What Standards Should a Good Laboratory Pure Water System Brand Meet?

There are many brands of laboratory pure water systems, but which are the best? Laboratory pure water is essential in every stage of the laboratory process and directly determines the quality of the results. What standards are there for evaluating laboratory pure water systems? Different international organizations, such as ASTM, CLSI, and ISO, have different water quality standards, making it difficult to definitively classify laboratory water quality levels. Generally,

we select different water quality levels based on the application of the experiment, and then choose a pure water system that meets the specific water requirements. We typically consider the following factors:

1. Water Quality Monitoring Methods

A high-quality pure water system must include real-time online ion level monitoring (resistivity meter) and continuous online organic contamination monitoring (TOC meter). Traditional pure water systems only have a resistivity meter, which reflects water purity by monitoring the movement of ions between the cathode and anode in the liquid solution. The measurement results are related to ion concentration, charge number, and ion mobility, making it a good parameter for indicating water purity. The theoretical resistivity of ultrapure water at 25℃ is 18.2 MΩ·cm.

However, using only a resistivity reading of 18.2 MΩ·cm (25℃) as an indicator of ultrapure water is far from sufficient. Many organic molecules do not exist in ionic form and therefore do not affect resistivity measurements. The result is that even with a good resistivity reading, organic contamination may still exist in the water. Similarly, detecting a TOC value only indicates the amount of organic matter in the water, not the concentration of anions and cations. Therefore, both must be combined; selecting a pure water system equipped with both resistivity and TOC detectors is essential to ensure higher quality ultrapure water.

2. Microbial Contamination

Microbial contamination is a frequently overlooked problem. Once it occurs, it quickly spreads throughout the pure water system and gradually forms a biofilm, which is difficult to remove. Microorganisms continuously release organic matter and ions into the water, leading to ghost peaks in ion chromatography and high-performance liquid chromatography (HPLC) analyses.

Microorganisms are particulate matter, which can cause increased column back pressure during HPLC analysis and damage the ICP nebulizer during ICP-MS experiments. Microorganisms also release a certain number of macromolecules, such as enzymes (nucleases, alkaline phosphatases, etc.) and endotoxins. The presence of nucleases is highly detrimental to molecular biology experiments involving DNA and RNA, and the alkaline nucleases released by microorganisms can cause false positive results in enzyme immunoassays. Endotoxins can activate cells to produce signal transduction substances and inhibit enzyme activity.

To prevent microbial contamination, we should choose a water tank with no dead volume, a circulating pipeline, a sterile overflow outlet, an air filter, and UV sterilization (such as Millipore's Tank), which can maximally inhibit the growth and proliferation of microorganisms within the system.

3. Ease of Use Ultrapure water must be used immediately after use because highly purified water has a higher affinity for chemicals and is more likely to absorb chemicals from storage containers (such as glassware and test tubes) and chemical vapors compared to ordinary water. Common laboratory polyethylene glass bottles and wash bottles can also release organic and inorganic substances into ultrapure water, contaminating it. Therefore, if experiments require ultrapure water, it's best to choose an ultrapure water system with remote water intake and multiple water outlets. Firstly, this allows for simultaneous satisfaction of different researchers' needs for various water qualities. For example, a Millipak system with a microfiltration membrane at the water outlet can be chosen for general laboratory experiments or chromatography applications, while a Biopak system with pyrogen removal and RNase functions is suitable for cell culture or molecular biology applications. Secondly, multiple remote water outlets offer greater flexibility and convenience compared to traditional methods. For instance, a remote water outlet can be placed inside a clean bench, reducing the chance of contamination and making experiments more convenient.

4. Easy Maintenance

The ion exchange resins, activated carbon, and ultrafiltration membranes used in ultrapure water systems have a limited lifespan. Many people believe that ion exchange resins can be continuously regenerated and recycled, but this is not the case. Expired or regenerated resins release high concentrations of contaminants into the water. Repeated use of regenerated resins can also lead to resin particle breakage, releasing particles and organic matter into the water and accelerating microbial growth. Therefore, the regular replacement of consumables is a crucial matter that must be implemented effectively. The built-in warning function of the instrument is therefore very important; it can remind you to prepare to replace consumables before they run out and the water quality deteriorates completely. Furthermore, when purchasing a water purifier, we should choose one where consumables can be easily and conveniently replaced. Even if an engineer cannot provide on-site service promptly, we won't have to worry about poor water quality delaying our experiments.

5. Ultrapure Water is Not a Panacea

We often wonder if we can use high-quality ultrapure water for all experimental operations. However, in some cases, using expensive ultrapure water is not only unnecessary but can also lead to poor experimental results. This is because trace elements are beneficial and essential in certain biological applications.

A good example is when raising zebrafish for experiments. The water used to raise the fish must absolutely not contain chlorine, ammonia, pesticides, or nitrites, and the fish are very sensitive to changes in the pH of the water. Although the pH of ultrapure water is 7.0, it readily absorbs carbon dioxide from the air, forming carbonic acid in the water and lowering the pH to around 5.6. Furthermore, because ultrapure water removes all metal ions so thoroughly, it also removes trace elements essential for fish growth, which is detrimental to the fish's health and development. Therefore, reverse osmosis water is generally more suitable for zebrafish than ultrapure water.

In other situations, using Grade II or Grade III water is a better choice than ultrapure water. For example, prolonged contact between ultrapure water and metal can gradually corrode the metal probe. Therefore, choosing the right water quality for the specific application is crucial, and we cannot rely solely on a single water purifier for all purposes.