When you need consistent and reproducible results - for either critical or sensitive applications - you need reliable ultrapure water. Ultrapure water is mainly defined by its conductivity or resistivity, 0.055µS/cm or 18.2MΩ*cm, respectively. When this value is achieved, the water is essentially free from ions which can impact sensitive analytical applications, showing as ghost peaks or affecting separation of the ions in the sample. The higher the sensitivity of the analytical application, the higher water quality you will need.

When producing ultrapure water, other impurities such as TOC, endotoxins, enzymes and particulates are removed in the process to some degree.  This may affect life science applications (by degrading RNA and DNA, for example) in addition to affecting sensitive analytical applications. If you have an application that requires low TOC or RNase | DNase-free water for example, make sure your purification system has additional purification steps to achieve the needed quality.

The UV-lamp breaks down and oxidizes organic material into charged particles with either a negative or positive charge. Using a dual wavelength, 185 and 254 nm, the UV-lamp can oxidize and break down organics. After the UV-lamp, the ions and organics need to be removed from the water; this is usually done with a deionization process.

For critical analyses in analytical research where a low organic content is required (e.g. HPLC or ICP-MS), a water purification system that includes a UV-lamp is needed. After the UV-lamp, a deionization cartridge is needed to remove the ions that have been oxidized. Additionally, it is highly recommended to use activated carbon because of its high surface area and adsorption properties that allow it to efficiently remove organics from the product water.

This is usually achieved by using a mixed bed resin cartridge that contains both deionization resin and activated carbon after the UV-lamp instead of a pure deionization resin cartridge. The final TOC value, however, is completely dependent on your feed water. If your feed water has a high initial TOC value, you will need to pretreat it first, otherwise you may not be able to achieve the low TOC value you need. In addition, the higher value TOC that goes into the system, the faster you will exhaust the consumables, leading to shorter change intervals and higher total cost of ownership.

There are two methods widely used to measure total organic content (TOC value) - with the help of:

• built-in TOC monitor - for the TOC monitor the water is then moved through the UV-lamp and DI cartridge. After the DI cartridge, the water is directed to a separate chamber where the organic content is oxidized and then proceeds to drain. The oxidation releases CO2, which in turn raises conductivity, which is measured.
• TOC indicator - the TOC indicator method measures the second conductivity measurement on-line (instead of in a separate chamber) after the UV-lamp but before the DI cartridge. The difference is again calculated and converted to a TOC value. However, after the second measurement, all effects on downstream components are considered on a theoretical level; therefore, the accuracy is not as good as with the TOC monitor, which determines the TOC value at point of use. If the TOC value is important to your application, you should choose a system that uses a TOC monitor to measure the TOC level.

Both methods use the difference in conductivity to determine organic content in the water. 

This depends on your application and analysis. The more sensitive your instrument and method, the lower the TOC value you should have in your sample to minimize interference and remove anomalies like ghost peaks. As an example, if you are using ICP-MS, you will need a lower TOC value than HPLC to get reliable results, as the ICP-MS is a more sensitive analytical method. However, too high a level of organics (high TOC value) might, in addition to e.g. showing as ghost peaks in your results, clog the columns and other consumables, resulting in shorter lifetime for your equipment and leading to a higher total cost of ownership.

An ultrafilter should be used when doing life science-related research or applications, such as cell culture or any work with DNA or RNA. Ultrafiltration removes endotoxins, which is important when working with mammalian cell cultures. The ultrafilter also removes critical enzymes such as RNase/DNase and proteases that play a part in the degradation of RNA/DNA and proteins, respectively.

For applications such as mammalian cell culture that need to be low in endotoxins, a system that produces ultrapure water (i.e. Type 1 water) with either an internal or external ultrafilter should be used. Ultrafilters use pores that are so small that it is measured as Molecular Weight Cuf-off (MWCO), which uses Daltons as its units. Because of the MWCO, endotoxins, nucleases, proteases, etc. are removed from the product water.

There are two processes widely used to remove RNase and DNase from laboratory water:

• the addition of chemical DEPC (diethyl polycarbonate) to the water - this inactivates the nucleases that degrades RNA and DNA, i.e. RNase and DNase
• the use of an ultrafilter - except for avoiding the use of a suspected carcinogen, the advantage of ultrafilters is that it removes the RNase and DNase from the water instead of inactivating the nucleases. Inactivated nucleases, if not removed, contribute to the TOC (Total Organic Carbon) level. Another advantage ultrafiltration has over DEPC is that the DEPC/water mixture needs to be autoclaved to eliminate excess DEPC. In this process, DEPC produces ethanol and carbon dioxide. Ethanol also contributes to the TOC level, and carbon dioxide raises the conductivity.

pH is measured by how many ions there are in the water - specifically the number of H+ ions. If the water is ion free, which is the case if the water has a conductivity of 0.055 µS/cm (resistivity 18.2 MΩ*cm), the pH is 7, i.e. neutral.

When working with ultrapure water, it is important to make sure the quality is as expected. Therefore, you should always dispense some water before collecting and check the quality. When dispensing, you should use containers that are extractable-free (e.g. glass), and use them directly when possible. The quality of ultrapure water does not stay when stored, as it absorbs impurities from the air (such as CO2). Therefore, ultrapure water should be used fresh whenever possible. It is also important to do regular maintenance and check-ups to ensure the quality of the water. One should also not extend the point of use by adding tubing, as this will cause impurities to build up in the tubing.

The feed water quality for water purification systems that produce ultrapure water has a big role in the lifetime and the total cost of ownership of the system. It is recommended to pretreat feed water (to RO water or pure water). This will ensure the lifetime of the consumables in the system can be used for the duration of their intended lifetime. Otherwise, impurities will exhaust the consumables faster, which will lead to shorter lifetime and change. You should always check the system specifications for information regarding the feed water that is recommended or consult with the manufacturer’s experts.