Grading Standards for Laboratory Pure Water

Laboratory pure water can be classified into four conventional grades: pure water, deionized water, Class II pure water, and ultrapure water.

I. Pure Water: The lowest level of purification, typically with a conductivity between 1-50 μS/cm. It can be produced using a single weakly basic anion exchange resin, reverse osmosis, or single distillation. Typical applications include glassware cleaning, autoclaving, temperature and humidity control chambers, and water for cleaning machines.

II. Deionized Water: Conductivity is typically between 1.0-0.1 μS/cm. Produced using mixed-bed ion exchange with a strong anion exchange resin, it has relatively high levels of organic matter and bacterial contamination, meeting various needs such as cleaning, preparing analytical standards, preparing reagents, and diluting samples.

III. Class II Pure Water: Conductivity <1.0 μS/cm, total organic carbon (TOC) content less than 50 ppb, and bacterial content less than 1 CFU/ml. Its quality is suitable for a variety of needs, from reagent preparation and solution dilution to preparing nutrient solutions for cell cultures and microbiological research. This type of pure water can be produced through double distillation, or by integrating multiple technologies such as RO and ion exchange/EDI. It can also be combined with adsorption media and UV lamps.

IV. Ultrapure Water: This level of pure water approaches the theoretical purity limit in terms of resistivity, organic matter content, particle size, and bacterial content. It is obtained through pre-purification via ion exchange, RO membranes, or distillation, followed by nuclear-level ion exchange purification. Typically, ultrapure water has a resistivity of up to 18.2 MΩ-cm, TOC <10 ppb, filters out particles of 0.1 μm or even smaller, and has a bacterial content below 1 CFU/ml. Ultrapure water is suitable for various precision analytical experiments, such as high-performance liquid chromatography (HPLC), ion chromatography (IC), and ion capture-mass spectrometry (ICP-MS). Low-pyrogen ultrapure water is suitable for biological applications such as eukaryotic cell culture. Ultrafiltration technology is typically used to remove large molecular bioactive substances, such as pyrogens (resulting in <0.005 IU/ml) and undetectable nucleases and proteases.

Currently, the most widely used pure water standards in the world include: the International Organization for Standardization (ISO), the American College of Clinical Pathology (CAP) standards for pharmaceutical-grade water, the American Society for Testing and Materials (ASTM), the National Committee on Clinical Trial Standards (NCCLS), and the American Pharmaceutical Association (USP). my country also has its own pure water standards: the Chinese National Standard for Electronic-Grade Ultrapure Water GB/T 11446-1997 and the Chinese National Standard for Laboratory Water GB/T 6682-2008.