The Myths and Truths of Measuring Pure Water

Applications Lab Manager at Thermo Fisher Scientific

Techniques such as liquid chromatography and mass spectrometry require ultrapure water (UPW) for trace-level analysis. Contaminants such as organic and inorganic compounds, particulates, bacteria and endotoxins have been removed from highly purified UPW. Conductivity and total organic carbon (TOC) are two important criteria for monitoring reagent water quality, as per ASTM Standard D1193. Some impurities, such as ions, can be easily measured by conductivity or resistivity. Total organic carbons (TOCs) can be measured by some UPW systems. Other impurities, such as particulates, bacteria and endotoxins, do not register on system displays at all. Finally, pH is not an important criterion for UPW. Because water-purity measurement is so important yet is a complex, multifaceted process, some clarifications are in order.

Myth 1: Ultrapure water conductivity after dispensing should match the water system display

Conductivity and resistivity are inverse values that provide an indication of the concentration and mobility of ions in the UPW. One might assume that the conductivity of water dispensed from a system should agree with the value on the system’s display. However, once water is exposed to the external environment, it begins to absorb CO2 and its conductivity increases (according to the atmospheric concentration of CO2, temperature, etc.). Therefore, if conductivity is displayed as 0.055 μS/cm (18.2 MΩ∙cm), it is likely that the conductivity of the dispensed water has risen to 1 or 2 μS/cm (or dropped to 1 or 0.5 MΩ·cm) within seconds of collection. For ambient air, 300–500 ppm CO2 is typical, which causes the conductivity of water at 0.055 μS/cm (18.2 MΩ∙cm) to rise to approximately 1 μS/cm (1 MΩ∙cm). Small, crowded areas can have CO2 concentrations between 1000 and 3000 ppm, in which case the conductivity will possibly increase to around 2 μS/cm (0.5 MΩ∙cm).

Temperature can also have a significant impact—the higher the temperature, the higher the conductivity (see Figure 1). Variations in UPW conductivity are likely a result of the natural environment as opposed to faulty UPW equipment and should be taken into consideration when measuring water quality.

 Figure 1 – The relationship between temperature and conductivity of UPW.

Myth 2: The pH of dispensed ultrapure water should be 7.0

One might expect the pH of the water inside a UPW system to be 7.0; however, as with conductivity, once water is dispensed and exposed to CO2 in the air at typical ambient conditions, the pH is expected to drop to approximately 5.7 (see Figure 2), a significant change from the predispensed UPW. Once CO2 is dissolved, it forms carbonic acid, which dissociates to hydrogen and bicarbonate ions, leading to changes in conductivity and pH. Measurements of pH can be unstable and inaccurate due to the low Waterionic strength and low buffering capacity of UPW. These phenomena are anticipated, however, and are not indicative of a faulty UPW system. As a rule, the pH of the final solution should be monitored and adjusted instead of monitoring the pH of UPW. Since pH is not considered an indication of water quality, a pH of 5–8 is not cause for concern.

 Figure 2 – The relationship between the CO2 levels in UPW and pH.

Myth 3: The ultrapure water system will measure all the impurities in the water

While UPW systems remove many impurities, they do not measure all relevant parameters. Water pH, for instance, is not a measure of water purity, and although purification processes remove impurities to trace levels, a particular pH cannot be guaranteed. The most common measurement of water purity is resistivity/conductivity. TOC detection is less common and is generally done only on systems with that particular feature.

UPW systems are also unable to identify or quantify the presence of bacteria. Bacterial contamination can lead to issues with the measurement of ions in the water, that is, biofilms can form around the instrumentation, or the flow from point-of-use final filters can be reduced. By-products and metabolites from bacteria can cause interferences in certain applications. Since UPW systems are not designed to alert the laboratory to the presence of bacteria, prevention is important. Similarly, particles such as debris or rust that are not ionized are unmeasurable and can obstruct the system’s internal components.

Myth 4: Accessories will improve UPW quality

Accessories like cartridges, UV lamps, ultrafilters and extra tubing can be added to UPW systems, but do not always improve water purity. UV lights, for example, provide not only germicidal protection against microorganisms, but break down organic compounds and are often included as standard with UPW systems, but it is important that any by-products from the UV oxidation process are removed before they can affect purity. Some systems include cartridges containing particular resins optimized to remove impurities.

Most systems use a final filter at the point of dispensing that screens the water for bacteria or particulates. Because filters can pick up contaminants, rinsing the final filter with UPW with 500 mL of water before use is recommended. Systems with ultrafiltration have a much smaller pore size than point-of-use final filters, that is, they are able to remove endotoxins and nucleases. Internal ultrafilter systems are preferred to external ultrafilters, which can amass contaminants. Some laboratories attach additional tubing to the outlet of the system for convenient dispensing, but UPW outside of the water system is unstable and is likely to absorb impurities. Furthermore, anything that UPW contacts has the potential to leach contaminants into the water, including residues from tubing. Inert, clean tubing that is replaced often is recommended if tubing is used.

Conclusion

The integrity of reagent water is often taken for granted, yet purity issues can be serious. Understanding the water purification process, the results it generates and the limitations of purification systems can help ensure that water is at the correct purity for use in sensitive laboratory protocols.

Gayle Gleichauf is application lab manager, Water Analysis, Thermo Fisher Scientific ([email protected]), and Kim Knepper is global application specialist, Water Purification, Thermo Fisher Scientific, 22 Alpha Rd., Chelmsford, Mass. 01824, U.S.A.; tel.: 800-225-1480; e-mail: [email protected]www.thermofisher.com

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