Laboratory Water Requirements and Grading

Introduction to Laboratory Water Use

In today’s laboratory environment, water, as the fundamental element in the majority of experiments, holds a paramount position. The quality of water often dictates the authenticity and reproducibility of experiment results. For many experts engaged in experiments, the norm is to seek ultra-pure water with impurity elements and compound concentrations in the parts per billion (ppb) range or even lower. High-performance liquid chromatography (HPLC), used for trace analysis, demands ultra-pure water with extremely low organic pollutant content as the mobile phase. Typically, trace analysis requires water that is free of detectable substances, with any potential interfering agents separated from the water. Hence, the selection of a scientifically effective laboratory water system is of utmost importance.

Characterization Parameters of Water Quality

• Inorganic Content – Resistivity/Conductivity
• Organic Content – Total Organic Carbon (TOC)
• Microbial Content – Bacterial Count
• Particle Content – Particle Count (with specified particle size, usually less than 0.2μm)

Methods of Water Purification

• Distillation Method:

  • An ancient and commonly used method to transform drinking water into pure water. The purity increases with the number of distillations, categorized as single, double, or triple distillation. Another less common but higher purity method is sub-boiling distillation.

• Filtration Method:

  • Reverse Osmosis (RO):

    Used to filter out pollutants smaller than 1nm, removing 90% of ion pollutants, most organic pollutants, and almost all particulate pollutants.

  • Ultrafiltration (UF):

    A continuous filtration method with a molecular cutoff of 5000 Daltons, employed for purifying or removing impurities from biological macromolecules. In ultra-pure water systems, it is primarily used to eliminate biomolecules such as nucleases and endotoxins.

  • Microfiltration (MF):

    Filtration with pore sizes between 0.1μm – 5μm, referred to as microfiltration. Applied to remove particles and microbial bodies from pure water. Pore sizes above 5μm are termed pre-filtration, applied at the inlet of pure water systems to remove large impurities from tap water.

• Adsorption Method:

  • Utilizes activated carbon with high porosity to adsorb and remove certain microbes, free chlorine, and other impurities.

• Photooxidation Method:

  • Utilizes 185nm or 254nm ultraviolet light to kill and oxidize microorganisms in water, controlling the Total Organic Carbon (TOC) levels.

• Ion Exchange Method:

  • With the continuous advancement of industrial water production, ion exchange resins also evolve. Three main methods include:

    • Classic Ion Exchange (SDI):

      Anion and cation exchange resins placed separately; professional regeneration is required. Generally produces deionized water with a purity level of approximately 1 MW-cm.

    • Nuclear-Grade Resin Ion Exchange:

      The most efficient method, providing Grade I ultra-pure water with a resistivity of 18.2 MW-cm. The ion exchange resin is a one-time-use, non-regenerable product.

    • Electrodeionization (EDI):

      An upgraded form of ion exchange involving membrane technology and an electric field for instant regeneration.

Laboratory Water Grading

Laboratory pure water is classified into four standard grades:

• Pure Water:

  • Lowest purification level, typically with a conductivity range of 1-50μs/cm. Produced through single weak anion exchange resin, reverse osmosis, or single distillation. Commonly used for glassware cleaning, autoclaves, constant temperature and humidity chambers, and washing machine water.

• Deionized Water:

  • Conductivity usually between 1.0-0.1μs/cm. Produced using mixed-bed ion exchange with strong anion exchange resin. Suitable for cleaning, preparing analytical standard samples, reagent preparation, and diluting samples.

• Laboratory Grade II Water:

  • Conductivity <1.0μs/cm, TOC <50ppb, and bacteria count <1CFU/mL. Applicable for various purposes, from reagent preparation and solution dilution to providing nutrient solutions for cell culture and microbiological research. This type of pure water can be produced through double distillation, or by integrating various technologies such as RO, ion exchange, and UV light.

• Ultra-Pure Water:

  • Approaches theoretical purity limits in terms of resistivity, organic content, particle, and bacteria levels. Produced through pre-purification methods such as ion exchange, RO membrane, or distillation, followed by final purification through nuclear-grade ion exchange. Typically, ultra-pure water has a resistivity of 18.2 MΩ-cm, TOC <10ppb, and removes particles smaller than 0.1μm and bacteria count below 1CFU/mL. Ideal for precise analytical experiments such as HPLC, IC, and ICP-MS. Ultra-filtration techniques are commonly employed for biological applications, like eukaryotic cell culture, to remove large biomolecules, ensuring water purity (<0.005 IU/mL).

Major Water Standards in Laboratories

Internationally recognized standards for pure water include:

• International Organization for Standardization (ISO)
• College of American Pathologists (CAP)
• ASTM International
• Clinical and Laboratory Standards Institute (NCCLS)
• United States Pharmacopeia (USP)

Chinese water standards:

• GB/T 11446 for electronic-grade ultra-pure water
• GB 6682 for laboratory water
• WS/T 574 for clinical laboratory reagent water