The quality of laboratory water plays a crucial role in the ability to conduct accurate and efficient experiments. The polishing step that produces Type 1, or ultrapure water, often receives a lot of attention, since this water is used in critical laboratory applications. However, the overall water purification process involves not only the final polishing step, but also a pretreatment step which typically removes 95 to 99 percent of the contaminants originally present in water (Fig. 1).
Since the pretreatment step removes the bulk of the contaminants in tap water, optimum water quality in the lab can only be achieved if the pretreatment step can be relied upon to consistently produce pure water that meets specifications. In many laboratories, however, this process is neglected. An inefficient pretreatment process will compromise the efficiency and productivity of the laboratory. To illustrate this point, the quality of ultrapure water from identical polishing systems fed with two different sources of pure water was compared by HPLC (Fig. 2).
Pretreatment technologies
Tap water contains a wide array of contaminants in high concentrations, including particulates, ions, organics, and bacteria. It is necessary to use a combination of technologies to remove all contaminants to the desired levels.
Distillation — Distillation may be useful as a first purification step as it removes a broad range of contaminants; however, it consumes large amounts of tap water (for cooling) and electrical energy (for heating). Also, some contaminants are carried into the condensate, and careful maintenance is required to ensure purity. The use of distillation for water purification in the laboratory is on the decline.
Reverse osmosis (RO) — Reverse osmosis is capable of removing the bulk of a wide range of contaminants, and is therefore useful as a first step in the purification process. The efficiency of RO varies among contaminant types.
Ion exchange resins — Ion exchange resins remove dissolved inorganics (ions) and charged organics effectively, but not neutral organics, bacteria, and particles. Once all ion binding sites on the resins are occupied, ions are no longer retained (except when operating in an electrodeionization process). In service deionization (SDI) systems, the resins can be regenerated using strong acids and bases (tanks containing ion exchange resin are swapped for tanks with freshly regenerated resin that has been processed off-site). The chemically regenerated DI beds can introduce organics and particles into otherwise pure water.
Electrodeionization (EDI) — This technology is a combination of ion exchange resins, selective semi-permeable anionic and cationic membranes, and direct electrical current. The EDI process effectively deionizes water, while the ion exchange resins are continuously regenerated by the electric current in the unit.
Activated carbon — There are two forms of activated carbon used in water purification: natural and synthetic. Natural activated carbon is fine powder made of irregularly shaped grains. It contains a high concentration of ionic contaminants and is used only as a preliminary step to remove excess chlorine from tap water and to reduce organic contamination. Synthetic activated carbon is made by the controlled pyrolysis of polystyrene spherical beads, and is a much cleaner material. It is used for the removal of trace organics of low molecular weight, typically in the polishing step for the production of ultrapure water.
Germicidal UV — Ultraviolet radiation is widely used as a germicidal treatment for water. UV lamps emitting light at 254 nm inactivate microorganisms and prevent microbial growth and contamination. Absorption of UV light leads to DNA modification in the bacterial cells, inhibiting their metabolism, thereby preventing their multiplication.
No one single technology can efficiently remove all the contaminants in water and therefore a combination of technologies must be used in water purification systems. Table 1 compares the ion removal efficiency of single technologies (RO, distillation, SDI), and a combination of technologies (RO-EDI). Table 2 compares the reduction in total oxidizable carbon (TOC) levels using single (2a) and combination technologies (2b). A study compared the efficiency of mixedbed deionization (DI) cartridges and RO-EDI in reducing TOC levels. When 2000 L of tap water were purified, DI water exhibited unstable TOC values, while the water purified through the RO-EDI technologies had TOC values that were mostly <50 ppb (Fig. 3). While EDI cannot be expected to have any effect on neutral molecules, charged organics behave similarly to inorganic ions and are rejected. Consequently, the TOC level decreases slightly from RO water to EDI water.
Table 1. Ion removal efficiency of RO, RO-EDI, distillation (still), and SDI (values in ppb).
Cation | Tap | RO | RO-EDI | Still | SDI |
Sodium | 12904.74 | 667.59 | 0.46 | 0.51 | 0.05 |
Ammonium | 168.08 | 13.06 | 0.04 | 3.63 | 0.04 |
Potassium | 1713.15 | 109.07 | 0.01 | 0.28 | 0.06 |
Magnesium | 2525.18 | 5.55 | 0.00 | 0.02 | 0.01 |
Calcium | 70501.02 | 117.03 | 0.01 | 0.17 | 0.17 |
Anion | |||||
Fluoride | 102.57 | 2.05 | 0.01 | 0.32 | 0.01 |
Chloride | 30033.42 | 280.97 | 0.34 | 4.65 | 0.03 |
Nitrite | 17.53 | 3.76 | – | 0.28 | – |
Nitrate | 26508.46 | 1758.45 | 0.09 | 3.67 | – |
Sulfate | 31423.55 | 989.08 | 0.79 | 1.10 | – |
Table 2a. Reduction of TOC levels by RO and distillation (still) (values in ppb).
Tap |
RO |
Still |
1410 |
64 |
104 |
1180 |
38 |
75 |
Table 2b. Reduction of TOC levels by RO-EDI, distillation (still), and SDI (values in ppb).
Tap |
RO-EDI |
Still |
SDI |
859 |
20 |
58 |
77 |
977 |
14 |
47 |
58 |
Maximizing the quality of ultrapure water In the laboratory, pure water (Type 2 or Type 3) is pretreated and polished to produce ultrapure water for sensitive applications such as LC-MS or ICP-MS. Therefore, a reliable pretreatment process for pure water is ultimately critical for ultrapure water quality.
In one study, DI and RO-EDI pretreatment systems (A and B, respectively) were connected to the same tap water source and the purified water from each was polished using identical Milli-Q units. System A product water had high and fluctuating TOC levels, whereas System B showed TOC concentrations consistently around 10 ppb (Fig. 4).
In another study, the SDI-Distillation and RO-EDI pretreatment systems (A and B, respectively) were connected to the same tap water source and the purified water from each was polished using identical Milli-Q units. System A product water had fluctuating TOC levels, whereas the ultrapure water polished from the RO-EDI system consistently showed TOC levels below 10 ppb (Fig. 5).
In the polishing step, three technologies are typically combined: ion exchange resins (single use), synthetic activated carbon, and UV photooxidation. These three technologies are necessary to obtain and maintain TOC levels less than or equal to 5 ppb in ultrapure water.
Conclusions
Optimal water quality in the laboratory requires the best source of pure water. Even though there are a number of purification technologies available, no single technology will remove all the contaminants to levels low enough for laboratory use. Thus, a combination of technologies is recommended. The combination of RO and EDI is the best choice for producing Type 2 water. The ion and TOC levels of RO-EDI water are consistently low, meeting the needs of many general laboratory applications, and for use in clinical analyzers. Also, the consistent quality of RO-EDI water makes it most suitable for producing ultrapure water with a polishing system such as a Milli-Q system. The result is ultrapure water with consistently low levels of ions and TOC. Other sources of pure water, such as distillation, SDI, and even the combination of SDI and distillation have been shown to produce polished water with fluctuating contaminant levels. Such fluctuation compromises the quality of data obtained from modern analytical instruments where trace and ultra trace analyses are possible. Finally, in addition to choosing the best technologies based on a sound understanding of water quality, it is important that laboratory personnel be trained in the proper use and maintenance of the systems, and that appropriate water quality parameters be monitored on a regular basis. Using a system equipped with resistivity and TOC monitoring capabilities allows users to check the quality of water being delivered.
Dr. Estelle Riche has been Senior Scientist in the Application group of the Lab Water business field of Millipore SAS for 9 years. Cecilia Devaux joined Millipore SAS over 13 years ago, and is currently Head of Laboratory and Analytical Group Manager in the R&D of the Lab Water business field. Dr. Stephane Mabic has been Application Manager for the Lab Water business field at Millipore SAS for more than 10 years, and is also Worldwide Training Manager. www.emdmillipore.com
This article appeared in the November/December 2015 issue of Controlled Environments.