In the laboratory, tap water is not pure enough for experimental use. Analytical researchers are commonly concerned with elements and compounds in the parts per billion range. Life science research is often very sensitive to many contaminants, particularly heavy metals and dissolved organics. High performance liquid chromatography (HPLC) requires ultrapure water for calibration of detector base lines and elution of reverse phase columns. HPLC needs water essentially free from dissolved or colloidal ionic and organic contaminants. Atomic absorption spectrometry (AAS) requires pure water that is very low in inorganic, organic or colloidal contaminants. Trace element analysis requires water that is free of the elements being examined.