Water used in a lab can in general be classified as tap-water, deionized water (dH₂0) or distilled/deionized water (ddH₂0).

1. Tap water is most frequently used for cleaning. For making media and solutions dH₂0 and ddH₂0 are frequently used. Special applications such as HPLC my require higher grade water.

2. Deionized water (dH₂0) is obtained by a filtration system (e.g. Milli-Q (Millipore) ). The system removes ions and organic materials. The purify of the dH₂0 produced is measured by resistivity (which measures residue ion concentrations) and total organic carbon (TOC). A system may have built-in resistivity and TOC monitors. Users should pay attention to these monitors to recognize failure of the deionization system.

3. Filters for the filtration system should be checked regularly and changed if necessary.

4. UV photo-oxidation is available for some systems, which further reduces TOC to produce "HPLC-grade" water.

5. Deionized water (dH₂0) can be further distilled to produce ddH₂0.