Water grade is dependent on the nature of impurities one is considering. Water impurities are generally minerals, chemicals and organic, and biological. Average city water will contain all these impurities in different proportions. City water supplies and their level of acceptable impurities is highly dependent on where one lives and the nature of local rules and regulations.
Depending on the nature of water use, commonly used terminologies for water grades are molecular grade water, DNase, and RNase, and nuclease free water, DEPC treated water, proteomic grade water, endotoxin free water, distilled water, deionized water, water for injection (WFI), pharma grade water, biotech grade water, biological grade water, cell culture grade water, HLPC grade water, Ultapure grade water.
There are several key factors that determine water quality and their applicable usage.
Conductivity
Conductivity refers to the ease with which water conducts electricity. Measured as micro-Siemens per centimeter (µS/cm) at 25oC, this property indicates the ionic content in the water. This is generally used for less pure water types (raw water to drinking water).
Resistivity
Resistivity is a measure of the difficulty by which electric current travels through a water sample. Generally, water samples with higher resistivity levels have less ionic content and are hence purer. This property is inversely proportional to conductivity. Thus, water with high resistivity will have low conductivity.
Resistivity values are reported as Mega-Ohms per centimeter (MO-cm) at 25oC and are typically used for water types with higher purity levels.
Total Organic Carbon (TOC)
Currently, TOC is considered the most accurate indicator for the presence of organic impurities (microorganisms and other organic materials) in water samples. In measuring TOC, the organic compounds present in the sample are oxidized to forms that can be quantified.
Note: There are various oxidation and detection methods used in quantifying TOC content. Options include subjecting samples to high-temperature combustion or high-temperature catalytic oxidation in an oxygen-rich environment, thermochemical oxidation, photochemical oxidation, or photo-oxidation by ultraviolet light. The most appropriate option is determined by the nature and concentration of the TOC and analytical requirements.
Turbidity
Turbidity refers to the relative clarity of a liquid and is a measurement of the amount of light scattered by suspended particles in the water when a light source is shined through it. Thus, the more turbid the sample, the higher the intensity of scattered light. These colloidal materials are typically less than 0.5 µm in size and may include iron, silica, aluminum, clay, silt, minute organic and inorganic compounds, and other microscopic organisms.
Ultrapure Water (Type I)
Recognized as the purest type of water, ultrapure water is used for the most critical applications and advanced analytical procedures in the laboratory, which include cell and tissue culture preparation, gas, and liquid chromatography, inductively coupled plasma mass spectrometry (ICP-MS), and molecular biology applications.
Ultrapure water has a resistivity of more than 18.2 MΩ-cm at 25°C, conductivity of less than 0.056 µS/cm, and less than 50 ppb TOC.
General Laboratory Grade Water (Type II)
While it doesn’t have the same level of purity as ultrapure water, this type of water can be ideally used as feed water for clinical analyzers as well as for general laboratory practices. Moreover, it can be used for various applications including microbiological analysis and preparation, electrochemistry, general spectrophotometry, FAAS, and as feed for creating Type I water.
Type II water has a resistivity of more than 1 MΩ-cm, conductivity of less than 1 µS/cm, and TOC of less than 50 ppb.
Primary Grade Water (Type III)
Type III water (also known as RO water) may have a lower purity level compared to Types I and II, but it is still good enough for numerous non-critical laboratory applications such as glassware rinsing, media preparation, heating baths, and other secondary tasks. It can also be used as feed water to produce Type I and Type II water.
Produced and purified using reverse osmosis technology, RO water is about 90% to 99% free of contaminants and has a resistivity greater than 4 MΩ-cm, a conductivity of less than 0.25 µS/cm, and less than 200 ppb of TOCs.
Feed Water (Type IV)
Like Type III, feed water is produced through reverse osmosis and is commonly used to create pure water. It has a resistivity of 200KΩ and a conductivity of less than 5 µS/cm.
Other water grades and purity that life sciences, biotech, biopharma, and bioprocess industry refer to are, as follows.
Molecular grade water, DNase, and RNase free water, and DEPC-treated water; these nomenclatures signify that water is suitable for molecular biology applications to protect DNA and RNA from degrading contaminating nucleases.
Endotoxin free water signifies ultrapure water purified to remove bacterial cell was components lipopolysaccharides. Endotoxin free water is suitable for cell culture work as well as for production of pharmaceutical, bioprocessing, testing, and injectable materials.
Proteomic grade water is suitable where water must be free from environmental protein molecules such human skins, household mites, and other proteins.
HPLC grade water is suitable from performing analytical work using HPLC, LC-MS and other highly sensitive analytical tools.
Water for injection (WFI) is ultra-pure water suitable for pharmaceutical and biotech manufacturing, such water may be suitable for injectable if they are qualified for such uses.
Distilled and Deionized water are generally steam distilled and sterile filtered for use as a research laboratory reagent.
Different grades of water have different purity levels, so you really need to distinguish between them to understand their suitable applications in the lab.