Preparation & Testing of Reagent Grade Water

Reagent grade water is used in the laboratories where instruments can measure chemicals in parts-per-trillion, making the chance of any contaminant fouling the testing very high. Reagent grade water needs to be ultrapure; that is, it has to be devoid of anything that can potentially interfere with the lab equipment or reagents (the materials used to perform the tests). Even starting with distilled water is not enough.

Start with the Water System

• Choosing the right tubing for the water to run through is crucial.

  No matter how pure the water starts out, if there are contaminants introduced into the water system, the result will be an unacceptably impure grade of water. An ultrapure water system needs to be made from a single plastic like polypropylene that has been produced without fillers, plasticizers or substances used in manufacturing to make it easier to get the parts out of their molds. In addition, an adsorbtion system (such as running the water through an activated charcoal filter) that will attract and remove gasses and liquid or solid contaminants also should be included in the water system. Utraviolet oxidation can be used in the system to eliminate bacteria and other microorganisms.

Filtering

• A 0.2-micron absolute filter is used as the final stage to purify water in all of these water systems to make it reagent-grade. The challenge in designing or choosing this filter is to make certain that the final filtering itself is not introducing any contaminating substances, which would defeat the entire purpose. Sources of contamination can come from filtration particles (the activated charcoal, for instance) and any wetting agents that are used to make the water flow through the filter more smoothly.

Rinsing the System

• It might seem obvious, but an important step in getting ultrapure water that can be used in the lab is to rinse the water system before using the water for mixing with other chemicals or running any tests. The lab technicians need to do enough testing to determine how long the water needs to be run through the system at the start of the day, whenever a filter or other component needs to be replaced and whenever taking water for a sample. Running water through the system for long enough is the only way to ensure that the water coming out of the tap is as pure as it should be.

Testing

• Two kinds of contaminants need to be tested for: chemicals and microorganisms like bacteria. For chemical contamination, the testing method of choice is using chromatography, which separates the materiel being tested into its component parts. That can be done in a variety of ways: column chromatography mixes the sample with a solvent and runs it through adsorptive materials that different substances will cling to, while others pour through to the next level. Gel-permeation chromatography separates compounds by the size of the molecules. Other methods include gas chromatography (commonly seen on TV shows like CSI) and ion-exchange chromatography.

  For microorganisms, the best method available until the late 1990s was to run the water through an extremely fine filter and culture the residue for a few days to a week, then count the number of bacterial colonies that grow. In 1998 a group of Japanese scientists developed a method using antibodies, attached to an enzyme that produces light in a chemical reaction, to attach to DNA. The method makes live and dead organisms visible because they glow.