Deionized vs. Distilled Water
By Gregory Salata, Ph.D., Kelso, WA
Many sampling programs include collection and analysis of an equipment blank to ensure there is no contribution of contaminants from the sampling equipment and associated process. To establish that sample collection procedures are contaminant free, an equipment blank is often collected. Equipment blanks are collected by passing water through the sample collection apparatus or utensil and collecting the water into the appropriate containers. To ensure that the water itself is contaminant free, the laboratory will supply the field crew with deionized (DI) water.
Also known as demineralized water, DI water is prepared by passing a water source through specially-manufactured ion exchange resins, which bind to and remove minerals (i.e. salts). The minerals include both cations and anions including (but not limited to) sodium, calcium, iron, copper, chloride, sulfate, and bromide. Ion-exchange systems are commonly used as residential water softeners, normally as cation exchange systems to remove hardness. More sophisticated systems, such as the one used at Columbia Analytical Services, incorporates a reverse osmosis membrane, a carbon pre-treatment cartridge to remove organic analytes, and a UV post treatment to kill bacteria. DI water is dispensed throughout the laboratory through a segregated plumbing system to specially marked faucets.
In instances where no DI water is available, commercially available distilled water may be used as a substitute. Distilled water is produced by boiling the water and then condensing the steam, leaving residual material (salts, etc.) behind. Because most commercially available distilled water is stored in plastic containers, care must be taken if the water is to be used for semi-volatile and/or volatile organic analysis due to phthalate contamination or elevation of volatile organics due to diffusion through the plastic.
Tags: anions, bacteria, bromide, calcium, carbon, cations, Chloride, contaminant, contamination, copper, deionized, demineralized, DI, distilled, equipment blank, ion exchange, iron, membrane, minerals, organic, osmosis, phthalate, semi-volatile, sodium, sulfate, UV, volatile, water, water softeners