Water is the most commonly used reagent in the laboratory. Scientists use it to prepare other reagents, dilute samples and standards, and to prepare eluents and buffers. It is, of course, also used as an experimental blank, and for many other everyday laboratory tasks, such as glassware rinsing. As analytical techniques become increasingly sensitive, the potential impact of water quality on experimental results cannot be overlooked.
Here are a few tips to help you obtain the highest water quality from a water purification system and avoid common pitfalls when handling high purity water.
1. Monitor water quality.
Many ultrapure water systems display an ‘18.2’ value, while some also display a ‘TOC’ value. Both values are water quality monitoring outputs that concern the water delivered by the purification system. Resistivity is commonly used to assess the ionic purity of water. If the ultrapure water delivered by your system has a resistivity of 18.2 MOhm.cm, it is free of any ionic impurity. Certain systems also monitor the organic contamination of ultrapure water, and provide a Total Oxidizable Carbon (TOC) value. The TOC of ultrapure water is usually at or below 5 ppb (or µg/L).
2. Flush the system before collecting water.
High purity water can readily absorb chemicals from its surroundings. If a water purification system has been idle overnight or for a few days, volatile molecules from the laboratory air (organic solvents, cleaning products, etc.) may eventually contaminate the water at the system's point-of-use. Therefore, it is good practice to first discard a small volume of water (one or two liters), in order to collect high quality, freshly purified water for use with sensitive experiments.
3. Change the water purification system’s cartridges regularly.
It is not good practice to keep using water purification system cartridges beyond the manufacturer's recommendations – even if the system continues to deliver good quality water. Unfortunately, some purification technologies, such as ion exchange resins, will suddenly and unpredictably release large amounts of impurities when exhausted. This may occur on the day of a crucial experiment, potentially affecting results. Changing cartridges as recommended by the manufacturer will protect you from variations in water quality.
4. Avoid using plastic tubing at the outlet of the system.
Placing a piece of plastic tubing at the outlet of your water system may have several drawbacks. First, plasticizers may leach into the ultrapure water dispensed by the system. In addition, water drops remaining inside the tubing may foster the development of bacteria or algae, and contaminate the next batch of ultrapure water collected. A number of water purification systems have a point-of-delivery device that can easily be adjusted to fit your glassware, thereby avoiding splashes and eliminating the need for additional tubing.
5. Do not store ultrapure water.
When stored, ultrapure water quickly absorbs carbon dioxide from the air, leading to the formation of carbonic acid, carbonate and bicarbonate. In addition, bacteria and algae may develop in stagnant water, especially near a window or a source of heat. Bacteria may release byproducts such as nucleases, endotoxins and other organics, which can have an impact on experiments. Storage containers may also contaminate the water, especially large carboys, whose inner walls can be difficult to clean. Whenever possible, it is best to draw freshly purified water from the system just before use.
6. Select the proper container.
When preparing eluents or reagents for analyses, you may need to collect ultrapure water in a container. In these situations, you should choose the container very carefully. Borosilicate bottles are preferred for organic analyses (e.g. HPLC), since they will not release any organic molecules into the water. For ionic analyses, polyethylene or polypropylene containers are often recommended, as they release fewer ions than glass; while for ICP-MS, fluoropolymers such as PFA are best. Capping the container to limit direct contact with laboratory air, particles and bacteria is also recommended.
7. Make sure to clean containers properly.
The containers used to collect high purity water should be very clean to avoid contaminating the water. The cleaning procedure depends on the type of analyses being performed (diluted acid, acetone, etc.), but in all cases it is very important to rinse the containers with ultrapure water several times. For trace analyses, avoid both detergents and mixing glassware used for these experiments with the glassware used for other types of work in the lab. Also, don’t forget to clean and rinse the caps with the same care as the containers.
8. Do not generate bubbles when dispensing water.
When dispensing ultrapure water into a container for trace analyses, it is important to avoid generating air bubbles. Increasing the surface of contact between the air in the laboratory and the ultrapure water will increase the risk of introducing airborne contaminants into the water. These may be volatile organics, or inorganics such as ammonia or chloride, which are ubiquitous in laboratories. It is best to tilt the container, so that the stream of water slides along the side wall of the flask, instead of flowing straight to the bottom and generating bubbles.
9. Watch out for extractables and other contaminants.
When performing trace or ultra-trace analyses, everyday laboratory items can become a source of contamination. For example, gloves, paraffin film, marker pens, and even new furniture or floor cleaning products may release contaminants, affecting water quality.
10. Location, location, location.
If you perform trace analyses, you can help prevent contamination by making sure you place your source of ultrapure water in a clean part of the laboratory – away from heavy foot traffic, areas where many chemicals are used, and from windows and air conditioning vents. On the other hand, you may wish to place your source of pure water close to your laboratory’s washing area, or the weighing station where you prepare common reagents. Modern water purification systems give you the flexibility to place the pretreatment system (delivering pure water) and the polishing system (delivering ultrapure water) in different locations, for greater efficiency.
To learn more practical tips about your lab’s water purification system watch this informative webinar.
This article was written by Estelle Riché and Stephane Mabic, Lab Water, Millipore S.A.S., France.