Different types of purified water required in the laboratory: What you need to know

Deciding which type of water you need to meet your application can be challenging. Xylem can help.

According to the specific situation you are dealing with on the experimental bench, there are various types of water purity, ranging from Type III used for general purposes (such as rinsing beakers) to Type I+suitable for sensitive applications such as graphite furnace atomic absorption spectroscopy (GF-AAS). Deciding which type of water you need to meet your application can be challenging.

How to evaluate and define the purity of laboratory water quality?

In order to achieve a coherent water quality classification system, we utilized key factors that describe various properties of water.

electrical conductivity of water

The conductivity report is measured in microsiemens per centimeter (μ S/cm) at 25oC and is the reciprocal of the resistivity, providing a measure of fluid conductivity. Conductivity is commonly used to evaluate the water quality from "raw water" to "drinking water", providing valuable and non-specific water quality indicators that reflect the ion content in the water.

The electrical resistivity of water

At 25 o'clock, it is reported as megaohms per centimeter (M Ω - cm) oC, and the resistivity is related to conductivity: high resistivity equals low conductivity. Therefore, it can also measure the ion content of water. Unlike conductivity, resistivity is mainly used to evaluate ultrapure water.

Organic compound content in water

Organic compounds can exist in multiple forms in water, so measuring each one individually is impractical. The indicator used is the total organic carbon (TOC) content of the solution. Measure by oxidizing existing organic compounds and quantifying the resulting oxidation products. TOC is an organic impurity presence indicator that we can achieve close to a "universal indicator".

In addition, chromatographic techniques can be used to determine the specific situation of organic content, but this is often considered costly and time-consuming, and is not suitable for general monitoring workflows.

Biological pollution of water

The presence of biological pollutants such as bacteria and other microorganisms in untreated water is a common problem. The bacterial level reported in colony forming units per milliliter (CFU/ml) is maintained at a low level through filtration, UV treatment, and sterilization solution. After incubation in a suitable culture medium, the individual bacterial species and total number of surviving cells can be determined. The number of bacteria can also be monitored through fluorescence detection to quickly detect and distinguish between dead and live bacteria.

In addition to the bacteria themselves, endotoxins produced by the cell walls of Gram negative microorganisms (endotoxin units reported per milliliter, EU/ml; 1 EU/ml (approximately equal to 0.1 ng/ml) can also be evaluated using standard assays based on Limulus cell lysis activity.

The presence of colloids in laboratory water

Suspended particles can cause water turbidity (measured as Nephelometric Turbidity Units, NTU), so it is recommended to filter them out of laboratory water as much as possible. This colloidal material has a size of less than 0.5 microns and may contain iron, silicon, aluminum, or organic materials. The fouling index (FI) is commonly used to estimate the potential of water blocking filters under 0.45 micron filtration conditions.

Establish pure water quality standards

Multiple international committees have been working to establish consistency in water purity standards - the more people agree with these standards, the easier it is to generate reproducible data. Some laboratories may also adopt standards set by regulatory agencies in their regulatory regions, such as pharmacopoeias in Europe, the United States, or Japan. However, these standards rarely have specific applications for specific applications.

Clinical and Laboratory Standards Institute (CLSI) - formerly known as NCCLS

As of 2006, CLSI had abandoned the typical Class I, II, and III classifications and instead recommended that water should only be "suitable for its intended use" and only have a detailed description of one grade: clinical reagent laboratory water. CLSI also briefly introduced other grades, such as Special Reagent Water (SRW) and Instrument Water Supply.

International Organization for Standardization (ISO)

The ISO standard is based on ISO 3696:1987, which specifies three levels of water: Level 1, Level 2, and Level 3, with Level 1 being the purest water quality (see below):

ISO grade water quality parameters

American Society for Testing and Materials (ASTM)

ASTM adopts the D1193-06 standard and has four grades of water (see table below):

Choose the appropriate type of pure water quality

Knowing what type of water supply system you need can be overwhelming. We have local sales representatives who can help you choose the purification system that suits your needs. Welcome to contact us for more information.