. Different filtration steps will remove, depending on the cutoff of the filtration membrane, starting from large particles, bacteria and viruses, down to endotoxin and nucleases. However, while microporous filters like e.g. the ones used for the removal of bacteria with a cutoff of 0.2 micron can remove 100% of particles above the cutoff diameter, ultrafiltration filters that are used for smaller particles and organic compounds like endotoxin and nucleases remove most, but not all contaminants above the cutoff diameter.

Both microporous filters and ultrafiltration membranes can clog when to many contaminants are deposited on the surface. If this happens they need to be replaced. Ultrafiltration membranes clog much easier than microporous filters, so that they are usually placed behind other filtration steps in a water purification system. To further reduce TOC a UV-oxidizer needs to be used.

Since UV oxidizers increase the amount of dissolved ions and therefore reduce resistivity in the water, the most logical position of a UV-oxidizer in a water purification system is before the deionzing step that produced water with very high resistivity. The UV-oxidizer also sterilizes the water reducing the risk of bacterial contamination of the subsequent ion-exchanger resin in this configuration. The low pressure mercury lamps in UV-oxidizers/sterilizers have a limited life span and need to be replaced from time to time. In many systems another ultrafiltration step is used at the very end to ensure the sterility of the water.